Chemostratigraphy and sedimentary facies analysis of the Permian Lucaogou Formation in the Jimusaer Sag, Junggar Basin, NW China: Implications for tight oil exploration

Accepted Manuscript Chemostratigraphy and sedimentary facies analysis of the Permian Lucaogou Formation in the Jimusaer Sag, Junggar Basin, NW China: Implications for Tight Oil Exploration Chang Liu, Keyu Liu, Xiaoqi Wang, Luya Wu, Yuchen Fan PII: DOI: Reference:

S1367-9120(18)30135-4 https://doi.org/10.1016/j.jseaes.2018.04.013 JAES 3467

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Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

2 February 2018 21 March 2018 14 April 2018

Please cite this article as: Liu, C., Liu, K., Wang, X., Wu, L., Fan, Y., Chemostratigraphy and sedimentary facies analysis of the Permian Lucaogou Formation in the Jimusaer Sag, Junggar Basin, NW China: Implications for Tight Oil Exploration, Journal of Asian Earth Sciences (2018), doi: https://doi.org/10.1016/j.jseaes.2018.04.013

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Chemostratigraphy and sedimentary facies analysis of the Permian Lucaogou Formation in the Jimusaer Sag, Junggar Basin, NW China: Implications for Tight Oil Exploration

Chang Liu1, Keyu Liu2,3*, Xiaoqi Wang1, Luya Wu2 and Yuchen Fan2 1 Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China 2 China University of Petroleum (East China), Qingdao, Shandong 266580, China 3 Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266071, China

Abstract: The middle Permian Lucaogou Formation (P2l) in the Jimusaer Sag of the southeastern Junggar Basin, NW China hosts China’s first commercial tight (shale) oil production. Two tight-oil sweet spot intervals have been identified within the P2l formation. Coupled chemostratigraphic and sedimentary facies analysis reveals that the sweet spot intervals were deposited in deep–shallow saline lacustrine to nearshore environments under an overall dry climate setting. The sweet spot intervals in the P2l formation comprises several chemostratigraphically and lithologically distinct units deposited under the influence of subtle climatic and environmental changes that have previously not been recognized. A total of 11 depositional units have been identified within the two sweet spot intervals based on an Integrated Prediction Error Filter Analysis (INFEFA) of Gamma Ray logs and environmental parameters derived from chemostratigraphic data. The tight oil reservoir sweet spot intervals were found to be controlled by the spatial and temporal distribution of total organic carbon (TOC) and reservoir properties (e.g., porosity and permeability) and source-reservoir coupling. Two potential tight oil exploration plays are recognised, including those depositional units with porous reservoir beds interbedded with high-TOC source beds (a self-generation play), and those units with porous reservoir beds adjacent to high-TOC source beds (a near-source play). Keywords: Cheomostratigraphy, tight oil, palaeoenvironment, Lucaogou Fm, Jimusaer Sag, Junggar Basin

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1. Introduction

Tight oil reservoirs have become an important emerging reservoir play for hydrocarbon exploration in recent years (Sonnenberg and Pramudito, 2009; Sun et al., 2016). Tight oil is mainly accumulated in fine-grained reservoir formations with low porosity and low permeability, such as fine sandstone, siltstone, shale, mudstone and carbonate, etc. (IEA, 2013; BGR, 2012). These types of reservoir formations are usually deposited under relative low-energy environmental settings (Schieber and Zimmerle, 1998; Jiang et al., 2013) at relative slow depositional rates, often as thin beds. These fine-gained rock formations often have experienced complex palaeo-environmental changes. This is especially true for the continental lacustrine systems, which are much more sensitive to high-frequency climate changes down to seasonal scales (Katz and Lin, 2014). Therefore the changes in accommodation space and sediment supply are much more frequent compared with marine depositional systems (Permutter and Matthews, 1990; Bohacs et al., 2012). The variations of palaeo-environmental settings have direct impact on the sedimentary and lithological facies, organic matters (total organic content (TOC)), mineral types, porosity and permeability of the tight reservoir formations.

To date, over one billion tons (about 7 billion bbls) of proven oil reserves have been found within the middle Permian Lucaogou Formation tight oil plays in the Jimusaer Sag, southeastern Junggar Basin (Kuang et al., 2012). Tight oil in the area has been discovered mainly in two fairways or “sweet spot” intervals, where single well production from the intervals amounts up to 60 tons (about 420 bbls) per day via horizontal drilling and fracturing, showing an excellent oil production potential (Kuang et al., 2012).

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The Lucaogou Formation (P2l) was previously regarded as a set of source rock in the Junggar Basin for conventional petroleum resources. It has relatively high TOC, ranging mainly from 2.8% to 4.1%. The kerogen types are dominated by Type I and Type II organic matters. Vitrinite reflectance (Ro) mainly ranges from 0.7% to 1.0%, within the oil generation window (Kuang et al., 2014; Cao et al., 2016a). Organic matter occurs widespread within almost all types of lithofacies including mudstone, dolomitic mudstone, silty fine sandstone, muddy siltstone, etc. (Cao et al., 2016a; Qu et al., 2017). The main mineral types include quartz, K-feldspar, albite, calcite, dolomite, hematite, pyrite, gypsum and clay, etc. (Xi et al., 2015; Wu et al., 2016). The Lucaogou Formation is believed to have been deposited in a hyper saline lake and is characterised by predominantly lacustrine deposition including: deep lacustrine, semi-deep lacustrine, shallow lacustrine to nearshore, deltaic and beach bar sedimentary facies (Zhang et al., 2002; Wu et al., 2015; Wu et al., 2016; Qu et al., 2017). However, published works on the depositional environments of the Lucaogou Formation so far are quite general, and lack elaborated delineation of the spatial variations of the complex lithofacies. Oil accumulation in the Lucaogou Formation displays near-source charging characteristics, typical of self-generation and near-source reservoir plays in the two sweet spot intervals (Cao et al., 2016a).

Here we report a detailed chemostratigraphic and sedimentary facies analysis of a continuous cored well, J-305, recently drilled in the central part of the Jimusaer Sag using 1D elemental (XRF) measurements down to cm scales (with 3-5 cm sampling interval) and 2D elemental mapping on selected core slabs with up to 50 m spatial resolution (spot size), in conjunction with detailed lithofacies analysis from visual inspection, optical and scanning electron microscopy (SEM) imaging. TOC and reservoir physical properties were also analysed to aid the reconstruction of the 3

elaborated vertical variations of the depositional environments. The work was designed to primarily focus on the two oil-producing sweet spot intervals, and to understand the impact of depositional environments on the tight oil accumulations in the study area.

2. Geology setting

Junggar Basin, which is located in the northwestern part of China and covers an area of approximately 130x103 km2, is a large intracontinental superimposed basin located at the intersection of the Kazakhstan, Siberian, and Tarim plates (Fig. 1). The Jimusaer Sag is located in the southeastern part of the basin with an area of about 1278 km2, surrounded by the North Santai Uplift to the northwest, the Shaqi Uplift to the northeast, the Guxi Uplift to the east and the Fukang Fault Zone to the south. The sag is bound by the Laozhuangwang and Jimusaer faults to the north, major pinch-outs to the east, the Houbaozi and Santai faults to the south, and the Xidi Fault to the west (Fig. 1). The Jimusaer Sag comprises a Middle Carboniferous flexure basement, and Permian to Quaternary sedimentary sequences (Fig. 2).

The late Middle Permian Lucaogou Formation is sandwiched in between the Upper Permian Wutonggou Formation at top and the Lower Permian Jinjingzi Formation below (Fang et al., 2006; Kuang et al., 2012; Fig. 2). The formation consists mainly of mixed fine-grained rock formations of mudstone, dolomitic mudstone, silty fine sandstone, muddy siltstone, etc. (Cao et al., 2016a; Qu et al., 2017). The thickness of the Lucaogou Formation is generally greater than 200 m over the bulk (approximately 725 km2) of the sag with a known maximum thickness up to 350 m. The formation generally thins from east to west (Fig. 2a). The Lucaogou Formation can be further subdivided into a lower member (P2l1) and an upper member (P2l2) using the regional maximum

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flooding surface as a reference (Kuang et al., 2013a; Fig. 3). They were deposited in a deltaic, shoreface to lacustrine settings dominated by fine-grained sediments (Fig. 4). Each member contains one relatively porous and permeable interval with relatively high oil saturation, named respectively the upper (sweet spot) interval and lower (sweet spot) interval (Kuang et al., 2013b; Fig. 3), from which the tight oil in the sag was primarily produced (Kuang et al., 2012).

3. Samples and Methods

In the Jimusaer Sag, a total of 21 wells have penetrated through the Lucaogou Formation (Fig. 4). Continuous coring of the formation was obtained in Well J-174 (Kuang et al., 2012; Wu et al., 2016) with most of the publications on the Lucaogou Formation being based on this well. As the core had been intensively sampled, there is not much core sample left for detailed facies analysis. In 2016 a neighboring well, Well J-305, was drilled with continuous coring over the depth intervals of the two sweet spot intervals in the Lucaogou Formation. This provided a rare opportunity for us to conduct a systematic investigation over the entire cored interval. Well J-305 penetrates almost the entire sedimentary sequence of the Lucaogou Formation (Fig. 2).

A Thermo Scientific Niton XL hand-held energy-dispersive X-ray fluorescence (ED-XRF) analyser was used to measure elemental compositions (22 elements at each sampling point) on clean and smooth core surfaces with a spatial resolution of 3-5 cm. The ED-XRF analyser has two measurement modes: Soil Mode and Mining Mode. During the measurement, the Soil Mode was firstly used and then switch to the Mining Mode to collect elemental data. The detection time used for the Soil Mode is set to 60 seconds, which detect elements with concentrations lower than 1%, whereas the detection time used for the Mining Mode is set to 120 seconds, for detecting elements

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with concentrations higher than 1%. Elemental data from both modes are merged during the data processing. The instrument has a built-in calibration system and the measurement unit is in ppm The ED-XRF has been shown to provide a robust “first look” at drill core chemostratigraphy (Harry et al., 2012). A total of 2109 sampling points were measured in the upper sweet spot interval (29.45 m long core) and the lower sweet spot interval (44 m long core). Two-dimensional element mapping was carried out on 34 polished core samples using a Bruker Micro X-ray Fluorescence (Micro-XRF) analyser (Tornado M4) with a spatial resolution of 50 μm. Total Organic Carbon (TOC) measurements and Rock Eval Analysis were conducted on 28 source rock samples using an IFP Rock Eval Analyser VI. Key organic geochemical parameters including TOC, petroleum generation potential (S1+S2), pyrolysis temperature (Tmax) and hydrogen index (HI) were obtained. A total of 50 samples were examined using a Field Emission Scanning Electron Microscope (FE-SEM) from Carl Zeiss (Supra 55).

All these measurements and analyses were undertaken at the laboratory of the Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, except for the 2D micro-XRF mapping, which was conducted in China University of Petroleum (East China).

4. Results

4.1 Elemental distribution

Major elements in the core samples analysed include: Mg, Al, Si, P, S, K, Ca, Ti, Mn and Fe, whereas the trace elements measured consist of Ba, V, Cr, Ni, Cu, Zn, Th, Rb, U, Sr, Zr, Nb and Mo (Table 1). Si, Ca and Al are the predominant elemental constituents with average weight 6

percentages (wt %) of 21.35, 6.51 and 3.86, respectively, in the upper sweet spot interval, and 20.01, 7.37 and 3.60 in the lower sweet spot interval (Table 1). In the upper sweet spot interval, the weight percentages (wt %) of Mg range from 0.19 to 6.56 (avg. 1.37), while that of Ca ranges from 0.19 to 31.93 (avg. 6.51). In the lower sweet spot interval, the weight percentages (wt %) of Mg ranges from 0.26 to 4.48 (avg. 1.67), while that of Ca ranges from 0.33 to 38.04 (avg. 7.37%).

In the upper sweet spot interval, the weight percentages (wt %) of Al range from 0.41 to 7.03 (avg. 3.86); Si ranges from 1.97 to 36.91 (avg. 21.35); and K ranges from 0.13 to 3.24 (avg. 1.58). In the lower sweet spot interval, the weight percentages (wt %) of Al range from 0.18 to 5.91 (avg. 3.6); Si ranges from 0.73 to 29.31 (avg. 20.01); and K ranges from 0.24 to 4.44 (avg. 1.76).

Mg and Ca are the main elemental constituents of dolomite and calcite; whereas Al, Si and K are the main elemental constituents for terrigenous sediment components such as quartz, K-feldspar, albite and clay, etc. More dolomite and calcite, and less terrigenous components are present in the lower sweet spot interval compared with that in the upper sweet spot interval.

Compared with the elemental compositions of the Upper Continental Crust (UCC) (Yan et al., 1997), elements in the upper sweet spot interval of the Lucaogou Formation has the following characteristics (Table 1): (1) V has enrichment factors of 0.42-4.77 (avg. 1.46); (2) Ni has enrichment factors of 1.27-10.18 (avg. 3.44); (3) Cu has enrichment factors of 0.65-8.31 (avg. 2.31); (4) U has enrichment factors of 2.29-33.18 (avg. 5.87); (5) Sr has enrichment factors of 0.0895-20.30 (avg. 1.40); (6) Mo has enrichment factors of 2.61-76.85 (avg. 8.71); (7) Rb has depletion factors of 0.03-1.32 (avg. 0.62); and (8) Ba, Cr, Zn, Th and Zr show minor enrichment or slight depletion.

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Elements in the lower sweet spot interval has the following characteristics compared to the elemental compositions of UCC (Table 1): (1) V has enrichment factors of 0.18-3.77 (avg. 1.33); (2) Ni has enrichment factors of 1.29-24.31 (avg. 3.33); (3) Cu has enrichment factors of 0.80-11.52 (avg. 2.10); (4) U has enrichment factors of 2.583-23.17 (avg. 5.56); (5) Sr has enrichment factors of 0.1421-34.28 (avg. 1.72); (6) Mo has enrichment factors of 2.56-127.66 (avg. 7.16); (7) Rb has depletion factors of 0.04-1.52 (avg. 0.58); (8) Ba, Cr, Zn, Th and Zr show either minor enrichment or slight depletion.

4.2 Lithofacies

On the basis of detailed core observation, the two sweet spot intervals mainly contains mudstone, dolomitic mudstone, silty fine sandstone, muddy siltstone, dolomitic siltstone, siltstone and silty fine sandstone, etc. (Figs 3 and 5). The lithological profiles show frequently variations, sometimes at centimetre scales.

In the J-305 well, the upper and lower sweet spot intervals have different lithological combinations and oil saturation levels (Fig. 3). The upper sweet spot interval in the depth range from 3398 m to 3425 m is dominated by mudstone (51.2%), muddy siltstone (23.8%) with varying amounts of silty mudstone (4.1%), marl (1.8%), dolomitic mudstone (6.7%), muddy dolomite (7.8%), tuffaceous siltstone (1.6%), dolomitic siltstone (1.2%), and silty fine sandstone (1.8%). The lower sweet spot interval occurs in the depth range from 3542 m to 3585 m, and is dominated by muddy siltstone (49.8%), mudstone (21%) and silty mudstone (23.4%) with varying amounts of dolomitic mudstone (1.4%), dolomitic siltstone (2.4%), and silty fine sandstone (2%) (Fig. 3).

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4.3 Petrophysical properties

Petrophysical analyses indicate that the porosity and permeability of different lithologies or lithofacies from the upper and lower sweet spot intervals vary significantly (Fig. 6). In the upper sweet spot interval (Fig. 6a), mudstones and muddy siltstones have relatively wide ranges of porosity values of 0.6%-9.8%, and 8.44%-19%, respectively; while dolomitic mudstones are characterized by a relatively narrow porosity range of 5.6%-9.5%. Mudstones and dolomitic mudstones have relatively low porosities with mean values of 3.8% and 7.3%, respectively; while muddy siltstone is characterized by relatively high porosities with an average value of 14.3%. Mudstones and muddy siltstones have wide ranges of permeability values of 0.001 mD-11.3 mD and 0.001 mD-9.32 mD, respectively; while muddy siltstones is characterized by a relatively high permeability with an average value of 0.94 mD. The permeability values of dolomitic mudstones range from 0.014 mD to 0.065 mD with a relatively low average value of 0.045 mD.

In the lower sweet spot interval (Fig. 6b), the porosity values of silty fine sandstone, muddy siltstone, dolomitic siltstone, dolomitic mudstone spread over wide ranges of 12%-23.8%, 0.9%-18.9%, 10.72%-18.9% and 0.93%-13.2%, respectively; while the porosities of siltstone, silty mudstone, mudstone are characterized by relatively narrow porosity ranges of 12.2%-14.01%, 6.2%-12.9%, and 2.4%-4%, respectively. Overall, silty fine sandstone, siltstone, muddy siltstone and dolomitic siltstone are dominated by relatively high average porosity values of 16.43%, 13.11%, 11.53%, 14.62%, respectively; whereas mudstone, dolomitic mudstone and silty mudstone have low mean porosity values of 3.27%, 6.06% and 9.55%, respectively.

The permeability ranges of silty fine sandstone, muddy siltstone, dolomitic siltstone,

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dolomitic mudstone also spread over relatively wide ranges: 0.001 mD-0.762 mD, 0.001 mD-18.1 mD, 0.0047 mD-0.726 mD, 0.001 mD-137 mD, respectively; while the permeability values of siltstone, silty mudstone, mudstone are characterized by relatively narrow ranges of 0.08 mD-0.127 mD, 0.001 mD-0.02 mD, 0.001 mD-0.006 mD, respectively. Overall, silty fine sandstone, muddy siltstone, dolomitic siltstone, siltstone have relatively high average permeability values of 0.263 mD, 0.577 mD, 0.162 mD and 0.104 mD, respectively; whereas silty mudstone, mudstone, dolomitic mudstone are characterized by relatively low average permeabilities of 0.011 mD, 0.003 mD and 0.068 mD, respectively. The main favourable reservoir rock types include muddy siltstone, dolomitic mudstone, siltstone and silty fine sandstone.

In general, different lithofacies show quite different permeability and porosity relationships (Fig. 6). In the upper sweet spot interval (Fig. 6a), permeability of muddy siltstone is positively correlated with porosity. For the mudstone and dolomitic mudstone, permeabilities and porosities appear to be uncorrelated. In the lower sweet spot interval (Fig. 6b), permeabilities of silty fine sandstone, siltstone, muddy siltstone and dolomitic siltstone appear to be positively correlated with porosities. For the mudstone and dolomitic mudstone lithofacies, there appears to be no relationships between permeabilities and porosities. Samples with high permeabilities and low porosities are probably caused by the presence of micro fractures (Fig. 5k) and thin parallel siltstone lamina in the samples (Fig. 5h).

4.4 Pore Structures

On the basis of detailed microscopic analysis and SEM imaging, the reservoirs in the Lucaogou Formation appear to have complex pore types and structures. They are dominated by

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three major pore types including: primary pores, secondary pores and fractures. The primary pores comprise primary inter-granular and primary inter-crystalline pores, whereas the secondary pores consist of inter-granular, inter-crystalline and intra-granular dissolution pores (Wu et al., 2016). The primary inter-crystalline pores are mainly developed in the mudstone and dolomite mudstone lithofacies between illite, pyrite and dolomite, characterized by relatively small pores sizes of <2 μm in diameter (Fig. 7a, b, c). The primary inter-granular pores are mainly developed in the muddy siltstone, dolomitic siltstone and siltstone with relatively large pore sizes of >5 μm in diameter (Fig. 7d), and those developed in mudstones have small pore sizes of <1 μm in diameter. The secondary inter-crystalline pores are mainly developed in the dolomitic mudstone from dissolution (Fig. 7e) with pore sizes ranging from <1 μm to >10 μm in diameter, larger than the primary inter-crystalline pores. The secondary inter-granular pores are mainly developed in the dolomitic siltstone and muddy siltstone, from dissolution of dolomite and detrital clasts, with relatively large pore sizes of >100 μm in diameter (Fig. 7f, g). The secondary dissolution intra-granular pores are mainly formed by dissolution of dolomite and detrital grains (Fig. 7h).

4.5 Organic geochemical characteristics

Hydrocarbon generation potentials in the Lucaogou Formation were evaluated for the mudstone and dolomitic mudstone in the upper sweet spot interval, and the mudstone, silty mudstone, dolomitic mudstone and muddy siltstone in the lower sweet spot interval. Data from Rock-Eval analysis indicates that the kerogen types are dominated by Type I and II (Fig. 8). In the upper sweet spot interval, the kerogen in mudstone, marl and dolomitic mudstone is mainly of Type I (Fig. 8a). In the lower sweet spot interval, the kerogen type in the mudstone and silty mudstone lithofacies is primarily of Type I, while the kerogen in muddy siltstone and dolomitic 11

mudstone is primarily of Type I with minor component of Type II1 (Fig. 8b). All the potential source rocks have suitable kerogen types for oil generation.

In the upper sweet spot interval (Fig. 9), TOC in mudstone ranges from 3% to 9.57% with an average of 5.83%, whereas TOC in dolomitic mudstone ranges from 5.98% to 13.35% with an average of 9.72%. In the lower sweet spot interval (Fig. 10), the TOC values for various lithofacies are as the following: 6.08-6.47% for mudstone with an average value of 6.28%, 0.63-10.3% for dolomitic mudstone with an average value of 4.83%, 5.43-10.34% for silty mudstone with an average value of 8.78%, and 2.31-6.12% for muddy siltstone with an average value of 3.98%. The results show that all the mudstone facies in the two sweet spot intervals have high TOC values greater than 2%, and some of the muddy siltstone facies also have relatively high TOC values. However, TOC values vary considerably within the interval (Figs 9 and 10).

5. Discussion

5.1 Depositional environment deduced from chemostratigraphic analysis

For fine-grained sedimentary formations, depositional environments not only affect the organic matter distribution but also the reservoir petrophysical properties. Climate is considered as a major controlling factor for lake level fluctuations, lake water compositions such as salinity levels and biogenic materials and sediment types (Yan and Zheng, 2015).

Some elemental ratios in fine-grained sediments such as Sr/Cu, Fe/Mn, Mg/Ca, Rb/Sr have been suggested to be effective proxies for climate changes (Ratcliffe et al., 2010; Xiong et al., 2011). High Sr/Cu, high Mg/Ca, low Fe/Mn ratios and low Rb/Sr ratios are indicative of dry climate (Roy and Roser, 2013; Gao et al., 2016). In the upper sweet spot interval (Fig. 9), the 12

relatively high Sr/Cu (avg. 10.41) and Mg/Ca (avg. 0.48) ratios, and low Fe/Mn (avg. 44.45) and Rb/Sr (avg. 0.45) ratios indicate a dry climatic setting. In the lower sweet spot interval (Fig. 10), the relatively high Sr/Cu (avg. 16.06) and Mg/Ca (avg. 0.3) ratios, low Fe/Mn (avg. 33.52) and Rb/Sr (avg. 0.19) ratios also suggest a dry climate setting. The lower sweet spot interval was deposited in a comparatively drier climatic setting.

The salinity of lake water is also sensitive to climate changes. Previous investigations indicated that some elemental ratios, such as Ca/(Ca+Fe), can be used to reflect water salinity levels. High Ca/(Ca+Fe) ratio was shown to reflect enhanced salinity in fresh to brackish lake water ( Lan et al., 1987; He et al., 2017). In the upper sweet spot interval (Fig. 9), the relatively high Ca/(Ca+Fe) (avg. 0.95) ratios indicate a brackish-water depositional setting. In the lower sweet spot interval (Fig. 10) the relatively high Ca/(Ca+Fe) (avg. 0.75) ratios also indicate a brackish-water depositional environment. Comparatively, the upper sweet spot interval was deposited under a more saline condition than the lower sweet spot interval. Highly saline lake water can result in density contrasts in water column and form a stable saline water stratification (Wu et al., 2016). In the surface water, high dissolved oxygen can lead to the thriving of euryhaline algae, plankton and halophilic organisms. The predominant Type I and II kerogens in the Lucaogou Formation are indicative of the presence of similar biological sources. The deeper part of the water column may become hyper saline and anoxic, favorable for the preservation of organic matters (Lee, 1992; Meyers and Ishiwatari, 1993).

Certain elemental ratios may be used to indicate water depths, including (Al+Fe)/(Ca+Mg) and Rb/K (Xiong et al., 2011). Higher ratios indicate relatively deep water. In the upper sweet spot interval (Fig. 9), the (Al+Fe)/(Ca+Mg) (avg. 2.6) and Rb/K (avg. 2.86‰) ratios are higher than 13

the corresponding average ratios of 0.96 and 2.12‰ in the lower sweet spot interval (Fig. 10), indicating that the upper sweet spot interval was deposited in a comparatively deeper water depth.

The redox condition of a depositional environment can influence the preservation of organic matters. The trace elements of U, Th, V, Ni, Cr, Co and Mo are considered to be redox-sensitive (Dean et al., 1997; Nicolas et al. 2006). Palaeo-redox conditions can thus be determined using some elemental ratios, such as U/Th, V/Cr and V/(V+Ni). Reduced conditions are characterised by U/Th>1.25, V/Cr>4.25 and V/(V+Ni)>0.84, whereas oxic conditions are usually marked by U/Th<0.75, V/Cr<2 and V/(V+Ni)<0.6 (Dell et al., 1988, Jones and Manning, 1994). According to the criteria, both the upper sweet spot interval (Fig. 9) with U/Th (avg. 1.11), V/Cr (avg. 2.96) and V/(V+Ni) (avg. 0.6) and the lower sweet spot interval (Fig. 10) with U/Th (avg. 1.18), V/Cr (avg. 2.73) and V/(V+Ni) (avg. 0.6) were under anoxia and reduced conditions at the time of deposition.

Overall, the depositional environment of both the upper and lower sweet spot intervals are largely characterised by relatively saline, anoxia to reduced conditions under a relatively dry climatic setting. However, there are considerable fine-scale variations in the depositional environments within the two intervals (Figs 9 and 10). The organic matter and petrophysical properties also show complex variations in the vertical profile.

Spectral trend attribute analysis is a stratigraphic cycle identification technique that makes use of the spectral analysis of well logs or time series data (Nio et al., 2005). With cyclostratigraphy as the theoretical basis, this technique converts well logs into one INPEFA (INtegrated Prediction Error Filter Analysis) curves by employing digital signal processing means to manifest the characteristics of stratigraphic cycles hidden in well log data. An INPEFA curve

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can be constructed in three steps. Firstly, MESA (Maximum Entropy Spectral Analysis) is performed on the well log data to obtain a MESA curve; Secondly, PEFA (Prediction Error Filter Analysis) is performed to obtain numerical errors between the MESA predicted value and the true value of well log data at corresponding depths, named the PEFA curve (at this point, the error value is the result of actual data value minus the filtered value); Finally, an integrated processing is performed on the PEFA curve to obtain the INPEFA curve that can be used for sequence subdivision, cycle recognition based on its positive and negative trend and the turning points. We used the U/Th ratio as a redox parameter, Ca/(Ca+Fe) as a salinity parameter, (Al+Fe)/(Ca+Mg) as a water depth parameter and Sr/Cu as a climate parameter, and converted these parameter logs into INPEFA cures to highlight the depositional cycles (Figs 11 and 12). We also converted the Gamma Ray (GR) log into an INPEFA cures, which produced the same cycles as that from the elemental ratio data.

Based on the INPEFA curve from the elemental ratio (parameter) logs, the upper sweet spot interval (Fig. 11) can be subdivided into 5 distinct environmental units, while the lower sweet spot interval (Fig. 12) can be subdivided into 6 environmental units. In the upper sweet spot interval (Fig. 11), the No. 1 unit mainly comprises mudstone, dolomitic mudstone and muddy dolomite. The depositional environments are mainly of a reduced, saline and shallow water setting under a semi-humid climate. The No. 2 unit consists mainly of siltstone and muddy siltstone in its lower part, and mudstone interbedded with muddy dolomite. The depositional environment of this unit is characterised by a reduced, semi-saline to saline, shallow water setting under a dry climate. The No. 3 unit mainly comprises mudstones in the top and base with muddy siltstone and dolomitic siltstone in the middle part. Compared with the No. 2 unit, the depositional environment of this 15

unit is of a less reduced, less saline water but in a deeper lake water setting under a more humid climate. The No. 4 unit consists mainly of muddy siltstone in the top and mudstone interbedded with muddy dolomite in the lower part. The depositional environment of this unit from base to top changed from a weakly reduced to a weakly oxidized setting, a fresh to saline water, a deep to shallow water setting under a humid to a dry climate. The No. 5 unit is mainly composed of muddy siltstone interbedded with mudstone in the top and mudstone in the lower part. The depositional environment is mainly of a weakly reduced, fresh and deep water setting under a dry to a humid climate.

In the lower sweet spot interval (Fig. 12), the No. 1 unit consists mainly of dolomitic mudstone interbedded with silty mudstone. The depositional environment of this unit from base to top changes from a weakly reduced to a weak oxidised, a semi-saline to saline, a shallow to deep water settings under a dry climate. The No. 2 unit mainly comprises mudstone and dolomitic mudstone in the top, and dolomitic siltstone and muddy siltstone in the lower part. The depositional environment of this unit changed from a weakly reduced to a reduced, a fresh to semi-saline, a deep to shallow water setting under a humid to dry climate. The No. 3 unit consists mainly of muddy siltstone interbedded with mudstone and dolomitic siltstone in the upper part, and dolomitic mudstone in the lower part. The depositional environment is mainly of weakly reduced, saline to fresh-water, shallow to deep water settings under a dry to humid climate. The No. 4 unit mainly comprises dolomitic siltstone at the top, muddy siltstone interbedded with mudstone and dolomitic mudstone in the middle, and muddy siltstone interbedded with a meter of silty fine sandstone at the base. The depositional environment of this unit is mainly of a semi-saline, a reduced to weakly reduced, and a shallow to deep setting under a dry to humid 16

climate. The No. 5 unit consists mainly of dolomitic siltstone, silty fine sandstone, muddy siltstone and silty mudstone. The depositional environment changed from a weakly oxidised to reduced, a semi-saline to saline and a deep to shallow water setting under a humid to dry climate. The No. 6 unit is composed mainly of mudstone interbedded with dolomitic mudstone in the upper part, and muddy siltstone in the lower part. The depositional environment of this unit is mainly of a weakly oxidized, a fresh to semi-saline and a deep to shallow water setting under a humid to dry climate.

5.2 Effect of depositional environment on organic matter enrichment

Previous investigations indicated that oils accumulated in the two sweet spot intervals of the Lucaogou Formation were mainly derived from source rocks interbedded in the reservoir units or stacked in the vicinity of the tight reservoir rocks (Cao et al., 2016). Therefore it is important to investigate the distribution of organic matters (OM) and the controls on the enrichment of OM. Multiple factors can influence OM accumulations, including high OM inputs (Gallego et al., 2007), oxygen-deficiency in bottom waters under a stratified water column (Meyers and Ishiwatari, 1993) and a combination of both factors (Tyson, 2005; He et al., 2017). These factors are strongly influenced by the paleaoenvironment, including climate, redox conditions, salinity and water depth. A humid climate can result in a biological bloom, high salty of lake water can influence the biological distribution and develop a reduced condition in the bottom waters favoring the preservation of OM.

In the upper sweet spot interval, there are slight positive correlations between the U/Th (Redox Index) ratio and the TOC content (R2=0.52; Fig. 13b) and between the Ca/(Ca+Fe) (Salinity Index) ratio and TOC content (R2=0.58; Fig. 13c). However, there are virtually no

17

correlations between the Sr/Cu (Palaeoclimate parameter) ratio and the TOC content (R2=0.014; Fig. 13a), nor the (Al+Fe)/(Ca+Mg) (Palaeo-water depth parameter) ratio and the TOC content (R2=0.0002; Fig. 13d). The relationships between OM and the palaeo-environmental parameters indicate that a high saline and a strongly reduced condition are favorable to OM enrichment, while water depths and climatic settings only weakly influence the enrichment of OM. In the lower sweet spot interval, there appears to be virtually no correlations between all the palaeo-environmental indicators and the TOC contents with all correlation coefficients being less than 0.2 (R2=0.0095, R2=0.0079, R2=0.1801, R2=0.0002; Fig. 13). This suggests that the controls on the OM enrichment in the Lucaogou Formation are much more complex.

The palaeo-environments of the Lucaogou Formation appear to have changed quite frequently over the period of its deposition and the two sweet spot intervals can be largely subdivided into 11 distinct units (Figs 11 and 12). Each unit was deposited under a distinct palaeo-environmental setting with a distinct TOC range. The entire upper interval was deposited under an overall reduced environment, favorable for OM preservation.

In the upper sweet spot interval (Figs 9 and 11), the No. 1 unit has relatively high TOC of 7.17%-9.82% with an average value of 8.27%. During its deposition the climate changed from a dry to a semi-humid setting, which was beneficial to mineral nutrient supplies and phytoplankton growth (Talbot and Kelts, 1990). This led to high primary production with OM primarily derived from plankton (Type I and II kerogen). Although the water depth was shallow, the highly saline lake water formed a stable stratified-water column (Wu et al., 2016) with the bottom waters developing a strongly reduced condition, favorable for OM preservation. With an overall shallow water depth, enormous OM can be deposited relatively rapidly to the lake bottom to be preserved 18

under the anoxic environment developed. The No. 2 and No. 3 units have lower TOC of 3.94%-5.98% with an average value of 5% compared with the No. 1 unit. The climate condition was almost the same among the three units. They should have the same high primary productivity. However, the water depths in the No. 2 and No. 3 units were much deeper. This means that OM may take a relative longer time to settle down to the lake bottom, during which some OM may be consumed/oxidised. The No. 4 unit has the highest TOC of 9.57%-13.35% with an average value of 11.46% in its upper part. It was deposited during the most humid climate setting among the first 4 units, which would be conducive to primary productivity. The highest primary productivity was chiefly related to both autochthonous OM from algae and aquatic plants and allochthonous OM transported from land under a humid climate (Wu et al., 2017). The presence of higher proportions of silty mudstone in the No. 4 unit compared with that in the first 3 units indicates that more sediments were carried into the lake from land, including detrital sediments, aquatic plants and allochthonous OM. In the No. 5 unit, mudstone was only developed at the bottom part with a relatively low TOC of 3%. During the deposition of the mudstone in the No. 5 unit, the climate was relatively dry and the water depth was relatively shallow. Therefore it is characterised by relatively lower primary productivity compared with other units.

In the lower sweet spot interval (Figs 10 and 12), the redox conditions for the first five units are generally in the range of a weakly reduced to a reduced setting with subtle variations within the interval, favorable for OM preservation. Salinity, water depth, climate and TOC values changed quite frequently within the interval. The No. 1 and No. 3 units have lower average TOC of 5.43% and 6.12%, respectively, compared with that in the No. 2 and No. 4 units, which have TOC values of 5%-10.34% (avg. 8.53%), and 6.08%-10.3% (avg. 8.19%), respectively. The first 19

four units were deposited under a similar humid climate setting with similar primary productivities, but during the deposition of the No. 1 and No. 3 units, the lake water became relatively deeper and fresher. Therefore, OM in the No. 1 and No. 3 units may have been subjective to comparatively longer oxidation during settling and subsequent preservation. This may explain the relatively low TOC values in the two units compared with that in the No. 2 and No. 4 units. In the No. 5 unit, the TOC value decreases from 9.02% to 0.63% from the middle to the top as the climate changed from a semi-humid to a dry setting, lake water changed from deep to shallow, salinity changed from fresh-water to saline, and redox condition changed from a weakly reduced to a reduced setting. The change from a semi-humid to a dry climatic setting caused a decrease in the primary productivity and thus a reduction of TOC despite of the presence of a favorable redox condition. The No. 6 unit has relatively a low TOC value of 2.31%, which may be attributed to a weak oxidation condition during its deposition.

In general, the entire Lucaogou Formation is relatively enriched with TOC (Fig. 13). The variabilities in TOC within the formation (Figs 9-12) were controlled by the subtle changes in depositional environments, which influenced the enrichment of OM in a variety of ways. The most favorable depositional environment for OM enrichment is the combination of a semi-humid climate, a semi-deep to shallow brackish lake water.

5.3 Effects of depositional environment on reservoir petrophysical properties

The tight reservoirs in the Lucaogou Formation are characterized by fine-grained lithofacies, containing mainly dolomite and terrigenous siliceous sediments with low primary porosity (Wu et al., 2016; Cao et al., 2016). Reservoir rock types consist mainly of muddy siltstone and dolomitic

20

siltstone. Secondary dissolution pores in dolomite and feldspar grains are usually well developed in tight reservoirs, and are the major reservoir storage spaces in many reservoirs around the world (Mazzullo and Harris, 1990; Yuan et al. 2015). This is also the case in the Lucaogou Formation (Wu et al., 2016). Previous investigation indicated that the tight reservoirs in the Lucaogou Formation were formed in a hyper saline lacustrine depositional environment. The presence of dolomitic material may have prevented compaction. In addition, organic acids generated from the interbedded TOC-rich beds may cause widespread dissolution of dolomite and feldspar grains (Wu et al., 2016). However, there have been no detailed investigation to document how the palaeo-depositional environment can influence the petrophysical properties of the tight reservoirs.

In the upper sweet spot interval, the (Al+Fe)/(Ca+Mg) ratio, a proxy for Palaeo-water depth, is positively correlated with porosity of the muddy siltstone with a correlation coefficient (R2) of 0.62 (Fig. 14a). The Sr/Cu ratio, a proxy for palaeoclimate, has a slightly negative correlation with porosity of the muddy siltstone with a correlation coefficient (R2) of 0.67 (Fig. 14b). The Ca/(Ca+Fe) ratio, a proxy for palaeo-salinity, has a slightly negative correlation with porosity of the muddy siltstone with a correlation coefficient (R2) of 0.58 (Fig. 14c).

In the lower sweet spot interval, the (Al+Fe)/(Ca+Mg) ratio (palaeo-water depth indicator) has slightly positive correlations with the reservoir porosity with a correlation coefficient (R2) of 0.66 for dolomitic siltstone and 0.29 for muddy siltstone (Fig. 14d). The Sr/Cu ratio (palaeoclimate indicator), has a slightly negative correlation with porosity of the muddy siltstone with a correlation coefficient (R2) of 0.35, but has an arch-shaped correlation with porosity of the dolomitic siltstone (Fig. 14e). The Ca/(Ca+Fe) ratio, a proxy for palaeo-salinity, has a slightly negative correlation with porosity with a correlation coefficient (R2) of 0.73 for the dolomitic 21

siltstone and 0.44 for the muddy siltstone (Fig. 14f). Porosities appear to be weakly affected by palaeo-water depth and palaeo-salinity.

The controlling factors on petrophysical properties of the Lucaogou Formation reservoir are complex. In the vertical sections (Figs 9-12), the changing patterns of petrophysical properties and the corresponding environmental parameters are quite variable. In the upper sweet spot interval (Figs 9 and 11), muddy siltstone, dolomitic siltstone, siltstone and silty fine sandstone are mainly distributed in the No. 2–5 units. The No. 1 unit is dominated by mudstones with poor petrophysical properties and a mean porosity of only 1.2%. The No. 2 unit has a porosity range of 8.44%-19% with an average value of 11.82%. The No. 3 unit has a porosity range of 6.7%- 17% with an average value of 11.07%. The No. 4 unit has a porosity range of 0.7%- 19% with an average value of 9.87%. The No. 5 unit has a porosity range of 12.03%- 17.8% with an average value of 16.05%. Among the five units, the No. 5 unit appears to have the best petrophysical properties (Fig. 11), as the palaeoclimate during the deposition of the No 5 unit was of the most humid setting, which may have developed a high energy regime (strong hydrodynamics). The relatively deep lake water would also provide ample accommodation space for the high terrestrial influx. The strong hydrodynamics can carry relatively coarse-grained terrigenous sediments into the lake, forming extensive high-energy sedimentary structure such as parallel beddings (Fig. 5e, f). The coarse-grained terrigenous sediments would form relatively larger inter-grain pores and the some coarse sedimentary grains such as albite, feldspar can also be easily dissolved during burial (Fig. 7g, h). The parallel beddings and associated good sorting can also result in good permeability. Reservoir porosities in the No. 2 and No. 3 units are also quite high, reaching 19% and 17%, respectively. This is because the prevailing semi-humid to dry climate at the time and the hyper 22

saline water are conducive to dolomite deposition. Dolomite can prevent compaction and can also be easily dissolved (Fig. 7e, f; Wu et al., 2016). Some dissolution pores were observed in samples from the No. 2 and No. 3 units with high dolomite contents (Fig. 5m, n, q, r).

In the lower sweet spot interval (Figs 10 and 12), muddy siltstone, dolomitic siltstone, siltstone and silty fine sandstone are present in all 6 units. In the No. 1 unit, porosities range from 8.94% to 16.1% with an average value of 12.01%. In the No. 2 unit, porosities range from 4% to 13.4% with an average value of 9.85%. In the No. 3 unit, porosities range from 0.9% to 10.72% with an average value of 6.22%. In the No. 4 unit, porosities range from 6.2% to 23.8% with an average value of 11.04%. In the No. 5 unit, porosities range from 0.93% to 18.6% with an average value of 12.34%. In the No. 6 unit, porosities range from 2.8% to 18.9% with an average value of 14%.

The No. 5 and No. 6 units have the best petrophysical properties among all the units (Fig. 12). Reservoir porosity increases as climate changed from a semi-humid to a dry setting, lake water changed from deep to shallow, and salinity changes from fresh to saline settings. Such a unique environmental setting also caused an increase in the dolomite content, and thus increasing secondary dissolution pores (Fig. 5i, j, o, p). The petrophysical properties become worsened from the No. 4 unit to No. 3 unit (Fig. 12), as the dolomite content was reduced due to the climate changed from a dry to a humid setting, the lake water changed from shallow to deep and saline to fresh. This resulted in a decrease in secondary dissolution pores. The petrophysical properties of the interval between the No. 3 and No. 2 units are better than that in the upper part of the No. 2 unit and the lower part of the No. 3 unit (Fig. 12), as the climate setting during the deposition between the No. 3 and No. 2 units was mostly humid, which may lead to a strong hydrodynamic 23

regime. The deep water setting also offered ample accommodation for the high terrestrial influx. The strong hydrodynamic regime can carry coarse-grained terrigenous sediments into the lake, provide better sorting and form high-energy sedimentary structures such as cross or parallel beddings. From the upper part of the No. 2 unit to No. 1 unit, the petrophysical properties become better (Fig. 12), as the climate changed from a semi-humid to a humid setting, the lake water changed from shallow to deep, which may lead to a strong hydrodynamic regime.

It can be seen that depositional environments can influence the petrophysical properties in two aspects. Firstly, a humid climate may lead to a high-energy regime or a strong hydrodynamic setting. Strong hydrodynamics can carry large quantities of coarser terrigenous sediments and develop sedimentary structures such as cross or parallel beddings, which would lead to high porosity and permeability. A deep lake water can offer ample accommodation space for the high terrestrial influx. Secondly, a dry climate and a hyper saline water can lead to an enhanced dolomite deposition, which can prevent severe compaction, and can also be easily dissolved to form secondary dissolution pores.

5.4 Implications for tight oil exploration

Based on the discussion above, the OM contents and reservoir porosities of the two sweet spot intervals in the Lucaogou Formation were strongly influenced by the palaeo-environments. In the upper interval (Fig. 11), the No. 1 and No. 3 units have high TOC, while the No. 2 and No. 4 units contain high porosity reservoir beds. As the No. 1 and No. 3 units are underlain by the No. 2 and No. 4 units, they form effective reservoir-seal couplets for oil accumulations. In the No. 2, No. 3 and No. 4 units, some reservoir beds with good porosities are interbedded with the high-TOC

24

source beds, thus oil generated from the source beds can charge directly into the neighboring reservoir beds without migration.

In the No. 1-4 units, the lower part of No. 5 (3570.8 m-3577 m) and the No. 6 units of the lower sweet spot interval (Fig. 12), the reservoir beds with good porosities are stacked next to the high-TOC source beds. Therefore oil generated from the source beds can charge into the adjacent reservoir beds via short migration. In the upper part of the No. 5 unit and the lower part of the No. 4 unit (3568 m-3570.8 m; Fig. 12), there are no high TOC source beds present. There would be no oil accumulations in those reservoir beds. The best exploration targets in the Lucaogou Formation are of two types: (1) those units with good reservoir beds intermingled with high-TOC source beds; and (2) those units with good reservoirs adjacent to high-TOC source beds.

6. Conclusions

On the basis of detailed chemostratigraphic and sedimentary facies analysis of the sweet spot intervals in the Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin, NW China the following conclusions can be drawn:

(1) The sweet spot intervals were mainly deposited in a shallow lake to shoreface saline lacustrine environment under an overall dry climatic setting, and a dominantly reduced depositional environment. The depositional environments changed quite frequently during the deposition of the Lucaogou Formation. The upper sweet spot interval can be subdivided into five distinct units, while the lower sweet spot interval can be divided into six units based on chemostratigraphic analysis.

(2) TOC in the two sweet spot intervals ranges from 0.63% to 13.35% with an average value 25

of 6.24%, and exhibits strong variabilities vertically. The best depositional setting for OM enrichment is of a semi-shallow and semi-saline lake water under a semi-humid climate.

(3) The main reservoir rock types are muddy siltstone, dolomitic siltstone, siltstone and silty fine sandstone. Secondary dissolution pores form the bulk reservoir storage spaces.

(4)

Palaeo-environmental changes influenced the tight oil reservoir mainly in two ways.

Firstly, a humid climate may lead to a high energy regime or strong hydrodynamics. Strong hydrodynamics can carry coarse-grained terrigenous sediments into the lake, providing better sorting and developing high-energy sedimentary structures such as cross or parallel beddings, which can lead to high porosity and permeability, and also provide ample accommodation for high terrestrial influx. Secondly, a dry climate and hyper saline lake water can lead to an enhanced dolomite deposition, which is more resilient to compaction and can also be easily dissolved to form secondary dissolution pores.

(5) There are two potential tight oil plays including those depositional units with porous reservoir beds interbedded with high-TOC source beds (a self-generation play), and those units with porous reservoir beds adjacent to high-TOC source beds (a near-source play).

Acknowledgements

This research is financially supported by the National Basic Research (973) Program of China (No. 2014CB239004) on “Formation and Enrichment of Chinese Continental Tight Oil (Shale Oil) Resources”. We are grateful to the Chief Scientist of the “973” programme, Prof. Caineng Zou of the Research Institute of Petroleum Exploration and Development (RIPED), PetroChina for his support. A number of people also contribute to this research including Songtao Wu of RIPED, Prof. 26

Yingchang Cao, Dr Kelai Xi and Dr Jianliang Liu of China University of Petroleum (East China). The Xinjiang Oil Company of PetroChina is acknowledged for providing background geological data and for accessing to the core samples. This work was also supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA14040401).

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U.S. Energy Information Administration (EIA), 2013. Technically Recoverable Shale Oil and Shale Gas Resources: an Assessment of 137 Shale Formations in 41Countries outside the United States. http://www.eia.gov/analysis/studies/worldshalegas/pdf/fullreport.pdf. Wu, H.G., Hu, W.X., Cao, J., Wang, X.L., Wang, X.L., Liao, Z.W., 2016. A unique lacustrine mixed dolomitic-clastic sequence for tight oil reservoir within the middle Permian Lucaogou Formation of the Junggar Basin, NW China: reservoir characteristics and origin. Marine and Petroleum Geology 76, 115-132. Wu, L.Y., Pang, X.Q., Zhou, L.M., Pang, H., 2015. The quality evaluation and hydrocarbon generation and expulsion characteristics of Permian Lucaogou Formation source rocks in Jimusar sag, Junggar Basin. Acta Geologica Sinica-English Edition 89, 283-286. Xi, K.L., Cao, Y.C., Zhu, R.K., Shao, Y., Xue, X.J., Wang, X.J., Gao, Y., Zhang, J., 2015. Rock types and characteristics of tight oil reservoir in Permian Lucaogou Formation, Jimusaer Sag. Acta Petolei Sinica 36, 1495-1507 (in Chinese with English abstract). Xiong, X.H., Xiao, J.F., 2011, Geochemical indicators of sedimentary environments—A summary. Earth and Environment 39, 405-414 (in Chinese with English abstract). Yan, L.J., Zhang, M.P., 2015. The response of lake variations to climate change in the past forty years: a case study of the northeastern Tibetan Plateau and adjacent areas, China. Quaternary International 371, 31-48. Yan, M.C., Chi, Q.H., Gu, T.X., Wang, C.S, 1997. Chemical compositions of continental crust and rocks in Eastern China. Geophysical and Geochemical Exploration 21, 451-459. Yuan, G.H., Cao, Y.C., Jia, Z.Z., Gluyas, J., Yang, T., Wang, Y.Z., Xi, K.L., 2015. Selective dissolution of feldspars in the presence of carbonates: the way to generate secondary pores in buried sandstones by organic CO2. Marine and Petroleum Geology 60, 105-119. Zhang, J., Liu, L.J., Huang, Y., 2003. Sedimentary characteristics of middle upper Permian in Jimusaer sag of Junggar Basin. Xinjiang Geology 21, 412-414 (in Chinese with English abstract).

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Table and figure captions Table 1. Summary of elemental compositions of core samples from the upper and lower sweet spot intervals showing minimum, maximum, average elemental concentrations Fig. 1. Location map of the study area and key wells in the Jimusaer Sag. (a) Geographic location of the Junggar Basin in western China; (b) Location of the Jimusaer Sag in the southeastern part of the Junggar Basin; (c) Structural framework of the Jimusaer Sag showing major faults and key wells drilled. Well J-305 is located in the southeastern quarter of the sag (modified after Kuang et al., 2012). Fig. 2. Stratigraphic architecture of the Jimusaer Sag. (a) Stratigraphic cross section showing the overall sag-wide stratigraphic framework; and (b) generalized Paleozoic-Mesozoic stratigraphic column showing various formations and their thicknesses. The target stratigraphic unit, the Lucaogou Formation (P2l), is about 200-350 m thick in the sag. See Fig. 1 for the location of the cross section (modified after Wu et al., 2016). Fig. 3. Depth profiles of wireline logs (Gamma ray, Sonic, Density, Neutron Porosity and Resistivity), lithofacies and oil saturation of Well J-305. The positions of two sweet spot intervals (the upper and lower sweet spot intervals) are shown. Fig. 4. Generalised sedimentary facies maps of two regressive sequences within the Lucaogou Formation. (a) Sedimentary facies map of the lower interval; (b) sedimentary facies map of the upper interval (modified after Qiu et al., 2016; Wu et al., 2016) Fig. 5. Composite (Al, K, Ca, Mg and Si) elemental and core images of samples from Well J-305. Note the extreme lithological variabilities at cm-mm scales. (a) and (b): dark mudstones interbedded with gray dolomitic mudstone from 3568.47-3568.55 m; (c) and (d): dark muddy siltstone with parallel bedding from 3563.43-3563.52 m; (e) and (f): brownish muddy siltstone with parallel bedding from 3418.84-3418.87 m. (g) and (h): gray mudstone with white calcite stripe in the upper interval, gray dolomitic mudstone with thin dark muddy siltstone lamina and oil-bearing lenses from 3575.46-3575.53 m; (i) and (j): brownish muddy 31

siltstone, dolomite-rich in the upper interval with large dissolution pores from 3575.11-3575.2 m; (k) and (l): brownish mudstone with horizontal and vertical fractures from 3418.84-3418.87 m; (m) and (n): brownish muddy siltstone with abundant dolomite and dissolution pores from 3408.73-3408.79 m; (o) and (p): brownish muddy siltstone with high dolomite content at the bottom from 3576.99-3577.04 m; (q) and (r): grey dolomitic siltstone with dissolution pores from 3413.99-3414.06 m. Fig. 6. Relationships between porosities and permeabilities for different lithofacies in the upper (a) and lower (b) sweet spot intervals of the Lucaogou Formation. Fig. 7. Varieties of pores within the Lucaogou Formation. (a) SEM image of mudstone from 3421.71 m of Well J-305, showing primary inter-crystalline pores between illite; (b) SEM image of mudstone from 3551.47 m of Well J-305 well, showing primary inter-crystalline pores between fibrous pyrite and primary inter-granular pores between clasts; (c) SEM image of dolomitic mudstone from 3414.01 m of Well J-305, showing primary inter-crystalline pores between dolomite; (d) Photo micrograph of dolomitic siltstone from 3144.86 m of Well J-174, showing primary inter-granular pores between dolomite and clast; (e) SEM image of dolomitic mudstone from 3421.71 m of Well J-305, showing secondary dissolution inter-crystalline pores between dolomite; (f) BSE image of dolomitic mudstone from 3575.17 m of Well J-305, showing secondary dissolution inter-granular pores between dolomite and clast; (g) SEM image of muddy siltstone from 3418.8 m of Well , J-305, showing secondary dissolution inter-granular pores between clasts; (h) Photo micrograph of muddy siltstone 3142.13 m of Well J-174, showing secondary dissolution intra-granular pores of K-feldspar: InterC=inter-crystalline pores, Pyr=pyrite, Dol=dolomite, InterG=inter-granular pores, D-InterC= dissolution inter-crystalline pores, D-InterG=dissolution inter-granular pores, D-IntraG=dissolution intra-granular pores. Fig. 8. Kerogen type delineation based on Rock-Eval parameters for different lithofacies in the upper (a) and lower (b) sweet spot intervals. Fig. 9. Depth profiles of TOC, porosities and environmental parameters derived from chemostratigraphic data of the upper sweet spot interval in the Lucaogou Formation in Well 32

J-305 Fig. 10. Depth profiles of TOC, porosities and environmental parameters derived from chemostratigraphic data of the lower sweet spot interval in the Lucaogou Formation in Well J-305 Fig. 11. Depth profiles of palaeoenvironmental parameters, TOC and porosity of the upper sweet spot interval in the Lucaogou Formation and depositional unit subdivision based on INPEFA analysis in Well J-305 Fig.12. Depth profiles of palaeoenvironmental parameters, TOC and porosity of the lower sweet spot interval in the Lucaogou Formation and depositional unit subdivision based on INPEFA analysis in Well J-305 Fig. 13. Relationships between Sr/Cu ratios and TOC content (a), U/Th ratios and TOC content (b), Ca/(Ca+Fe) ratios and TOC content (c), (Al+Fe)/(Ca+Mg) ratios and TOC content (d). Fig. 14. Relationships between (Al+Fe)/(Ca+Mg) ratios and porosity in the upper sweet spot interval (a); Sr/Cu ratios and porosity in the upper sweet spot interval (b); Ca/(Ca+Fe) ratios and porosity in the upper sweet spot interval (c); (Al+Fe)/(Ca+Mg) ratios and porosity in the lower sweet spot interval (d); Sr/Cu ratios and porosity in the lower sweet spot interval (e); Ca/(Ca+Fe) ratios and porosity in the lower sweet spot interval (f).

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Table 1 Lower Sweet Spot

Upper Sweet Spot

UCC

Min

Max

Ave

Min

Max

Ave

/

Mg (%)

0.19

6.56

1.37

0.26

4.48

1.67

/

Al (%)

0.41

7.03

3.86

0.18

5.91

3.60

/

Si (%)

1.97

36.91

21.35

0.73

29.31

20.01

/

P (%)

0.03

13.91

0.24

0.033

7.64

0.14

/

S (%)

0.13

22.87

0.94

0.067

48.32

0.78

/

K (%)

0.13

3.24

1.58

0.24

4.44

1.76

/

Ca (%)

0.19

31.93

6.51

0.33

38.04

7.37

/

Ti (%)

0.02

1.56

0.26

0.03

1.03

0.25

/

Mn (ppm)

54.39

2832.81

601.053

104.4

2924.37

586.72

/

Fe (ppm)

11913

200441.2

19069.58

2029

736084.3

17628.3

/

Ba (ppm)

239.8

3935.81

641.77

83.15

9022.75

677.48

640

V (ppm)

29.59

333.66

101.87

12.39

264.02

93.13

70

Cr (ppm)

13.34

222.46

42.06

11.16

401.98

52.46

44

Ni (ppm)

26.57

213.71

72.16

27.06

510.53

70.02

21

Cu (ppm)

11.12

141.33

39.28

13.57

195.78

35.63

17

Zn (ppm)

12.15

258.19

63.13

13.81

1299.64

58.3

63

Th (ppm)

2.75

77.63

9.33

2.59

74.05

9.13

9.5

Rb (ppm)

3.17

125.14

58.68

3.76

144.82

54.91

95

U (ppm)

4.13

59.72

10.57

4.65

41.7

10

1.8

Sr (ppm)

26.85

6090.9

418.8

42.63

10285.3

514.97

300

Zr (ppm)

13.94

1112.71

160.03

8.55

542.68

168.21

170

Mo (ppm)

3.13

92.22

10.451

2.71

153.19

8.597

1.2

UCC refers to elemental compositions of the Upper Continental Crust

34

Highlights Detailed chemostratigraphic analysis is applied for the first time to determine palaeoenvironmental changes in the Jimusaer Sag, Palaeoclimatic settings control the enrichment of organic matters and reservoir petrophysical properties Two tight oil play types have been identified in the Lucaogou Formation in the Jimusaer sag

35