sequence stratigraphy and sedimentary facies in the...

Journal of Earth Science, Vol. 24, No. 1, p. 075–088, February 2013 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-013-0308-3

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 strati-

graphic 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, northeast-

This 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 sub-division 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

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 forma-tion’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

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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 depos-ited 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 deltaic- fluvial system. Zhang et al. (2011) suggested that the Lower Shanxi Formation was deposited in an epicon-tinental environment as is evidenced by marine fossils.

For the study area, many different sequence sub-division 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 low-stand 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 sedi-mentary 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.


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 micro-facies 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). 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 succes-sions. 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.

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Figure 3. Lithology, sedimentary facies, and sequence stratigraphic divisions of the lower member of Shanxi Formation at Heidaigou outcrop (see Fig. 1a for location).


The deposits of Section 2 are 8.5 m-thick mud-stone (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 mud-stone, 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 succes-sion 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 submem-ber 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 Heidai-gou 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.


Section 4 is gravelly coarse sandstone that is de-posited 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 sedimen-tary structure (Fig. 4f). These characteristics indicate a braided river, and the succession may represent

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.

Table 1 Trace-element content in mudstone samples at Heidaigou outcrop, see Fig. 3 for location

Ba Ca Cr Ga Mn Ni Pb Sr Zn Sr/Ba Sr/Ca

(μg/g) (wt.%) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g)

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 54.13 45.12 60.0 109.2 0.13 272.6

5 65.22 0.17 28.26 1.95 48.91 913.1 100.1 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 mud-

stone, 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 succes-sion 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 Heidai-gou 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 repre-sent terrestrial braided-channel deposits.

The uppermost section of the deposits at Heidai-gou 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 rec-ognize the first, fourth, and seventh parts of the depos-its as a braided-river facies. These three parts of sand-stone are braided channel with fine-upward succes-sions. 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 For-mation indicates a sequence boundary of SQ1 (Fig. 4a). The 18 m thick braided-channel sandstone is in-terpreted 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


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 depo-sition 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 out-crop 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 out-crop represents the TST in SQ2. The uppermost de-posits 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 depos-its is typical of the braided channel that formed the lowstand systems tract of SQ3. The uppermost depos-its of the Heidaigou outcrop are 2 m thick gray silty sandstone, which is eroded by the overlying conglom-erate 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 sand-stone, 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

sandstone and displays a coarsening-upward succes-sion. 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 moder-ately 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 characteris-tics indicate that the mudstone was deposited in a ma-rine environment. The sea level rose, and the mud-stone 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 suc-cession ranging from 2 823.5 to 2 815 m. This sand-stone erodes the carbonaceous mudstone of P1s1-1 (Fig. 5h). The main features of the sandstone are the mas-sive 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 gray-ish green, gray, and chromocratic mudstone with little silty mudstone (Fig. 5j). On the basis of an electron-


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 carbo-naceous 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 fining- upward succession. The fourth part of the sandstone forms a fining-upward succession.

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 sand-stone is the same as that of the first part, forming a coarsening-upward succession, followed by a fining- upward 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 fining- upward 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.


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 depos-ited 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 succes-sions indicate mouth-bar deposits, and the fining- upward successions indicate subaqueous braided- channel deposit. In addition, the carbonaceous mud-stone 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 interdis-tributary, mouth bar, swamps, and shelf.

Braided delta is delta with megaclast and con-trolled by a braided-river system rich in sand and gravel (Jiang, 2003). A mouth bar was mainly depos-ited at the end of the subaqueous braided channel. Subaqueous braided channels represent the most ac-tive part of the distributive channel network and are intimately associated with mouth bars (Cornel and Janokp, 2006). In the study area, the mouth bar con-tains 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 Forma-tion 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

of the sandstones in P1s1-1 are mainly subaqueous braided-channel deposits. The mutation of the lithol-ogy indicates a sequence boundary of SQ1. Gravelly coarse-grained sandstones are interpreted as having developed during a relative fall in sea level and repre-sent 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 mud-stone 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 en-vironment, 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 terri-genous 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 accom-modation must therefore have approximately equaled the production rate of peat at that time. This aggrada-tional interval represents the HST of SQ1. The entire swamp deposit forms the HST in SQ1.

A subaqueous braided channel erodes the under-lying 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 en-vironment (Zhang et al., 2011). The location where


dark gray mudstone first appeared is considered the FFS in SQ2. The location where the mudstone disap-peared 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 coal-bed 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 sand-stone, 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 en-vironment is also considered the TST in SQ3. Com-pared with SQ1 and SQ2, the marine depositional en-vironment 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 third- order 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 depo-sitional 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 car-bonaceous 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 seis-mic 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 sub-members forms an obvious progradational shape.

The progradational shape of the three submem-bers P1s1-1 to P1s1-3 is also apparent in other ways. On the basis of core observations, we find that the granu-larity 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 sedi-ments 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).


Figure 6. Sedimentary facies stacking patterns and lateral trends (NW-SE). (a) The cross-well profile ori-ented northeast-southwest is perpendicular to the subaqueous braided channel; (b) seismic-profiles re-sponses; 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

P1s1-2

P1s1-3

74.69

72.16

66.49

1.32

3.87

2.95

23.27

27.53

31.20

0.30

0.45

0.52

0.45

0.43

0.38

Based on the regular changes in lithology, suc-

cession, clastic composition, and the vertical facies stacking patterns shown in Fig. 6, we propose that the braided delta is progradational in the three submem-bers 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 se-quences 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 sedimen-tary 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).


Sy

ste

ms

tract

Th

ick

ness

Lit

ho

log

y

(m)

10

20

30

40

50

60

0

70

LST

TST

HST

LST

TST

HST

LSTSQ3

SQ2

SQ1

TST

Seq

uen

ce

Lit

ho

log

y

Sedimentary facies

Sy

ste

ms

tract

Sea level

Fall Rise Seq

uen

ce

Sea level

Fall Rise

LST

TST

HST

LST

TST

HST

LST

TST

HST

SQ1

SQ2

SQ3

Depth

2 780

2 790

2 800

2 810

2 820

2 830

2 840

2 850

2 860

(m)

CoalbedSilty

mudstone

Carbonaceous

mudstoneMudstone

Fine-grained

sandstone

Medium-grained

sandstone

Coarse-grained

sandstoneConglomerate

Gravelly coarse

sandstone

Braided delta

Braided river

Shelf

Braided delta

Braided river

Braided river

Braided delta

SwampShoreline

LakeMarine

Lake

LakeSwamp

MarineSwamp

Daniudi Gas Field Heidaigou outcrop

NE 0 40 km

Sedimentary facies

NE

0 40 km

2 780

2 790

2 800

2 810

2 820

2 830

2 840

2 850

2 860

Figure 7. Sedimentary facies lateral trends from Heidaigou outcrop to Daniudi Gas Field in the lower member of Shanxi Formation.

Figure 8. Sedimentary facies and gas-pool distribu-tion in the LST of SQ1 in the lower member of Shanxi Formation in the Daniudi Gas Field.

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. 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 interdis-tributary, 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 out-crop. Each sequence consists of three systems tracts: the lowstand systems tract, the transgressive systems tract and the highstand systems tract. Detailed sedi-mentological and stratigraphic analyses indicate that the entire sequence is characterized by a regional re-gression with braided-channel deposits marking the


bases of each retrogradational sequence, shelf mud-stone 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. 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. REFERENCES CITED

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