WIRELINE LOG-BASED IDENTIFICATION OF FACIES AND THE MID-SAN ANDRES UNCONFORMITY
IMPLICATIONS FOR TARGETING ADDITIONAL RECOVERY FROM THE SAN ANDRES FORMATION
Although cores provide the best data for characterization of facies, stratigraphy, and identification of the mid-San
Andres unconformity, two wireline logs can supplement core data: borehole imaging logs and spectral gamma ray logs.
PREDICTIVE MODEL OF HYDROCARBON DISTRIBUTION AND FLOW REGIME
San Andres-Grayburg Reservoirs, Andrews County Area
North
South
Depth
ft
m
Fuhrman-Mascho Field
Emma Field
FMI-BASED
FMI-BASED CORE
CORE DESCRIPTION
DESCRIPTION
0
Arrow
Arrow Fuhrman-Mascho
Fuhrman-Mascho Unit
Unit #307
#307
1 in
-1000
Andrews
Andrews County,
County, Texas
Texas
GR
(API)
0
Depth
(ft)
125
N–D crossplot porosity
(%)
30
20
10
0
-10
-1100
SGR
CGR
Flow regime 4
Flow Regime 1
Permeable
porosity zones
-350
Emma oil/water: -1165'
Flow regime 2
-1200
4150
Tepee
structure
Grayburg
-300
Grayburg Fm.
Upper San Andres
-1300
4200
-400
Lower San Andres
Flow regime 3
FMU oil/water: -1340'
21. Although the upper San Andres is productive in many reservoirs in the Permian Basin, in inner platform settings like that of the
Fuhrman-Mascho field area, the succession is dominated by low permeability, tidal-flat facies, and locally porous, cycle base
subtidal facies. Although basal Grayburg siltstone/sandstone cycles are locally productive in these areas, the lower San Andres,
because it consists of more stratiform and continuous, unconformity-related porosity, should be considered a primary target.
The potential of reservoirs developed at the lower San Andres unconformity may not have been realized throughout its extent.
In Fuhrman-Mascho field, the lower San Andres is a major contributor to production as it is in the Means field to the north.
Elsewhere it may have not been fully evaluated. In the nearby Emma field, for example, the reported oil/water contact is
approximately 100 ft higher than it is in Fuhrman-Mascho. This may be a result of capillary effects associated only with the
recrystallization below the mid-San Andres unconformity. The oil-water in Emma is placed in fusulinid wackestones of the upper
San Andres that display low permeabilities and apparent high water saturations despite their moderate porosity. It is possible
that the better rock fabrics in the lower San Andres may contain higher oil saturations than those in the upper San Andres
despite their being some 100 ft below the apparent oil/water contact.
QA9980c
Permeable siltstone
Tidal flat
Outer ramp wackestone
Inner ramp mudstone
Subtidal middle ramp
Recrystallized dolostone
Flow Regime 1. Transgressive, high continuity siltstones; highly productive and floodable
4250
Flow Regime 2. Inner ramp. low permeability carbonates; flow baffle; poor floodability
Flow Regime 3. Outer ramp, high continuity, permeable carbonates; floodable
Flow Regime 4. Middle ramp, permeable carbonate grainstones; highly productive and floodable
QAc1890c
QAc1886c
17. Paired core photograph and FMI log image: upper San Andres tidal-flat rocks. Tidal-flat facies are
characterized on FMI by thin laminated character. Note tepee structure.
0
upper San Andres Fm.
4300
4350
0
1 in
4400
1 in
CONCLUSIONS
Regional outcrop and subsurface studies show that a major unconformity, caused by a major sealevel fall with accompanying nondeposition and possible erosion, separates upper and lower San
Andres platform carbonate successions. Studies of the San Andres in the Fuhrman-Mascho area
indicate that this unconformity is the locus of significant porosity and permeability development.
The continuity of this unconformity porosity suggests that it should be a major target in reevaluating
existing San Andres reservoir completions and in subregional exploration.
lower San Andres Fm.
4450
4500
Cycle
boundary
4550
ACKNOWLEDGEMENTS
This study was funded by the University of Texas System as part of the University Lands Advanced
Recovery Initiative Program. Thanks go to Steve Hartman and Wallie Gravitt of the University West
Texas Operations Office, Midland, Texas, for their support. I am grateful to Stan Decker of Arrow
Operating Company for freely and promptly providing reservoir data for the study. Steve Hill, also
with Arrow, readily shared data, interpretations, and insights into reservoir geology and operations.
Jerry Lucia contributed to the petrophysical analysis of the reservoir. Peter Swart performed stable
isotope analyses.
4600
Tidal-flat carbonate
Mixed shallow subtidal carbonate
Siltstone–sandstone
Fusulinid wackestone–packstone
Cycle
QAc1892c
QAc1893c
18. Paired core photograph and FMI log image: lower San Andres fusulinid facies. Fusulinid facies are
characterized by vermiform FMI pattern.
QAc1889c
19. With proper core calibration, borehole imaging logs can provide
high resolution information on facies type, rock fabrics, and
fracture distribution. Vertical facies successions and cycle
boundaries are easily definable for this well using the FMI,
providing a basis for stratigraphic correlation and rock-fabric
determination.
QAc1948c
QAc1894c
20. Paired core and FMI log image: upper San Andres. FMI has very high resolution that can be used to discriminate even the finest
scale cycles and facies successions. Note: Core depths are about 5 ft higher than log.
Bureau
of
Economic
Geology
QAc1895c