Sedimentary environment and model for lacustrine organic matter enrichment:lacustrine shale of the Early Jurassic Da'anzhai Formation, central Sichuan Basin, China

Zheng-Lu Xiao , Shi-Jia Chen ＊, Shao-Ming Zhang ,Rui Zhang , Zhi-Yong Zhu , Jun-Gang Lu , Yong Li ,Xiang-Dong Yin , Long-Xiang Tang , Zhang-Hao Liu ,Zong-Hui Lin

a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu 610500, China

b School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China

c PetroChina Southwest Oil & Gas Field Company, Chengdu 610051, China

d Xi'an Branch of PetroChina Materials Co., Ltd Xi'an 710000, China

e State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China

Abstract Based on the analysis of element geochemistry and total organic carbon (TOC), this study investigates the main factors controlling organic matter(OM)enrichment,reconstructs the evolution process of the sedimentary environment, and proposes a dynamic OM enrichment model of the Jurassic Da'anzhai (D)Formation,Sichuan Basin.The results indicate that the Sichuan Basin was generally dominated by a warm and oxidizing sedimentary environment, but with some peculiarities, such as a hotter climate in the D1 member and more anoxic lake water in the D2a member.The sedimentary evolution of the Da'anzhai Formation can be divided into a fluctuating sedimentary stage, a stable sedimentary stage and a reef-building stage. The D2a member showed the strongest hypoxia,the weakest weathering,the largest amount of terrestrial inputs,and the highest TOC content. The TOC is positively correlated with reducing conditions and terrestrial inputs,negatively correlated with weathering. Based on these findings, it is suggested that the global climate in the Early Jurassic period had a complex regional effect and the global oceanic anoxic events of the Toarcian did not spread to the Sichuan Basin. Thus, the anoxic deep water, high terrestrial inputs, and weak weathering were conducive to rapid deposition and preservation of lacustrine OM.

Keywords Lacustrine shale, Early Jurassic Toarcian, Organic matter enrichment model, Da'anzhai For

1.Introduction

Previous studies suggested that the global climate was warm during the Jurassic (Hallam, 1991; Frakes et al., 1992). However, in-depth studies of different regions revealed a complex regionalisation of the global climate. For example, Rogov and Zakharov(2010) proposed that a continental ice sheet existed in the Circum-Arctic Basin during the Jurassic. Korte et al. (2015) showed that the surface seawater temperature fluctuated significantly in the Jurassic. Sun et al. (2007) found a significant change in atmospheric CO2content during the Jurassic. The global oceanic anoxic events in the Early Jurassic Toarcian have also been widely studied (Hesselbo et al., 2000;McElwain et al., 2005; Svensen et al., 2007; Burgess et al., 2015). Generally, there is no concurrence on the climate change mechanism during the Jurassic period, and conclusions drawn from different climate parameters are somewhat contradictory.

In addition,the dominant factors controlling organic matter (OM) enrichment are often different due to differences in sedimentary environments.For example,Harris et al.(2004)and Sun et al.(2020)proposed that biological productivity is the most important factor in OM enrichment. However, Mort et al. (2007) and Zeng et al. (2015) suggested that the redox condition is the dominant factor. Lash and Blood (2014) expressed the opinion that terrestrial inputs could dilute OM. Carroll and Bohacs (2001) emphasized the influence of basinfilling methods on OM enrichment. Zonneveld et al.(2010) concluded that weathering and clay mineral distribution are closely associated with OM enrichment.Furthermore,compared with that of marine shale,the sedimentary environment of lacustrine shale is more complex,and its OM enrichment mechanism is less understood(Xu et al.,2017).

The most typical Jurassic lacustrine shale in southern China developed in the Da'anzhai Formation,which has a high OM abundance and considerable sedimentary thickness(Yang et al., 2005;Chen et al.,2015; Su et al., 2018). Moreover, this formation is the key potential target for shale oil development in the Sichuan Basin(Sun et al.,2021).However,due to a lack of research on the factors controlling OM enrichment in lacustrine basins, it is difficult to determine the dominant OM enrichment intervals, and this has significantly hindered progress of shale oil exploration.

Using multiple geochemical methods, this study investigates the main factors controlling OM enrichment, reconstructs the evolution of the sedimentary environment, and proposes a model for the enrichment of lacustrine OM in the Early Jurassic Da'anzai Formation.The results provide a better understanding of the relationships between environmental evolution of the Early Jurassic terrestrial system and the factors controlling lacustrine OM. Additionally, this study serves as an important reference for shale oil exploration and development in the Sichuan Basin and other similar lacustrine basins.

2.Geological setting

The Sichuan Basin,located in southwest China,is a large inland cratonic basin formed after the collision between the Indian and Eurasian plates and the uplift of the Yangtze Plate (Fig. 1b) (Yang et al., 2014; Liu et al., 2016; Li et al., 2019, 2020). The Sichuan Basin is the largest natural-gas-producing area in China,providing oil and gas resources to the surrounding cities (Ma, 2017). Owing to its suitable depth and temperature, the Jurassic sediments form the only unit in the Sichuan Basin rich in liquid hydrocarbon resources (Wang et al., 2013; Chen et al., 2015; Su et al., 2018). The study area is located in the central Sichuan Basin, between Chengdu and Chongqing, two National Central Cities of China (Fig. 1b). Most of the oil and gas fields are developed in this area (Chen et al., 2014, 2015;Su et al., 2018).

During deposition of the Early Jurassic Da'anzhai Formation,tectonic activities in the Sichuan Basin and its surrounding areas gradually weakened, and the sedimentary environment gradually became stable(Yang et al.,2014;Li et al.,2019).This resulted in the formation of the largest Jurassic lake and the deposition of TOC-rich shale (Chen et al., 2015; Su et al.,2018). Based on the lithological characteristics, the Da'anzhai Formation can be divided vertically from bottom to top into three members: D3, D2, and D1respectively(Fig.1b).The thickness of the D3member is 5-15 m, and its lithology is dominated by shelly limestone with thin black shale.The thickness of the D2member is 35-60 m,and its lithology is generally thick massive black shale.The thickness of the D1member is 20-30 m, and its lithology is mainly grayish-brown massive limestone, argillaceous shelly-limestone, and black shale(Xu et al.,2017;Su et al.,2018).

Fig. 1 Location map of the study area and lithology column of the sampling well: a- Global palaeogeographic world map during the Middle Jurassic,modified from Li and Jiang(2013);The inset map of China is modified after the Standard Map Service of the National Administration of Surveying, Mapping and Geoinformation of China (http://bzdt.ch.mnr.gov.cn/) (No. GS (2019)1825). b- Outline and sampling location of this study in Sichuan Basin; c- Stratigraphic scheme and lithology of the Jurassic Da'anzhai Formation in Sichuan Basin.

Surrounding the lake basin margin,a large number of shelly banks developed in the D1member, which is considered to be the most important reef-building stage in the Early Jurassic in the Sichuan Basin (Chen et al.,2020).Previously,the D2member was considered to be the most effective set of source rocks in the Jurassic in the Sichuan Basin(Chen et al.,2015;Su et al.,2018).At present,the D2member is regarded as the most favorable shale oil exploration target(Sun et al.,2021).For the convenience of petroleum exploration, the D2member has been divided into three sub-members:D2c,D2b,and D2a,from bottom to top(Fig.1c).

3.Samples and experimental methods

3.1. Samples

Twenty-five shale samples were obtained from Well G10 (longitude 106°0′8′′, latitude 31°16′23′′, and drilled by PetroChina Southwest Oil & Gas Field Company) in the Gongshanmiao structure in the central Sichuan Basin for TOC measurement (Table 1). Thirtysix samples were obtained to determine trace and major elements(Tables 1 and 2),of which 8,7,10,7,and 4 samples were obtained from D1,D2a,D2b,D2c,and D3members, respectively.

3.2. Experimental methods

TOC measurements were performed at the Exploration and Development Research Institute of Shengli Oilfield, Sinopec, using a USA Leco CS-200 carbon and sulfur analyzer(Table 1).The samples were ground to particle sizes less than 0.2 mm. After removing the inorganic carbon from the samples (>10 g) with dilute hydrochloric acid, the samples were burned under high-temperature oxygen flow to convert the TOC to CO2. The TOC content of the samples was then determined using an infrared detector.

The major elements were analyzed at the Bohai Research Institute of CNOOC-Tianjin Branch. The samples were dissolved using anhydrous lithium tetraborate.Ammonium nitrate was utilized as theoxidant,and lithium fluoride and a small amount of lithium bromidewere utilized as the co-solvent and release agent,respectively.The sample-to-reagent ratio was 1:8.The glass sample was produced using a sample melting machine at 1150-1250°C. A wavelength dispersive x-ray fluorescence spectrometer (ZSX PrimusII) was utilized for the measurements, and 10 major elements were obtained for each sample.The content of the six metal ions was determined via conversion(Table 1).

Table 2 Trace elements(ppm)of the Da'anzhai Formation shale samples from Well G10 in the central Sichuan Basin.

Trace elements were also tested at the Bohai Research Institute of CNOOC-Tianjin Branch.First,the samples were dissolved in hydrofluoric and nitric acid inside a closed dissolver. The hydrofluoric acid was then removed via evaporation using a hot plate.Next,the samples were dissolved in nitric acid under sealed conditions, and the diluted liquid was directly determined using the external standard method of inductively.Coupledplasmamassspectrometry(PerkonElmer NexION 350X). A total of 40 trace elements were detected in each sample (Table 2).

4.Results

4.1. Organic geochemistry

The TOC content of the Da'anzhai Formation shale is widely distributed, ranging from 0.65% to 3.72% (Table 1), with the largest proportion (24%) of TOC at 1.5%-2.0% (Fig. 2a). The TOC content of the different members of the Da'anzhai Formation varies considerably.The D2amember has the highest TOC content with an average of 2.69%(～2.0%-3.7%).All D2amember samples have a TOC content of more than 1.5%(Table 1 and Fig.2b).The D2band D3members have moderate TOC content with a mean of 1.69% (～0.65%-2.64%) and 1.76% (～1.32%-2.32%)respectively.The D1and D2cmembers have lower TOC content with an average of 0.92% (～0.84%-1.03%)and 1.26%(～0.9%3-1.88%)respectively.

4.2. Major element geochemistry

The main enriched oxides in the Da'anzhai Formation are SiO2(48.75%), Al2O3(17.67%), CaO (7.14%),Fe2O3(6.50%), K2O (2.94%), and MgO (2.14%). The MnO,P2O5,Na2O,and TiO2contents are less than 1.0%(Table 1). Compared to the average shale (AS)(Wedepohl, 1971, 1991; Daniel and Bustin, 2009), the Da'anzhai Formation shale is generally higher in CaO,Fe2O3and P2O5, similar in Al2O3, and lower in other oxides(Fig.3a).

The D1member has the highest CaO content(12.91%) of all members, while the terrestrial input minerals (Al2O3, Fe2O3, MgO, K2O and SiO2) are lower than in other members. In contrast, the D2aand D3members have the lowest CaO content (1.99% and 2.55%, respectively), while the terrestrial input minerals are generally higher. The D2band D2cmembers have moderate CaO content and terrestrial input minerals(Fig. 3b).

4.3. Trace element geochemistry

Fig. 2 TOC distributions in Da'anzhai Formation shale: a- Cumulative for all members; b- Proportion with TOC greater than 1.5% in the different members.

Ba (827.33 ppm), Mn (518.78 ppm), Sr(237.14 ppm), Zn (192.45 ppm), V (184.34 ppm), Zr(140.91 ppm),Cr(140.38 ppm),Rb(121.73 ppm),and Ni (59.37 ppm) are the most enriched trace elements in the Da'anzhai Formation.The content of other trace elements is below 50%. Compared with AS (Fig. 4),Da'anzhai shale is more enriched in V, Cr, Zn, Ga, Cd,Cs, Ba, Hf, and Bi, has similar Be, Cu, and Al, and has lower contents of other trace elements.

When comparing the different members, the Cr,Mn,Co,Ni,Zn,Rb,Sr,Ba,Hf,Pb,Bi,and Th contents in the D1member are generally highest. The D2aand D2bmembers have higher V,Se,and Mo contents but lower Mn,Y,and La contents.The D2cmember has higher Mn,Cd,La,Bi,and Th contents.The D3member has higher Al and Cd contents, and the D2cand D3members have the lowest Mo of all the members (Fig. 4).

5.Discussion

5.1. Biological productivity and its impact on OM enrichment

Fig. 3 Distribution of major elements in Jurassic Da'anzhai Formation shale in the Sichuan Basin:a-Degree of mineral enrichment of the Da'anzhai Formation shale compared with that of average shale;b-Mineral assemblage proportions of the different members.

Fig.4 Degree of trace element enrichment relative to those of average shale(Changliang Mountain area)in the Jurassic Da'anzhai Formation in the Sichuan Basin.

Biological productivity plays an important role in OM enrichment (Pedersen and Calvert, 1990; Harris et al., 2004; Zhang et al., 2005). Generally, there is a positive correlation between biological productivity and shale OM content (Pedersen and Calvert, 1990;Harris et al.,2004;Yang et al.,2019;Sun et al.,2020).Biogenic Ba (Babio= Batotal- [Al × Ba/Alalusilicate],typically calculated as Ba/Alalusilicate= 0.0075) in sediments can reflect primary productivity (Suess, 1980;Brummer, 1992; Dymond et al., 1992; Bonn et al.,1998). Phosphorus (P) is also an important component in organisms, and the P/Ti is often utilized as an index of paleoproductivity. Generally, P/Ti values of>0.79, 0.34-0.79, and <0.34 indicate high, medium,and low productivity,respectively(Algeo et al.,2011).

The average Ba(bio)of the Da'anzhai Formation is 290.64 ppm(～26.18-489.26 ppm),and the average P/Ti is 0.45(～0.10-3.04).These values indicate that the overall biological productivity of the Da'anzhai Formation is medium.In contrast,the D1member has the highest biological productivity,with average Ba/Al and P/Ti values of 145.60 and 0.77, respectively. The average Ba/Al and P/Ti values of the D2bmember are 102.02 and 0.51, respectively, indicating medium biological productivity.The biological productivities of the D2a,D2c,and D3members are low,with average Ba/Al and P/Ti values lower than 92.3 and 0.34, respectively (Table 2).

There was no correlation between TOC and Ba/Al,however, TOC was negatively correlated with P/Ti(Fig. 5).So, the biological productivity is not the main factor controlling OM enrichment in the Da'anzhai Formation.

5.2. Sedimentary environment and its impact on OM enrichment

5.2.1. Climate conditions

The composition and relative content of elements in lake sediments can reflect the climate conditions.Under a humid climate,elements such as Fe,Mn,Cr,V,Ni,and Co are easily eroded and transported from the source area to the lake basin (Cao et al., 2012; Qiu et al., 2015; Hu et al., 2016; Moradi et al., 2016).However, due to evaporation and consequently increased salinity under a dry climate, Ca, Mg, K, Na,Sr, and Ba precipitate. Therefore, the ratio (i.e., Cvalue) of these two element groups can effectively reflect the climate conditions. The climate is correspondingly categorized into arid, semi-arid, semihumid and semi-arid, semi-humid, and humid, according to C-value of<0.2,0.2-0.4,0.4-0.6,0.6-0.8,and >0.8. Additionally, the ratio of Sr to Cu is used to determine climate conditions.Generally,Sr/Cu values of >10 and 1-10 indicate dry-hot climate and warmhumid climate, respectively (Reheis, 1990; Xu et al.,2017).

Fig.5 Crossplots of biological productivity parameters(Ba(bio)and P/Ti)versus TOC.There is no correlation between TOC and Ba(bio)and TOC is negatively correlated with P/Ti.

The Sr/Cu and C values range from 9.77 to 103.22 and 0.04-1.06 respectively, indicating frequent climatic changes during the deposition period of the Da'anzhai Formation.The mean Sr/Cu and C values for the D1member are 25.64 and 0.48 respectively, indicating a semi-arid climate condition. The mean Sr/Cu and C-values for the D2aand D2cmembers are 3.57 and 0.70,and 3.96 and 0.75 respectively,indicating a semihumid climate condition.The mean Sr/Cu and C values of the D2bmember are 8.46 and 0.65 respectively,between the D1and D2cmembers, corresponding to semi-arid and semi-humid climate conditions. The mean Sr/Cu and C-values of the D3member are 2.36 and 0.83 respectively, indicating a humid climate condition (Table 2).

No evident correlation is observed in the crossplot between the climatic indices (Sr/Cu and C-value) and TOC (Fig. 6). Therefore, climate is not a major factor controlling OM enrichment in the Da'anzhai Formation.

5.2.2. Redox conditions and water depth

The ratio of V to Ni can be used to reflect the redox conditions.Generally,V/(V+Ni)>0.77 indicates water stratification and a reducing sedimentary environment, while V/(V + Ni) < 0.6 indicates weak water stratification and an oxygen-rich sedimentary environment (Hatch and Leventhal, 1992; Miao et al.,2004). Moreover, Jones and Manning (1994) proposed that V/Cr values of <2, 2-4.25, and >4.25 indicate oxygen-enriched, transitional, and anoxic conditions,respectively. The Rb/K values give an accurate indication of the water depth, with high value indicating deep water (Jin et al., 2006; Sun et al., 2020).

In this study,V/Cr and V/(V+Ni)values range from 0.36 to 2.38 and 0.53-0.82 respectively, indicating that the Da'anzhai Formation was a transitional sedimentary environment of oxidation to reduction. The average V/Cr and V/(V+Ni)of the D1,D2a,D2b,D2c,and D3members are 1.11 and 0.69,1.85 and 0.79,1.70 and 0.78, 1.34 and 0.74, and 1.20 and 0.73, respectively(Table 2). Based on these results, the D2aand D2bmembers were deposited in a relatively reducing sedimentary environment,the D2cand D3members in a weakly oxidized sedimentary environment,and the D1member in an oxygen-rich sedimentary environment.The mean Rb/K×104values for members D1,D2a,D2b,D2cand D3are 63.80, 52.79, 52.98, 46.69 and 40.72,respectively (Table 2), which indicate a gradual increase in water depth from D3to D1member.

Fig. 6 Crossplots between climatic parameters (Sr/Cu and C-value) and TOC. There is no correlation between TOC and climate conditions.

Fig. 7 Crossplots between TOC and (a, b) redox (c) water depth, and (d) salinity. There is a positive correlation between TOC and redox conditions and no correlation between TOC and water depth and salinity.

From the crossplots, evident positive correlations between TOC and V/Cr and V/(V + Ni) are observed(Fig.7a and b).However,no correlation was observed between Rb/K and TOC (Fig. 7c). These results show that redox condition is one of the main factors controlling OM enrichment in the Da'anzhai Formation.

5.2.3. Salinity conditions

Strontium (Sr) and barium (Ba) are widely distributed elements in the Earth's crust, and both exist in lakes in the form of bicarbonate. When lake water salinity increases, Ba first precipitates in the form of BaSO4. With a further increase in salinity, Sr precipitates in the form of SrSO4. Therefore, the Sr/Ba often reflects the change in salinity. Generally, Sr/Ba values of<1 and>1 represent freshwater and saltwater environments, respectively (Dymond et al., 1992;Jarvis et al., 2001; Kimura and Watanabe, 2001).

The mean Sr/Ba for the D1, D2a,D2b, D2cand D3members are 0.65, 0.19, 0.36, 0.22 and 0.18, respectively (Table 2), indicating an overall freshwater sedimentary environment for the Da'anzhai Formation.In comparison,the D1and D2bmembers were deposited with higher water salinity. No correlation is observed between salinity indexes and TOC(Fig.7d),therefore,salinity is not a major factor controlling OM enrichment in the Da'anzhai Formation.

5.2.4. Weathering conditions

The Chemical Index of Alteration (CIA=Al2O3/(Al2O3+Na2O+CaO+K2O)×100)is used to reconstruct paleoclimate and weathering conditions (Fedo et al.,1995; Price and Velbel, 2003). Nesbitt and Young(1984) suggested that CaO should be excluded from the CIA equation due to the uncertainty of whether CaO is derived from carbonate. Therefore, they proposed the Chemical Index of Weathering(CIX=(Al2O3)/(Al2O3+Na2O + K2O)] × 100) to reconstruct paleoweathering conditions (Liang et al., 2018). Additionally, the (La/Yb)N(La/Sm)N(Gd/Yb)N, and (Ce/Yb)Nvalues(where N is the chondrite normalized value)are positively correlated with light rare-earth elements and weathering (Wright et al., 1984; Murray et al.,1990; Ross et al., 1995; Roy and Smykatz, 2007).

Fig.8 Crossplot between TOC and weathering parameters(CIX and(La/Yb)N).There is a negative correlation between the weathering index and TOC.

The mean CIX and(La/Yb)Nvalues for the Da'anzhai Formation are 83.42 (～80.26-85.75) and 7.11(～2.90-9.23) respectively, indicating that it has undergone moderate to high weathering. The mean CIX and (La/Yb)Nvalues for the D1, D2a, D2b, D2c, and D3members are 83.94 and 7.54, 81.18 and 6.92, 83.05 and 6.68, 84.64 and 7.09, and 85.13 and 7.69,respectively (Table 2). Based on these results, the weathering intensity of the Da'anzhai Formation was first weakened and then strengthened, with the D2amember being the weakest.As seen from the crossplot in Fig. 8, there is an evident negative correlation between the weathering indexes and TOC, therefore,weathering conditions affect OM enrichment in the Da'anzhai Formation.

5.2.5. Terrestrial inputs and hydrodynamic conditions

SiO2,TiO2and Al2O3are stable terrestrial elements and are widely used to determine terrestrial inputs.Besides, Fe + K content is a practical index for determining terrestrial inputs(Arthur and Dean,1998;Wang et al., 2020). Zr is a typical inert element with a prominent terrestrial orientation, and the high Zr content reflects high-energy hydrodynamic conditions.Rb occurs mainly in sedimentary rocks in the form of silicates preserved in clay,mica,and other fine or light minerals, and is easily deposited under low-energy hydrodynamic conditions (Dypvik and Harris, 2001;Chen et al., 2006). Thus, high Zr/Rb value indicates relatively high-energy hydrodynamic condition(Dypvik and Harris,2001; Chen et al., 2006).

In this study, the D1and D2bmembers have relatively low terrestrial input minerals, with the sum of Al2O3,SiO2and TiO2contents being 55.96%and 65.40%respectively.The D2cmember has a medium content of terrestrial input minerals,with a combined Al2O3,SiO2and TiO2contents of 70.35%.The content of terrestrial input minerals in the D2aand D3members are relatively high, with a total Al2O3, SiO2, and TiO2contents of 74.37% and 74.70%, respectively (Table 1). The above results indicate that the Da'anzhai Formation was generally characterized by low to moderate levels of terrestrial input, but the amount of terrestrial inputs varied frequently between members of the same depositional period. The average Zr/Rb values for D3,D2c, D2b, D2a, and D1members are 1.63, 1.53, 1.00,1.04, and 0.99, respectively (Table 2), which indicate that the hydrodynamic force gradually weakened from the D3to D1member.

There is a weak positive correlation between TOC and terrestrial inputs and no correlation between TOC and hydrodynamic conditions (Fig. 9). Therefore,terrestrial input is one of the factors controlling OM enrichment in the Da'anzhai Formation.

5.3. Evolution of sedimentary environment

5.3.1. Sedimentary environments of the Early Jurassic Sichuan Basin

Based on the results of the above analysis,it is not difficult to determine the sedimentary environment of the Early Jurassic Da'anzhai Formation in the Sichuan Basin (Table 3). Palaeoenvironmental conditions include: humid to semi-arid climate, oxygen-poor freshwater environment, shallow to moderate water depth, weak to moderately hydrodynamic conditions,low to moderate terrestrial input, and moderate to high weathering. The climate of the Early Jurassic in the Sichuan Basin was generally warm,but there were also exceptions, such as a hotter climate during the deposition of the D1member, which support the view that the global climate of the Jurassic was complex and regional. In addition, the Sichuan Basin was generally oxic in the Early Jurassic,suggesting that the global oceanic anoxic event of the Early Jurassic Tolstoyan did not spread to the terrestrial lake basins.

Fig. 9 Crossplots of TOC versus the terrestrial input minerals SiO2 (a), TiO2 (b), and Fe + K (c) and hydrodynamic parameter (Zr/Rb) (d).There is a weak positive correlation between TOC and terrestrial inputs and no correlation between TOC and hydrodynamic indexes.

Based on the longitudinal variation of geochemical parameters(Fig.10),the sedimentary environment of the Da'anzhai Formation can be generally divided into three depositional stages:1)A fluctuating stage(2712-2691 m): humid climate, strong weathering, but all indexes characterizing the water environment show large-scale fluctuations; 2) A stable stage (2691-2664 m): humid climate, weak weathering, large water depth and high reducibility; 3) A reef-building stage (2664-2659.5 m): biological productivity,climate,water salinity and terrestrial input conditions all show significant anomalies. Organisms are more favoured to grow in a hotter,more oxygen-rich,saltier and low-input sedimentary environment.

5.3.2. Sedimentary environment comparison of each member

Due to different purposes (e.g. the delineation of strata in an oilfield is based on the selection of target layers for hydrocarbon exploration),the boundaries of sedimentary environments are often not entirely consistent. For a particular member, the sedimentary environment may have to be studied independently to serve the corresponding purpose. According to the results presented in Section 5.2, the sedimentary environment of the Da'anzhai Formation is summarized with the different members as the boundary.

During the deposition of the D3member, the climate was humid,the lake was weakly oxidized,the water depth was shallow, the salinity was low, the hydrodynamic force was moderate, the weathering was intense,and the terrestrial inputs were relatively high (Fig. 11a). Compared with the D3member, the climate of the D2cmember was slightly hot and evolved into a semi-humid climate. The water depth, reduction, and salinity increased, however, the hydrodynamic force, weathering, and terrestrial inputs decreased (Fig. 11b). The D2bmember continued the trend of the D2cmember,wherein the climate changed to semi-humid and semi-arid, the water depth,reduction,and salinity continued to increase,and the hydrodynamic force, weathering, and terrestrial inputs continued to decrease(Fig. 11c).

The sedimentary environment of the D2amember is unique. The water depth, reduction, weathering, and hydrodynamic force continued to follow the trend ofD2bmember, however, the climate became relatively humid, the salinity decreased, and the terrestrial inputs increased sharply during this period (Fig. 11d).The sedimentary environment of the D1member sharply contrasts that of D2amember. The climate rapidly became hot, the lake basin became oxygenrich, the water depth, salinity, and weathering increased,and the terrestrial inputs and hydrodynamic force decreased during this period (Fig. 11e).

Table 3 Summary of sedimentary environments in different members of the Da'anzhai Formation.

It should be emphasized that the above conclusions are based on the analysis of only one well.Well G10 is located in the center of the lake basin, where the development of shelly banks is more limited. If other wells at the edge of the lake basin are selected for the same analysis, the conclusions obtained may be slightly different. Therefore, the conclusions of this paper on the overall sedimentary environment of the Early Jurassic in the Sichuan Basin are generally applicable. However, the conclusions of this paper on the sedimentary environment of each member of the Da'anzhai Formation may not be completely accurate,which is influenced by the sedimentary zone.

5.4. Dynamic enrichment model of lacustrine OM

Based on the vertical variation characteristics of the various geochemical indexes (Fig. 10), the enhanced development of organisms(high Ba/Al)is characterized by high Sr/Cu,Sr/Ba,Rb/K and low TiO2and Zr/Rb.The corresponding sedimentary environment is arid climate,deep water conditions with relatively high-salinity,and low terrestrial inputs and hydrodynamic force. It is inferred that most of the shelly organisms in the Da'anzhai Formation preferred to live in hot, oxygenrich, relatively high salinity, quiet and deep water environments and are probably an assemblage of Youshashania-Hemicryptotus-Eucypris (Yang et al., 2006;Chen et al.,2019).

Section 5.2 shows that the TOC of the Da'anzhai Formation is positively correlated with reducing conditions and terrestrial inputs and negatively correlated with weathering. Biological productivity,climate, water depth, salinity, and hydrodynamic force were not major factors affecting OM enrichment in the Da'anzhai Formation. The correlation shows that although the biological productivity of the D2amember was low,this layer exhibited the highest TOC content (Fig. 10). Therefore, the later preservation conditions of OM are highly significant. A dynamic model of the OM enrichment in the Da'anzhai Formation is established based on the sedimentary environment characteristics of a high TOC well section (Figs. 10 and 12).

Fig. 10 Geochemical indexes versus depth of the Da'anzhai Formation obtained from Well G10 in the Sichuan basin.

Fig. 11 Environment evolution model during the different periods of the Jurassic Da'anzhai Formation (a-e) in the Sichuan Basin.

Fig.12 OM enrichment model of the Jurassic Da'anzhai Formation in the Sichuan Basin.The reducing environment at the bottom of the lake basin is conducive to OM preservation.High terrestrial inputs and weak weathering increase the OM deposition rate in shallow water,reduce OM oxidation and decomposition times, inhibit the OM loss, and facilitate rapid deposition and preservation of OM.

During the deposition of the Da'anzhai Formation,the orogenic activity of Sichuan Basin was weakest and the sedimentary environment was the most stable.This facilitated the development of deep lacustrine facies over a large area(Yang et al.,2014;Feng et al.,2015;Su et al., 2018). A stable water environment is conducive to OM enrichment.In humid climates,evaporation is weak, water salinity is low, and the salinity stratification interface is deep.A reducing environment forms in the bottom of the deep water,which is favorable for OM preservation.In shallow water,high terrestrial input and weak weathering increased the OM deposition rate,reduced OM oxidation and decomposition times,inhibited OM loss, and facilitated rapid deposition and preservation of lacustrine OM (Leythaeuser, 1973;Müller and Suess, 1979; Henrichs and Reeburgh, 1987;Petsch et al.,2000).

6.Conclusions

The climate of the Early Jurassic in the Sichuan Basin was generally warm, but there were also short hot periods, which support the view that the global climate in the Jurassic period had complex regionalization. The overall bias towards oxidation in the Sichuan Basin during the Early Jurassic suggests that the global oceanic anoxic event of the Early Jurassic Toarcian did not spread to the continental lake basins.

The sedimentary environment of the Da'anzhai Formation can be generally divided into three depositional stages: fluctuation, stable and reef-building stages. Under the overall sedimentary environment evolution system of the Da'anzhai Formation, the D2amember exhibited abnormal characteristics,including the strongest reducibility, the weakest weathering,and the highest terrestrial inputs.

The shelly organisms in the Da'anzhai Formation lived in a hot, oxygen-rich, relatively high salinity,deep and quiet water environments. The TOC of the Da'anzhai Formation is positively correlated with reduction and terrestrial inputs,negatively correlated with weathering, and has no evident correlation with other indexes.

The reducing environment formed at the bottom of the lake basin was favorable for OM preservation.High terrestrial inputs and weak weathering increased the OM deposition rate in shallow water, reduced the OM oxidation and decomposition times, and inhibited OM loss. These factors are conducive to rapid deposition and preservation of lacustrine OM.

Availability of data and materials

All data generated or analyzed during this study are included in this published article [and its supplementary information files].

Funding

This research comes from the Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance(NO.2020CX050000),and the Research and Innovation Fund for Graduate Students of Southwest Petroleum University (NO. 2019cxyb005).

Author contributions

Xiao Zhenglu, Software, Writing original draft;Chen Shijia, Project administration, Funding acquisition; Zhang Rui & Zhang Shaoming, Resources, Supervision;Zhu Zhiyong,Methodology;Lu Jungang,Project administration, Conceptualization; Li Yong, Writing revised draft; Yin Xiangdong, Writing revised draft;Tang Longxiang,Data curation;Lin Zonghui,Software,Data curation; Liu Zhanghao, Formal analysis.

Competing interest

The authors declare that they have no competing interests.

Acknowledgements

The authors would like to express their gratitude to the PetroChina Southwest Oil and Gas Field Company for providing the resources required to collect the samples used in this study.Additionally,we would like to thank everyone who provided valuable comments and suggestions during the interpretation and revision processes.