Controlling Factors and Formation Models of Organic Matter Accumulation for the Upper Permian Dalong Formation Black Shale in the Lower Yangtze Region, South China: Constraints from Geochemical Evidence.

The black shale in the upper Permian Dalong Formation is considered as an excellent source rock in the Lower Yangtze region. However, mechanisms of organic matter (OM) accumulation in such a setting are scarcely understood. Here, the characteristics of total organic carbon (TOC) and elemental geochemistry of 33 rock samples from GD1 well are systematically investigated to characterize the paleoenvironmental conditions and OM accumulation mechanisms. Results show that the lower and middle parts of Dalong Formation (section A) display high TOC contents ranging from 1.19 to 6.45% (average 3.19%), whereas the upper part (section B) exhibits medium TOC contents varying from 1.18 to 4.90% (average 2.09%). These data also elucidate that the target shales were deposited in a complex paleoenvironment with moderate to strong water-mass restriction that was characterized by warm and semiarid-semihumid paleoclimate, high biotic productivity, fluctuating plaeoredox conditions, and a relatively high sedimentary rate. Compared to the organic-rich shales from section A mainly developed under an anoxic condition, shales from section B formed in an oxic-to-dysoxic water environment exhibited a comparatively higher sedimentary rate. Moreover, among all these factors that might affect OM accumulation, the paleoredox conditions appear to be the dominant controlling factors for section A, whereas the biotic productivity, paleoredox conditions, and sedimentary rate are the main controlling factors for section B. Finally, two formation models for OM accumulation in Dalong Formation shale in the Lower Yangtze region are proposed. The "preservation model" for OM accumulation in section A emphasizes that the reducing deep-water environment, which was mainly caused by the regional sea level rise, is favorable for OM preservation. The "integrated model" for OM accumulation in section B stresses that greater biotic productivity intensifies respiratory oxygen consumption in a water column and a higher sedimentary rate can greatly shorten OM exposure time for respiration by oxygen, both of which cause OM accumulation under an oxidizing water environment. These findings also add to our knowledge that despite the oxygenated water environment during shale deposition, TOC contents are not necessarily lower.

Introduction

Organic matter (OM) accumulation is a complicated physical and chemical process, involving many factors, such as biotic productivity, paleoredox condition of bottom water, and sedimentary rate as well as post-depositional degradation process.[1−10] In the past few decades, the controlling factors of OM enrichment in modern and ancient sediments have been extensively explored.[1,4,5,11] The general consensus has been reached that OM accumulation in marine environments calls for specific geological conditions which are generally thought to be connected with an enhanced biotic productivity in the overlying water column,[2,7,12,13] and nonoxidizing water environment,[1,4,8] or an integration of both.[3,5,6] Besides, it has been accepted that no single process or model can explain OM accumulation in all sedimentary environments owing to the complicated geological history and that each sedimentary environment may mirror the effects of several factors contributing to OM accumulation in the organic-rich sediments.[14]

The upper Permian Dalong Formation is not only regarded as an excellent source rock but also as the target for shale gas exploration in the Lower Yangtze region.[15−18] Existing studies have revealed that Dalong Formation black shale has favorable conditions for shale oil and gas enrichment with high abundances of total organic carbon (TOC), medium thermal maturity, and high brittle mineral contents.[16−19] Specially, shale oil and gas discoveries have been obtained in the upper Permian strata from well GD1, which was implemented by Nanjing Geological Survey Center of China Geological Survey in 2016.[20] In spite of these achievements, shale gas exploration in the Lower Yangtze platform has not made an actual breakthrough by far, although it is considered to possess a huge shale gas potential.[15−18] In such a system, the paleoenvironmental conditions and OM accumulation during the black shale deposition play crucial roles. However, previous studies of the Dalong Formation have mainly focused on stratigraphy, reservoir characteristics, source rock evaluation, and shale gas resource potential prediction,[15,17,19] and most of them are based on outcrop samples, while the study of OM accumulation in such a setting is quite scarce.[21] In addition, OM plays a vital role in controlling the hydrocarbon generation potential, pore structure, and adsorption capacity in organic-rich shale systems.[22,23] In order to adequately comprehend the Dalong Formation, controlling factors and mechanisms of OM accumulation in such a circumstance must be clarified and serve as a valid geologic foundation for shale gas exploration in the Lower Yangtze region in the future.

Elemental geochemistry is the most extensively employed method for quantitatively reconstructing the paleoenvironment.[14,24−27] Generally speaking, organic carbon, phosphorus (P), and biogenic barium (Babio) are the popular proxies for qualitatively evaluating the paleomarine productivity.[28−32] Several redox-sensitive elements (e.g., V, U, and Ni) and their ratios (e.g., U/Th, V/Cr, Ni/Co, and V/(V + Ni)) have been widely applied to interpret the paleoredox conditions over the past couple of decades.[14,23,24,33−38] REE distribution patterns and ratios of (La/Yb)N, Th/U, and Na/K are the important geochemical indices to qualitatively characterize the sedimentary rate.[39−43]

In this study, we present TOC, major element (ME), trace element (TE), and rare earth element (REE) studies on the Dalong Formation black shale samples from well GD1 in the western area of the Lower Yangtze platform. Based on these data, paleoclimate, biotic productivity, paleoredox conditions, and sedimentary rate are systematically characterized. Then, controlling factors of OM enrichment in such a setting are comprehensively analyzed by examining co-variations between TOC contents and multiple geochemical indicators. Finally, mechanisms and formation models of OM accumulation for the Dalong Formation black shale in the Lower Yangtze region are presented.

Geologic Setting and Well Description

The Lower Yangtze region, which covers an area of 36 × 104 km2, is tectonically located in the east of the Yangtze Plate and is bounded by the Tanlu fault zone on the northwest, the Qinling-Dabie mountain tectonic zone on the west, the Ganjiang fault zone on the southwest, the Jiangshao fault zone on the south and southeast, and the Yellow Sea and the East China Sea on the east.[17,44] It has undergone multistage complicated tectonic movements and brought about forming a range of structures in the northeast-southwest direction (Figure ). From north to south, the Lower Yangtze region can be further divided into six secondary structural units, namely, Jiaonan orogenic belt, Chuquan Depression, Yanjiang Depression, southern Anhui-southern Jiangsu Depression, Jiangnan uplift, and Qiantang Depression[18] (Figure ).

Figure 1

Location map of the studied area showing the tectonic units in the Lower Yangtze region and the location of the sampling well.

Generally speaking, the Lower Yangtze region has mainly experienced three stages of evolution: the marine sedimentary period from Late Sinian to Middle Triassic; the continental sedimentary period from Late Triassic to Early Cretaceous; and the structural transformation period from Late Cretaceous to Neocene.[44] It is noteworthy that marine-continental transitional facies strata (Longtan Formation) were deposited during the Permian period. Vertically, the Lower Yangtze area has a distinct double-layer structure, which was separated by a regional unconformity between the Late Triassic and Early Jurassic strata. Beneath the interface, the strata are relatively flat without the evident thrusting characteristics. However, above the unconformity, the strata show obviously structural deformations with an overturned fold.[44]

The upper Permian Dalong Formation and Longtan Formation in the Lower Yangtze region have been considered as not merely high-quality source rocks but as potential shale gas reservoirs.[18,19,44] The Dalong Formation black mudstone and shale are widely distributed in the Lower Yangtze region and well-preserved with a maximum thickness of 200 m. It is in conformable contact with the underlying Longtan Formation and in parallel unconformity contact with the overlying Yinkeng Formation.[45] There are obvious lithological variations among the Longtan Formation, Dalong Formation, and Yinkeng Formation. The Longtan Formation consists of grayish black mudstone, sandstone, and limestone intercalated with thin coal seams deposited in a marine-continental transitional environment, and the Yinkeng Formation predominantly consists of limestones developed in a carbonate platform.

The well GD1 is located in the southern Anhui-southern Jiangsu Depression (Figure ). The Dalong Formation of well GD1 is in the depth of 915.4–985.6 m, with a total thickness of approximately 70 m, having experienced a complete sea level rise and fall process during deposition (Figure ). Based on the regional sea level fluctuations, Dalong Formation can be further subdivided into two parts: the lower and middle Dalong Formation (section A) consists of black siliceous shale and carbonaceous shale interbedded with thin silty mudstone and siltstone laminae, exhibiting high TOC content and abundant pyrite, which is considered to be formed in deep-water shelf to basin facies (Figure ); the upper Dalong Formation (section B) is mainly composed of gray black siliceous shale and silty mudstone intercalated with siltstone, exhibiting medium TOC content and abundant pyrite, which is believed to be developed in shallow-water shelf facies (Figure ).

Figure 2

Stratigraphic column of the Dalong Formation according to well GD1.

Results

TOC

TOC contents for the 33 rock samples are listed in Table S1 and Figure . For well GD1, samples from section A exhibit relatively high TOC contents ranging from 1.19 to 6.45%, on an average of 3.19%. Shale samples from section B show medium TOC contents varying from 1.18 to 4.90%, with an average of 2.09%, obviously lower than that of section A. This phenomenon can be explained that OM is conducive to be enriched when the regional sea level rises and tends to be depleted when the regional sea level falls (Figure ).

Major Elements

The ME oxide contents of Dalong Formation rock samples from well GD1 display unapparent differences between section A and section B (Table S1). The major chemical compositions are dominated by SiO2 (34.38–68.03%; average, 54.60%), followed by Al2O3 (8.87–19.66%; average, 14.18%), CaO (0.43–18.52%; average, 6.65%), Fe2O3 (1.93–6.74%; average, 4.46%), K2O (1.05–2.96%; average, 2.09%), MgO (1.13–4.95%; average, 1.77%), FeO (0.55–4.53%; average, 1.54%), and Na2O (0.71–2.44%, average, 1.13%). The remaining ME oxide contents show less than 1.0% and include TiO2 (0.20–0.63%; average, 0.38%), P2O5 (0.05–0.77%; average, 0.15%), and MnO (0.02–0.11%; average, 0.05%). Generally speaking, marine organic-rich sediments tend to be regarded as a mixture of three kinds of oxides: SiO2 (quartz), Al2O3 (clay minerals), and CaO (carbonate minerals).[23]Figure illustrates that the shale samples from section A and section B are variably enriched in SiO2 and CaO relative to Al2O3, which is likely associated with the stable terrestrial detrital inputs.

Figure 3

SiO2–Al2O3–CaO ternary diagram for the Dalong Formation shale samples from well GD1. Average shale is shown as a black triangle.

Trace Elements

The selected TE concentrations of the Dalong Formation shales from well GD1 are listed in Table S2. On average, the most abundant TEs in Dalong Formation are V (378.0 ppm), Sr (353.5 ppm), Ba (280.0 ppm), Cr (119.1 ppm), and Zr (113.0 ppm), followed by Ni (68.9 ppm) and Mo (20.3 ppm). The TEs with concentrations lower than 20 ppm include U (14.7 ppm), Th (13.3 ppm), and Co (10.8 ppm) (Table S2). Particularly, the TE concentrations in Dalong Formation are different between section A and section B. For example, shales from section A display higher average concentrations of V, Ni, Mo, Ba, and U, whereas somewhat lower contents of Cr, Co, and Th than these corresponding values of shales from section B (Table S2).

The enrichment factor (EF) values can be used to characterize the TE enrichment degree. For well GD1, the obviously enriched TEs (EF > 1) incorporate Mo (EF = 9.80–44.65, mean 9.80), U (EF = 1.57–9.47, mean 4.77), V (EF = 1.62–5.56, mean 3.46), Cr (EF = 0.81–3.02, mean 1.59), Sr (EF = 0.73–2.35, mean 1.46), Th (EF = 0.68–1.82, mean 1.27), and Ni (EF = 0.60–2.06, mean 1.23) (Table S3), being likely associated with the enrichment of OM or clay minerals,[31,46,47] which is different from the enrichment pattern as V > Ni > Mo > Cr for redox-sensitive TMs in the Bakken shales.[48] The depleted TEs (EF < 1) mainly include Zr (EF = 0.44–2.17, mean 0.85), Co (EF = 0.34–1.02, mean 0.69), and Ba (EF = 0.31–1.47, mean 0.59) (Table S3). As shown in Figure , shale samples from section A exhibit higher EF values for the majority of the TEs except for Cr, Co, and Th relative to section B.

Figure 4

EF diagram of the selected TEs for the Dalong Formation shale. A horizontal line (EF = 1) emphasizing the enrichment or depletion of an element.

Rare Earth Elements

No pronounced differences are present for REE concentrations and their associated parameters of the Dalong Formation shales from section A and section B (Tables S4 and S5). The total REE concentrations (ΣREE) are in the range of 87.24–288.94 ppm, with an average of 156.42 ppm, being slightly higher than that of the Upper Continental Crust (UCC, 146.40 ppm[49]). However, the average value of REE concentrations for the Dalong Formation shale is obviously lower than that of the North American Shale Composite (NASC, 173.21 ppm[50]) and the Post-Archean Average Australian Shale (PAAS, 184.77 ppm[49]), mirroring that the ΣREE is influenced by the detrital influxes to some extent.[41,51] Here, the UCC was adopted to normalize the REEs.

As shown in the diagram of UCC-normalized REE distribution patterns (Figure ), the curves seem to exhibit extremely uniform trends, probably demonstrating that the REEs in the Dalong Formation shales from section A and section B have originated from a similar source of terrestrial detrital inputs (provenance).[52] The REE distribution patterns are characterized by relatively flat LREE (from La to Eu) and HREE (from Gd to Lu) (Figure ). The ratios of LREE to HREE range from 5.40 to 10.58, with an average of 7.85 (Table S5), being slightly lower than that of the UCC (9.54), suggesting a comparatively weak REE fractionation. The Ce anomaly (δCe) and Eu anomaly (δEu), defined as δCe = CeN/(LaN × PrN)1/2 and δEu = EuN/(SmN × GdN)1/2,[49] are in the range of 0.74–0.99 and 0.65–1.24, with an average of 0.89 and 0.94 (Table S5), respectively, implying weakly negative Ce and Eu anomalies. The (La/Yb)N ratios vary from 0.58 to 1.04, with an average of 0.82.

Figure 5

UCC-normalized REE distribution patterns of the Dalong Formation shale samples from core GD1.

Discussion

Paleoclimate

The chemical index of alteration (CIA) was extensively applied to reconstruct the paleoclimatic conditions[53−56] and characterize the paleoweathering degree.[31,57,58] The CIA was put forward by Nesbitt and Young[59] and defined as follows

It needs to be emphasized that the unit of ME oxides in the abovementioned expression is mole, and CaO* merely represents the CaO originated from silicate minerals.[59] In this case, it is essential to make a calibration to the absolute CaO contents for the existence of carbonate minerals and apatite. In this work, CaO contents were primarily calibrated for phosphate by means of the measured P2O5 contents,[60] namely,

When the “remaining number of moles” is more than that of Na2O, the mole of CaO* was supposed to be equivalent to that of Na2O. Otherwise, the mole of CaO was regarded as that of CaO*.[60−62]

Previous studies have elucidated that fine-grained siliciclastic rocks deposited in a cold and arid climate with weak chemical weathering normally have CIAs in the range of 50–65 while those formed in a warm and humid climate with medium chemical weathering generally have CIAs in the range of 65–85 and those deposited in a hot and humid climate with strong chemical weathering commonly have CIAs in the range of 85–100.[59] In this study, the calculated CIAs of Dalong Formation shales vary from 56.25 to 79.25 (Table S5 and Figure ), with an average of 70.25, being close to that of PAAS (CIA = 69.0, referred from Kasanzu et al.[57]), mirroring a warm and humid paleoclimate with weak to medium chemical weathering (Figure ). Although the paleoweathering tends to lead to depletion in TE and REE to some extent,[63] the fresh samples from borehole and weak to medium chemical weathering together suggest that paleoweathering seems to have little effect on the obtained results and possible interpretation.

Figure 6

Ternary diagram of molecular proportions Al2O3–(CaO* + Na2O)–K2O. Shown at the left side is the CIA scale. The Dalong Formation shale samples display slight chemical weathering to medium chemical weathering. Post-Archean Average Australian shale (PAAS) is shown as a black pentagram.

Apart from CIA, the C-value [Σ(Fe + Mn + Cr + Ni + V + Co)/Σ(Ca + Mg + Sr + Ba + K + Na)] is also regarded as a useful and reliable proxy to surmise the paleoclimatic conditions.[64] Growing numbers of researchers[31,64,65] have concluded that C-values of 0–0.2, 0.2–0.4, 0.4–0.6, 0.6–0.8, and 0.8–1.0 indicate arid, semiarid, semiarid-to-semihumid, semihumid, and humid climate, respectively. In the present study, the C-values for Dalong Formation shale samples vary from 0.13 to 1.00, with an average of 0.46 (Table S5 and Figure ), likely deciphering a semiarid to semihumid paleoclimate. Besides, no obvious differences are present for CIAs and C-values of the Dalong Formation shales from section A and section B (Figure ).

Figure 7

Stratigraphic distributions of TOC, productivity-related proxies (P, P/Ti, Ba, and Ba/Ti), paleoredox indicators (EFU and EFMo), sedimentary rate proxy ((La/Yb)N), and paleoclimate indicators (CIA and C-value) in core GD1. Dotted lines in boxes represent the averages.

The aforementioned plaeoclimate proxies (CIA and C-value) together suggest that the Lower Yangtze region was undergoing a warm and semiarid-to-semihumid paleoclimate during the Late Permian period. Furthermore, as shown in Figure , the vertical variations of CIA and C-value do not share a similar trend with TOC fluctuations, demonstrating that the paleoclimate was not the dominant controlling factor of OM accumulation in the Lower Yangtze area.

Biotic Productivity

The biotic productivity generally plays a vital role in OM enrichment, which is closely related with the abundant supply of nutrient elements such as P[28,32,39,66] and Ba.[24,29,30,67] In this work, these two elements are adopted to qualitatively estimate the biotic productivity.

P values of the 33 rock samples from GD1 well are mostly in the range of 300–700 ppm (except for DL-2, DL-3, DL-5, DL-8, and DL-11; Table S5 and Figure ), which is typical of average shales and shows no P enrichment. It is noteworthy that P values of DL-2, DL-3, DL-8, and DL-11 samples are more than 1000 ppm, showing significant P enrichment. However, such an enrichment may be a signal of short-term oxygenation event.[68] Conversely, the DL-5 sample has relatively lower P concentration because anoxic bottom seawater or porewaters have triggered P recycling, not owing to low primary productivity.[68] In general, OM and authigenic minerals probably play a dilutive effect on P concentrations. In order to exclude the dilutive effect, the P/Ti ratio has been proven as the reliable biotic productivity proxies instead of the absolute P content because Ti is one of the stable detrital elements.[49] For well GD1, the P/Ti ratios of Dalong Formation shales are in the range of 0.10–1.51 (average 0.30; Table S5), which is above that of PAAS (0.12; Taylor and Mclennan[49]), probably indicating a relatively high biotic productivity in the Lower Yangtze region during the Late Permian period. Specially, the average P/Ti ratio (0.35; Table S5) of shale samples from section A is slightly higher than the corresponding value (0.21; Table S5) of shale samples from section B. This phenomenon can be explained in part by the sea-level fluctuations, a higher average P/Ti ratio from section A was basically compatible with the rising sea level, during which the upwelling nutrient fluxes have the potential to boost the biotic productivity. Moreover, as presented in Figure , the patterns of vertical variations of the P/Ti ratio are somewhat different from TOC curves, mirroring that OM accumulation in Dalong Formation strata in the Lower Yangtze region is not merely influenced by the biotic productivity but also may be affected by the preservation conditions (e.g., paleoredox condition and sedimentary rate) and other factors.

The Ba/Ti ratio is also a reliable geochemical proxy used in analyzing the fluctuations of biotic productivity in marine sediments. For well GD1, the Ba/Ti ratios of Dalong Formation shales range from 0.06 to 0.30, with an average of 0.13 (Table S5), being close to that of PAAS (0.11; Taylor and Mclennan[49]) and thus indicating a medium biotic productivity. This conclusion is not in accordance with the scenario inferred from P/Ti ratios as discussed above. Furthermore, as shown in Figure , Ba concentrations of the investigated samples are uniformly in the range of 200–400 ppm, which are typical values for shales providing no indication of elevated primary productivity. However, the productivity-related Ba tends to undergo reductive dissolution in anoxic sediments, making its preservation unlikely.[69] Accordingly, we counsel caution in qualitatively evaluating the biotic productivity in organic-rich sediments formed in an anoxic water environment by means of Ba concentration.

Paleoredox Conditions

Paleoredox conditions of the water column generally have a significant influence on OM accumulation.[1,4,8,11] The paleoredox-sensitive indicators (e.g., U/Th, V/Cr, Ni/Co, and V/(V + Ni) ratios) have been widely employed for interpreting the paleoredox conditions.[14,23,24,33,34,70,71] Notably, existing studies have established the reference standards for U/Th (0.75, 1.25), V/Cr (2.0, 4.25), Ni/Co (5.0, 7.0), and V/(V + Ni) (0.46, 0.6) ratios to discriminate oxic, dysoxic (suboxic), and anoxic (reducing) grades.[25,33−35,72,73] However, the latest research achievements from Algeo and Liu[74] and Algeo and Li[75] demonstrated that it is impossible to set a strict proxy value for each paleoredox threshold that can be applied to all sedimentary systems. In other words, paleoredox threshold values determined on the basis of one formation cannot be applied directly and uncritically to other formations of different ages, depositional backgrounds, and dynamic paleoredox conditions, as has been extensively done to date. More importantly, Algeo and Liu[74] concluded that trace-element EFs are particularly reliable proxies than others. In this regard, the EFU and EFMo are adopted to characterize the paleoredox conditions in the present study.

As shown in Figure , the EFU and EFMo for section A of well GD1 are in the range of 3.09–9.47 (average 6.04) and 1.17–44.65 (average 11.19), being close to those corresponding values of Black Sea[75] (10–30 and to ∼300, respectively) and Saanich Inlet[75] (to ∼3 and to ∼80, respectively), indicating that the organic-rich shales in the lower Dalong Formation were predominantly developed under an anoxic condition and that a small portion was deposited in a dysoxic water environment. Section B in the upper Dalong Formation has the EFU and EFMo in the range of 1.57–3.29 (average 2.24) and 2.46–11.80 (average 7.01), being obviously lower than those of section A and closer to those corresponding values of California Margin (<2 and <6, respectively), elucidating an oxic-to-dysoxic water environment.[75] Both of the conclusions are also supported by the cross plot of EFU versus EFMo (Figure ), where samples from section A plot toward the zone between the dysoxic end to the anoxic one of unrestricted marine trend of Algeo and Tribovillard[27] and samples from section B plot in areas near the dysoxic end. Besides, the cross plot of Mo versus TOC contents shows a moderate to strong water-mass restriction for well GD1 based on the comparison with four modern marine systems (Figure ).

Figure 8

Cross plot of EFU versus EFMo, showing that Dalong Formation shale samples from core GD1 mainly plot toward the zone between the dysoxic end and the anoxic one (Base map adapted with permission from ref (27). Copyright 2009 Elsevier.) The information of shale samples offers no evidence for operation of a particulate shuttle. The diagonal dashed lines represent Mo/U molar ratios of the seawater (Sw).

Figure 9

Cross plot of TOC vs Mo contents. Dotted lines represent the regression lines for four modern marine systems (Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet), mirroring moderate to strong degrees of hydrographic restriction (Adapted with permission from ref (76). Copyright 2006 John Wiley and Sons.)

Furthermore, cross plots of TOC contents versus EFU and EFMo show a complicated correlation (Figure a,b). For section A, EFU and EFMo show positive correlations with TOC contents with the fitting coefficient R2 = 0.31 and 0.67, respectively, which demonstrates that the anoxic bottom water environment played a critical role in OM accumulation in the lower Dalong interval. However, there are weakly positive co-variations between TOC contents and EFU and EFMo for section B with the fitting coefficient R2 = 0.48 and 0.25, respectively, deciphering that the paleoredox condition of the water column played a less important role in OM accumulation in section B relative to section A.

Figure 10

Correlations between TOC contents and paleoredox indicators (EFU and EFMo), productivity proxy (P/Ti), and sedimentary rate proxy (La/Yb)N. (a) TOC vs EFU; (b) TOC vs EFMo; (c) TOC vs P/Ti; and (d) TOC vs (La/Yb)N.

Sedimentary Rate

The sedimentary rate also has a significant effect on OM enrichment.[5,41,71,77] REE distribution patterns and (La/Yb)N ratios have been successfully employed for qualitatively evaluating the sedimentary rate.[40,41] Generally, a high sedimentary rate can lead to a weak REE fractionation because the duration between REEs and clay minerals is reduced[78,79] and thus results in the (La/Yb)N ratio close to 1 (the subscript “N” represents the normalization to UCC in this study).[40] As shown in Figure , REE distribution patterns for the Dalong Formation shales from section A and section B are relatively flat, reflecting that the sedimentary rate is comparatively stable during the Dalong Formation shale deposition in the Lower Yangtze region. Additionally, the (La/Yb)N ratios of all 33 rock samples vary from 0.58 to 1.04, with an average of 0.82 (Table S5 and Figure ), mirroring a high sedimentary rate. Specially, the (La/Yb)N ratios for section B are in the range of 0.70–0.98 (average, 0.86), being slightly higher than those of section A (0.58–1.04; average, 0.79; Table S5). Besides, the Th/U ratio can also be used as an indicator of the sedimentary rate.[43] In general, higher values of Th/U indicate a higher sedimentary rate. As shown in Table S2, Th/U ratios for section B are in the range of 1.14–2.88 (average 2.09), being obviously higher than those corresponding values of section A (0.46–1.02, average 0.71). Both of the (La/Yb)N and Th/U ratios imply that the sedimentary rate during shale deposition in section B was somewhat higher than that of section A.

Furthermore, as shown in Figure d, there are clearly negative co-variations between TOC contents and (La/Yb)N ratios for section B, demonstrating that the sedimentary rate plays a significant role in OM accumulation in the upper Dalong Formation. In other words, the OM in section B has experienced a short residence time in the bacterial decomposition zone, which may be favorable for OM preservation in an oxidizing water environment. However, there are no obvious correlations between TOC contents and (La/Yb)N ratios for section A (Figure d), suggesting that the sedimentary rate is not a main controlling factor on OM accumulation during the organic-rich shale deposition in the lower Dalong Formation.

Controlling Factors of OM Accumulation

As discussed above, the Dalong Formation shale in the Lower Yangtze region was deposited in a complex paleoenvironment with moderate to strong water-mass restriction, which was mainly characterized by warm and semiarid–semihumid paleoclimate, relatively high biotic productivity, and a relatively high sedimentary rate. Moreover, the paleoredox conditions of the water column during shale deposition have experienced a transition from a reducing environment in section A to an oxidizing environment in section B. All these factors controlling OM accumulation are evaluated by examining the correlations between TOC contents and the aforementioned geochemical indicators.

For section A, TOC contents show clear co-variations with paleoredox indicators (e.g., R2 = 0.67 in the TOC-EFMo diagram; Figure b) and weak or no obvious correlations with the biotic productivity proxy (Figure c), paleoclimate proxies (Figure ), and sedimentary rate proxy (Figure d), probably reflecting that the paleoredox conditions are the dominant controlling factors of OM accumulation in the lower Dalong interval in the Lower Yangtze region. Consequently, OM accumulation in section A can be explained as a “preservation model”.[3,5,6] Similarly, for section B, TOC contents exhibit comparatively good co-variations with the biotic productivity proxy (e.g., R2 = 0.73 in the TOC-P/Ti diagram; Figure c) and sedimentary rate proxy (e.g., R2 = 0.71 in the TOC-(La/Yb)N diagram; Figure d), weak correlations with the paleoredox indicators (e.g., R2 = 0.48 in the TOC-EFU diagram; Figure a), and no correlations with the paleoclimate proxies (Figure ), demonstrating that the biotic productivity and preservation conditions (paleoredox conditions and sedimentary rate) together play the vital roles in OM accumulation in the upper Dalong Formation in the Lower Yangtze region. Therefore, OM accumulation in section B can be explained as an integration of the “productivity model”[2,7,12,13] and “preservation model”,[3,5,6] named “integrated model” here. This model can be interpreted as follows: the formation of Dalong Formation shale in the Lower Yangtze region is not determined by a single factor but is the result of the mutual configuration and coupling of multiple factors such as paleoclimate, paleoredox, biotic productivity, and sedimentary rate. All these factors would directly or indirectly affect the supply or preservation of OM.

Mechanisms and Formation Models of OM Accumulation

Based on the regional sea level fluctuations, Dalong Formation can be divided roughly into two parts in the evolution of the paleoenvironment (Figure ). During the deposition of section A, the Lower Yangtze region was deposited in a reducing deep-water open marine environment, which is favorable for OM preservation. Meanwhile, the regional sea level rise could have carried the nutritious materials from bottom waters to surface waters, accelerating the blooms of algae, bacteria, and phytoplankton, and thus enhanced the biotic productivity. Moreover, the Lower Yangtze region was under a warm and semiarid–semihumid paleoclimate during this period, which was generally beneficial to boosting the biotic productivity. Greater biotic productivity and an anoxic water environment could together create abundant OM. On the basis of these analyses, a sketched diagram of the “preservation model” for OM accumulation in the lower and middle Dalong Formation in the Lower Yangtze region is established (Figure a).

Figure 11

Sketched diagram of the “preservation model” (a) and “integrated model” (b) for OM accumulation in the upper Permian Dalong Formation shale in the Lower Yangtze region.

During the deposition of section B, the Lower Yangtze region was accompanied by a well-oxygenated environment owing to the falling sea level. The warm and semiarid–semihumid paleoclimate in the Lower Yangtze region during this period is conducive to accelerating the degree of chemical weathering of the parent rock and increasing the input of nutrients to the water body and promoting the blooms of organisms such as algae and bacteria, which can directly contribute to enhancing the primary productivity. On the one hand, greater biotic productivity increases the sinking flux of organic carbon, which intensifies respiratory oxygen consumption in water columns, thus creating a positive feedback loop.[9,80] On the other hand, a higher sedimentary rate can greatly shorten OM exposure time in the degradation region of aerobic bacteria and thereby reduce OM degradation under an oxidizing water environment.[41,77] All these pieces of information confirmed that in spite of the oxidizing water column, TOC contents in the upper Dalong Formation are not necessarily lower as observed in Table S1 because the enhanced biotic productivity and a higher sedimentary rate could together create abundant OM. According to these analyses, a sketched diagram of the “integrated model” for OM accumulation in the upper Dalong Formation in the Lower Yangtze region is proposed (Figure b).

Conclusions

Based on our comprehensive analyses of TOC contents and geochemical indicators of the upper Permian Dalong Formation black shales from GD1 well in the Lower Yangtze region, the following conclusions can be obtained:

The Dalong Formation shale in the Lower Yangtze region was deposited in a complex paleoenvironment with moderate to strong water-mass restriction, which was mainly characterized by warm and semiarid–semihumid paleoclimate, relatively high biotic productivity, and a comparatively high sedimentary rate. Besides, the paleoredox conditions of the water column during shale deposition have experienced a transition from a reducing environment in the lower Dalong interval (section A) to an oxidizing environment in the upper part (section B).

The formation of Dalong Formation organic-rich shale in the Lower Yangtze region is not determined by a single factor but is the result of the mutual configuration and coupling of multiple factors such as paleoclimate, paleoredox, biotic productivity, and sedimentary rate. All these factors would directly or indirectly affect the supply or preservation of OM.

Two major formation models for OM accumulation are proposed. The “preservation model” for OM accumulation in section A emphasizes that the reducing deep-water environment, which was mainly caused by the regional sea level rise, is favorable for OM preservation. The “integrated model” for OM accumulation in section B stresses that the relatively high biotic productivity accelerates respiratory oxygen consumption in the water column and a higher sedimentary rate can significantly reduce OM exposure time in the degradation region of aerobic bacteria, both of which cause OM accumulation in an oxidizing water environment.

Samples and Methods

Samples

A total of 33 rock samples at a depth of 917–984 m were collected from well GD1 using the sampling interval of 1–3 m for the analysis of TOC and elemental geochemistry. Concretely speaking, there were 22 rock samples (DL-1 to DL-22) collected from section A, and the remaining 11 samples (DL-23 to DL-33) were taken from section B. All samples were stored in sample sacks to prevent external contamination. Table S1 and Figure show more information of these target samples, mainly involving lithology, depth, and sample number.

Experimental Methods

Prior to analysis, all the target samples were initially washed and dried and then were crushed and ground into powder (less than 200 meshes) in an agate mortar for TOC, ME, TE, and REE measurement. All these experiments in this study were performed on whole-rock powdered samples and tested at the Beijing Research Institute of Uranium Geology, China.

For TOC analysis, aliquots (200 mg) of powdered samples were first processed with dilute hydrochloric acid (HCl) at 60 ± 5 °C for 24 h to dissolve the inorganic carbon (carbonate minerals) and then rinsed repeatedly with distillated water to remove the HCl. Subsequently, the samples were desiccated several hours at 60–80 °C and whereafter analyzed using a LECO CS-400 analyzer. The analytical precision was better than 0.1%.

For ME analysis, samples (powder) were first heated to 105 °C to take out the adsorbed water and further baked at 920 °C to remove OM completely. Afterward, the ashed powders and the mixtures of Li2B4O7 and BLiO2 were fused at 1150 °C into a glass disc. MEs were measured on the fused glass disc with the help of an X-ray fluorescence spectrometer. The analytical precision for MEs was better than 5%. The detailed experimental procedure for ME analysis was performed by following Cao et al.[41]

For TE and REE analysis, the powdered samples were primarily digested using the mixed acids (HF/HNO3/HClO4 = 1:1:3) for 12–24 h at 200 °C in a pressure-tight Teflon bomb. Then, the resulting liquid was measured on an inductively coupled plasma–mass spectrometry (ICP–MS). The detailed experimental procedure was performed by following Liu et al.[81] The analytical precision for TEs and REEs was better than 5%.

Proxy Calculations

To analyze the chemical compositions of the black shales, the enrichment factor (EF) has been widely applied to characterize the enrichment degree of each element or its oxide.[23,41,82] EF is calculated by normalizing an element or its oxide to Al, which is regarded as one of the stable proxies of the terrestrial detrital influxes, and afterward comparing these ratios to their corresponding values of a standard shale, such as Average Shale or PAAS.[14,23,31,82] Here, Average Shale is used as the standard shale, so EF is defined as shown below/where x represents the concentration of an element or its oxide. The ratio in the numerator is for the studied samples, whereas that in the denominator is for the Average Shale (data from Wedepohl[83]). When EF > 1, the sample is relatively enriched in the element or its oxide relative to Average Shale, while EF < 1 indicates depletion.[25]