Scholarly article on topic 'Architecture and quantitative assessment of channeled clastic deposits, Shihezi sandstone (Lower Permian), Ordos Basin, China'

Architecture and quantitative assessment of channeled clastic deposits, Shihezi sandstone (Lower Permian), Ordos Basin, China Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Chengye Jia, Ailin Jia, Xin Zhao, Jianlin Guo, Haifa Tang

Abstract Lower Permian Shihezi sandstone in Ordos Basin is the largest gas reservoir in China. Architecture elements of channel, overbank and floodplain facies of braided channel deposits were identified through an outcrops survey, and their proportion of channel facies have been quantitatively estimated from well logging. Characteristics of architecture elements, such as sand thickness, bounding surfaces and lithofacies were investigated through outcrops and core. Petrology of Shihezi sandstone has also been studied in detail. Analysis on sandstone components shows that monocrystalline quartz with approximately 76% bulk volume, and lithic up to 5%–45% bulk volume, are the two main components. Litharenite and lithic quartz sandstone are the main rock types. Compaction is concluded by former researchers as the control factor of low permeability. Examination through thin section reveals that secondary pores developed well in coarse sand. Inter-granular dissolution is included as the positive effect to increasing porosity, and is concluded as the control factor to the generation of net pay. Scale of coarse grained channel fills and channel bar sandstone bodies are quantitatively estimated. Strike-oriented, dip-oriented, and vertical distribution of channel fills and channel bar sandstone bodies have been investigated. The geometry of sand bodies can be depicted as an elongated lens. Subsurface mapping reveals that channel sandstone bodies distribute widely from both lateral and longitudinal cross section profiles, and are poorly connected.

Academic research paper on topic "Architecture and quantitative assessment of channeled clastic deposits, Shihezi sandstone (Lower Permian), Ordos Basin, China"

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ADVANCING RESEARCH EVOLVING SCIENCE

Available online at www.sciencedirect.com

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Journal of Natural Gas Geoscience xx (2017) 1—10

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Original research paper

Architecture and quantitative assessment of channeled clastic deposits, Shihezi sandstone (Lower Permian), Ordos Basin, China

Chengye Jia*, Ailin Jia, Xin Zhao, Jianlin Guo, Haifa Tang

Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China

Received 7 September 2016; revised 16 December 2016 Available online ■ ■ ■

Abstract

Lower Permian Shihezi sandstone in Ordos Basin is the largest gas reservoir in China. Architecture elements of channel, overbank and floodplain facies of braided channel deposits were identified through an outcrops survey, and their proportion of channel facies have been quantitatively estimated from well logging. Characteristics of architecture elements, such as sand thickness, bounding surfaces and lithofacies were investigated through outcrops and core. Petrology of Shihezi sandstone has also been studied in detail. Analysis on sandstone components shows that monocrystalline quartz with approximately 76% bulk volume, and lithic up to 5%—45% bulk volume, are the two main components. Litharenite and lithic quartz sandstone are the main rock types. Compaction is concluded by former researchers as the control factor of low permeability. Examination through thin section reveals that secondary pores developed well in coarse sand. Inter-granular dissolution is included as the positive effect to increasing porosity, and is concluded as the control factor to the generation of net pay. Scale of coarse grained channel fills and channel bar sandstone bodies are quantitatively estimated. Strike-oriented, dip-oriented, and vertical distribution of channel fills and channel bar sandstone bodies have been investigated. The geometry of sand bodies can be depicted as an elongated lens. Subsurface mapping reveals that channel sandstone bodies distribute widely from both lateral and longitudinal cross section profiles, and are poorly connected. Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Shihezi sandstone; Lower Permian; Ordos Basin; Architecture elements; Petrology; Quantitative assessment

1. Introduction

Modern fluvial sedimentology had its beginnings during the World War II, with the work of H. N. Fisk [1] and his colleagues on the depositional framework of the Mississippi River [2]. Researches began on architecture of channeled reservoir in 1980s [3] and facies analysis techniques for the detailed description and interpretation of channel-fill architecture have been improved [4]. Over the past 20 years, outcrop analogs have also been integrated in reservoir characterization and reservoir modeling to reduce uncertainties

* Corresponding author.

E-mail address: cyjia@petrochina.com.cn (C. Jia). Peer review under responsibility of Editorial office of Journal of Natural Gas Geoscience.

and to understand heterogeneities of deposition units in three dimensions [5—7]. Geological characterization including elements, pattern and deposition mechanisms derived from outcrops have been proved to be valuable information which can be used as conditioning data in the recognition and description of subsurface fluvial architecture [5,8].

Ephemeral-fluvial braided systems constitute one of the main reservoirs in many oil and gas fields around the world [8—13]. Reservoir architecture elements and its characteristics are control factors in accumulation and recovery of hydrocarbon [14,15].

The widespread distribution of braided fluvial Shihezi sandstone bodies in the Ordos Basin and their potential as hydrocarbon reservoir warrant a better understanding of the fluvial processes involved in their development. Most published descriptions of the Ordos Basin Shihezi sandstone are

http://dx.doi.org/10.1016/j.jnggs.2017.01.002

2468-256X/Copyright © 2017, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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piecemeal and two-dimensional, and are insufficient to reconstruct adequately their sedimentary architecture as well as to infer the processes associated with bar formation and migration, and channel incision and filling. Outcrops located in Liulin County, eastern of Ordos Basin allowed a detailed, three dimensional description of one Shihezi sandstone body. With more exploration and development wells drilled, sufficient log and experimental data of core analysis makes reservoir characterization feasible.

The purpose of our study is to provide geoscientists and engineers with qualitative description and quantitative data from Lower Permian channeled sandstone deposits. This data set is of great importance to success rate of exploration well and field development strategy.

In this paper, we demonstrate a comprehensive approach to architecture elements analysis integrated with outcrop investigation and reservoir characteristics of different units including lithofacies, stratal geometries, and petrophysical properties derived from laboratory analysis on core samples.

2. Data and methods

Braided stream deposits consist of numerous interconnected channels, separated by bars [16] and dominated by coarse-grained sediments such as sands and gravel [17]. The entire channel complex may contain water and the bar may be submerged during high water period. During periods of drought, only one channel, or even no channel, is active. Thus, multi-cycle sandstone and gravel sheets are deposited in a braided channel and its width may vary widely with respect to its thickness. With respect to depth, an upper limit of width can be estimated [18].

By outcrops investigation, qualitative and quantitative data such as bedform geometry, bedset thickness, and lateral continuity can be obtained by determining the individual geobody dimensions of fluvial sandstone and are used to guide the reservoir characterization [5,19,20]. Through Field survey, thickness of different channel facies can be estimated and lithofacies can be identified.

Complete sets of logs (Latero Log Deep, Latero Log Shallow, Formation Density Compensated Log, Compensated Neutron Log, Bore Hole Compensated Sonic Log, Spontaneous Potential Log, and Gamma Ray Log) for the reservoir units were digitized. Eletrofacies zonation through well logging extrapolation helps to estimate scale of multi-storey channels and single-storey channels and log interpretation can be used to construct cross section maps.

Cores obtained from drilling provide direct and detailed information such as lithology, lithofacies, and sedimentary structures which are symbols of sedimentary environment and thin section analysis is the efficient way for pore structure analysis and pore space characterization.

Quantitative data for calculating the proportions of facies and scale of sandstone bodies were collected by different spacing well logs from cross section. Well logs are commonly close to actual measured sections [21]. Eighty well logs were used to make the calculations.

3. Geological setting and stratigraphy

Ordos Basin with area of about 32 x 104 km2 is one of the largest sedimentary basins in China. Although once wrongly regarded as a relatively stable cratonic sedimentary basin [22,23], it is now widely considered as a Gondwana-derived fragment of continental crust on the western edge of the North China block [24]. Complex tectonic and sedimentary evolutions of the basin have resulted in the formation of various structural units consisting of highly tectonic fold-thrust belts and horst-graben features forming mountainous outer rim of the Ordos Basin (Fig. 1). Six first-grade tectonic units can be identified: Weibei Uplift, Yimeng Uplift, Jingxi Flexing Belt, Tianhuan Depression, Western Margin Thrust Belt and Yishaan Slop. In contrast, inner part of Ordos Basin shows minor tectonic deformation. Microcontinental amalgamation from Middle to Late Paleozoic provided a broad, gently subsiding craton on which were deposited terrigenous shelf, deltaic and fluvial clastics and associated coals.

The regional stratigraphy of the Ordos Basin is illustrated in Fig. 2. The basement of the basin comprises Archaean and Lower Proterozoic crystalline rock. Middle and Upper Ordo-vician, Silurian, Devonian and Lower Carboniferous units are absent within the major part of the basin. According to fission

Fig. 1. Simplified geological map of the Ordos Basin and location of the outcrops (Modified from Ref. [25], 2003).

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Fig. 2. Regional stratigraphy of Ordos Basin (Modified from Ref. [25], 2003).

track studies, the basin underwent a regional uplift of almost 2000 m, during which Cretaceous and Palaeogene/Neogene sediments would have been eroded in most part of the basin [25]. Sedimentary sequence, with thickness of 6000 m in the center part of the basin, is composed of Lower Paleozoic marine carbonates, Upper Paleozoic shallow marine to continental clastic deposits and Cenozoic and Mesozoic continental clastics.

The Permian units of the study area are composed of a braided clastic depositional series which are subdivided into

Shiqianfeng Formation, Upper Shihezi Formation, Lower Shihezi Formation and Shanxi Formation [26]. Lower Shihezi is the main reservoir layer and distributed widely in the basin. It is comprised of numerous narrow and elongate fluvial sandstone bodies that fill channels incised into floodplain strata.

4. The hierarchy of depositional units and lithofacies

The concept of a hierarchy of depositional scales and the relationship of hierarchy to depositional processes on various time and physical scales was firstly initiated by Allen [27]. Miall developed a numerical ranking to the architectural subdivision of fluvial deposits [17] and compared the two classifications [28]. In our study, we employ the classification of Miall [28].

4.1. Bounding surface

The most prominent bounding surfaces in the outcrop profile described in this paper which extend for a hundred meters along the profile are termed as channel elements (or fifth-order surfaces). They are visible in Liulin outcrop because coarse sandstone above the surface and coal/mudstone below. In different locations, they may be thin, but the contrasts between a well-cemented sandstone or pebbly sandstone and underlying mudstone or coal are still obvious. Macro-forms are ranked as fourth-order architectural elements and constitute the major subdivisions of the fifth-order channel element and accretionary cross-bedding are the most distinctive macroforms [28]. The fourth-order surfaces are commonly convex-up, parallel to the accretionary bedding, or flat, truncated by erosion. First- to third-order rank surfaces have not been distinguished in this study because of its small scale.

4.2. Lithofacies

Observation and classification of lithofacies are now standard components of the facies-analysis methodology for studying sedimentary rocks. In the Shihezi Formation, seven lithofacies and subfacies are recognized on basis of study of outcrop and core. Shihezi Formation was subdivided into two lithofacies assemblages, including sand lithofacies and finegrained lithofacies. Individual lithofacies are listed in Table 1, modified on classification of Rust and Jones [29].

5. Architecture elements

Early attempts at the classification of architectural elements were developed by some researchers [30—33]. A revised classification of fluvial architectural elements was provided by Miall [17] based on the former works. Architecture elements analysis conducted in this study employ the classification of Miall [17]. Sandy Bedforms (Element SB), Gravel Bars and Bedform (Element GB), Laminated Sand Sheets (Element LS), and Biochemical deposits of overbank environment were identified through Liulin outcrops survey.

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Table 1

Lithofacies in Shihezi sandstone, Ordos Basin, China.

Lithofacies assemblages

Lithofacies

Sand Lithofacies Spe Matrix support, sand-granular-pebble sand (Fig. 3g) Sh Horizontal cross-bedded sand (Fig. 3d) Sp Planar cross-bedded sand (Fig. 3c) Sr Ripple, wavy and climbing-ripple sand (Fig. 3f) Sw Wedge cross-bedded sand (Fig. 3e) Sm Structureless to faintly laminated sand, massive sand (Fig. 3a) St Trough cross-bedded sand (Fig. 3b) Fine-grained Fm Mudstone (Fig. 3h and i)

Lithofacies Fs Fine grained sand (Fig. 4c)

5.1. Channel

In fluvial deposits, channels commonly have multistory fills with each story bounded by an erosion surface and major channels are bounded by fifth-order surfaces. The Channel element of Shihezi sand includes components of SB, GB, and

LS units. The bounding surfaces and channel margins of Shihezi sandstones are completely exposed with cutbanks observed in outcrop.

5.1.1. Sandy bedforms (element SB)

Element SB is part of channel fills and constitutes 39.47% of the channel facies. This element is composed of sand lithofacies of Sp, Sh, Sw and Sm which result from the transport of sand by traction currents as bed load and in intermittent suspension. Individual sandy bedforms observed through outcrops in this study are about 4.3 m—8.3 m in thickness (Fig. 4a and b).

The sandstones in this element vary from fine-grained and medium-coarse-grained to coarse-grained; and locally contain scattered pebble. Bedding in these facies consists of cross-bedding, ripple, massive, and faintly lamination.

5.1.2. Gravel bars and bedforms (element GB)

Facies channel bar constitute 11.31% of the channel facies. The gravel clastic sandstones are amalgamated into bars in several ways and three main methods of midchannel gravel bar

Fig. 3. Lithofacies of Lower Permian Shihezi sands identified from cores. (a) Grey medium-coarse massive sandstone, Well Zhao 15, 2950 m; (b) Ash grey trough cross-bedded coarse sandstone, Well Tong 11, 2825.5 m; (c) Ash grey planar cross-bedded coarse sandstone, Well Shan 171, 3123.5 m; (d) Ash grey horizontal cross-bedded coarse sandstone, Well Tong 11, 2879.8 m; (e) Green grey wedge cross-bedded coarse sandstone, Well Shan 144, 3241.78 m; (f) Dark grey ripple argillaceous siltstone, Well Zhao 11, 3143.18 m; (g) Ash grey matrix support pebble sandstone, Well Zhao 20, 3068.1 m; (h) Dark massive mudstone, Well Zhao 28, 3157 m; (i) Dark grey mudstone, Well Shan 246, 3061 m.

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Fig. 4. Architecture elements identified from outcrops which are located in Liulin County, Shaanxi Province, China. The exact location shown in Fig. 1. Lower Permian Shihezi Formation is exposed and bounding surfaces are also indicated by arrows with rank numbers. (a) and (b) Element of sand bedforms with thickness of 4.3 m (a) and 8.3 m (b); (c) Element of floodplain fines with thickness of 3.3 m; (d) Gravel bars and bedforms with thickness of 3.15 m.

construction were recognized by Ashmore [34] (1991). Element GB are usually inter-bedded with lenses of gravity-flow deposits and lithofacies such as Spe (Fig. 4d) and is characteristic of lithofacies. St matrix support conglomerates surveyed from the drilling core are commonly composed of centimeter-scale clasts, from 1 cm to 3.6 cm in diameter.

5.1.3. Laminated sand sheets (element LS)

Laminated sand sheets are composed of fine-grained sandstones and thin-bedded shale with high clay contents such as lithofacies of Sr (Fig. 3f). Horizontal bedding or wavy bedding develops. This element is interpreted as a variant of the channel element in which the channel is of low energy, possibly undergoing abandonment and is filled by fine-grained deposits [28,29]. Element LS constitute tiny contents of the channel elements, almost 0.22%.

5.2. Overbank

Facies overbank constitute 4.8% of the channel facies. Coal and paleosols are typical components of biochemical sediments, which are symbols of architectural elements of the overbank environment [17]. Coal seams are typically inter-bedded with fine-grained overbank sediments and may also overlie or underlie crevasse-splay deposits and fluvial channel-fill deposits (Fig. 4b and d).

Fossil soil yield regarding climates and the evolutionary patterns of fluvial floodplains. Root traces, soil horizons, and soil structures are the three main field features of paleosols [35]. Color and components of paleosols are related to climate

and source terrains [17]. Black and dark-green mudstones (Fig. 3i) indicate sedimentary environment of swamp and humid-semiarid climate.

5.3. Floodplain

5.3.1. Floodplain fines (element FF)

Facies floodplain constitutes 44.42% of the channel facies. Floodplain fine sand consists of sheet like units. It reflects the depositional surface as flat and may be traced laterally for more than 100 m [17]. Element FF in this study can be investigated in outcrops with thickness of 3.3 m and fine grained sand (Fig. 4c).

6. Characteristics of sands

6.1. Petrology and diagenesis

Diagenesis phases including compaction, siliceous cementation and carbonate cementation have been studied by former researches [26,36]. Compaction is concluded as the control factor of low permeability. Whereas, dissolution is regarded as the positive effect to porosity increasing and is concluded as the control factor to the genesis of net pay.

In our study, examination of Shihezi sandstones in thin section reveals a composition dominated by monocrystalline quartz (approximately 76% bulk volume) and lithic (up to 5%—45% bulk volume). Litharenite and lithic quartz sandstone with less quartz sandstone are the main rock types (Fig. 5). Sandy bedforms (Element SB) and gravel bars and

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Fig. 5. Constitutional diagram of sandstone components (Q: quartz; F: feldspar; L: lithic; Block 1: quartz sandstones; Block 2: litharenite quartz sandstones; Block 3: lithic sandstones; Block 4: feldspar lithic sandstones. Modified from Zhu [37] 2008).

bedforms (Element GB) are the channel elements of high energy and filled by bed-grained deposits including sand lithofacies of Sp, Sh, Sw, Sm, Spe. Poor-sorted deposits have less content of quartz and high content of lithic and are lith-arenite or lithic quartz sandstone (Fig. 5). Fine grained deposits have high content of quartz and are mainly quartz

sandstones. Laminated Sand Sheets (Element LS) and flood-plain fines (Element FF) including lithofacies of Fs are of low energy depositional environment and are well-sorted.

Primary pores are less developed hampered by compaction. Secondary pores are well developed in coarse sands. Intergranular dissolution pores and intergranular secondary pores caused by phase transformation of feldspar to kaolinite were observed in microphotograph which results in positive effect to porosity increase (Fig. 6). Coarse sands characterized by matrix supports have been less infected by compaction and primary pore partly remains. Remaining primary pores act as the accumulation space for sour fluids during the diagenetic process. This may result in the intergranular dissolution and development of secondary pores. Poor-sorted lithofacies of Sp, Sh, Sw, Sm and Spe are formed in high energy channel with high deposition rates. These sands have large scale grains which act as supporting matrix during compaction and then less hampered by compaction. Primary pores partly remains in these sands which result in relatively high porosity.

Whereas, fine grain sands with lithofacies of Fs and Sr do not have support from the grain matrix, and less primary pores left through compaction which result in less accumulation spaces for sour fluids and secondary pores are less developed due to less occurrences of dissolution. Pore volume reduced during compaction and no increase through secondary diagenesis. Thus, compared with fine grain sands, coarse sand has higher porosity and is favorable to be reservoir net pay.

Fig. 6. Examination of pore structure through thin section (Q: quartz; M: mica; K: kaolinite; P: pore; D: deformation of mica; E: enlargement of quartz). (a) Intergranular dissolution pore (plane polarized); (b) Concavo-convex grain-to-grain contact caused by deformation of mica (perpendicular polarized); (c) Pore reduction with quartz secondary enlargement (perpendicular polarized); (d) Phase transformation of feldspar to kaolinite with generation of intergranular pore (plane polarized).

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6.2. Electrofacies analysis

Well logging provide digitized information of full column and has been an effective method for geological study. In our study, there is a good correlation between lithofacies derived from core and well logging response. Coarse sands formed in point bar have typical well logging response of relative low value of gamma ray log (Fig. 7). As shown in Fig. 7, different lithofacies have different logging responses. Channel sands including lithofacies of Spe, Sh, Sp, Sr, Sw, Sm and St have relatively low GR response due to its poor content of shale volume. Channel bars and Bedforms (Element GB) composed of lithofacies including St and Spe shows even lower GR value. Mudstone and horizontal cross-bedded sands in of low energy currents have high shale volume and GR value in well logging.

In this work, well logging reveals that coarse sands can be classified into two different types. The first type of coarse sands has relative low value of GR, SP and DEN and relative low value of RLLD and RLLS. Low value of GR, SP and DEN can be interpreted as its higher porosity and permeability and low value of RLLD and RLLS should be ascribed to its less or no hydrocarbon content. They should be interpreted as sediments of channel fills (Figs. 8 and 9).

Besides characteristics of low values of GR, SP and DEN, the second type of coarse sands have relative high value of

RLLD and RLLS. According to well logging interpretation, these coarse sands have much higher porosity and permeability than type one and its high value of RLLD and RLLS indicates high content of hydrocarbon accumulated in pore. Based on former study on facies and petrology, they should be sediments of channel bar (Figs. 8 and 9).

6.3. Scale of sandstone bodies

Width and spacing of the sand bodies are key factors to success rate of exploration well, and degree of interconnec-tedness determines the strategies of field development [17] (Miall 1996).

In this study, mapping of sandstone bodies were conducted using data derived from a well-developed field (well spacing rang from 300 m to 800 m). Subsurface mapping reveals that channel sandstone bodies distribute widely from both lateral and longitudinal cross section profiles and have relatively poor connectivity (Figs. 8 and 9). This indicates a frequent diversion of braided stream. Channel sandstone is interbedded in floodplain deposits. Multistory channel fill sandstone bodies have thicknesses ranging from 7 m to 18 m with an average of 12 m. Channel width is estimated in range of 930 m—1465 m with an average of 600 m through the dip oriented (WE) cross section profile. Then width-to-thickness ratio is estimated to be 50:1.

Fig. 7. Correlation between lithofacies derived from core and well logging response, showing well section of 3064—3081 m, Well Zhao 20. Coarse sands have typical well logging response of low value of gamma ray log (VF = very fine; F = fine; M = medium; C = coarse). Lithofacies: (a) Horizontal cross-bedded sand (Sh), interlayered with ripple and climbing-ripple sand (Sr); (b) Upper, trough cross-bedded sand (St), lower, massive sand (Sm) to granular sand (Spe); (c) Sand-granular-pebble sand (Spe); (d) Planar cross-bedded sand (Sp); (e) Fine grained sand (Fs); (f) Mudstone (Fm).

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Fig. 8. Stratigraphic interpretation of sandstone bodies through Shihezi Formation Dip oriented (WE) cross section, showing lateral connectivity and scale of sandstone bodies. Q3

Fig. 9. Mapping of sand bodies, showing poor connectivity and widespread distribution. Strike-oriented cross-sections (NS) through Shihezi Formation, showing longitudinal connectivity and scale of sandstone bodies. (Channel fills composed of lithofacies including Spe, Sh, Sp, Sr, Sw, Sm and St; Channel bar composed of lithofacies including St and Spe. Modified from Miall [28], 2003).

Coarse-grained channel bars which have relative high porosity, permeability and high hydrocarbon content are the effective reservoir. Coarse-grained channel bar sandstone bodies have thickness ranging from 5 m to 8 m with an average of 6.2 m; and its width ranges from 70 m to 130 m with the mean value of 105 m. Strike-oriented scale of coarse grained channel bar sandstone bodies is estimated to be 230 m—650 m, with the mean length of 475 m. The geometry of channel bar sand bodies can be depicted as an elongated lens.

7. Conclusions

This work presents an approach to architecture elements analysis on basis of outcrops survey and quantitative assessment to channel fills and channel bar sandstone bodies. Architecture elements including sandy bedforms (element SB), gravel bars and bedform (element GB), and biochemical deposits of overbank environment and floodplain fines, were identified from outcrops or cores. The characteristics of different elements (thickness, bounding surfaces and

lithofacies) were studied. It can be concluded that the sedimentary setting of Lower Permian Shihezi Formation should be braided river.

Sandstone components and rock types were studied in detail. Monocrystalline quartz with approximately 76% bulk volume and lithic up to 5%—45% bulk volume are the two main components. Litharenite and lithic quartz sandstone with less quartz sandstone are the main rock types. Examination through thin section reveals that secondary pore developed well in coarse sand.

Scale of coarse grained channel fills and channel bar sandstone bodies are quantitatively estimated. Subsurface mapping reveals that channel sandstone bodies distribute widely from both lateral and longitudinal cross section profile, and are poorly connected. The geometry of channel bar sand bodies can be depicted as an elongated lens. Width-to-thickness ratio of channel fills is estimated to be at 50:1.

Lower Permian Shihezi sandstone is the main gas reservoir of Ordos Basin. This study did some work from the aspect of sedimentology and carried out some quantitative assessment for channel fills and channel bar sand bodies. Intensive study

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on reservoir heterogeneity and detailed reservoir model are recommended for further work.

Foundation item

Supported by China National Science & Technology Major Project (2016ZX05062, 2016ZX05015-006); PetroChina R&D Project (2014F-4701-03).

Conflict of interest

The authors declare no conflict of interest. Acknowledgments

The authors would like to thank Dr. Lijuan Wang, Dr. Na Luo, Dr. Hanqing Zhu and Dr. Qunming Liu for numerous discussions in the field and meaningful suggestions to this work.

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