Controlling Factors of Organic-Rich Lacustrine Shale in the Jurassic Dongyuemiao Member of Sichuan Basin, SW China

Organic-rich continental shale, widespread in the Sichuan Basin during the deposition of the Jurassic Dongyuemiao Member (J 1 d), is considered the next shale hydrocarbon exploration target in southern China. To identify a shale gas sweetspot and reduce exploration risk, it is of great signi ﬁ cance to determine the organic matter (OM) enrichment mechanism of J 1 d shale. In this study, based on sedimentological characteristics and organic matter content, high-resolution major and trace elements were systematically analyzed to demonstrate terrigenous in ﬂ ux, paleoredox, paleosalinity, paleoproductivity, and paleoclimate. The 1st section interval of the J 1 d 1st submember is dominated by shallow lake subfacies, while the other intervals have the characteristic of semideep to deep lake subfacies. The 1st submember interval of J 1 d lacustrine shale is characterized by the warmest-humid paleoclimate, strongest weathering degree, highest terrigenous input, moderate paleoproductivity, and paleoredox condition. Within the Dongyuemiao 1st submember, the 4th section interval has the highest paleoproductivity and the most oxygen-de ﬁ cient condition in bottom water. During the deposition period of the 2nd submember, the sedimentary environment turned to a cold-dry paleoclimate, weak weathering degree, low terrigenous input, low paleosalinity, and high paleoproductivity. Under the background of semideep and deep lake, the terrigenous OM input plays the most critical role in controlling OM enrichment. Moreover, the high primary productivity of lake surface water and the suboxic condition of lake bottom water contribute to the formation of relatively higher TOC lacustrine shale interval in the 4th section of 1st submember.

Previous studies have revealed that lacustrine shale in China has great exploration potential, and corresponding mechanismic investigations have been carried out in the Jurassic strata of the Sichuan Basin, Triassic Yanchang Fm. in Ordos Basin, Shahejie Fm. of Bohai Bay Basin, Cretaceous Shahai and Jiufotang Fm. in Fuxin Basin, and the Cretaceous Qingshankou Fm. of Songliao Basin [14][15][16].The Jurassic continental shale strata in the Sichuan Basin are considered the most realistic alternative exploration target [16,17].In order to achieve efficient exploration and development, it is first necessary to identify the "sweetspots" of lacustrine organic-rich shale.Nevertheless, the sedimentary characteristics and organic matter accumulation mechanism have yet to be unified.
Elemental proxies have been applied very often to reveal environmental effect including terrigenous input, paleoredox condition, paleosalinity, and paleoproductivity on the accumulation of organic matter in fine-grained sedimentary rocks [3,4].Based on the division of Dongyuemiao Member in the Sichuan Basin, sedimentary facies marker, petrological characterization, and high-resolution elemental analyses are integrated to determine environmental conditions of different intervals in Dongyuemiao lacustrine shale.This study is aimed at deepening understanding of origin of Dongyuemiao lacustrine organic-rich shale and provide guidance for shale gas development.

Geological Background
The Sichuan Basin is a typical craton basin at the western Upper Yangtze Block (Figure 1(a)), and the basin area is about 260,000 km 2 .The study area is located in the Eastern Sichuan Basin (Figure 1(a)), dominated by lacustrine environment during the Early to Middle Jurassic.The Dabashan, Longmenshan, and Micangshan on the periphery of the basin constitute the provenance areas of the Jurassic sedimentary period [17][18][19].Four times lake transgressions were identified during Jurassic system.From the bottom to the top, four continental shale strata are identified: Zhenzhuchong Member (J 1 z), Dongyuemiao Member (J 1 d), Da'anzhai Member (J 1 dn), and Lianggaoshan Fm. (J 1 l), respectively.OM is thought to accumulate in semideep lacustrine to deep lacustrine environment controlled by anoxic condition [18,19].The lithology of J 1 d is mainly shale, followed by shale limestone and argillaceous limestone.Three submembers can be identified based on lithology: 1st submember, 2nd submember, and 3rd submember.The 1st submember can be subdivided into four sections: 1st, 2nd, 3rd, and 4th sections (Figure 1(b)).Except for the 2nd submember, the other intervals mainly constitute shale.The total organic carbon (TOC) value of J 1 d shale is between 0.5% and 2.0%, and the average value is greater than 1% [20].The OM type is mainly type II and locally developed type III, and the vitrinite reflectance (R o ) exceeds 1.2%.Thus, it has entered the stage of high-maturity evolution and is during gas generation stage. 2 Geofluids

Macroscopic and Microscopic Sedimentological
Characteristics.The four wells shown in Figure 1(a) were continuously cored in the J 1 d.First, the macroscopic sedimentological characteristics of cores were observed.In order to observe the microscopic sedimentological characteristics, 200 thin sections from core samples were observed by the Carl Zeiss Imager 2 microscope.4(c)).Different from the normally stacked shell fragments in the shell shoal, the shell fragments inside the CDF are tightly squeezed to form massive bedding (Figure 4(e)).LTC is typically characterized by graded bedding, and multiple graded beddings overlay upward in the vertical direction (Figure 4(c)).Occasionally, erosional surfaces can be observed at the bottom of each graded bedding (Figure 4(f)).In addition to CDF and LTC, transitional flow deposits associated with LTC and CDF can also be identified [22][23][24].The transitional flow deposits internally exhibit frequent interbedding of discontinuous lenticular siltstone (or shell fragments) and matrix shale (Figure 4(g)).
4.1.3.Deep Lacustrine Subfacies.Deep lacustrine subfacies refer to the deepest part of lake.Wave can hardly affect this subfacies, so the sedimentary water is quiet, and the stratification of the water body forms an dysoxic-suboxic environment of the bottom water.Benthos cannot survive in this environment, so no shell fragments are observed on core surface (Figure 5 Since the extremely low terrigenous influx weakens the dilution effect of terrigenous debris on organic matter, organic matter can be well enriched.

Major Element Concentration.
As shown in Figure 6, the Ca concentration of the 1st section lacustrine shale varies widely, from 0.36% to 19.09%, averaging of 5.02% (n = 8).
For the 2nd section lacustrine shale, the content of Ca is between 0.20% and 21.80%, averaging 5.12% (n = 13).The Ca concentration of the 3rd section lacustrine shale varies narrowly, ranging from 0.24% to 4.62%, averaging 1.60% (n = 10).The Ca content of 4th section lacustrine shale varies widely, ranging from 0.12% to 22.08%, with an average of 5.73% (n = 10).The content of Ca for 1st submember is close to the upper continental crust.

Geofluids
The content of Ca for 2nd submember argillaceous limestone and calcareous mudstone is higher than the other sections and the upper continental crust, ranging from 2.92% to 25.54%, with average value of 15.10% (n = 12).The Ca concentration of 3rd Submember lacustrine shale varies narrowly (n = 3), from 4.25% to 6.85%, averaging 5.83%.
The variation of B/Ga is opposite to that of V/Sc (Figure 8).The B/Ga of the 1st submember lacustrine shale is the highest, ranging from 1.57 to 3.57, and the average value is 2.33 (n = 41).The B/Ga of the 2nd submember is relatively low, ranging between 1.35 and 2.52, averaging 1.90 (n = 12).The B/Ga for 3rd submember lacustrine shale ranges from 2.03 to 2.44, with an average of 2.19 (n = 3).

Discussion
5.1.Terrigenous Influx.The concentrations of Zr, Ti, and Al in fine-grained sedimentary rocks are hardly affected by weathering or diagenesis, thus these elements are used to evaluate terrestrial input [27,28].Al only exists in the clay minerals of fine-grained sedimentary rocks, while Ti and Zr are mainly assigned to clay, sand, and silt particles composed of ilmenite, rutile, and augite [29,30].The terrigenous input proxies represented by Al, Ti, and Zr has gradually decreasing trend in the 1st submember (Figure 6).Terrigenous input Long-term (second-order) Short-term (third-order) Figure 3: Integrated sedimentary column of J 1 d from the well B in the Eastern Sichuan Basin (the lake-level curves is modified from [19]).
5 Geofluids reaches lowest level in the 2nd submember.Into the 3rd submember, terrigenous input gradually increased.Zr/Al and Ti/Al ratios are thought to closely relate to the coarser part of sediments [29,31].The clear Ti-Al correlation suggests that Ti comes from the lattice of clay minerals or stable terrigenous clastic materials [3,4].The 1st-4th sections are characterized by clear Ti-Al correlation (Figure 9(a)), suggesting that the detrital influx is relatively stable.Zr usually exists in clay minerals or heavy minerals of silt size (e.g., zircons) [32].The correlation of Al and Zr is clear in 1st-4th sections (Figure 9(b)).The results suggest that terrigenous influx for 1st-4th sections is relatively stable.

Paleoclimate Conditions.
The warm-humid climate can contribute to the atmospheric water cycle by raising chemical weathering intensity.Under this environmental condition, nutrients are continuously transported to seawater or lake water, which helps to improve the primary productivity of surface water.
Chemical index of alteration (CIA) has been applied to determine paleoclimatic conditions.The specific calculation method is [33][34][35]] The enrichment factor (EF) of environment-sensitive trace elements has been widely applied to reveal environmental conditions.
The specific formula is as follows: 6 Geofluids Note: X represents the element X concentration, and (ðX/AlÞ CC means the X/Al ratio in the continental crust [36][37][38].
Moreover, the excess value of element was another sensitive proxy reflecting environment condition [39].The calculation formula is as follows: Note: X total represents the element X concentration, and X/Ti PAAS means the X/Ti ratio in the Australian postarchean average shale (PAAS) [39].
Previous studies suggest that high CIA * values reflect warm, humid paleoclimate, and chemical weathering.And low CIA * values represent dry and cold condi-tions, and chemical weathering is weak [33,40].CIA * values ranging from 50 to 65 reflect cold-dry paleoclimate and low chemical weathering degree [40].CIA * values ranging from 65 to 85 reflect the warm-humid paleoclimate and moderate chemical weathering.The CIA * values ranging from 85 to d 100 represent the hot and humid paleoclimatic condition with strong chemical weathering.The CIA * for the 1st submember is the highest, representing warm-humid climate.The CIA * for the 2nd submember is the lowest, reflecting relatively cold-dry climatic condition with low chemical weathering.The CIA * values of the 3rd submember are close to 1st submember, reflecting warm-humid climatic condition.The PIA * values for 1st submember are between 17 and 99, and the PIA * values for 2nd submember are between 15 and 50.The variation trend of the PIA * is similar to the CIA * , reflecting the same climate change (Figure 8).(2).Dashed lines refer to element contents of upper continental crust [25].8 Geofluids and it is generally cleaned by particles in seawater, resulting in the concentration of Ga in seawater that is generally much lower than that in freshwater system.Therefore, the content of Ga in marine sediments is generally lower than that in continental sediments [42,43].Therefore, B/Ga and B contents are often used to distinguish sedimentary environments.The results suggest that the paleosalinity during deposition of 1st submember is in the range of brackish water and has a gradual decreasing trend.The paleosalinity of the 2nd submember is very similar to freshwater.The paleosalinity of the water body during the deposition of the 3rd submember was converted to brackish water.Na and K are highly active elements in alkali metals.Usually Na and K are evenly distributed in water, and their  9 Geofluids levels can be used to directly reflect salinity [44].The variation trend of the (K+Na) curve is similar to that of B/Ga, suggesting the same paleosalinity variation characteristics.

Paleoredox Conditions.
Trace element proxies including U au , V/Sc, U EF , and Mo EF were widely used to determine redox conditions for paleowater, and the smaller of these proxies reflect the higher oxidation degree, and the larger the ratio reflects the stronger reduction degree [45][46][47].Authigenic Mo, authigenic U enrichment, and Mo-U covariant models have been applied to determine redox conditions and water mass limitation [36,37,48].In general, the oxic condition showed little or no enrichment of authigenic U and Mo, while the anoxic conditions showed strong enrichment of authigenic U and Mo [36,37].The data points of each section are all located within the dysoxic zone of the unrestricted marine trend (Figure 10), and only a few data points near the suboxic zone.The variation trend of U au and V/Sc suggests that the bottom water is dominated by suboxic condition during the deposition period of the 4th section and 2nd submember of J 1 d.The other sections are dominated by dysoxic lake bottom water (Figure 8).5.5.Paleoproductivity Proxies.The primary productivity is proposed to be a critical factor controlling organic matter accumulation in shale [3,4].The strong enrichment of Ba, Cu, and Zn suggests that there was a high content of organic matter that brought it to the sediments.Subsequently, Ba, Cu, and Zn in the sediments were preserved under reducing conditions [50].Therefore, they can be used as alternative indicators of paleoproductivity level.The results show that the 4th section and 2 submember of J 1 d have the largest Ba xs , Cu xs , and Zn xs , reflecting the highest level of paleoproductivity.
5.5.1.Organic Matter (OM) Accumulation Mechanism.During the J 1 d deposition, a complete transgressive-regressive sedimentary cycle was developed within the study area [19].The 1st section deposition represents to the early stage of the J 1 d deposition.The eastern study area is dominated by shallow lacustrine facies (Figure 11(a)), and the lake is relatively small.The shell shoals composed of a large number of benthos can be observed on the core intervals.The water body is in oxic-dysoxic condition, which is not favorable for the preservation of OM.
During the 2nd section deposition, it experienced a strong transgressive action, and the lake area expanded significantly.The study area has the characteristics of semideep and deep lacustrine environment.Sediments of the shell shoal can trigger large-scale slumping, forming shell interlayers of gravity flow origin within semideep lacustrine environment (Figure 11(b)).During this period, the water body is relatively stable and conducive to algae and other lower aquatic organisms multiply, reflecting significantly increased primary productivity.At the same time, appropriate terrigenous influx does not cause a strong dilution effect of OM but provides more terrigenous OM.In addition, the dysoxic conditions of water bodies will also be favorable for the preservation of OM, which is favorable for OM enrichment.
During the 3rd section deposition, the decrease in lake level leads to a decrease in the distribution range of deep lake areas.The water turbulence was not of benefit to phytoplankton reproduction (Figure 11(c)), and the primary productivity was at low level.During the 4th section deposition, it experienced strong lake transgression again, and the lake area reached the maximum.Meanwhile, the bottom water body of the lake was relatively stable, which was beneficial to the reproduction of aquatic phytoplankton, including algae, which was reflected in the significant increase in productivity indicators (Figure 11(d)).At the same time, appropriate terrigenous influx does not cause a strong dilution effect of OM but provides more terrigenous OM.In addition, the suboxic conditions of water body will contribute to the OM preservation and enrichment.
During the 2nd submember deposition, the study area has been restored to a semideep lacustrine environment, and gravity flow deposits were frequently developed.However, since the paleoclimate of this period converted to cold-dry climate, terrigenous influx almost stagnated, and the water body gradually changed to fresh water (Figure 11(e)).Meanwhile, the bottom water is dominated by suboxic condition.These conditions are beneficial to the reproduction and preservation of phytoplankton, by raising primary productivity.However, due to the lack of terrigenous organic matter input, the TOC content has not increased significantly.During the deposition period of the 3rd submember, the lake level gradually decreased, the lake basin shrank greatly, and the distribution range of the deep lake area gradually returned to the 1st submember deposition period.However, due to the turbulent water body and the large input of terrigenous debris (Figure 11(f)), these conditions were unfavourable for phytoplankton reproduction and the preservation of OM.  11 Geofluids

Conclusions
(1) In 1st submember, the 1st section interval of the Jurassic Dongyuemiao Member was dominated by shallow lacustrine subfacies, while the other sections have the characteristics of semideep and deep lacustrine environment.The 2nd and 3rd submembers also have the characteristics of semideep and deep lacustrine environment (2) The 1st submember interval of the Jurassic Dongyuemiao Member was characterized by the most warm-humid condition, strongest weathering degree, largest terrigenous influx, moderate paleoproductivity, and moderate paleoredox condition.Within this submember, the 4th section interval had the highest paleoproductivity, and the most oxygen-deficient condition in bottom water.During the period of the 2nd submember interval, the sedimentary environment turned to cold-dry climatic conditions, weak weathering degree, low terrigenous input, low paleosalinity, and high paleoproductivity (3) Under semideep lacustrine and deep lacustrine background, terrigenous OM input played a key role in controlling OM enrichment of Dongyuemiao lacustrine shale.Moreover, the high primary productivity of lake surface water and suboxic condition of lake bottom water are also beneficial for the formation of high TOC interval in the 4th section of 1st submember
(a)).The rock type of this subfacies is gray-black shale (Figure 5(b)).The laminae inside the shale are not obvious and the boundaries of each laminae (Figure 5(c)).The terrigenous influx in the deep lacustrine subfacies is very small (Figure 5(d)).The terrigenous debris is mainly composed of silt-size quartz particles (Figure 5(e)), which are dispersed in the shale (Figure 5(f)), and the bedding and orientation are poor (Figure 5(g)).

Figure 2 :
Figure 2: Macroscopic and microscopic sedimentary characteristics of shallow lacustrine subfacies for J 1 d from well A. (a) Core photograph of shallow lacustrine subfacies showing shell shoal and ripple bedding, 1 section, 2518.29 m-2518.50m.(b) Normal stack of shell fragments, 2518.49m.(c) Normal stack of shell fragments exhibiting wave fluctuation, 2518.39 m.(d) Silt laminae showing discontinuous planar lamination, 2518.36 m.(e) Shell fragments exhibiting strong wave fluctuation, 2518.34 m.(f) Normal stack of shell fragments, 2518.32 m.(g) Normal stack of shell fragments, 2518.29 m.

Figure 7 :
Figure 7: Environment-sensitive element concentrations for J 1 d from well C (2). Dashed lines refer to element contents of upper continental crust [25].

Figure 8 :
Figure8: Environment-sensitive elemental proxies for J 1 d from well C.The dotted lines represent the threshold value proposed by[26].

Figure 10 :
Figure10: Authigenic Mo-U covariation patterns of J 1 d.The diagonal dashed lines refer to the Mo/U molar ratios of the seawater (sw).Grey area refers to the "unrestricted marine" trend, characterization of depositional systems with no limited trace metal renewal.Yellow area refers to the "particulate shuttle" trend, a characterization of environments in which intense redox cycling of metals occurs within the water column[49,50].