Journal of Earth Science, Vol. 24, No. 1, p. 075–088, February 2013
Printed in China
DOI: 10.1007/s12583-013-0308-3
ISSN 1674-487X
Sequence Stratigraphy and Sedimentary Facies in
the Lower Member of the Permian Shanxi
Formation, Northeastern Ordos Basin, China
Wei Du* (杜伟), Zaixing Jiang (姜在兴), Ying Zhang (张颖), Jie Xu (徐杰)
School of Energy Resources, China University of Geosciences, Beijing 100083, China; Institute of
Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
ABSTRACT: The Lower Permian Shanxi (山西) Formation is one of the main gas-bearing stratigraphic units in northeastern Ordos (鄂尔多斯) Basin, China. Based on an integrated investigation of
well logs, cores, and outcrop, we delineated the sedimentary facies of the lower member of the Shanxi
Formation and divided the succession into three third-order sequences from base to top as SQ1, SQ2,
and SQ3. The lower region of Shanxi Formation was deposited in the following sedimentary facies or
subfacies: subaqueous braided channel, subaqueous interdistributary, mouth bar, swamp and shelf in
the Daniudi (大牛地) Gas Field and braided channel, and shelf and lake at Heidaigou (黑岱沟) outcrop.
Braided-river deposits form the lowstand systems tract (LST) in each sequence. Braided channels mark
the sequence boundaries at Heidaigou outcrop. A shelf and lake depositional environment with dark
gray mudstone forms the transgressive systems tract (TST). The location where dark gray mudstone
first appears above the braided channel marks the first flooding surface (FFS), and the end of that
marks the maximum flooding surface (MFS). The highstand systems tract (HST) deposits are
fine-grained sediments with an aggradational parasequence at Heidaigou outcrop and swamp coalbed
in the Daniudi Gas Field. Mouth-bar sand bodies in braided delta front, which form the LST in each
sequence, form excellent reservoirs in the Daniudi Gas Field.
KEY WORDS: sequence stratigraphy, sedimentary facies, braided channel, shelf, Ordos Basin.
INTRODUCTION
There are various viewpoints about the sequence
subdivision and sedimentary facies in the lower
member of the Permian Shanxi Formation, northeastThis study was supported by the China National Key Research
Project (No. 2011ZX05009-002) and the MOE Yangtze River
Scholar and Innovative Team Program of China (No. IRT0864).
*Corresponding author: [email protected]
© China University of Geosciences and Springer-Verlag Berlin
Heidelberg 2013
Manuscript received February 10, 2012.
Manuscript accepted April 25, 2012.
ern Ordos Basin, China. Many scholars described the
sedimentary facies by the cores in the subsurface and
others using the outcrops in the northeastern Ordos
Basin. The sequence subdivision and the sedimentary
facies have not been connected by the cores and the
outcrop until now. Different views cause a large
amount of difficulties in prediction and exploration.
This paper uses the outcrop and the cores from the
subsurface to build a new model of the sequence subdivision and the sedimentary facies in the northeastern
Ordos Basin.
Geological Background
In China, the Ordos Basin is the second largest
Wei Du, Zaixing Jiang, Ying Zhang and Jie Xu
76
sedimentary basin that contains huge proven geologic
reserves of natural gas. With an area of approximately
320 000 km2, the basin is located in the western part
of the North China Block. The Ordos Basin comprises
six structural units: Yimeng uplift, Western edge
overthrust belt, Tianhuan depression, Yishan ramp,
Jinxi flexural belt, and Weibei uplift (Hao et al., 2007;
Cao, 2005; Chang et al., 2004; Li and Lu, 2002). The
focus of this paper is the Daniudi Gas Field, which is
located in northeastern Yishan ramp and has an area of
approximately 2 003 km2 (Fig. 1).
The Lower Permian Shanxi Formation, which
lasted approximately 9 Ma, was deposited in a very
gentle paleo-topographic setting (high in the north and
low-lying in the south) after an overall regressive
107 o
109 o
(a)
succession of the Carboniferous Taiyuan Formation
(Chen et al., 2004; Wang et al., 2002; Wang and Shen,
2000). The Shanxi Formation is an important
gas-bearing stratigraphic unit, particularly the formation’s lower member, which forms one of the most
important gas plays in the Daniudi Gas Field (Hao et
al., 2006).
The thickness of the lower member of Shanxi
Formation varies from 70 to 90 m, and the formation
consists of three submembers, P1s1-1, P1s1-2, and P1s1-3
(Fig. 2). The first submember, P1s1-1, consists of
medium- to coarse-grained sandstone, thick coalbeds
and mudstone, whereas P1s1-2 and P1s1-3 consist mainly
of conglomerate and gravelly sandstones as well as
coarse-grained sandstones, thin coalbeds, and
111 oE
(c)
120 km
0
40 oN
N
Hangjinqi
0
15 km
D32
N
D47
D27
Heidaigou
35 o
36 o
A’
be lt
t bel t
D35
Daniudi
Gas Field
Yulin
Yishan ramp
D12
S
Ji nx i fl ex ur e
37 o
38 o
We ste rn edg e ove rth rus
A
Tian huan depr essio n
39 o
Yimeng uplift
A
D48
D10
D38
D25
D20
D15
D39
D29
D5
Qingyang
S’’
D26 A’’
D52
Weibei uplift
Basin boundary
City name
Tectonic boundary
Daniudi Gas Field
Outcrop location
D15
S
S’’
Well location A
Seismic profile
A’’
Cross-well profile
E
2
0
Western edge
overthrust belt
Daniud i Gas Field
Tianhuan
depressio n
-2
Jinxi flexural belt
Yishan ramp
2 A’
K1
J 1-2
0
C-P
O
∈
T3
-4
-2
-4
-6
-6
-8
Lower
Lower JurassicCretaceous Middle Jurassic
(J 1-2)
(K 1)
100 km
0
Altitude (km)
A
Altitude (km)
(b)
-8
Upper
Triassic
(T 3)
Carboniferous+
Ordovician
Permian
(C-P )
(O)
Cambrian
(∈)
Cambrian+
Ordovician
(∈-O)
Fault
Figure 1. (a) Structural divisions of the Ordos Basin (modified from Qiu and Gong, 1999); (b) geologic
structure cross section of the Ordos Basin (modified from Li and Lu, 2002); (c) well locations and cross-well
profiles shown in Fig. 6a and seismic profiles shown in Fig. 6b of the Daniudi Gas Field.
Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation
77
Figure 2. Lithology, sedimentary facies, and sequence stratigraphic divisions of the lower member of Shanxi
Formation in the Daniudi Gas Field. The lower member of Shanxi Formation can be divided into three
third-order sequences: SQ1, SQ2, and SQ3; LST. lowstand systems tract; TST. transgressive systems tract;
HST. highstand systems tract.
mud-stones. During the Late Paleozoic, siliciclastic
sediments were derived mainly from the Yimeng uplift
in the northern Ordos Basin (Dou et al., 2009).
Previous Research
Differing views exist concerning the sedimentary
models of the study area. He et al. (2001) suggested
that the clastic sedimentary system must have been
caused by the meandering river’s shallow marine delta.
Ye and Qi (2008) proposed that the delta was deposited on a shallow sea. Zhu et al. (2007) described a
wetlands-valley model with strong erosion in the
study area. Other researchers have proposed that the
sedimentary system in the study area is a deltaicfluvial system. Zhang et al. (2011) suggested that the
Lower Shanxi Formation was deposited in an epicontinental environment as is evidenced by marine fossils.
For the study area, many different sequence subdivision schemes have been proposed by different
researchers (Zhu et al., 2002; Fan et al., 1999; Zhai
and Deng, 1999). Zhang et al. (1997) interpreted the
Shanxi Formation as a single third-order sequence and
proposed a sequence stratigraphic model with the lowstand systems tracts (LST) consisting mainly of
braided-channel sandstones, the transgressive systems
tracts (TST) dominated by anastomosing deposits, and
the highstand systems tract comprising meandering
river deposits. However, this interpretation is too
broad to be useful in delineating the detailed sedimentary features observed or to adequately explain the
lateral distribution of reservoir sand bodies in the
study area. Li et al. (2003) interpreted the lower
member of the Shanxi Formation as a single
third-order sequence. Zhu et al. (2007) interpreted the
lower member of the Shanxi Formation as three
third-order sequences that are characterized by a
strong basinward regression.
Wei Du, Zaixing Jiang, Ying Zhang and Jie Xu
78
PP11Ss 1-3
1-3
(Ma)
Upper 275
(m)
8
70
Braided
river
1-2
PP11Ss1-2
5
Lower
Shanxi
Lower Permian
8
4
40
3
30
SQ3
LST
HST
Lake
7
6
5
Rise
TST
Lake
50
Sea level
Fall
7 60
6
Systems
System
tract
tract
Lithology
Sequence
Samples
Thickness
Age
Section
Submember
Member
Formation
Series
OUTCROP SEDIMENTARY FACIES AND
SEQUENCE ANALYSIS
Outcrop Sedimentary Facies
The Heidaigou outcrop is located in southern
Inner Mongolia, approximately 160 km from the
Daniudi Gas Field in northeastern Ordos Basin (Fig.
1a). The outcrop is approximately 70 m thick and can
be divided into 8 sections (Fig. 3).
The lower member of Shanxi Formation at
Heidaigou outcrop begins with a scour surface at the
base, indicating the erosion on the underlying Taiyuan
Formation. The first section of Heidaigou outcrop
consists of gravelly coarse-grained sandstones (Fig. 3).
These deposits comprise three fining-upward successions. In the first fining-upward succession, 4 m thick
gravelly coarse sandstone erodes the mudstone of the
Taiyuan Formation (Fig. 4a). The succession begins
with massive bedding (Fig. 4a), followed by planar
cross-bedding and inclined cross-bedding (Fig. 4b).
Two other fining-upward cycles also have scour
surfaces at their bases. Trough cross-beddings are the
major sedimentary structures. These characteristics
indicate braided-channel deposits.
Sedimentary
Sedimentary
facies
facies
DATABASE AND METHODOLOGY
This work was conducted using cores from 20
wells, 2 000 km2 of 3D seismic data and well-log and
gas-production data from 100 wells. Detailed cores
and precise measurement of a well-exposed outcrop at
Heidaigou allowed us to identify sedimentary microfacies within the three submembers of the lower
member of Shanxi Formation. Samples were analyzed
by ICP-AES to determine the trace elements in the
mudstone. As a result, a new sequence stratigraphic
framework was established, and facies types were
identified (Fig. 2).
Lake
SQ2
TST
Braided
river
LST
Shoreline
HST
4
3
1-1
PP11Ss1-1
2
20
SQ1
Braided
river
1 10
280
0
UC Taiyuan Formation
Conglomerate
Fine-grained
sandstone
LST
1
Gravelly coarse-grained
sandstone
sandstone
Silty
mudstone
TST
Shelf
2
Coarse-grained
sandstone
Mudstone
Medium-grained
sandstone
1
UC
Mudstone
sample
Upper
Carboniferous
Figure 3. Lithology, sedimentary facies, and sequence stratigraphic divisions of the lower member of Shanxi
Formation at Heidaigou outcrop (see Fig. 1a for location).
Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation
The deposits of Section 2 are 8.5 m-thick mudstone (Fig. 3). The color of the mudstone changes
from dark gray to black from bottom to top (Fig. 4c).
The Sr/Ba is 0.48 in sample 2 with a sharp rise to 1.57
in sample 3 (Table 1). In addition, the Sr/Ca increases
from 141.7 to 901.0 in the two samples. All these
characteristics show that the sea level rises sharply
upward in the mudstone. On the basis of the color of
the mudstone and the trace elements in the samples,
we suggest that the mudstone is a shelf deposit.
Section 3 at Heidaigou outcrop consists of mudstone, silty mudstone, siltstone, and fine sandstone.
79
These fine-grained sediments can be divided into three
reverse cycles. Each cycle begins with silty mudstone,
followed by siltstone and fine sandstone. The succession in the sandstone begins with parallel bedding,
followed by wavy cross-bedding (Fig. 4d). Compared
with the samples from the lower part, the Sr/Ba in
sample 4 decreased to 0.13. This decrease indicates a
sharp fall in the relative sea level. These fine-grained
sediments indicate a shoreline deposit.
The preceding three sections comprise submember P1s1-1 of the lower member of Shanxi Formation at
Heidaigou outcrop.
Figure 4. Photographs of the sedimentary structures of the lower member of Shanxi Formation at Heidaigou outcrop (see Fig. 1a for location). (a) Sandstones erode the underlying mudstone; (b) incline bedding; (c)
dark gray mudstone in P1s1-1; (d) parallel bedding; (e) sandstones erode the underlying mudstone in P1s1-2;
(f) planar cross-bedding in P1s1-2; (g) gray black mudstone in P1s1-2; (h) wavy cross-bedding; (i) sandstones
erode the underlying mudstone in P1s1-3; (j) wedge-shaped cross-bedding; (k) planar cross-bedding in P1s1-3;
(l) conglomerates erode the underlying silty mudstone of P1s1-3.
Wei Du, Zaixing Jiang, Ying Zhang and Jie Xu
80
Section 4 is gravelly coarse sandstone that is deposited in a fining-upward succession that begins with
a scour surface at the base (Fig. 3). Gravel diameter in
the coarse-grained sandstone is approximately 2 cm
(Fig. 4e). Planar cross-bedding is the major sedimentary structure (Fig. 4f). These characteristics indicate a
braided river, and the succession may represent
Table 1
braided-channel deposits.
Above Section 4, a 4.5 m thick layer of mudstone
exists whose color changes from gray to dark gray
from bottom to top (Fig. 3). The trace elements in
these three samples (samples 5–7) show that the sea
level rose (Table 1). The Sr/Ba in these three samples
indicates that the mudstone is lake deposit.
Trace-element content in mudstone samples at Heidaigou outcrop, see Fig. 3 for location
Ba
Ca
Cr
Ga
Mn
Ni
Pb
Sr
Zn
(μg/g)
(wt.%)
(μg/g)
(μg/g)
(μg/g)
(μg/g)
(μg/g)
(μg/g)
(μg/g)
Sr/Ba
Sr/Ca
1
275.4
0.4
47.2
3.38
133.6
77.6
116.6
234
213.7
0.85
585.0
2
103.1
0.35
67.28
3.93
153.1
202.9
106.1
49.6
136.9
0.48
141.7
3
120.4
0.21
32.35
0.92
20.71
56.05
30.81
189.2
39.6
1.57
901.0
4
456.9
0.22
49.2
1.09
24.48
5
65.22
0.17
28.26
1.95
48.91
54.13
45.12
60.0
109.2
0.13
272.6
913.1
100.1
24.4
53.0
0.37
143.7
6
70.9
0.19
27.69
3.26
124.6
342.7
33.99
28.1
39.7
0.40
148.1
7
328.3
0.33
73.35
1.31
34.49
45.05
91.62
104.2
168.8
0.32
315.8
Section 6 at Heidaigou outcrop consists of mudstone, silty mudstone, siltstone, and fine sandstone.
These fine-grained sediments can be divided into three
reverse cycles. Each cycle begins with silty mudstone,
followed by siltstone and fine sandstone. The succession in the sandstone begins with parallel bedding,
followed by wavy cross-bedding (Fig. 4h). These
fine-grained sediments indicate lake deposits with
more terrestrial detritus than the fifth part at Heidaigou outcrop.
Sections 4 to 6 comprise the second submember,
P1s1-2, in the lower member of Shanxi Formation at
Heidaigou outcrop.
Section 7 at Heidaigou outcrop is composed of
gravelly coarse sandstone and conglomerate that are
deposited in a fining-upward succession (Fig. 4i). The
gravel diameter in the gravelly coarse sandstone is
approximately 5 cm, and the gravels are directionally
arranged (Fig. 4j). The succession has large trough
cross-bedding (Fig. 4k). These characteristics represent terrestrial braided-channel deposits.
The uppermost section of the deposits at Heidaigou outcrop is gray silty sandstone.
Based on the sedimentary characteristics of the
eight parts of the deposits from P1s1-1 to P1s1-3 at
Heidaigou outcrop, we suggest that three braided-river
depositional systems exist, a shelf depositional system
and three lake depositional systems (Fig. 3). We recognize the first, fourth, and seventh parts of the deposits as a braided-river facies. These three parts of sandstone are braided channel with fine-upward successions.
Sequence Stratigraphic Analysis at Heidaigou
Outcrop
A braided-stream depositional system exists in
the lowermost part of submember P1s1-1 at Heidaigou
outcrop. The erosion on the underlying Taiyuan Formation indicates a sequence boundary of SQ1 (Fig.
4a). The 18 m thick braided-channel sandstone is interpreted as having developed during a relative fall in
sea level and represents the lowstand systems tract
(LST) of SQ1.
The braided-channel sandstone is covered by
gray mudstone, which forms the bottom layer of the
second part of Heidaigou outcrop. The deposits of the
second part are mainly mudstone and silty mudstone
whose color changes from gray to black from base to
top. These characteristics indicate that the sea level
underwent a sharp rise, and the input of terrigenous
sediments decreased, which can also be proved by the
Sr/Ba and Sr/Ca in the mudstone samples. We propose
that the gray mudstone at the beginning of the second
part marks the first flooding surface (FFS), and the
Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation
black mudstone at the end of second part marks the
maximum flooding surface (MFS). Therefore, the
second part of Heidaigou outcrop is attributed to the
transgressive systems tract (TST) in SQ2.
The uppermost deposits of P1s1-1 at Heidaigou
outcrop consist of mudstone, silty mudstone, siltstone,
and fine sandstone, which can be divided into three
reverse cycles. These fine-grained sediments suggest a
greater rate of sediment supply than during the deposition of the underlying mudstone. We infer that the
sea level had begun to fall and that this part of P1s1-1
represents the highstand systems tract deposition in
SQ1 (Chen et al., 2001).
The fourth part of the deposits at Heidaigou outcrop is a braided-river depositional system, which
erodes the HST in SQ1 (Fig. 4e). The erosion forms a
sequence boundary in SQ2. The entire fourth part is
considered the lowstand systems tract of SQ2.
On the basis of the changes in mudstone color
from gray to dark gray from base to top and the testing
of samples from above the LST in SQ2, we infer that
the sea level rose and the fifth part of Heidaigou outcrop represents the TST in SQ2. The uppermost deposits of P1s1-2 constitute several reverse cycles. These
progradational deposited cycles indicate that the sea
level began to fall and that this part of P1s1-2 represents
the HST deposition in SQ2.
The seventh part of the Heidaigou outcrop deposits is typical of the braided channel that formed the
lowstand systems tract of SQ3. The uppermost deposits of the Heidaigou outcrop are 2 m thick gray silty
sandstone, which is eroded by the overlying conglomerate in the upper member of Shanxi Formation (Fig.
4l). We suggest the uppermost part as the TST in SQ3
and that the HST of the SQ3 was eroded by the upper
strata.
SUBSURFACE SEDIMENTARY FACIES AND
SEQUENCE STRATIGRAPHIC ANALYSIS
Sedimentary and Well-Log Characteristics
We chose well D15 as a key well at which to
perform facies analysis. Deposits in P1s1-1 are mainly
sandstone, mudstone, and coalbed (Fig. 5a). The sandstone, which ranges from 2 840 to 2 857.5 m, can be
divided into three parts. The lower part, ranging from
2 853.7 to 2 857.5 m, is coarse- and medium-grained
81
sandstone and displays a coarsening-upward succession. The succession begins with low-angle
cross-bedding, followed by “S” foreset laminae (Figs.
5b and 5c). This part of the sandstone is considered a
mouth-bar deposit in braided delta front.
The middle of the coarse-grained sandstone
forms a fining-upward succession that begins with
massive beddings and is followed by a trough
cross-bedding. The grains are subangular and moderately sorted. The uppermost coarse-grained sandstone
is separated from the middle section by approximately
0.5 m thick dark gray silty mudstone (Fig. 5g), and the
main succession is massive bedding (Fig. 5h). The GR
log of the upper part of the coarse-grained sandstone
is typically cylinder-shaped (2 840.5–2 849.5 m, D15,
Fig. 5). The characteristics of these two fining-upward
parts of sandstone indicate a subaqueous
braided-channel facies.
The mudstone overlying the sandstone, ranging
from 2 835.5 to 2 840.5 m, contains a small amount of
carbonaceous clastics and plant stems (Fig. 5e). Some
crinoids and foraminifera debris are found in the dark
gray mudstone (Zhang et al., 2011). These characteristics indicate that the mudstone was deposited in a marine environment. The sea level rose, and the mudstone was created as the result of a shelf deposit.
The uppermost deposits in P1s1-1, which range
from 2 823.5 to 2 835.5 m, comprise three sets of thick
coalbeds and two sets of thin black carbonaceous
mudstones (Fig. 5f). These deposits formed in a stable
swamp environment.
The deposits in P1s1-2 range from 2 800 to
2 823.5 m and can be divided into three parts (Fig. 5a).
The lower part is sandstone with a fining-upward succession ranging from 2 823.5 to 2 815 m. This sandstone erodes the carbonaceous mudstone of P1s1-1 (Fig.
5h). The main features of the sandstone are the massive gravelly coarse sandstones, which are poorly
sorted but whose roundness is high. The particle size
of the gravels ranges from 2 to 3 mm. The gravelly
sandstone has little matrix and is particle-supported,
indicating a subaqueous braided-channel deposit.
The middle part of P1s1-2 is mudstone and ranges
from 2 803 to 2 815 m. The lithology is mostly grayish green, gray, and chromocratic mudstone with little
silty mudstone (Fig. 5j). On the basis of an electron-
82
probe analysis of the mudstone sample, Zhang et al.
(2011) determined that the content of the Mg/O in the
siderite of the mudstone sample is approximately
0.5%–7.8%, which indicates a shelf depositional
environment.
The uppermost part of P1s1-2, ranging from 2 800
to 2 803 m, comprises coalbeds and thin black carbonaceous mudstones (Fig. 5a). Different from P1s1-1, the
thickness of the coalbed in the upper part of P1s1-2 is 3
m, and it decreases sharply. These deposits indicate a
swamp depositional environment.
The deposits in P1s1-3 range from 2 779.5 to
2 800.5 m and are mainly conglomerate, sandstone,
mudstone and coalbed (Fig. 5a). These deposits can be
divided into four parts. The first three parts form a
coarsening-upward succession followed by a finingupward succession. The fourth part of the sandstone
forms a fining-upward succession.
Wei Du, Zaixing Jiang, Ying Zhang and Jie Xu
The first part of coarsening-upward succession,
which ranges from 2 796.5 to 2 800.5 m, is
coarse-grained sandstone with mudstone rips (Fig. 5a).
The grains are subangular and moderately sorted. The
upper sandstone, which ranges from 2 793.5 to
2 796.5 m, comprises of gravelly coarse sandstone and
conglomerate with a fining-upward succession.
The succession of the second part of the sandstone is the same as that of the first part, forming a
coarsening-upward succession, followed by a finingupward succession that ranges from 2 788.5 to 2 792.5
m (Fig. 5d). The fining-upward succession ranges
from 2 788.5 to 2 790.5 m with 5 cm thick gravelly
strips supported by particles and overlain by 2 m of
conglomerate with massive bedding (Fig. 5i).
The third part was also deposited in a finingupward succession, which ranges from 2 784.5 to
2 787.5 m. The fourth part of the deposit is a fining-
Figure 5. Detailed descriptions of lithology and sedimentary structures within the lower member of Shanxi
Formation from cores (well D15, from 2 780 to 2 858 m). (a) Lithologic log; (b) low-angle cross-bedding; (c)
“S” foreset laminae; (d) fining-upward succession; (e) mudstone in P1s1-1; (f) coalbed in P1s1-1; (g) scouring
surface and coarse-grained sandstone eroding the underlying mudstone in P1s1-1; (h) scouring surface in
P1s1-2; (i) massive bedding; (j) mudstone in P1s1-2; (k) mudstone in P1s1-3. Locations are shown to the left of
each photo.
Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation
upward succession that ranges from 2 782.5 to 2 783.5
m. The channel-floor conglomerate has little matrix
and is particle supported.
The middle section of P1s1-3, from 2 780.5 to
2 782.5 m, is mudstone with carbonaceous clasts (Fig.
5k). Some crinoids and foraminifera debris are found
in the mudstone, however, less than in P1s1-1. These
characteristics indicate that the mudstone was deposited in a marine environment.
The uppermost section of P1s1-3, from 2 779.5 to
2 800.5 m, is 1 m thick coalbed, which indicates a
swamp depositional environment.
All the deposits with coarsening-upward successions indicate mouth-bar deposits, and the finingupward successions indicate subaqueous braidedchannel deposit. In addition, the carbonaceous mudstone and coalbed formed in a swamp.
On the basis of our observations of cores and the
examination of well-log response data, we suggest that
the lower member of Shanxi Formation was deposited
in the following sedimentary facies or subfacies:
subaqueous braided channel, subaqueous interdistributary, mouth bar, swamps, and shelf.
Braided delta is delta with megaclast and controlled by a braided-river system rich in sand and
gravel (Jiang, 2003). A mouth bar was mainly deposited at the end of the subaqueous braided channel.
Subaqueous braided channels represent the most active part of the distributive channel network and are
intimately associated with mouth bars (Cornel and
Janokp, 2006). In the study area, the mouth bar contains fine-, medium-, and coarse-grained sandstone.
Sandstone is found in the mouth-bar deposits in a
coarsening-upward succession with cross-beddings
and “S” foreset laminae (Figs. 5b, 5c, and 5d). The
sandstone in the subaqueous braided channels was
deposited in a fining-upward succession with massive
bedding (Figs. 5g, 5h, and 5i).
Subsurface Sequence Stratigraphic Analysis
A braided delta-front depositional system formed
at the bottom of the lower member of Shanxi Formation in the Daniudi Gas Field. The lower part of the
deposits in P1s1-1 is coarse-grained sandstone that
forms a coarsening-upward succession and overlies
the mudstone in Taiyuan Formation (Fig. 5a). The rest
83
of the sandstones in P1s1-1 are mainly subaqueous
braided-channel deposits. The mutation of the lithology indicates a sequence boundary of SQ1. Gravelly
coarse-grained sandstones are interpreted as having
developed during a relative fall in sea level and represent the LST of SQ1 (Van Wagoner et al., 1990).
The braided delta-front sandstones are overlain
by dark gray mudstone in P1s1-1, which contains some
crinoids and foraminifera debris. These characteristics
indicate that the mudstone was deposited in a marine
environment. Compared with the lower part of the
braided-delta deposits, the sea level rose in these two
parts. The location where dark gray mudstone first
appeared is considered the FFS in SQ1. The location
where the mudstone disappeared is considered the
MFS in SQ1. The entire section of dark gray mudstone forms the TST in SQ1.
The uppermost deposits in P1s1-1 are coalbed and
black carbonaceous mudstones. Coal accumulation is
controlled by the tectonic setting, the depositional environment, the paleoclimate, and the availability of
plant material (Zhang, 2003; Han and Yang, 1980).
Areas in which subsidence rates are either too low or
too high are not favorable for coal accumulation (Zhu
and Wang, 2010; Shao et al., 2003). Bohacs and Suter
(1997) suggested that significant volumes of terrigenous organic matter can be preserved to form coal
only when the overall increase in accommodation is
approximately equal to the production rate of peat.
The sedimentary environment of coal is most likely
within the LST and HST when the rates of sea-level
change are moderate. The overall increase in accommodation must therefore have approximately equaled
the production rate of peat at that time. This aggradational interval represents the HST of SQ1. The entire
swamp deposit forms the HST in SQ1.
A subaqueous braided channel erodes the underlying carbonaceous mudstones of SQ1. The erosion
surface is considered the sequence boundary between
SQ1 and SQ2. We suggest that the deposit forms the
LST of SQ2.
Above the braided delta-front deposits is 12 m of
dark gray mudstone. The content of the Mg/O in the
siderite of a sample of the mudstone is approximately
0.5%–7.8%, which indicates a shelf depositional environment (Zhang et al., 2011). The location where
Wei Du, Zaixing Jiang, Ying Zhang and Jie Xu
84
dark gray mudstone first appeared is considered the
FFS in SQ2. The location where the mudstone disappeared is considered the MFS in SQ2. The entire shelf
deposit forms the TST of SQ2.
At the top of P1s1-2 is a 3.5 m thick layer of coalbed and black carbonaceous mudstones. These swamp
deposits are the HST of SQ2.
The HST of SQ2 is terminated by the overlying
braided delta-front sandstone in P1s1-3. The sandstones
are mainly conglomerate and coarse-grained sandstone, which deposits as subaqueous braided channel
and mouth bar. The braided delta-front sandstone is
interpreted as having developed during a relative fall
in sea level and represents the LST of SQ3.
An approximately 2 m thick layer of mudstone
overlies the LST in SQ3. The marine depositional environment is also considered the TST in SQ3. Compared with SQ1 and SQ2, the marine depositional environment is much thinner than in SQ1 and SQ2 (Fig.
3).
The uppermost deposits are 2 m thick coalbed in
P1s1-3, which can be considered the HST of SQ3.
Based on the vertical associations and
depositional-cycle characteristics, we divided the
lower member of Shanxi Formation into three thirdorder sequences: SQ1, SQ2, and SQ3 in the Daniudi
Gas Field. Each sequence consists of three systems
tracts: LST, TST, and HST. The LSTs in the three
sequences are deposited in a braided delta-front depositional environment, while the TSTs are marine and
the HSTs are swamp environment.
Stacking Patterns and Lateral Trends
As stated above, three sequences exist in the
lower member of Shanxi Formation. The vertical
characteristic of the lithology in each sequence is
overlapped sandstone, mudstone, and coalbed (Fig. 2).
The sandstone occurs mainly in the subaqueous
braided channel and mouth bar of a braided delta front
in the lower part of each sequence or submember.
Dark gray mudstone deposited in a shelf forms the
middle part of each submember. The coalbed and carbonaceous mudstones indicate a swamp depositional
environment as the uppermost layer of each submem-
ber.
The lateral facies trends shown in Fig. 6 indicate
that the subaqueous braided channels in the lower part
of each submember in a northeast-southwest direction
extend with the sway of the estuary in the braided
delta front, which can also be recognized in the seismic profile (Fig. 6b). The sandstone of the mouth bar
was deposited at the end of the subaqueous braided
channels. The braided delta-front depositional system
in P1s1-1 deposits a greater distance from the shoreline
than that in P1s1-2 and P1s1-3. As a result, the mouth
bars reworked by the waves are more developed in
P1s1-1 than in P1s1-2 and P1s1-3.
As shown in Fig. 6a, the width of the subaqueous
braided channel in P1s1-1 is only 2–3 km. In P1s1-3, the
width reaches 6–8 km. The stacking of the subaqueous
braided channels in the three submembers is reflected
clearly in the seismic profiles (Fig. 6a). The extension
of the subaqueous braided channels in the three submembers forms an obvious progradational shape.
The progradational shape of the three submembers P1s1-1 to P1s1-3 is also apparent in other ways. On
the basis of core observations, we find that the granularity and the size of succession in the sandstone vary
regularly from base to top. The sandstone in P1s1-1 is
coarse-grained with little gravel, and the successions
have cross-beddings and massive beddings. The sediments in P1s1-2 are gravelly with a massive succession.
The grain size of the gravel is 2–4 mm (Fig. 5h). The
sediments in P1s1-3 are conglomerate and gravelly
coarse sandstone with massive beddings (Fig. 5i). In
thin sections, from P1s1-1 to P1s1-3, the content of
quartz decreases, while the content of lithic increases.
In addition, the percentage of sandstone increases
while that of mudstone decreases from P1s1-1 to P1s1-3
(Table 2).
The braided delta in P1s1-1 is the smallest of the
three sequences or submembers.The braided channels
extend below the sea level for a short distance. The
terrigenous sediments deposit quickly and then form
mouth bar and interdistributary in the Daniudi Gas
Field. The braided delta extends longer in P1s1-2 and
P1s1-3 and grows increasingly larger (Fig. 6).
Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation
85
Figure 6. Sedimentary facies stacking patterns and lateral trends (NW-SE). (a) The cross-well profile oriented northeast-southwest is perpendicular to the subaqueous braided channel; (b) seismic-profiles responses; see Fig. 1c for location.
Table 2
Clastic composition in sandstone and lithology
percentage in the Daniudi Gas Field
Quartz
Feldspar
Lithic
Sandstone
Mudstone
(%)
(%)
(%)
(%)
(%)
P1s1-1
74.69
1.32
23.27
0.30
0.45
P1s1-2
72.16
3.87
27.53
0.45
0.43
P1s1-3
66.49
2.95
31.20
0.52
0.38
Based on the regular changes in lithology, succession, clastic composition, and the vertical facies
stacking patterns shown in Fig. 6, we propose that the
braided delta is progradational in the three submembers in northeastern Ordos Basin.
DISCUSSIONS
Both Heidaigou outcrop and the Daniudi Gas
Field can be divided into three sequences. The HST of
SQ3 at Heidaigou is eroded by the conglomerate in
the upper member of Shanxi Formation. All three sequences consist of three systems tracts: the lowstand
systems tract, the transgressive systems tract, and the
highstand systems tract at Heidaigou outcrop and in
the Daniudi Gas Field.
Braided stream forms the LSTs in SQ1 at
Heidaigou outcrop, while the LSTs became braided
delta front in the Daniudi Gas Field. The shoreline,
where braided river enters the water, is between
Heidaigou outcrop and the Daniudi Gas Field (Fig. 7).
Sandstones in the three sequences or submembers
are mostly contained in the LSTs. The depositional
environment of the LSTs of the lower member of
Shanxi Formation is braided delta front in the Daniudi
Gas Field (Fig. 8). As shown in Fig. 7, the sedimentary facies are subaqueous braided channel, mouth bar,
and interdistributary.
According to statistical analysis, the wells whose
gas production varies from (2–10)×104 m3/d are
mainly located in the mouth-bar sandstones. The wells
whose gas production varies (0.5–2)×104 m3/d are
mainly located in the subaqueous braided channel
sandstone (Fig. 8).
Wei Du, Zaixing Jiang, Ying Zhang and Jie Xu
86
LST 2 800
2 800
HST
SQ2 TST
LST
HST
22810
810
22820
820
Lake
Marine
Swamp
70
(m)
60
Braided river
Braided delta
Swamp
Marine
Braided river
Braided delta
22830
830
Shoreline
Swamp
Shelf
2 840
SQ1 TST 2 840
Systems
tract
Fall
Rise
TST
40
HST
SQ2 TST
LST
30
HST
TST
20
SQ1
Braided river 10
850
LST 22850
Sea level
SQ3 LST
50
Lake
Lake
Sequence
Sedimentary facies
Sedimentary
facies
HST 2 790
TST 2 790
SQ3
Heidaigou outcrop
Lithology
(m)
22780
780
40 km
Thickness
Rise
Depth
0
Lithology
Fall
Systems
tract
Sea level
Sequence
NE
Daniudi Gas Field
NE
LST
Braided delta
0
22860
860
Conglomerate
Gravelly coarse
sandstone
Coarse-grained
sandstone
Medium-grained
sandstone
Silty
mudstone
Mudstone
Carbonaceous
mudstone
Coalbed
0
Fine-grained
sandstone
40 km
Figure 7. Sedimentary facies lateral trends from Heidaigou outcrop to Daniudi Gas Field in the lower
member of Shanxi Formation.
The sandstone of braided delta front deposited in
the LSTs of the three sequences is a potential reservoir.
In addition, gas production in mouth-bar sandstones is
higher than in subaqueous braided channels.
Figure 8. Sedimentary facies and gas-pool distribution in the LST of SQ1 in the lower member of
Shanxi Formation in the Daniudi Gas Field.
CONCLUSIONS
Based on observations of cores and outcrop and
examination of well-log response data, we suggest that
the lower member of Shanxi Formation was deposited
in the following sedimentary facies or subfacies:
subaqueous braided channel, subaqueous interdistributary, mouth bar, swamp and shelf in the Daniudi
Gas Field and braided channel, shelf and shallow lake
at Heidaigou outcrop. The lower member of Shanxi
Formation deposits a progradational braided delta in
the northeastern Ordos Basin.
The lower member of Shanxi Formation can be
divided into three third-order sequences, SQ1, SQ2,
and SQ3, in the Daniudi Gas Field and Heidaigou outcrop. Each sequence consists of three systems tracts:
the lowstand systems tract, the transgressive systems
tract and the highstand systems tract. Detailed sedimentological and stratigraphic analyses indicate that
the entire sequence is characterized by a regional regression with braided-channel deposits marking the
Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation
bases of each retrogradational sequence, shelf mudstone as the TSTs and shallow lake sandstone deposits
as the HSTs. In addition, in the Daniudi Gas Field, the
braided delta-front deposits form the LSTs of each
sequence with shelf as the TSTs and swamp as the
HSTs.
In the Daniudi Gas Field, the sand bodies of
mouth bar in braided delta front, which form the LSTs
of each sequence, are excellent reservoirs.
87
Fan, T. L., Guo, Q. J., Wu, X. S., 1999. Features of Sequence
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ACKNOWLEDGMENTS
This study was supported by the China National
Key Research Project (No. 2011ZX05009-002) and
the MOE Yangtze River Scholar and Innovative Team
Program (No. IRT0864). We also thank Shiyue Chen
and Longwei Qiu for their assistance in the field work.
Hao, S. M., Li, L., You, H. Z., 2007. Permo-Carboniferous
Paralic Depositional Systems in the Daniudi Gas Field and
Its Near-Source Box-Type Gas Accumulation-Forming
Model. Geology in China, 34(4): 606–611 (in Chinese
with English Abstract)
Han, D. X., Yang, Q., 1980. Coalfield Geology of China. In:
Han, D. X., Yang, Q., eds., Coalfield Geology of China.
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