Outcrop-based reservoir
characterization of
a kilometer-scale
sand-injectite complex
AUTHORS
Anthony Scott University of Aberdeen,
Department of Geology and Petroleum Geology, King’s College, Aberdeen, United Kingdom;
present address: Statoil, Reservoir Modeling,
Sandslihaugen 30, Bergen, Norway;
[email protected]
Anthony Scott, Andrew Hurst, and Mario Vigorito
ABSTRACT
This study documents that Danian-aged sand remobilization
of deep-water slope-channel complexes and intrusion of fluidized sand into hydraulically fractured slope mudstones of the
Great Valley sequence, California, generated 400-m (1312 ft)–
thick reservoir units: unit 1, parent unit channel complexes for
shallower sandstone intrusions; unit 2, a moderate net-to-gross
interval (0.19 sand) of sills with staggered, stepped, and multilayer geometries with well-developed lateral sandstone-body
connectivity; unit 3, a low net-to-gross interval (0.08 sand) of
exclusively high-angle dikes with good vertical connectivity; and
unit 4, an interval of extrusive sandstone. Unit 2 was formed
during a phase of fluidization that emplaced on an average
0.19 km3 (0.046 mi3) of sand per cubic kilometer of host sediment. Probe permeametry data reveal a positive relationship
between sill thickness and permeability. Reservoir quality is
reduced by the presence of fragments of host strata, such as the
incorporation of large rafts of mudstone, which are formed by
in-situ hydraulic fracturing during sand injection. Mudstone
clasts and clay- and silt-size particles generated by intrusioninduced abrasion of the host strata reduce sandstone permeability in multilayer sills (70 md) when compared to that in
staggered and stepped sills (586 and 1225 md, respectively).
Post-injection cementation greatly reduces permeability in
high-angle dikes (81 md). This architecturally based reservoir
zonation and trends in reservoir characteristics in dikes and sills
form a basis for quantitative reservoir modeling and can be
used to support conceptual interpretations that infer injectite
Copyright ©2013. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received December 9, 2011; provisional acceptance February 28, 2012; revised manuscript
received April 3, 2012; final acceptance May 14, 2012.
DOI:10.1306/05141211184
AAPG Bulletin, v. 97, no. 2 (February 2013), pp. 309–343
309
Anthony Scott currently holds a position as a
senior sedimentologist in the Reservoir Characterization and Geomodeling Group at Statoil
(2009–2012). He obtained a B.Sc. degree from
the University of Durham and an M.Sc. degree
and a Ph.D. from the University of Aberdeen.
His current research interests are in the evolution of fluid-flow phenomena, sediment remobilization of deep-water depositional systems,
and large-scale clastic intrusions.
Andrew Hurst University of Aberdeen,
Department of Geology and Petroleum Geology, King’s College, Aberdeen, United Kingdom;
[email protected]
Andrew Hurst is professor of production geoscience in the Department of Geology and Petroleum Geology, University of Aberdeen, a post he
has held since 1992. Previously, he worked for
Statoil and Unocal in a 13-yr industry career.
Sand injectites have been a major area of his
research since 1998, and he leads the Aberdeenbased Sand Injectites Research Group. He was
coeditor of AAPG Memoir 87 (2007), Relevance
of Sand Injectites to Hydrocarbon Exploration
and Production.
Mario Vigorito University of Aberdeen,
Department of Geology and Petroleum Geology, King’s College, Aberdeen, United Kingdom;
present address: Statoil, Exploration, Forus Vest,
Stavanger, Norway; [email protected]
Mario Vigorito joined Statoil in 2009 and, since
then, has worked as a sedimentologist in the
exploration on the Norwegian continental shelf
(North Sea Basin). From 2005 to 2009, he held a
position as a lecturer in geology and petroleum
geology at the University of Aberdeen and previously worked as a surveyor for the Italian
Geological Survey and as a consulting sedimentologist for oil companies. He has work and
research experience in a variety of oil provinces
(North Sea [United Kingdom] and Norwegian Sea
[Norway], Sacramento and San Joaquin basins
[California], and western Mediterranean), basins,
and tectonic settings, with a strong focus on
carbonate, siliciclastic, and unconventional plays
(e.g., sand intrusions and fractured basement)
and basin-flow analysis. He obtained an M.Sc.
degree and a Ph.D. in carbonate sedimentology
from Università degli Studi di Napoli “Federico II,”
Italy.
ACKNOWLEDGEMENTS
This study was conducted as part of the Sand
Injectites Phase 2 at the University of Aberdeen,
funded by Department of Energy and Climate
Change (DECC) (formerly the UK Department of
Trade and Industry), Dong, Lundin, Marathon,
Statoil, and Total. We thank our colleagues in the
Injected Sands Group for their support. The
interpretations presented here are solely those
of the authors. We thank Ian D. Bryant and
two anonymous reviewers, and AAPG Editor,
Stephen E. Laubach, for the peer-review comments that enhanced the manuscript. We also
thank Frances P. Whitehurst for editing the final
manuscript. We would like to thank Statoil U.K.
and, in particular, Mariner Petroleum Technology, for financial support in the publication of
this manuscript.
The AAPG Editor thanks the following reviewers
for their work on this paper: Ian D. Bryant
and two anonymous reviewers.
DATASHARE 46
Appendices 1–16 are accessible in electronic
version on the AAPG Website (www.aapg.org
/datashare) as Datashare 46.
310
architecture in situations where sands in low net-to-gross intervals are anticipated to have well-developed lateral and
vertical connectivity.
INTRODUCTION
When translating static geologic models into dynamic reservoirengineering models that are designed to evaluate recovery
mechanism and field development strategy, geologically defined flow units are required, and their spatial distribution is
mapped. This is particularly relevant to sand-injectite reservoirs because they are typified by complex geometries (Duranti
and Hurst, 2004; Hamberg et al., 2007) and architectural hierarchies (Vigorito et al., 2008; Hurst et al., 2011) and are more
permeable than the strata into which they were emplaced
(Duranti et al., 2002; Scott et al., 2009). They form by sand
remobilization and injection of fluidized sand from an overpressured parent unit (e.g., channels that sourced the fluidized
sand) into a hydraulically fractured seal lithology (e.g., mudstone) (Cosgrove, 2001; Jolly and Lonergan, 2002; Cartwright,
2010), thereby creating a network of sills and high- and lowangle dikes (Taylor, 1982; Hurst et al., 2011). These networks
form conduits for aqueous and hydrocarbon fluids (Jenkins,
1930; Jonk et al., 2003, 2005a, b; Mazzini et al., 2003a, b) that
focus flow between reservoirs separated by hundred-meter–
thick low-permeability seal lithology (Huuse et al., 2005).
Sandstone intrusions may pose a risk to the seal capacity
in strata that overlie conventional traps (Hurst et al., 2003a;
Cartwright et al., 2007; Hurst and Cartwright, 2007), but their
mapping (Huuse et al., 2005; Frey-Martínez et al., 2007) and
the definition of a new trapping style, termed “intrusive traps”
(Hurst et al., 2005), led to the successful deliberate exploration
of sand injectites for hydrocarbons (de Boer et al., 2007).
Sandstone intrusions commonly form large-volume reservoirs
and contain substantial reserves of hydrocarbon in many oil
fields (Table 1). They may contain millions of cubic meters of
sandstone (5 × 104 m3 [1.77 × 106 ft3], Hurst et al., 2003a;
50–60 × 106 m3 [17.66–21.19 × 108 ft3] intrusion–1, Huuse
and Mickelson, 2004), crosscut hundreds of meters of stratigraphic section (Jolly et al., 1998; Vigorito et al., 2008), and
are regionally developed (Jackson, 2007).
Sandstone intrusions can cause unexpected (in the context
of stratiform depositional reservoirs) fluid-flow effects during
production—for example, deleterious effects such as unpredicted, early water breakthrough (Briedis et al., 2007) and
helpful effects such as fieldwide enhanced vertical permeability
Reservoir Characterization of a Sand-Injectite Complex
Table 1. Examples of Fields where Hydrocarbons are Reservoired in Post-Depositional Remobilized Sandstone and Sandstone Injectite Complexes*
Field/Structure
Reservoir Interval and Age
Depositional Environment
Reservoir Style and Scale of Intrusion
Harding (UK)
Mid-Eocene
Gryphon (UK)
Balder Formation (early Eocene)
Chestnut (UK)
Alba/Chestnut sequence
(upper Eocene)
Sele, Balder, Horda formations
(early Eocene)
Lista, Hermod, Balder formations Deep-water basin-floor setting Low-angle dikes at channel margins;
(Paleocene–early Eocene)
crestal intrusions; conical intrusions
Våle, Lista, Balder formations
Deep-water slope setting
Low-angle dikes at margins of remobilized
(late Paleocene–early Eocene)
sandstone; intrachalk sands; fault-zone
injectites
Alba Formation (upper Eocene) Deep-water slope setting
Low-angle dikes at channel margins;
crestal intrusions
Volund (Norway)
Balder (Norway)
Nini area (Denmark)
Alba (UK)
Sleipner Øst (Norway) Ty Formation (lower Paleocene)
Danica (UK)
Sele, Balder, Horda formations
(early Eocene)
Jotun (Norway)
Heimdal Formation (Paleocene)
Hamsun (Norway)
Scott et al.
Cecile (Denmark)
Sele, Balder formations
(early Eocene)
Våle Formation (late
Paleocene–early Eocene)
Deep-water basin-floor setting Intrareservoir shales crosscut by intrusions;
crestal intrusions
Deep-water slope setting
Intrareservoir shales crosscut by intrusions;
crestal intrusions
Deep-water slope and
Remobilized depositional sandstones; conical
basin-floor setting
intrusions in reservoir underburden
Deep-water slope setting
Saucer-shape sandstone intrusion
Deep-water basin-floor setting Intrareservoir shales crosscut by intrusions
Deep-water slope setting
Low-angle dikes at channel margins;
crestal intrusions
Deep-water basin-floor setting Intrareservoir shales crosscut by intrusions;
crestal intrusions
Deep-water slope and
Sandstone-injectite complex of low-and
basin-floor setting
high-angle dikes and sills
Deep-water slope setting
Dome-shape sandstone injection; crestal
intrusions
*Reserve estimates are compiled from Hurst et al. (2005, table 1) and may not represent present-day estimates.
**BOE = barrels of oil equivalent; NA = not applicable.
Reserves
(× 106 BOE**)
280
Key References
25
Dixon et al. (1995);
Bergslien (2002)
Purvis et al. (2002);
Lonergan et al. (2007)
Huuse et al. (2005)
NA**
Szarawarska et al. (2010)
186
Briedis et al. (2007);
Wild and Briedis (2010)
Svendsen et al. (2010)
117
NA
460
NA
NA
Duranti and Hurst (2004);
Fretwell et al. (2007);
Huuse et al. (2007)
Satur and Hurst (2007)
Szarawarska et al. (2010)
203
Guargena et al. (2007)
NA
De Boer et al. (2007)
NA
Hamberg et al. (2007)
311
and pressure communication in a field with regionally developed intra- and interreservoir mudstones (Hurst et al., 2007; Satur and Hurst, 2007).
Therefore, incorporating their sedimentologic and
petrophysical properties into 3-D reservoir models
is important (Briedis et al., 2007). This process has
been inhibited by the lack of outcrop analog data,
which limits accurate subsurface reservoir characterization and geologic modeling (Purvis et al.,
2002; Briedis et al., 2007); in particular, thickness,
detailed architecture, textures, and porosity and permeability (e.g., vertical-to-horizontal ratios; Hurst
and Buller, 1984; Duranti et al., 2002) are poorly
constrained. As a consequence, spurious reservoir
models may be generated, which in turn leads to
ambiguous and inaccurate reserve estimates and
inappropriate field developments.
Here, we provide an outcrop-based reservoir
characterization of the kilometer-scale Panoche giant
injection complex (Figure 1A), which extends over
an area of approximately 400 km2 (154 mi2) in the
San Joaquin Basin, central California. Reservoir architecture and characteristics are investigated so
that an enhanced appreciation is possible of their
significance in numerical geologic models. The focus of this article is on the relationship between
sandstone intrusion distribution, sandstone-body
connectivity, architecture, microtexture, and porosity and permeability. This study provides a comprehensive database whereby readers can access data
on reservoir architecture (geometry, lithologic character, and net to gross [N/G]) and characteristics
(qualitative and quantitative petrography, and porosity and permeability) of sandstone intrusions and
remobilized sandstones (depositional units modified by sand fluidization) and locate this within
the injectite complex and regional stratigraphy
(Appendices 1–16; see AAPG Datashare 46 at
www.aapg.org/datashare).
GEOLOGIC SETTING
Reservoir-scale sand-injectite outcrops including
the Panoche giant injection complex (PGIC) are
reported from California in different stratigraphic
312
Reservoir Characterization of a Sand-Injectite Complex
sections in the northern San Francisco area, Sacramento Basin, and San Joaquin Basin (Thompson
et al., 1999, 2007; Huuse et al., 2004; Vigorito et al.,
2008; Scott et al., 2009). Basin-scale overpressure
linked to eastward subduction has been measured
in oil and gas fields in both the San Joaquin and
Sacramento basins (Berry, 1973; McPherson and
Garven, 1999). The occurrence of sand injectites
within these basins may reflect tectonically induced
basin-scale fluid overpressure. The PGIC is found
within the upper Mesozoic strata of the Great Valley sequence (GVS). The GVS filled the forearc
basin from the east during the Cretaceous by erosion of abruptly exhumed plutonic rocks in the
southern Sierra Nevada magmatic arc from the
early to mid-Maastrichtian, and from the late to
the end of the Maastrichtian, sediment was derived
from the west from exhumation of the Franciscan
complex (Dickinson, 1976; Ingersoll, 1979, 1983;
DeGraff-Surpless et al., 2002; Mitchell et al., 2010).
These petrotectonic provinces formed concurrently
during Pacific plate subduction beginning in the
Late Jurassic until the transformation of the subduction zone into a transform margin during the late
Cenozoic (Atwater and Molnar, 1973; Dickinson
and Seely, 1979).
The PGIC extends across the Panoche and Moreno formations of the GVS (Figure 1B). The Panoche Formation is a sandstone-rich succession that
evolves from submarine turbidite-fan deposits to
amalgamated channelized-fan complexes in its uppermost part (Ingersoll, 1979, 1983). In the uppermost parts, the sandstone units exhibit evidence
of channeling (50–100 m [164–328 ft] thick and
500–2500 m [1640–8202 ft] long), suggesting deposition in a proximal fan environment (Ingersoll,
1979). The Moreno Formation unconformably overlies the Panoche Formation and consists of four
members. The Dosados Member is an approximately 60-m (197-ft)–thick sequence of isolated
base-of-slope channel deposits (10–40 m [33–131 ft]
thick and 100–200 m [328–656 ft] wide) crosscut by
sandstone dikes. The Tierra Loma Shale Member is
an approximately 350-m (1148-ft)–thick sequence
of organic-rich, dark-brown mudstone deposited
on the lower to middle slope under suboxic conditions (McGuire, 1988), and the overlying Marca
Figure 1. Regional petrotectonic provinces, lithostratigraphy, and architecture of the Panoche giant injectite complex (PGIC; Vigorito et al.,
2008). (A) Petrotectonic provinces and major faults cropping out in central California (modified from Ingersoll, 1979; Nilsen and Dibblee,
1981). The PGIC crops out on the eastern flank of a monocline of the Great Valley sequence along the western margin of the Central Valley of
California. Faults: H = Hayward; SA = San Andreas. (B) Lithostratigraphy and architecture of the northern PGIC (modified from Vigorito and
Hurst, 2010). (C) Geologic map of the Panoche Hills (modified from Bartow, 1996). Study sites are located with boxes. Locations of logged
sections (in Figure 3) are marked. See Appendices 1 to 10 for text, maps, and stratigraphic sections of the study areas. MG = Moreno Gulch;
NoC = No Name Canyon; Mc = Marca Canyon; Ca = Capita Canyon; Ro = Rosseta Canyon; Do = Dosados Canyon; Es = Escapardo Canyon;
RA = Right Angle Canyon; WT = West Tumey; Fm = Formation; Mbr = Member; Cima Sst. L. = Cima Sandstone lentil; En. = environment;
Res. = reservoir unit.
Shale Member is an approximately 100-m (328-ft)–
thick pale-gray diatomaceous mudstone deposited
on the upper slope during times of upwelling during
sea level highstands, the base of which is a flooding
surface. At the top of the sequence is the Dos Palos
Shale Member, which is an approximately 175-m
Scott et al.
313
Figure 2. Outcrop localities, reservoir units, and
sample sites (see Table 2
for sample characteristics).
Reservoir units (1–4) of the
northern and transitional
PGIC are marked. (A) No
Name Canyon (NoC). Here,
a stepped sill measuring
more than 6 m (20 ft) thick
with a multilayered lower
margin interbedded with
slope turbidites extends
more than 440 m (1444 ft)
laterally and connects to
remobilized slope-channel
sandstones located approximately 200 m (656 ft) below
by 0.5- to 1.0-m (1.6–3.3 ft)–
wide thickness sandstone
dikes. (B) Marca Canyon
(Mc). Multilayer sills with
individual thickness of
0.01 to 0.3 m (0.033–0.98 ft)
are found along the same
stratigraphic horizon as
thin-bedded upper slope
siltstones in unit 2. (C)
Capita Canyon (Ca). A highangle dike crosscuts more
than 500 m (1640 ft) of
stratigraphic section in
unit 1, thereby connecting
proximal-fan channelized
sandstone positioned at
different stratigraphic levels. (D) Dosados Canyon
(Do). A low-angle dike
(5–12 m [16–39 ft] thick)
crosscuts units 2 and 3.
(E) Escapardo Canyon (Es).
A stepped sill approximately
1.0 to 2.0 m (3.3–6.6 ft)
thick extends for 180 m
(591 ft) and connects to
remobilized isolated slopechannel sandstones in the
underlying Dosados Member by high-angle dikes. See
Appendix 11 (AAPG Datashare 46, www.aapg.org
/datashare) for raw data.
PUB = parent unit belt;
IB =intrusive belt; SEB =
extrusive belt.
314
Reservoir Characterization of a Sand-Injectite Complex
Figure 2. Continued.
(574-ft)–thick mudstone with minor siltstones deposited on the upper slope to the shelf edge and
intercalated with the Cima Sandstone Lentil sandstones that consist of sandstone extrudites and cold-
seep carbonates (Schwartz et al., 2003; Minisini and
Schwartz, 2006; Vigorito et al., 2008).
Overall, the stratigraphic section from the
lowermost Panoche Formation to the uppermost
Scott et al.
315
316
Reservoir Characterization of a Sand-Injectite Complex
Moreno Formation records a shoaling from a basinfloor setting in the middle Maastrichtian to a slope
and shelf setting in the Danian (Anderson and Pack,
1915; McGuire, 1988). The occurrence of genetically related sand extrudites dates injection to the
Danian (65–63 Ma) (Vigorito et al., 2008).
METHOD
The methods of outcrop and laboratory data acquisition were similar for all studied localities and
samples (Appendix 1; see AAPG Datashare 46
[www.aapg.org/datashare]). Geographic and stratigraphic positions of sandstones were located using
high-resolution satellite images followed by field
observations (Appendices 2–10; see AAPG Datashare 46 [www.aapg.org/datashare]). Sandstonebody geometry was mapped, photographed, and
logged. The stratigraphic intervals were identified
by logging vertical sections along canyons including
Moreno Gulch (MG), No Name Canyon (NoC),
Capita Canyon (Ca), Rossetta Canyon (Ro), and
Right Angle Canyon (RA) (Figure 1C). From these
sections, the net-to-gross (N/G) ratio and proportion
of parent sandstone units, sandstone intrusions, and
sandstone extrudites were calculated (Appendix 11).
Estimates of the N/G of high-angle dikes were made
using image-analysis software (Appendix 12). Detailed sampling of architectural elements was conducted at outcrops including Marca Canyon (Mc),
Dosados Canyon (Do), and Escapardo Canyon (Es).
Where samples of sandstone intrusion were col-
lected, their positions and orientations were recorded
and were then air dried for 24 hr to remove formation fluids.
Permeability data were acquired using a steadystate probe permeameter (Halvorsen and Hurst,
1990; Sutherland et al., 1993; Hurst and Goggin,
1995; Hurst et al., 1995) to determine the permeability of a small volume of sandstone intrusion and
compare this with its petrographic microtexture
(Hurst and Rosvoll, 1991; Antonellini and Aydin,
1994; Appendices 13–14). Oriented samples were
taken from the same locations as probe permeameter measurements. Samples selected for thin
sections were impregnated with blue araldite dye
to highlight porosity and were then mounted on
glass slides measuring 75 × 23 to 46 mm (3.0 ×
0.91–1.8 in.) wide, ground to a thickness of approximately 30 mm, and prepared without glass
cover slips. Mineralogy, grain- and pore-size distribution (Appendix 15), and visual (Øv) and minuscement (Øm) porosity estimates were determined by
point counting (∼200 counts per thin section) and
were compared with sill thickness (Appendix 16).
Here, visual porosity is a measure of the void spaces
highlighted by blue dye in thin section and is a
fraction of the point-counted volume of blue dye
over the total volume as a percentage between 0
and 100%, and minus-cement porosity is the percent of both void space (blue dye) and volume of
cements and is determined by the ratio of pointcount totals for void space + carbonate cement +
quartz cement to the total point counts. Selected
thin sections were polished and carbon coated. An
Oxford Instruments Limited cathodoluminescence
Figure 3. The lithostratigraphy, thickness, net-to-gross (N/G) ratio, and proportions of architectural elements in reservoir units (1–4) of
the northern and transitional Panoche giant injectite complex. Because the base of the Marca Shale Member is a flooding surface, it has
been used as a datum for correlating logged sections. Each unit has a different proportion of undeformed depositional units, remobilized
parent sandstone units, sandstone sills, low-angle dikes, sandstone extrudites, and host mudstones: Unit 1 is characterized by remobilized
depositional sandstones in the Panoche Formation and Dosados Member, which acted as parent units that source fluids and sand to the
overlying intrusive complex (Vigorito et al., 2008). Unit 1 is characterized by high N/G ratios; unit 2 is dominated by sandstone sills (4–
30%) and low-angle dikes (<1–17%) in the Dosados and Tierra Loma members and has high N/G ratios (0.04–0.30 sandstone); unit 3 is
dominated exclusively by high-angle dikes in the upper Tierra Loma Shale Member, Marca Shale Member, and the Dos Palos Shale Member
and has consistently low N/G ratios (<0.01–0.05, see Appendix 12 for raw data); and unit 4 is defined by extrusive sandstone associated with
carbonate cold-seep communities in the Dos Palos Shale Member (Schwartz et al., 2003). MG = Moreno Gulch; NoC = No Name Canyon;
Mc = Marca Canyon; Ca = Capita Canyon; Ro = Rosseta Canyon; Do = Dosados Canyon; Es = Escapardo Canyon; RA = Right Angle Canyon;
WT = West Tumey; PUB = parent unit belt; IB = intrusive belt; EB = extrusive belt; LAD = low-angle dikes; MLS = multilayer sill; STS =
staggered sill; SLT = thin-bedded slope turbidites; STP = stepped sill; RMB = remobilized slope channel; Fm = Formation; Mbr = Member.
Scott et al.
317
318
Reservoir Characterization of a Sand-Injectite Complex
detector on a LINK Analytical AN101055S electrondispersive system was used to determine the chemical grain mineralogy and morphology, pore shape
and fill, cement composition, and microtextures
under backscattered-electron detector mode.
Permeability maps were constructed by measuring permeability on samples collected along and
across intrusions on orthogonal grids. A preferred
orientation to gridding was applied by weighing data
points located along one axis with respect to those
located on another axis to ensure that no gridding
in the surrounding low-permeability host strata exists. Gridding was preferentially applied along the
vertical and horizontal axes in high-angle dikes and
sills, respectively. Smoothing (spline) functions were
used to remove angular contours and to eliminate
noise.
equivalent to EB. In log section, the upper contact
of unit 1 is defined by the top of the shallowest
parent sandstone unit encountered in the PUBb. In
reality, remobilized units may occur shallower in
the section but were simply not encountered in the
line of the logged sections. The lower contact of
unit 1 is not defined because evidence of remobilization and injection in the Panoche Formation
occurs more than 500 m (1640 ft) below the base
of the logged sections. The upper contact of unit 2
is defined by the top of the shallowest sandstone
sill encountered in the IBa. The upper contact of
unit 3 is defined by the first occurrence of the
sandstone extrudite and is equivalent to the upper
contact of the IBb. The upper contact of unit 4 is
defined by the upper contact of the sandstone extrudite that is interbedded with the Cima Sandstone Lentil unit and coincides with the upper contact of the EB.
RESERVOIR ARCHITECTURE
Unit 1
A genetically linked parent unit belt (PUB), intrusive belt (IB), and extrusive belt (EB) is present
throughout the PGIC (Figure 1B; Vigorito and
Hurst, 2010). The PGIC is divided into three geographical sectors: the northern (MG to RA), transitional (RA to West Tumey [WT]), and southern
(Tumey Gulch) sectors (Figure 1C). Based on the
PGIC lithostratigraphy of Vigorito and Hurst and
on logged sections measured along several canyon
sections in the northern and transitional sectors
(Figure 2), we define the architecture and quantify
the reservoir characteristics of four units (Figure 3):
unit 1 equivalent to PUBa and PUBb, unit 2 equivalent to IBa, unit 3 equivalent to IBb, and unit 4
This unit contains depositional sandstones of the
upper Panoche Formation and the lower Moreno
Formation (Dosados Member) (Figures 2, 3). Depositional sandstones are locally deformed in the
upper Panoche Formation and are more intensely
deformed in the Dosados Member and, together
with low-angle dikes, they constitute the dominant
proportion of sandstone. In the upper Panoche Formation, high-angle dikes are only occasionally present (Figure 4A), where they crosscut stratigraphy,
thereby connecting depositional sandstones that
were originally separated by mudstones. The contact between dikes and depositional sandstones is
Figure 4. The external geometry and internal sedimentary structure of remobilized depositional sandstones and sandstone intrusions
of unit 1; lithologies represented in the photographs are mudstone (dark brown) and sandstone (light brown). (A) A high-angle dike
approximately 600 m (197 ft) long crosscuts three vertically staked sandstone units separated by mudstone within the upper Panoche
Formation: (i) the dike (dashed black line) connects depositional sandstones separated by thick (30 m [98 ft]) host mudstones at different
stratigraphic levels; (ii) line-drawing interpretation of (i). The dike splits and rejoins along its course. Upper Panoche Formation, Capita
Canyon (Ca). (B) A closer view of a 1.0- to 2.0-m (3.3–6.6-ft)–thick high-angle dike from Capita Canyon. (C) Detail of the sharp, planar
dike margin (dashed white line). Fluid-escape structures occur at the margin. (D) Remobilized sandstones in unit 1: (i) sandstone bodies
displaying winglike geometries at their lateral margins (white arrows) separated by a raft of mudstone (gray arrow); (ii) line drawing of
(i). High-angle dikes (black arrows) are connected to the upper margins of the parent unit, resulting in very good vertical connectivity.
Dosados Member, Moreno Gulch (MG). (E) Internal structure of remobilized depositional sandstones: (i) structureless sandstones
crosscut by a high-angle dike. The sandstone body (0.03 m [0.098 ft] thick) has a sharp contact; (ii) interpretation of (i). PUB = parent unit
belt; IB = intrusive belt; EB = extrusive belt.
Scott et al.
319
320
Reservoir Characterization of a Sand-Injectite Complex
Figure 6. Detail of multilayer sills in Figure 5E. (i) Individual sills are separated by thin (0.05–0.2 m [0.16–0.66 ft] thick) blocks of
mudstone. (ii) Line-drawing interpretation showing that the sills locally amalgamate into a composite sill with a cumulative thickness of
approximately 1 m (3.3 ft).
commonly sharp (Figure 4B, C). In the Dosados
Member, channelized sandstones are extensively
remobilized where, at the margins and sometimes
from the top of the channelized unit, low-angle
dikes (30–40° relative to bedding) form 5- to 15-m
(16–49-ft)–thick winglike projections (white arrows, Figure 4Di, ii), resulting in isolated rafts of
host mudstone (gray arrow). These dikes enhance
the lateral and vertical connectivity of channelized
sandstones (cf. black intrusions in Figure 4Dii).
Internally, the depositional sandstones are crosscut
by dikes (Figure 4Ei, ii).
Unit 2
This unit is a 100- to 290-m (328–951-ft)–thick
mudstone-rich interval characterized by sandstone
Figure 5. Unit 2. (A) Staggered sills defined by sharp planar margins that are laterally and vertically offset by high-angle dikes: (ii) interpretation of (i). Tierra Loma Shale Member, Marca Canyon (Mc). (B) A stepped composite sill (steps, >0.3 m [0.98 ft] in height; black
arrow). This sill arrangement is shown in Hurst et al. (2007, their figures 18, 19). Tierra Loma Shale Member, Escapardo Canyon (Es). (C)
Overview of a stepped sill with an erosional (scalloped; Figure 5D) upper margin and multilayered lower margin interbedded with thinbedded slope turbidites. In places, the sill is more than 6 m (19.7 ft) thick and, at outcrop, extends more than 440 m (1444 ft) laterally.
Tierra Loma Shale Member, No Name Canyon (NoC). (D) Convex-upward sandstone intrusions (white line) that cut into the overlying host
mudstone, termed “scallops” (SC; sensu Hurst et al., 2005): (i) a similar projection is shown in Vigorito et al. (2008, their figure 3B); (ii) line
drawing of a scallop; (iii) detail of the discordant margin. The SC are characterized by a sandstone intrusion that forms steep (70–80°),
discordant, and erosional flanks and an upper crest that cuts as much as 2 m (6.6 ft) (white line) of overlying host mudstone. (E) Multilayer
sills with individual thickness of 0.01 to 0.3 m (0.033–0.98 ft). Individual sills are separated by thin (0.05–0.2 m [0.16–0.66 ft] thick) rafts of
host mudstone. Tierra Loma Shale Member, Marca Canyon (Mc). Black boxes locate the permeability maps shown in Figure 12.
Scott et al.
321
Figure 7. Unit 3. (A) A high-angle dike that crosscuts approximately 200 m (656 ft) of the Marca Shale Member. The black box locates
the permeability map shown in Figure 16A. The locality is the No Name Canyon (NoC). (B) High-angle dikes crosscutting the Marca Shale
Member (Marca Canyon [Mc]). (C) A high-angle (80–90°), large thick (0.5 m [1.6 ft] wide) dike crosscutting the Marca Shale Member in
Marca Canyon. (D) Detail of the steep high-angle dike in C. The smaller dike on the left deviates from the main dike at an angle of
approximately 30°.
sills within the Dosados and Tierra Loma members (Figures 2, 3); the average N/G ratio is 0.19.
The greatest proportions of sandstone-rich reservoir facies are sills (average, 13%) and low-angle
dikes (average, 5%). Low-angle dikes are locally
present and tend to depart at high angles (>40°)
from remobilized channelized sandstones in unit 1
and become lower angle (5–17°) upward in reser322
Reservoir Characterization of a Sand-Injectite Complex
voir units 2 and 3. The sills have three geometries,
from stratigraphically deeper to shallower, staggered (Figure 5A), stepped (Figure 5B, C, D), and
multilayer (Figure 5E).
Staggered sills have intermediate thickness
(<1.0 m [3.3 ft]) and extend laterally <10 m (33 ft)
(Figure 5A). Furthermore, upsection stepped composite sills that are thicker (as much as 11.0 m
Figure 8. Extrusive sandstones in unit 4, the Dos Palos Shale Member, Marca Canyon (Mc). (A) Extrusive sandstones: (i) a high-angle dike
terminating at the base of a layer of extruded sandstone (dashed black line); (ii) interpretation showing the high-angle dike connected to
the base of the sandstone extrudite. (B) Internal sandstone extrusion geometry. (i) mounded morphology with internal subvertical
laminae that represent the vents of sand volcanoes with surrounding draped low-angle bedded sandstone, which commonly display softsediment deformation; (ii) interpretation showing steeply inclined laminae dipping away from a central conduit. IB = intrusive belt; EB =
extrusive belt.
[36 ft]) and more laterally continuous (>400 m
[1312 ft] long), with stepped, erosive upper and
lower margins (Figure 5B, C, D), occur. The sill
complex is composed of an approximately 1- to
2-m (3.3–6.6-ft)–thick upper main sill and several lower centimeter-thick (0.01–0.1 m [0.033–
0.33 ft]) multilayered sills. The main sill is connected to dikes (black lines, Figure 5B). High-angle
Scott et al.
323
324
Table 2. Outcrop Localities (Canyon Sections) and Architectural Elements Sampled in the Northern Panoche Giant Injection Complex*
Reservoir Characterization of a Sand-Injectite Complex
No Name Canyon (NoC)
A, <1 (31)%
B, 2 (22)%
C, 15 md
D, 30 (30)%;
1937 md
E, 15 (15)%;
384 md
F, 27 md
G, <1 (1)%
H, 3 (3)%
I, 1 md
HAD**
HAD
HAD
LAD**
LAD
HAD
HAD
HAD
MLS**
K, 3–30
(16–30)%;
1780–4894 md
STP**
L, 20 md
M, 5 (5)%;
34 md
O, 3 (3)%;
16 md
P, 3 (3)%;
25 md
Q, 13 (13)%;
130 md
R, 20 (20)%
S, 4 (4)%;
46 md
U, 85 md
HAD
HAD
N, 5–6
(6–8)%;
48–52 md
HAD
HAD
HAD
HAD
HAD
HAD
HAD
V, 56 md
W, <1 (19)%;
3 md
MLS
HAD
J, <1–5
(5–19)%;
4–13 md
MLS
Marca Canyon (Mc)
A, 0.5 (0.5)%;
B, 1.5–4
3 md
(25–34)%;
3–17 md
HAD
PFC**
C, <1
(14–16)%;
3–17 md
PFC
D, 7 (14)%;
11 md
G, 2 (2)%;
19 md
H, 4 (4)%;
9 md
E, <1–13 (3–8)%;
14–196 md
F, 2–10 (4–10)%;
11–117 md
I, 3 (26)%
J, 3 (20)%;
21 md
HAD
HAD
HAD
MLS
MLS
MLS
MLS
Capita Canyon (Ca)
A, 2 (20)%;
B, <1 (35)%;
50 md
52 md
PFC
PFC
C, 4 (26)%;
33 md
PFC
D, 2 (28)%;
63 md
PFC
E, 1 (32)%;
42 md
PFC
F, 5 (26)%;
63 md
PFC
G, <1 (1)%;
1 md
PFC
H, <1 (28)%;
4 md
PFC
I, 3 (33)%;
66 md
PFC
J, 5 md
HAD
N, 4 (20)%;
122 md
HAD
O, <1 (10)%;
5 md
HAD
P, <1 (719)%;
4–6 md
HAD
Q, 4 (18)%;
16 md
HAD
R, 52 md
S, 6 md
T, 1 md
HAD
HAD
RLC**
X, 32 (32)%;
4068 md
STS**
Z, 14 md
AA, 2 md
BB, 21 md
CC, 182 md
HAD
HAD
HAD
MLS
DD, 3 (9)%;
1 md
HAD
EE, 10 (10)%;
76 md
HAD
K, <1 (21)%;
5 md
HAD
L, 25 md
M, 1 md
HAD
HAD
U, 22 (31)%;
1147 md
LAD
V, 14 (14)%;
296 md
HAD
W, 9 md
FF, 3–34 md
HAD
GG, 43–72 md
HAD
HH, 10 md
HAD
HAD
Dosados Canyon (Do)
A, 5–14 (5–14)%; B, 3 (3)%;
45–231 md
268 md
C, 3 (3)%;
65 md
D, 6 (11)%;
65 md
E, 5–8 (5–8)%;
91 md
F, 4 (4–18)%;
458 md
HAD
HAD
HAD
HAD
HAD
K, 8 (9)%;
56 md
L, 14 (15)%;
294 md
M, 53–57 md
STS
STS
STS
N, 5–32
(5–32)%;
190–1655 md
STS
U, 6305 md
W, 27 (27)%;
4948 md
X, 40 md
LAD
LAD
EE, 179 md
FF, 2 (2)%;
24 md
HAD
HAD
Escapardo Canyon (Es)
A, 1 (22)%;
B, 0 (22)%;
19 md
3 md
PFC
PFC
K, 19–31
(20–31)%;
260–3048 md
STP
H, 7–26
(7–26)%;
466 md
STS
I, 2 (2)%;
8 md
STS
G, 4–10
(4–10)%;
32–105 md
STS
O, 11–28
(28–31)%;
727 md
LAD
P, 18 (19)%;
220 md
Q, 18 (18)%;
302 md
R, 4–5 (4–5)%;
131–302 md
S, 367 md
T, 13 (13)%;
131 md
LAD
LAD
LAD
LAD
LAD
Y, 17 md
Y, 30 md
Z, 75 md
AA, 40 md
BB, 298 md
LAD
STS
HAD
HAD
HAD
HAD
CC, 3–6
(3–6)%;
20–38 md
HAD
DD, 16
(16)%;
268 md
HAD
C, 26 (26)%;
1331 md
STP
D, 13 (13)%;
266 md
STP
E, 12 (12)%;
2132 md
STP
F, 18 (18)%;
287 md
STP
G, 27 (27)%;
1517 md
STP
H, 432 md
I, 12 (12)%
J, 432 md
STP
STP
STP
L, 634 md
M, 228 md
STP
STP
STS
J, 5–10
(10–12)%;
270 md
STS
Scott et al.
*In each canyon, the samples are assigned a letter. The numbers outside the brackets represent visual porosity (%) and permeability (md), whereas minus-cement porosity (%) is given within the brackets. Locations of samples of
sandstone are labeled with the corresponding letter along canyon sections in Figure 2, allowing the reservoir characteristics of the sample to be located within the injectite complex and regional stratigraphy.
**HAD = high-angle dike; LAD = low-angle dike; MLS = multilayer sill; STP = steeped sill; PFC = proximal-fan channelized sandstones; RLC = remobilized lower slope channelized sandstones; STS = staggered sill.
325
Table 3. Overview of the Mean Mineral Composition and Petrographic Characteristics of the Panoche Giant Injection Complex Sandstones*
Element
Remobilized Parent Units
Sample number (n)
Grain size
mm
phi
Detrital composition (%)
Qn, Fn, Ln
Mica
Mud
Glauconite
Cements (%)
Quartz
Carbonate
Mud matrix (%)
Minus-cement porosity (%)
Sandstone Intrusions
28
121
Sandstone Extrudites
7
0.2
2.4
0.2
2.4
0.2
2.4
Q42F40L18
1.8
3.5
0.9
Q43F45L12
1.7
12.9
0.8
Q47F42L12
1.6
7.9
0.4
1.7
11.7
5.0
16.0
1.1
1.7
5.3
12.9
2.4
20
0.8
23.5
*Mineral composition and minus-cement porosity are given in percentage of whole-rock volume. Relative percentages of quartz (Q), feldspar (F), and lithics (L) exclude
detrital mudstone clasts.
dikes crosscut the stratigraphy and connect to the
lower and upper margins of the sill (black lines,
Figure 5C). Close to the upper contact of unit 2,
multilayer sills are located along or near the stratigraphic horizon where thin-bedded, finer grained
depositional sandstone units are found (Figure 2).
The sills tend to be thin (0.01–0.3 m [0.033–
0.98 ft] thick) and laterally continuous (10–100 m
[33–328 ft] wide), emanate from a high-angle dike
(black lines, Figure 5E), and are separated by
mudstone units. Individual sills propagate along
bedding horizons and locally amalgamate into approximately 1-m (3.3-ft)–thick units (Figure 6).
High-angle sandstone dikes range in thickness
from less than 0.1 to more than 0.6 m (0.33–1.97 ft),
crosscut host mudstones at 80 to 90° (relative to
bedding), and are laterally continuous, in places
crosscutting more than 200 m (656 ft) of stratigraphic section, thereby producing excellent vertical but limited lateral connectivity (Figure 7A, B).
The dikes are en echelon in the lower 60 to 80 m
(197–263 ft) and bifurcate in the upper 140 to
160 m (459–525 ft) (Figure 7C, D). In the upper
100 m (328 ft), the dikes eventually taper out or
terminate beneath sandstones in unit 4.
Unit 4
Unit 3
Unit 3 is a 330- to 440-m (1083–1444-ft)–thick
mudstone-dominated interval of exclusively highangle sandstone dikes within the Tierra Loma, Marca
Shale, and Dos Palos Shale members (Figures 2, 3).
Net-to-gross ratio is very low (average, 0.02) when
derived from logged sections; the N/G determined
from the image analysis of high-angle dikes in outcrop cross sections indicates a significantly higher
N/G (average, 0.08). Locally, thick (5–12 m [16–
39 ft]) low-angle dikes originating in unit 2 crosscut
this unit, for example, the dike studied in Do (black
box, Figure 2D).
326
Reservoir Characterization of a Sand-Injectite Complex
An interval of extruded sandstone of as much as
30 m (98 ft) thick that overlies an unconformity
within the Dos Palos Shale Member comprises
unit 4 (Figures 2, 3). Because unit 4 is defined by
the presence of sandstone, the N/G ratio is typically 1. High-angle dikes connect to the base of unit
4 (Figures 2, 8Ai, ii), thus establishing connectivity
among sandstones in units 1 to 4.
Sandstones in unit 4 can be as much as 30 m
(98 ft) thick and extend laterally for at least 100 m
(328 ft), but in Marca Canyon, the sandstone units
are locally thin (∼5 m [16 ft] thick) and laterally
continuous (>30 m [98 ft] wide) (Figure 8Ai, ii).
Scott et al.
327
Figure 9. Visual and minus-cement porosity distribution of sandstone intrusions. (A) Total sill and dike population. (B) Total sill population. (C) Staggered sills. (D) Stepped sills.
(E) Multilayer sills. (F) Total dike population. (G) High-angle dikes. (H) Low-angle dikes. av = average; sd = standard deviation. See Appendix 14 (AAPG Datashare 46 at www.aapg
.org/datashare) for sample data.
Table 4. Comparison of Low-, Moderate-, and High-Permeability (Injected) Sandstones, Showing Mean Values*
Petrographic Variable
Grain size
mm
phi
Mudstone clast (%)
Cements (%)
Quartz
Carbonate
Mud matrix (%)
Visual porosity (%)
Minus-cement porosity (%)
Mean permeability (md)
Intrusive element (%)
Low (Poor) Permeabilities
(<0.1–100 md)
0.2
2.4
13
2
1
8
4
7
30
54% are high-angle dikes
Sample number (n)
53
Moderate (Good to Very Good)
Permeabilities (100–1000 md)
0.2
2.4
11
<1
<1
4
14
15
266
53 and 35% are low-angle
dikes and sills, respectively
40
High (Excellent)
Permeabilities (>1000 md)
0.2
2.3
9
<1
0
1
28
29
2457
31 and 69% are low-angle
dikes and sills, respectively
13
*The mineral compositions are given in percentages of whole-rock volume. Consult Appendix 14 for sample data.
Mounding is common and locally contains steeply
inclined laminae (Figure 8Bi) that dip away from
central conduits, the sandstone-filled vents of sand
volcanoes.
RESERVOIR CHARACTERISTICS
Description
Sandstones were investigated from all reservoir units
with sampling focused on units 2 and 3 (Figure 2;
Table 2). All sandstones are arkosic to lithic arkosic
in composition (Table 3). A broad range of visual
porosity is present in the sandstone intrusions (2–
33%; Figure 9A). Permeability is variable (sd =
884 md). High-permeability data (>1000 md) are
predominantly from sills, whereas lower permeabilities (<0.1–100 md) are mainly from high-angle dikes
(Table 4).
Forty-nine thin sections were described. The total sill population has mean visual and minus-cement
porosities of 13 and 15%, respectively (Figure 9B).
Staggered and stepped sills tend to be moderately
sorted and loosely packed (Figure 10A) and have
open pore space in which some grains appear to
float without obvious contact with adjacent grains
328
Reservoir Characterization of a Sand-Injectite Complex
(Figure 10B). The pore network is well connected
(range of length, 0.1–0.3 mm [0.004–0.012 in.]
wide), and large pore-throat diameters (0.05–
0.1 mm [0.002–0.004 in.] wide) with high visual
porosity (12–19%; Figure 9C, D) and a broad distribution of pore-throat diameters (dashed line;
Figure 11A) are predominant. Individual quartz
grains are pervasively crosscut by fractures and are
sometimes fragmented into arrays of smaller clasts
(Figure 10B). Fine to medium sand-size mudstone
clasts are common constituents of multilayer sills
(Figure 10C) along with clay- and silt-size quartz
grains that occlude pores (Figure 10D). This produces poor sorting and a poorly connected pore
network with small pore-throat diameters (average,
0.04 mm [0.002 in.] wide), low visual porosity (5%;
Figure 9E), and narrow pore-throat diameters (solid
line, Figure 11A). The abundance of silt-size and finer
grained clay particles in multilayer sills (Figure 10C,
D), when compared with similar grains in stepped
sills (Figure 10A, B), explains the lower permeability of more than 2 orders of magnitude in the
multilayer sills.
Permeability measurements were taken on 57
samples, and a positive relationship between sill
thickness and permeability is noted (Table 5). Acquisition of permeability data from three localities
Figure 10. Microtextures of sandstone sills from unit 2. Porosity is green to blue and black in plane-polarized light and backscatteredelectron microscopy modes, respectively. (A) A high-permeability sandstone (4893 md) from a stepped sill (location in Figure 12B). Note
the presence of fine to medium sand-size mudstone clasts (Md; black grains). Plane-polarized light. (B) Backscattered-electron microscopy image of the pore structure in a stepped sill that includes fractured grains. (C) A tightly packed, poorly sorted grain fabric from a
low-permeability sandstone (24 md) from a multilayer sill (location in Figure 12C). Grains of quartz (Qz) and Md are surrounded by finer
silt-size quartz grains and fine sand-size Md (black grains). Plane-polarized light. (D) Backscattered-electron microscopy displaying pores
between Qz and feldspar (F) grains in a multilayer sill. Clay-size particles occlude pores.
with sills of differing thickness shows that stepped
sills have high permeability (green to blue areas;
Figure 12A, B) relative to multilayer sills (red
shades, Figure 12C). Note the lower permeability
sandstone present toward the sill margins (286–
299 md; red and orange areas, Figure 12A). The total
sill population has a mean of 629 md (Figure 13B),
and the mean permeability of the staggered and
stepped sills is 586 (Figure 13C) and 1225 md
(Figure 13D), respectively. Multilayer sills have
a mean permeability more than seven times less
(70 md; Figure 13E). When thickness is plotted
against permeability, a modest positive correlation
exists (r 2 = 0.69; Figure 14).
Sandstone Dikes
Seventy-two thin sections were characterized. The
total dike population has mean visual and minuscement porosities of 7 and 11%, respectively (Figure 9F). High-angle dikes are tightly packed and
poorly sorted, with elongate grains displaying subvertical alignment (dashed circles, Figure 15A). Fractured quartz and feldspar grains are ubiquitous
Scott et al.
329
Figure 11. Pore-throat diameter distributions in sandstone sills (A) and dikes (B) from point counting versus cumulative percent visual
porosity. (A) The mean porosity within stepped sills (dashed black line; average, 0.63-mm [0.03-in.] pore diameter) is greater than in
multilayer sills (solid black line; average, 0.04-mm [0.002-in.] pore diameter). (B) The mean porosity in low-angle sandstone dikes
(dashed black line; average, 0.10-mm [0.004-in.] pore diameter) is larger than in high-angle sandstone dikes (solid black line; average,
0.05-mm [0.002-in.] pore diameter). See Appendix 15 (AAPG Datashare 46 at www.aapg.org/datashare) for data.
(dashed circles, Figure 15B). Where pores are occluded by carbonate cement, a disconnected pore
network is created with small pore-throat diameters
(0.02 mm [0.0008 in.]), low porosity (Øv = 5.0%,
average; Figure 9G), and a narrow distribution of
pore-throat diameters (solid line, Figure 11B).
Low-angle dikes are typically moderately sorted
and have loose grain packing with high visual porosity (Figure 15C, D); this creates a highly connected pore network with large pore-throat diameters (>0.15 mm [0.005 in.]), moderate porosity (Øv =
11%, average; Figure 9H), and a broad pore-throat
diameter distribution (dashed line, Figure 11B).
One-hundred and fifteen permeability measurements were made by sampling along and across
dikes. Detailed sampling of dikes at three outcrop
localities shows that high-angle dikes have pervasive low permeability throughout (<1–122 md; red
shades, Figure 16A, C). By contrast, a very thick,
low-angle winglike dike that originates near the top
of unit 2 and extends upward into unit 3 is characterized by high permeability (4947–6304 md;
purple shades, Figure 16B). The total dike population has a low average permeability (202 md;
Figure 13F), but high- and low-angle dike populations have significantly different average permeability, 81 and 529 md, respectively (Figure 13G, H).
DISCUSSION
Creation of quantitative static models that are realistic representations of the reservoir architecture
and pay distribution is strongly dependent on the
interpretation and integration of seismic and borehole data. This process is enhanced by the use of
Table 5. Comparison of Thin Multilayer (<0.1 m [0.33 ft] thick) and Thick Stepped (>0.1 m [0.33 ft]) Sandstone Sills*
Petrographic Parameter
Average grain size (mm)
Mean visual porosity (%)
Permeability (md)
Microtextures
Sandstone Sills (<0.1 m [0.33 ft])
Sandstone Sills (>0.1 m [0.3 ft])
0.16
7
121
Abundant silt-size grains, tight
packing, and poor sorting
0.20
16
911
Infrequent silt-size grains, loose
packing, and well sorted
*The thin sills contain tightly packed and poorly sorted grains, resulting in a low visual porosity and low permeability. Consult Appendix 19 for sample data.
330
Reservoir Characterization of a Sand-Injectite Complex
Figure 12. Permeability maps of sandstone sills in unit 2; the lithology represented is mudstone (gray), and sandstone permeability is
represented by a red-to-purple (low-to-high) color scale. (A) A thick stepped sill characterized by high-permeability sandstones (1561–
3047 md; green to blue areas). The gridding method used was natural neighbor (anisotropy of 0.1 and angle of 5°). (B) A very thick
stepped sill with a multilayered lower margin characterized by very high permeability sandstones (1780–4893 md; green to purple
areas). Note the low-permeability sandstone toward the sill margins (4–36 md; red and orange areas). The gridding method used was
natural neighbor (anisotropy of 0.1 and angle of 5°). (C) Multilayer sills characterized by low-permeability sandstones (10–20 md; red
areas). Note the area of higher permeability (58–117 md; yellow to green areas). The gridding method used was point krigging.
knowledge from subsurface and outcrop examples
that are geologically similar. As sandstone intrusions
create cross-stratal permeable networks in otherwise low-permeability strata, they are a significant
challenge when modeling their distribution and behavior during hydrocarbon production. The data
we present from the PGIC form a basis for subsurface analogy and quantitative modeling (Figure 17;
Table 6) that can enhance conceptual interpretations that infer vertical connectivity in situations
where dikes create fieldwide pressure communication (Briedis et al., 2007).
Reservoir Architecture
The tripartite organization of parent units (unit 1),
intrusive units (units 2 and 3), and sandstone extrusions (unit 4) is used as the basis for defining a
reservoir architecturally based zonation (Figures 1–
3). The intrusive unit is further subdivided into res-
ervoir units 2 and 3. Lateral and vertical variations
in sandstone geometry and N/G occur, and in
general, the abundance of depositional sandstones
and their proximity to the base of the sill zone, the
lithostatic equilibrium surface (Vigorito and Hurst,
2010), determine the N/G of the sandstone intrusions. Identification of unit 2 is important in subsurface modeling because the main variations in
reservoir volume occur in unit 2 where sandstonerich features (N/G of as much as 0.30 sandstone)
that are commonly identified in the subsurface such
as sills and saucer-shape and conical intrusions occur (Hurst et al., 2003b; Szarawarska et al., 2010).
At a smaller scale, sandstone-body geometry
varies within unit 2 where sill geometry is organized stratigraphically (Figure 5). When examining
borehole data, the challenge is to identify different
classes of sill, to differentiate between sills and depositional sandstones, and to differentiate even between sill and dike margins simply because so little
Scott et al.
331
Figure 13. Permeability distribution in sandstone intrusions. (A) Total sill and dike population. (B) Total sill population. (C) Staggered sills.
(D) Stepped sills. (E) Multilayer sills. (F) Total dike population. (G) High-angle dikes. (H) Low-angle dikes. See Appendix 14 (AAPG Datashare
46 at www.aapg.org/datashare) for sample data. kh = horizontal permeability; kv = vertical permeability; n = number of samples; av =
average; sd = standard deviation; cv = coefficient of variation; kh = horizontal permeability; kv = vertical permeability.
rock is sampled. Diagnostic features of sand injection such as feeder dikes (black lines, Figure 5),
upward erosive surfaces (white line, Figure 5D),
clasts of host mudstone derived from the injectite
margins (Figure 6), and meter-scale stepped erosional margins (black arrow, Figure 5B, D) can be
used collectively to differentiate sandstone intrusions from depositional units.
The general relevance of the tripartite organization to other sand injectites is discussed by VigFigure 14. Permeability (md) plotted
against sill thickness (m). Line of best fit is
plotted as a linear trend line. See Appendix
16 (AAPG Datashare 46 at www.aapg.org
/datashare) for data.
332
Reservoir Characterization of a Sand-Injectite Complex
orito and Hurst (2010; Table 1), but comparison
with other sand injectites is compromised by the
less extensive exposure elsewhere. However, in several cases, the tripartite organization is identified
despite substantial differences in sandstone content.
Our experience with the interpretation of subsurface sand injectites is that it is similar to the architecture in the PGIC, although the observation of
unit 1 (Figure 4) is typically poor (de Boer et al.,
2007; Wild and Briedis, 2010).
Figure 15. Microtextures in dikes from units 2 and 3. (A) A tightly packed and poorly sorted texture with faint subvertical grain
alignment (dashed circles) in low-permeability sandstone (50 md) from a high-angle dike; Capita Canyon (location in Figure 16A).
(B) Backscattered-electron microscopy of a poorly sorted sandstone in a high-angle dike with fractured quartz and feldspar grains
(dashed circles) and pores that are pervasively carbonate cemented. (C) A moderately to loosely packed grain fabric in a high-permeability
sample (4947 md) from a low-angle dike; Dosados Canyon (Do) (location in Figure 16B). Large pores (>0.15 mm [0.006 in.] in diameter) are
common. (D) A loosely packed grain fabric in a low-angle dike; Dosados Canyon (Do). Md = mudstone clasts.
Reservoir Characteristics
In sills, permeability has a moderate positive lognormal correlation with intrusion thickness (Figure 14). These correlate with variations in grain
sorting, packing, and abundance of clay- and siltsize particles (Figure 10), which result in variations in visual porosity and pore-size distribution
(Figures 9C, D, E; 11). Origins of clay-size particles in sills include entrainment at the injectite
margins as fluidized sand penetrated host mudstone and disintegration of mudstone rafts through
shear-induced abrasion during injection (Kamakami
and Kawamura, 2002; Diggs, 2007). An alternative
origin is their elutriation from the remobilized parent sandstone units (Duranti and Hurst, 2004).
Many of the clay-size particles were likely derived
from shear-induced abrasion of host mudstone clasts,
as fluidized sand entered the hydraulically fractured
strata (cf. Scott et al., 2009).
In contrast, the origin of the silt-size grains in
multilayer sills is more obvious and restricted to a
single origin. The silt-size grains likely originated
from the depositional turbidite beds located along
or near the stratigraphic horizon at which the multilayer sills intruded. The occurrence of thin-bedded
Scott et al.
333
Figure 16. Permeability maps of sandstone dikes from a stratigraphically deep (Panoche Formation) high-angle dike (A), a low-angle
dike (B), and a high-angle dike (C) (location in Figure 2). Lithologies represented are mudstone (light gray) and depositional sandstone
(gray); sandstone permeability is represented by the color scale. (A) Low-permeability sandstones characterize the entire dike (10–
20 md; red areas); Panoche Formation. (B) A thick low-angle dike in the Tierra Loma Shale Member, which originates in unit 2 and
crosscuts unit 3. (C) A high-angle dike characterized by low-permeability sandstones (40–80 md; green areas); Marca Shale Member.
PUB = parent unit belt.
sandy and silty turbidites appears to have had an
important function in promoting the formation
of multilayer sills (see maps A–E in Figure 2), as
these depositional units represent heterogeneities
within an otherwise mudstone-rich background of
unit 3. The incorporation of silt-size grains would
occur through hydraulic mixing of fine to medium sand-size injected grains with depositional
silt beds, thereby resulting in a lower permeability in
the multilayer sills (Figures 12C; 13E) when com334
Reservoir Characterization of a Sand-Injectite Complex
pared to that in the staggered and stepped sills
(Figures 12A, B; 13C, D).
Carbonate cement contributes to the occurrence of very low visual porosity and permeability
(Figures 9G; 13G; 15A, B) and a narrow poresize distribution in high-angle dikes (solid line,
Figure 11B). Jonk et al. (2005a, b) emphasize the
importance of sandstone-intrusion geometry, dimensions, and proximity to host strata in controlling carbonate cementation and suggest that cations
Scott et al.
Figure 17. Conceptual three-dimensional (3-D) diagrams that synthesize the salient reservoir architectural elements described in this study. (A) Panoche giant injectite complex (PGIC)
reservoir geometry and volumetrics: (i) an interpretation of the 3-D geometry showing differing reservoir connectivity among architectural elements in units 1, 2, 3, and 4; (ii) estimates
of sandstone-intrusion volume in units 2 and 3 (Table 6). (B) Summary of the 3-D geometry and trends in reservoir characteristics of architectural elements in the PGIC: (i) low-angle
dike, (ii) staggered sill, (iii) stepped sill, (iv) multilayer sill, and (v) high-angle dike. PUB = parent unit belt; IB = intrusive belt; EB = extrusive belt; N/G = net to gross.
335
336
Reservoir Characterization of a Sand-Injectite Complex
Table 6. Summary of the Reservoir Characteristics and Their Trends of Architectural Elements
Architectural
Element*
HAD
LAD
STS
STP
MLS
Dimensions**
y
<10–500 m
(33–1640 ft)
10–100 m
(33–330 ft)
1–10 m
(3.3–33 ft)
10–400 m
(33–1312 ft)
10–50 m
(33–164 ft)
t
0.01–8 m
(0.03–26 ft)
<1–15 m
(3.3–49 ft)
<1 m (3.3 ft)
<1–12 m
(3.3–39 ft)
0.05–0.3 m
(0.16–0.98 ft)
Petrographic
Characteristics***
f-s-s to m-s-s, Md, Mm,
CC, Tgp, Pgs, VGA
f-s-s to m-s-s, Mgp,
Pgs to Mgs
f-s-s to m-s-s, Tgp to
Mgp, Mgs, HGA
f-s-s to m-s-s, Md, Mgp
to Lgp, Ggs, HGA
s-s to m-s-s, Md, Mm,
Tgp, Pgs
Porosity Characteristics†
Øv
Øm
Average, 5%;
sd, 5%
Average, 11%;
sd, 7%
Average, 12%;
sd, 11%
Average, 19%;
sd, 10%
Average, 5%;
sd, 4%
Average, 5%;
sd, 5%
Average, 13%;
sd, 9%
Average, 14%;
sd, 11%
Average, 22%;
sd, 22%
Average, 6%;
sd, 5%
Permeability Characteristics††
k
Average, 81 md;
sd, 282 md
Average, 529 md;
sd, 1389 md
Average, 586 md;
sd, 1058 md
Average, 1224 md;
sd, 1300 md
Average, 70 md;
sd, 64 md
kv/kh
1.4
1.2
1.3
1.8
1.4
Trends in Reservoir
Characteristics
Lateral continuity of low
porosity and permeability
Lateral continuity of high
porosity and permeability
Lateral continuity of moderate
porosity and permeability
Lateral continuity of high
porosity and permeability
Lateral continuity of very low
porosity and permeability
*HAD = high-angle dike; LAD = low-angle dike; STS = staggered sill; STP = stepped sill; sd = standard deviation; MLS = multilayer sill.
**y = lateral extension; t = thickness.
***f-s-s = fine sand-size grains; m-s-s = medium sand-size grains; s-s = silt-size grains; Md = mudstone clasts; Mm = mud matrix; CC = carbonate cement; Tgp = tight grain packing; Pgs = poor grain sorting; VGA = vertical grain
alignment; Mgp = moderate grain packing; Mgs = moderate grain sorting; HGA = horizontal grain alignment; Lgp = loose grain packing; Ggs = good grain sorting.
†
Øv = average visual porosity of element; Øm = average minus-cement porosity of element.
††
k = average permeability; kv/kh = average ratio of vertical-to-horizontal permeability.
*Estimates assume that the the Panoche giant injection complex extends 11.1 km (6.9 mi) into the San Joaquin Basin (i.e., the same dimensions as the northern and transitional outcrop belt).
**RU = reservoir unit; t = average thickness (m); l = length of outcrop exposure of reservoir unit (m); w = inferred extension of Panoche giant injection complex into the San Joaquin Basin (m); N/G = average net to gross of reservoir
unit; Øv = average visual porosity of injected sandstones within the reservoir unit (%); GRV = gross rock volume (m3); ISV = intrusive sand volume (m3); IPV = intrusive pore volume (m3).
0.02 km3
0.001 km3
6.33 × 107
0.83 × 107
5.43 × 108
1.10 × 108
179
346
2
3
16,000
16,000
1000
1000
0.19
0.02
12
7
2.85 × 109
5.54 × 109
0.19 km3
0.02 km3
IPV per cubic
kilometer of GRV;
IPV**
ISV per cubic
kilometer of GRV
ISV**
GRV**
Øv (%)**
N/G**
w**
l**
t**
The PGIC constitutes large volumes of sandstone
intrusion within a predominantly mudstone-rich
(low N/G) background. By using average N/G
and thicknesses derived from the logged sections,
the volumes of sandstone intrusion in reservoir
units 2 and 3 in the northern and transitional PGIC
are estimated as 0.19 km3 (0.046 mi3) and 0.02 km3
(0.005 mi3), respectively, of intruded sandstone per
cubic kilometer of gross rock volume (Table 7).
In the subsurface, sandstone intrusions are likely
to have much greater volume than the PGIC; for
example, in the North Sea, the Paleogene sandinjectite play has hydrocarbon reserves estimated
to be approximately 2.5 billion bbl of oil equivalent
(BOE) (Hurst et al., 2005), all of which tend to
be reservoired only in the upper parts of injectite
complexes (figure 15 in Jonk, 2010; figure 2 in
Vigorito and Hurst, 2010). Hence, the total volume of these subsurface sandstone intrusions is
much greater. The upper part corresponds to units
2 and 3, but in the North Sea Paleogene, unit 2 is
typically considerably thicker relative to unit 3 and
is more sandstone-rich than the PGIC. To the best
of our knowledge, sand extrudites (unit 4) have not
been positively identified as significant reservoirs in
any subsurface examples (Hurst et al., 2006).
Mudstone distribution within PGIC sandstone
intrusions has a major influence on reservoir quality. Large rafts of mudstone are common in sills
and reduce their N/G (Figure 6Ai, ii, B, C). Some
of the large clasts are in situ and formed during
the hydraulic fracturing of the host mudstone
RU**
Reservoir Volumetrics
Table 7. Injected Sandstone and Pore-Volume Estimates in Units 2 and 3 Based on Averaged Sedimentary Log Data in the Panoche Giant Injection Complex (cf. Figure 2)*
partly derived from the host mudstones contributed
to the formation of carbonate cement. The high
angle of discordance between the sandstone intrusions and bedding in the host strata may create a
contact that enhances fluid flux along the bedding
planes (i.e., higher permeabilities along bedding
compared to across bedding). Enhanced flux from
the surrounding host mudstones into a dike would
carry with it cations that are conducive to carbonate
cementation. It is reasonable that a continued influx
of cations would pervasively reduce sandstone permeability in high-angle dikes (Figure 16A, C).
Scott et al.
337
(Cosgrove, 2001); these large clasts commonly
have sandstone-filled microfractures that contribute to hydrocarbon storage and enhance intraclast
permeability. Dikes and sills both have varied contents of fine- to coarse-grained mudstone particles (Md; Figures 10A, C; 15A) probably derived
from hydraulic fracturing and erosion of host strata
(Archer, 1984; Diggs, 2007; Macdonald and Flecker,
2007; Scott et al., 2009).
Volumetric models are based on maps of N/G,
porosity, and saturation, and we demonstrate spatial variations in N/G and porosity, which are summarized in Figure 17. However, when making reservoir volumetric estimates, several major challenges
exist. Although significant elements of sills are approximately conformable to bedding, they step and
erode upward (Figure 5Di, ii), features that are
foreign to depositional sandstones with approximately stratiform geometries. In the PGIC, even
the most sandstone-rich parts of the sill-dominated
unit 2 rarely exceed N/G = 0.3 sandstone; however, the levels of lateral and vertical connectivity
and storability are probably far better (Figures 5,
17) than most depositional sandstones with similar
N/G. The Balder field (Norway) is an example
where higher reserves and better-than-expected
hydraulic continuity between thin sandstones are
encountered (Briedis et al., 2007).
Because independent interpretation of sandstone distribution in Balder field demonstrated schematically how continuity of pore pressure was possible if sand-injectite architecture is invoked (figure 9
in Hurst et al., 2005). Despite no obvious interwell
correlation among sandstones, fieldwide pressure
communication during production prevails in sandinjectite fields (Briedis et al., 2007; Guargena et al.,
2007; Satur and Hurst, 2007). This reflects the high
level of connectivity between sandstone intrusions
even in conditions where the N/G of the reservoir
interval is very low (<0.01).
As many sandstone intrusions in oil fields are
neither cut by boreholes nor detected on seismic
data (Briedis et al., 2007; Huuse et al., 2007), the
proportion of reservoir sandstone actually present
is likely to be underrepresented in models based
only on subsurface data. Estimating the proportion
of unidentified pay simply because it cannot be re338
Reservoir Characterization of a Sand-Injectite Complex
solved using seismic data can be reconciled by reference to outcrop analogs that show the abundance
of small-scale sandstone intrusions and by consideration of production data that show substantially
that more hydrocarbon is present than was predicted from preproduction reservoir models.
IMPLICATIONS FOR SUBSURFACE
RESERVOIR MODELS
The PGIC allows elucidation of several problems
facing subsurface characterization of sand-injectite
reservoirs. Many of these problems are directly associated with the laterally and vertically discontinuous character of sandstone intrusions and the difficulty of detecting and mapping reservoir geometry
using subsurface data. Although data from PGIC
have been used to support sand-injectite reservoir
models (e.g., Briedis et al., 2007), we are unaware
of examples where outcrop data are used to populate subsurface models or where the distribution
of sandstones is recreated using inferences that are
based on relationships documented in this study
and observed in the PGIC (Smyers and Peterson,
1971; Vigorito et al., 2008; Vigorito and Hurst,
2010). In light of our outcrop observations, it appears inevitable that populating subsurface models
with data derived only from the subsurface (e.g.,
Fretwell et al., 2007) will invariably make a substantial underestimation of reservoir volume and
connectivity.
Our study demonstrates a basis for quantitative use of sand-injectite outcrop-analog data in
subsurface models. If key characteristics of sand
injectites can be recognized, such as the paleo–sea
floor at the time of sand injection (unit 4; Figure 8),
the base of unit 2 (base sill zone and lithostatic
equilibrium surface; Figures 2, 3), and the geometry of individual sandstone intrusions, then a reservoir zonation can be inferred. If the stress regime
at the time of sand injection is known, the orientation of intrusions in units 1 and 3 can be predicted
(Boehm and Moore, 2002). Recognizing the orientation of intrusions in unit 3 is relevant because
this is commonly the part of a sand injectite first
encountered during drilling.
An example illustrating the problem of using
subsurface data as the sole input for reservoir models
of sand injectites is the misinterpretation of smallscale sandstone intrusions. Individually small (subseismic-scale) sandstone intrusions are commonly
abundant and volumetrically significant parts of
large injectites, for example, the numerous minor
intrusions associated with sills (Figures 5E, 6A),
that, when observed in subsurface data, could easily
be misinterpreted as a depositional unit (the sill)
with minor sandstone intrusions instead of as a
small part of a very large sand-injectite complex.
The Paleocene Ty Formation in the Sleipner Øst
field (Satur and Hurst, 2007; Figure 6) is an example where dikes that individually constitute tiny
volumes of sandstone but are laterally extensive
enhance vertical sweep efficiency.
A common challenge to all subsurface studies
of sand injectites is that sandstone intrusions and,
specifically, dikes crosscut stratigraphy. Any biostratigraphic, sequence-stratigraphic, and chronostratigraphic boundary can lie above, within, or
below a specific sandstone intrusion (de Boer et al.,
2007; Szarawarska et al., 2010). Reservoir correlations based on conventional stratigraphic methods
will not capture the geometry of sandstone intrusions or place them in an appropriate spatial framework (Hurst et al., 2005), and geosteering wells
using biostratigraphy may consequently be misleading when applied to reservoir correlation. Unexpectedly good reservoir performance may be
attributable to the misidentification of thick sandstone intrusions as depositional sandstone units by
creating improved lateral and vertical continuity
with other sandstone units. Disappointing reservoir performance and volumetrics may occur where
lateral extent and thickness distribution are overestimated. For example, if a well was located at the
northwestern end of the main sandstone body in
Figure 5E and a depositional origin was inferred,
the reservoir quality and volume will be underestimated. If, however, a well was located toward the
southeast, reservoir volume would be overestimated.
Upscaling from a geologic model to a full-field
reservoir-simulation model presents some very specific challenges, in particular, preserving high vertical permeability in low (<0.01%) N/G stratal units
(Purvis et al., 2002; Briedis et al., 2007; Fretwell
et al., 2007). Fine-scale reservoir heterogeneities
(dimensions, <<0.1 m [0.3 ft]) recognized in PGIC
similar to those known from the subsurface are typically coarsened into grid cells with volumes between
0.6 and 3 × 104 m3 (0.22 and 1.06 × 106 ft3), which
is recognized to be a major limitation to the validity
of full-field models (Purvis et al., 2002; Briedis et al.,
2007; Guargena et al., 2007). The problem of averaging fine-scale permeability heterogeneity in
reservoir models is exemplified by the occurrence
of abundant small-scale (<0.1-m [0.33-ft] thickness) sandstone intrusions that locally enhance the
permeability of otherwise low-permeability intervals by many orders of magnitude (Figures 12, 16).
If, in full-field simulation models, reservoir volume is increased to facilitate vertical connectivity
(transmissibility), unreasonably high storage will
be generated in the models, and the localized effect of injectites on fluid flow during production
will be misrepresented. As well as producing a
misleading distribution of hydrocarbons, the fullfield models will be compromised or invalidated
with respect to their value for the prediction of
well production rates, production profiles, and
field lifetime.
When sand injectites are part of mudstonedominated units (unit 3, cf. Figure 7B [low N/G]
and Figure 8A [very low N/G]), their presence
may be rarely detected in boreholes and may be
elusive on seismic data but may have a deleterious
impact on seal capacity (Hurst et al., 2003a, b).
Where such units are evaluated as intervals into
which gases (for example, CO2) or cuttings may
be sequestered, a failure to assess the influence of
sand injectites will have potentially disastrous effects if the low N/G of the sandstones is not associated with a high level of connectivity.
CONCLUSIONS
Despite approaching two decades of studies on the
broad significance of sand injectites to hydrocarbon
exploration and production, before this study,
outcrop studies that published porosity and permeability data from the architectural elements of
Scott et al.
339
sandstone intrusions did not exist. The key conclusions of this study are as follows.
1. A tripartite organization of parent units (unit 1),
intrusive units (units 2 and 3), and sandstone
extrusions (unit 4) has been identified and is the
basis for defining distinct reservoir characteristics.
2. Evaluation of the intrusive complex reveals two
reservoir units: a lower sill-dominated moderate
N/G (0.19) interval (unit 2) and an upper low
N/G (0.08) interval dominated by high-angle
bifurcating dikes (unit 3).
3. Three distinct classes of sill are identified: staggered, stepped, and multilayer geometries. Mudstone clasts, clay-, and silt-size grains derived from
host strata generate lower visual porosity (average, 5%) in multilayer sills than in staggered and
stepped sills (12–19%).
4. Two dike classes are identified: winglike lowangle (30–40° relative to bedding) and highangle (80–90°) intrusions. Carbonate cement
contributes to the occurrence of very low visual
porosity in high-angle dikes (5%) when compared to low-angle dikes (11%).
5. This study indicates that sandstone-intrusion
permeability is heterogeneous. Staggered and
stepped sills are seven times more permeable
(average, 586 and 1225 md, respectively) than
multilayer sills (70 md).
6. Volumetric calculations indicate that unit 2 has
the greatest volume of injected sand and could
store as much as 6.33 × 107 m3 (2.24 × 109 ft3)
of fluid. Mudstone raft and clast distribution has
a major influence on N/G and volumes.
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