The Early Cretaceous Qiangtang Basin, Tibet - ACS Publications

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Mechanism of Organic Matter Accumulation in Residual Bay Environments: The Early Cretaceous Qiangtang Basin, Tibet Jian Cao,*,† Ruofei Yang,† Wei Yin,‡ Guang Hu,†,§ Lizeng Bian,† and Xiugen Fu∥ †

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, Jiangsu 210023, China ‡ SINOPEC Petroleum Exploration and Production Research Institute, Beijing 100083, China § School of Geoscience and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China ∥ Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan 610081, China S Supporting Information *

ABSTRACT: The controlling mechanisms for the accumulation and preservation of organic matter in residual bay environments during the transition from marine to continental settings are not well understood, although oil−gas source rocks can form in this setting. In this study, we develop a case study for the Early Cretaceous black rock series in the northern Qiangtang Basin, Tibet (i.e., the Upper Member of the Suowa Formation), by conducting a combined organic and inorganic geochemical analysis of micritic limestone, marl, and shale samples from an outcrop section. Results show that total organic carbon (TOC) contents of the studied samples are between 1.74% and 7.71%, with the organic matter being Type II/III kerogen. Of the three factors that could influence the observed TOCs and organic matter types, including paleoproductivity, preservational environment, and sedimentation rate, the preservational environment appears to be the dominant factor, independent of lithology. This is typically supported by the relatively modest covariance between redox-sensitive parameters and TOC contents, e.g., R2 = 0.625 in the Mn/Ca-TOC diagram and R2 = 0.690 in the U/Th-TOC diagram. This suggests that the suboxic−anoxic environment in the lagoon at the residual bay area promoted favorable conditions for organic matter preservation. In contrast, the other two factors, i.e., paleoproductivity and the rate of sedimentation, differed between three types of lithologies. For shales and micritic limestones, the effect of paleoproductivity was limited on the abundance of organic matter, and no significant effect of sedimentation rate was detected. In contrast, the paleoproductivity has a definite effect on the amount of organic matter preserved in the marls. These findings also add to our knowledge of the depositional environment that existed during the Early Cretaceous marine−continental transition in the Qiangtang Basin and further built our understanding of the potential hydrocarbon resources of the basin.

1. INTRODUCTION Developing an understanding of the properties and genesis of hydrocarbon source rock is of the utmost importance in oil-gas exploration.1−3 Generally, depositional environments of source rock can be divided into two end-member settings, i.e., shallowto-open marine and continental lacustrine,4−10 while during the change from marine to continental deposition, a residual bay basin can develop, whereas, during the transition from continental to marine deposition, a transgressive lacustrine basin (a coastal lacustrine basin) can form.11−13 Such kinds of modern examples/analogues include the Bohai Bay Basin in eastern China and the bay of Brest in France, which were both influenced by the Holocene marine transgression.14,15 They have become research highlights for decades and set the controlling mechanisms related to the accumulation, and preservation of organic matter in these transitional settings are not well understood due to the complex compositions of bioprecursors, although oil−gas source rocks can form in this setting.1,3,16,17 The Qiangtang Basin is a Mesozoic−Cenozoic sedimentary basin located in the north-central area of the Qinghai−Tibetan Plateau, western China (Figure 1a). This basin has been the focus of many geological and geochemical studies because the © XXXX American Chemical Society

sedimentary sequence preserves a record of the formation and evolution of Tethys and the collision of India with Eurasia.18−21 Recently, near the Shengli River in the northern Qiangtang Basin, a widely distributed Early Cretaceous black rock series with oil shales with constant thickness that includes a mudstone−limestone sedimentary sequence was discovered (Figures 1b and 2).22 Prior studies mainly focused on the formation timing of the basin, the organic geochemical characteristics including bioprecursors and biomarkers, and hydrocarbon resource potential.16,23,24 On the basis of the results of these previous studies and the geological background of the depositional environment, the study region is classified as a semiclosed bay open to the northwest,25,26 thus providing an invaluable opportunity to study the characteristics and accumulation mechanism of organic matter in transitional settings, especially a residual bay.27−29 In this study, to establish a better understanding of organic matter accumulation in transitional (i.e., residual bay) environments, we developed a case study of the Lower Cretaceous of Received: August 2, 2017 Revised: January 23, 2018 Published: January 24, 2018 A

DOI: 10.1021/acs.energyfuels.7b02248 Energy Fuels XXXX, XXX, XXX−XXX

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volcanic arcs and causing a marked marine regression. Therefore, the Qiangtang Basin was mostly subaerial during the Early Cretaceous and transformed into a semiclosed bay open to the northwest, while open marine facies remained only in some parts of the Northern Qiangtang Depression.30,32 During this period, several stages of transgression occurred intermittently, and major regression coincided with the uplift of the central Qiangtang Basin.22,25,26 As a consequence of transgression, a set of black shale series was formed. Subsequently, during the deposition of Upper Cretaceous Xueshan Formation, because a hot arid climate remained, the residual sea continued to shrink southwestward. Finally the entire basin was filled with alluvial river and continental lake facies sediments and transformed into a continental basin during the Late Cretaceous to Early Cenozoic.30,33 The Lower Cretaceous black rock series in the present study occur in the Upper Member of the Upper Jurassic−Lower Cretaceous Suowa Formation (Figure 2).34,35 The Upper Member of the Suowa Formation was deposited in a transitional environment from marine to continental and consists of a combination of fine-grained clastic rocks and carbonates, which occurs mainly in the central−western part of the Northern Qiangtang Depression. Deltaic mudstone, siltstone, and sandstone were deposited in the area east of the Quemocuo−Dazhuoma, while sandstone, micritic limestone, marl, limestone, shale and oil shale of the tidal-flat− lagoon facies were deposited in the central Northern Qiangtang Depression, overlain by thick layers of gypsum (Figures 2 and 3).34,35 The Lower Cretaceous black rock series are not only present in the Shengli River but also widely distributed in Changshe Mountain, Nadge Kangri, and Tuonamu areas, forming a west− east stretched black/oil shale belt with the size of approximately 30 km × 80 km (Figure 1b). Although the stratigraphic thickness of the black shales shows a general decrease from the west to the east (Figure 3),34 the black rock series could be the primary marks of contrast in the region.

Figure 1. Geological map of the study area. (a) Structural units of the Qiangtang Basin. (b) Simplified geological map of the sampling area (Adapted with permission from ref 23. Copyright 2010 Elsevier).

the Qiangtang Basin. The results also contribute to constraining the depositional environment and potential source rocks of the Qiangtang Basin during Early Cretaceous sea level changes, which is a contentious issue.17,23,24

2. GEOLOGIC SETTING The Qiangtang Basin, which is located in the central north of the Qinghai−Tibet Plateau at 32−35° N and 83−93° E, covers an area of ∼184,000 km2. The basin lies between the Hoh Xil− Jinsha River suture zone to the north and the Pangong Lake− Nu River suture zone to the south and can be further divided into three secondary structural units: the Northern Qiangtang Depression, the Central Uplift, and the Southern Qiangtang Depression (Figure 1a). The Qiangtang Basin is a superimposed basin filled with Paleozoic to Mesozoic marine sediments, which was formed on a pre-Devonian crystallized basement and Upper Paleozoic shallow metamorphic folded basement. The formation of the present basin involved the following stages, including the occurrence of a pre-Devonian basement, Devonian−Permian rifting, formation of a Triassic−Permian foreland basin, transition to a Jurassic passive continental margin, and the development of a Cretaceous−Cenozoic residual basin.30,31 During the Early−Middle Triassic to Late Jurassic, the main part Qiangtang Basin was submarine, and thousands of meters of carbonate sequences, interbedded with clastic rocks, were deposited.30 During the Late Jurassic to the Early Cretaceous, the Gangdise−Nyenchen Tanglha block moved northward and amalgamated with the Qiangtang block, resulting in closure of the Bangonghu−Nujiang Ocean. The subduction of the Gangdise−Nyenchen Tanglha block under the Qiangtang block resulted in the uplift of the central area to form into

3. SAMPLES AND METHODS The sampling outcrop in this study was located on the west bank of the Shengli River in the Northern Qiangtang Depression, at 33° 44′ N, 87° 25′ E (Figure 1b). A total of 28 samples were collected, including gray to dark gray marls, black shales, and micritic limestones. Note that shales represent a rock combination series that refer to fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite.36 The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than 1 cm in thickness, called fissility. In contrast, the marls and micritic limestones are massive and are composed of various amounts of clay minerals and silt-sized particles such as quartz and calcites. This means that all three types of samples have been divided in this study lithologically into shales, micritic limestones, and marls mainly based on their occurrence (i.e., fissility and laminated structure) rather than chemical compositions, although they have similar chemical compositions of elements Ca, Si, and Al, and are rich in Ca (see section 4.2.1 later for detail). The detailed sample numbers and lithologies are presented in Figure 2. In order to determine the characteristics and accumulation mechanism of organic matter of the Lower Cretaceous black rock series in the Qiangtang Basin, systematic analyses of organic and inorganic geochemistry were conducted. The organic geochemical analyses included total organic carbon (TOC) and Rock-Eval analyses. The inorganic geochemical analyses included major, trace, and rare earth elements (REEs) analyses. B


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Figure 2. Stratigraphic column of Middle Jurassic to Upper Cretaceous in the northern Qiangtang Depression, with black rock series and sampling location of the Shengli River outcrop.

Figure 3. Stratigraphic correlation of the Upper Member of the Suowa Formation in the Northern Qiangtang Depression (Adapted with permission from ref 34. Copyright 2012 Geoscience).

C


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Figure 4. Stratigraphic column of the bulk organic geochemistry in the Lower Cretaceous black rock series from the Shengli River outcrop. Data are from Table S1. The standard of organic matter (OM) abundance is based on data from ref 38. 3.1. Organic Geochemical Analyses. The organic geochemical analyses were performed at the Wuxi Research Institute of Petroleum Geology, SINOPEC. All 28 samples were analyzed. TOC values were measured on a LECO−CS-200 carbon/sulfur analyzer. Before the analysis, all the powdered samples were treated with HCl at 60 °C for 24 h to remove carbonates and then washed with distilled water to remove the HCl. Rock-Eval pyrolysis was performed on a Rock-Eval VI instrument, following the method of Chinese National Standard of rock pyrolysis analysis GB/T18602-2012. The powdered samples were heated to 300 °C in a helium atmosphere to measure the S1 value, and then heated linearly from 300 to 600 °C at a rate of 25 °C/min to measure the S2, S3, and Tmax values, where the S1 value is the amount of free hydrocarbon that can be volatilized from the rock sample (mg of HC/ (g of rock)), the S2 value is the amount of hydrocarbon produced by the cracking of organic matter (mg of HC/(g of rock)), the S3 value is the amount of CO2 produced during the initial isothermal heating step and the programmed heating phase up to 390 °C (mg of CO2/(g of rock)), and Tmax (°C) is the temperature at which the maximum S2 yield is reached. The hydrogen index (HI), oxygen index (OI), and production index (PI) values were calculated by using the formulas: HI = 100S2 /TOC, OI = 100S3 /TOC, and PI = S1/(S1 + S2). 3.2. Inorganic Geochemical Analyses. The inorganic geochemical analyses of major, trace, and rare earth elements were undertaken at the Institute of Geochemistry, Chinese Academy of Sciences. A total of 19 samples were selected to be analyzed. The selection criteria mainly included two points: (1) the samples were selected evenly by distance along the outcrop; and (2) all the lithologic types of samples were covered. This aims at a relatively comprehensive study as best. For major element analysis, powdered samples were baked at 105 °C to remove the adsorbed water prior to analysis and then heated to 920 °C to analyze the LOI (loss on ignition). Then, 0.5000 g of resulting powder and 4.0000 g of Li2B4O7 were fused at 1250 °C into a glass disc using an Analymate V8C instrument. Major elements were measured on the glass disc using a Rigaku 100e X-ray fluorescence (XRF). The analytical precision for major elements was within 1%, and

the detection limits were generally lower than 30 mg/kg. For trace element analysis, the inductively coupled plasma mass spectrometer (ICP-MS) was used. The analytical procedure followed Qi et al.37 Briefly, 0.0500 g powdered samples were placed in a Teflon bomb. To each sample was added 0.5 mL of HF and 1 mL of HNO3. The sealed bombs were then placed in an electric oven and heated to 185 °C for 12 h. After cooling, the bombs were then opened, evaporated to dryness on a hot plate, and followed by a second addition of 1 mL of HNO3 and evaporation to dryness. Then 2 mL of HNO3 and 1 mL of 500 ng/mL Rh solution was added as an internal standard, sealed, and placed in an electric oven (at 140 °C) for 5 h to dissolve the residue. After cooling, 0.4 mL of the final solution was placed into a 15 mL centrifuge tube and diluted to 8−10 mL with distilled deionized water for ICP-MS analyses. The analytical precision was better than 5%.

4. RESULTS 4.1. Organic Geochemistry. The bulk organic geochemical data for the 28 samples are shown in Supporting Information Table S1 and Figure 4. The total organic carbon contents range from 1.74% to 7.71%, suggestive of a set of hydrocarbon source rock with a relatively high organic matter abundance.38 Through the sequence, the TOC contents initially increase and subsequently decrease from bottom to top, peaking at samples SLR-07−SLR-08. TOC content differs according to sample lithology: marl samples SLR-01 to SLR-04 have TOC contents of 1.74%−3.77% (average, 2.80%); micritic limestone samples SLR-11 and SLR-12 have TOC contents of 5.61%−6.46% (average, 6.03%); and the shale samples have TOC contents of 4.12%−7.71% (average, 5.51%). In general, the values for the micritic limestone samples and shale samples are significantly higher than those of the marl samples. The hydrocarbon-generation potential values (PG = S1 + S2) vary from 2.90 to 16.11 mg/(g of rock) (average, 10.70 mg/(g of rock)), with most samples reaching good source rock D


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Figure 5. Diagrams of geochemical parameters showing the organic matter type of the Lower Cretaceous black rock series from the Shengli River outcrop: (a) HI versus OI; (b) HI versus Tmax.

Figure 6. Diagram of enrichment factors (EFs) of elements in the Lower Cretaceous black rock series from the Shengli River outcrop: (a) major elements; (b) trace elements. Data are from Tables S2 and S3.

standard38 (6−12 mg/(g of rock)). The variation trend of PG values among different lithologies is similar to that of TOC values (Figure 4). The hydrogen index values of the 28 samples range from 152 to 252 mg/(g of TOC) (average, 200 mg/(g of TOC)), and the oxygen index values range from 31 to 70 mg/(g of TOC) (average, 47 mg/(g of TOC)) (Table S1). According to the general standard established by Peters and Cassa,39 these values imply derivation from Type II−III kerogen (50−300 mg/(g of rock)). In diagrams showing HI−OI and HI−Tmax correlations (Figure 5), all the samples plot in the range between Type II and Type III organic matters. The marl samples which have relatively lower HI values (152−181 mg/(g of TOC)) lie quite close to the Type III area (50−200 mg/(g of TOC)), which could reflect algal material that has been oxidized during deposition as well as the oxidation of the organic matter by exposure at the outcrop.39 Moreover, the organic matter types

of the micritic limestone and shale samples are marginally better at oil generation than those of the marl samples. The Tmax values are 441−444 °C for the 28 samples, indicating mature thermal evolution. At this thermal maturity level, there might be some HI reduction.39 No obvious correlation is observed between rock types and locations in the succession, implying a normal evolution of organic matter without any effects from anomalous thermal events.38 The production index values range from 0.1 to 0.3, indicating similar degree of thermal evolution based on the Tmax values.38 4.2. Inorganic Geochemistry. 4.2.1. Major Elements. The major element oxides concentrations in the black rock series of the Shengli River area are listed in Table S2. The major chemical components are CaO (28.58%−38.45%; average, 34.51%), SiO2 (4.22%−25.23%; average, 17.64%), Al2O3 (0.58%−6.67%; average, 4.13%), MgO (0.83%−12.95%; average, 2.25%), Fe2O3 (0.77%−3.63%; average, 2.40%), and K2O (0.17%−1.66%; average, 1.06%). The contents of other E


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Figure 7. Stratigraphic column of the representative rare earth elements (REEs) parameters in the Lower Cretaceous black rock series from the Shengli River outcrop. Data are from Table S4. Note: δCe = Cen/(Lan × Prn)1/2. The subscript n indicates normalization to PAAS (PAAS data are from Taylor and McLennan41).

The EF values of the major element oxides are relatively low for the marl samples, for which they approach the EF value of PAAS (EF = 1), and the EF values of shale samples lie between those of the marl samples and micritic limestone samples. To evaluate the weathering degree of the source area, the chemical index of alternation (CIA) was used, which is defined as CIA = 100 × Al2O3/(Al2O3 + CaO* + Na2O + K2O), where all the compounds are calculated in molecular proportions and CaO* stands for CaO in silicate fraction only.42,43 The CIA value has a positive correlation with weathering degree for which the higher CIA value indicates the stronger weathering. As shown in Table S2, the CIA value of all the samples in this study ranges from 70.07 to 76.44, while marl samples SLR-01 to SLR-04 have relatively high CIA values (74.97−76.44), indicating a warm and humid period with strong weathering. In contrast, the micritic limestone sample SLR-12 has a low value of CIA (70.07), indicating weak weathering and arid climate. This can be supported by the high MgO content in sample SLR-12.44 In the other shale samples, the CIA value ranges from 72.88 to 75.20, suggesting a weathering intensity intermediate between those of marl samples and micritic limestone samples. 4.2.2. Trace Elements. The concentrations of trace elements in the black rock series from the Shengli River area are listed in Table S3. Like the major elements, EF values were also used to represent the enrichment characteristics of the elements during the formation of the black rock series (Table S3). Elemental abundance data show that abundant trace elements in this rock series include Sr (average concentration, 484.00 ppm) and Ba (95.62 ppm), followed by V (47.25 ppm), Ni (43.46 ppm), Rb (40.08 ppm), Zn (38.70 ppm), Zr (31.07 ppm), Cr (30.92 ppm), and Cu (26.15 ppm). Significantly enriched elements are

major element oxides are all less than 1% and include P2O5 (0.04%−0.87%; average, 0.46%), TiO2 (0.03%−0.30%; average, 0.20%), Na2O (0.03%−0.09%; average, 0.08%), and MnO (0.03%−0.06%; average, 0.04%). As for different lithologies, the marl samples, SLR-01−SLR-04, are very rich in Si, Al, and Ti, which are closely related to the amount of terrigenous input into a sedimentary system. In contrast, one of the micritic limestone samples, SLR-12, is poor in Si, Al, and Ti and rich in Ca and Mg, especially in MgO (12.95%). In addition, the Si, Al, Ca, and Mg contents of the shale samples lie between those of the marl samples and the micritic limestone samples. To further characterize the chemical composition of the black rock series, the enrichment factor (EF) was used to represent the enrichment of each element and its oxides. This factor is defined as EFX = (X/Al) sample/(X/Al) PAAS, where X represents the content of an element or its oxide and PAAS represents the value for post-Archean Australian shale.40 An EF value greater than 1 commonly implies that an element or its oxide was enriched during the sedimentary process, whereas a value less than 1 indicates depletion.40 In the samples from the Shengli River black rock series, the major element oxides that are enriched relative to PAAS include CaO (EF = 62.79− 747.31), P2O5 (EF = 0.84−35.02), MgO (EF = 2.11−165.23), MnO (EF = 0.72−15.03), and TFe2O3 (EF = 0.87−3.01) (Table S2 and Figure 6a). The oxides with relatively low EF values that are still greater than 1 include SiO2 (1.06−1.88) and K2O (1.26−1.37). The oxide with an EF value most similar to that of PAAS (EF ≈ 1) is TiO2 (0.85−0.99), and Na2O (0.18− 0.73) is a depleted oxide. In micritic limestone sample SLR-12, all the elemental oxides are enriched and display maximum EF values except for TiO2. In particular, Ca, Mg, Mn, and P oxides have much higher EF values than those of the other samples. F


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study area during the Early Cretaceous.30,31 With respect to rock type, the (La/Yb)n values are 1.06−1.19 (average, 1.15) for the marl samples, which are obviously higher than the values of the shale samples (0.96−1.17; average, 1.00) and the micritic limestone sample (1.01). This indicates that the fractionation of REEs in marl samples is higher than that of shale and micritic limestone samples, which further implies different sedimentation rates. In contrast, the (La/Sm)n and (Tb/Yb)n values in the three rock types are indistinguishable, indicating that all the samples have a similar provenance,47,50 consistent with the REE patterns (Figure 8).

Sr (EF, 4.74−36.95), Ni (1.41−10.51), U (1.13−5.00), Cu (0.66−4.33), Zn (0.90−6.02), Sc (1.30−4.31), Co (0.76− 2.35), V (0.87−5.08), and Cr (0.91−3.12). The intermediate elements (EF ≈ 1) include Rb (1.05−1.22), Ga (1.03−1.17), and Th (0.84−1.05). The obviously depleted elements are Cs (0.68−0.88), Nb (0.72−0.86), Hf (0.73−0.85), Ba (0.48− 1.55), and Zr (0.61−0.75) (Figure 6b). With regard to rock type, the micritic limestone sample SLR-12 shows the highest EF values for Sc, V, Cr, Co, Ni, and Sr and the shale samples are enriched with Cu and Zn. In the marl samples, SLR-01− SLR-04, all the trace elements are depleted, especially those (such as V, Cr, Co, and Cu) that are strongly enriched in other samples. 4.2.3. Rare Earth Elements. The concentrations and major parameters of the rare earth elements (REEs) of the black rock series in the Shengli River area are presented in Table S4 and displayed in Figure 7. The ΣREE concentrations range from 9.36 to 54.53 ppm, which are much lower than those of the PAAS value (184.77 ppm), reflecting that the lithologies are mudstones and micritic limestones and indicating that terrigenous input controlled the REE concentrations.45,46 With regard to lithology, ΣREE concentrations are lowest for micritic limestones and highest for marls, with shales displaying intermediate values. This pattern is consistent with the variations of elements reflecting terrigenous inputs, such as Si, Al, and Ti, further suggesting that the ΣREE concentration was controlled mainly by terrigenous inputs.47 In the diagram of PAAS-normalized REE patterns (Figure 8), the trend of the curves representing the three sample lithologies

5. DISCUSSION Based on the above results, the geochemistry of the samples in this study shows some uniqueness, which mainly points to elemental compositions of the three lithologies in the study area. The compositions obviously vary from the characteristics of normal open marine settings and continental brackish lacustrine settings, e.g., higher EF value of redox-sensitive elements than open marine settings51 and higher EF value of elements Ca, Mg, and Sr than continental brackish lacustrine settings.52 In addition, there are also differences among the three lithologies themselves. These uniquenesses may be attributed to different depositional processes among different settings, which will be discussed below. This might be a general characteristic of organic matter accumulation in transitional (residual bay) environments. 5.1. Conditions of Organic Matter Accumulation. The accumulation (i.e., abundance and type) of organic matter in the black rock series is affected by multiple factors, the most important of which are paleoproductivity, preservational environment, and sedimentation rate.1,53,54 The characteristics of these three factors in the Early Cretaceous black rock series in the Shengli River area are discussed below to determine the main factors affecting the abundance and type of organic matter found in these rocks. 5.1.1. Paleoproductivity. The initial productivity in marine or lacustrine settings is thought to be a significant factor for the formation of organic-rich hydrocarbon source rocks.1,55,56 For example, high-productivity offshore continental shelf and upper slope areas are important regions for the development of highquality source rocks,57,58 which is commonly related to the plentiful supply of biologically essential elements such as P,59,60 Ba,61,62 and Mn.53,63,64 In the present study, results show that P and Mn are generally enriched in all the samples (Table S2, Figure 6a), implying that productivity was high.40 The EF values of P and Mn in the shale and micritic limestone samples are much greater than 1, which also indicates higher productivity.40,65 In contrast, the EF values of P and Mn in the marl samples are less than or equal to than 1, which implies relatively lower productivity. After eliminating the abiotic effects of P and Ba from the terrigenous input, the ratios of P/Al and Ba/Al can be applied to represent paleoproductivity,66 using the ratio of Mn/Ca as a reference value.64 In the Shengli River black rock series profile, except for the micritic limestone sample (SLR-12), the patterns of vertical variation of P/Al and Ba/Al ratios for marl and shale samples both display an increasing trend from bottom to top, with the increasing trend of P/Al ratios slightly stronger than that of Ba/Al (Table S5, Figure 9). The ratios of P/Al and Ba/ Al for the marl samples (SLR-01−SLR-04) are low, likely reflecting low paleoproductivity during sedimentation. In

Figure 8. Diagram of PAAS-normalized REE patterns of samples from the Shengli River outcrop.

are generally similar, reflecting a similar source of terrigenous input (provenance).48 The ratios of light REEs (LREEs) to heavy REEs (HREEs) of all the samples range from 7.00 to 9.32, which are close to the PAAS value (9.49), implying relatively weak fractionation of the REEs. The values of (La/ Yb)n are 0.91−1.19, those of (La/Sm)n are 0.90−1.13, and those of (Tb/Yb)n are 1.10−1.30. These results plot as horizontal lines that are slightly positively inclined in the diagram of the PAAS-normalized REE pattern, implying that seawater did not significantly affect the REE pattern. The Ce anomalies, defined as δCe = Cen/(Lan × Prn)1/2, are 0.83−0.92, reflecting weakly negative anomalies or normal values and implying a continental-margin provenance (δCe = 0.79− 1.5449). This finding is consistent with the geological background having been a passive continental margin in the G


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Figure 9. Stratigraphic column of geochemical parameters indicative of paleoproductivity, preservational condition and sedimentation rate of samples from the Shengli River outcrop. Data from Table S5.

contrast, the corresponding ratios for the micritic limestone and shale samples are much higher than those of the marl samples, indicating higher paleoproductivity during the formation of the micritic limestones and shales. The high ratio of P/Al of the micritic sample can reflect the high paleoproductivity and also might be attributed to the low content of Al2O3. Furthermore, the Mn/Ca ratio for the micritic limestones is slightly higher than that of the shales and much higher than that of the marls. This might partly be attributed to other forms of carbonates (i.e., magnesium carbonate) in this sample; calcium cannot represent all the carbonates in this sample.67 The curve representing this ratio is slightly different from the curves of the P/Al and Ba/Al ratios and similar to that of TOC (Figure 4). These variations in the ratios reflect the combined effects of paleoproductivity, preservational environment, and sedimentation rate and are consistent with the theoretically expected effects. Given the above, we conclude that productivity was generally high during the formation of the Early Cretaceous black rock series in the Shengli River area, although the productivities associated with the three different rock types remain distinct: the paleoproductivity of the marls was relatively low compared with the identical productivities of the micritic limestones and shales. 5.1.2. Preservation of Organic Matter. Preservational conditions exert an important control on the abundance of organic matter.1,68,69 For example, many high-quality hydrocarbon source rocks have been discovered in anoxic environments,70−72 such as lacustrine settings of the Qingshankou Formation in the Songliao Basin of China73 and the Lagoa Feia Formation in Campos Basin, Brazil,74 and the marine settings in the Cretaceous Eagle Ford in Texas75 and the Silurian Longmaxi in NW Guizhou Province of China’s Upper Yangtze Block.76

During sedimentation, different elements display different geochemical properties because of variations in hydrodynamic and redox conditions, which can cause dispersion, assembly, and/or precipitation of these elements.76−78 Therefore, the geochemical behaviors of different elements can be used to describe the sedimentary environment and the redox conditions of sediments.78,79 Tribovillard et al.40 proposed that under anoxic conditions, U, V, and Ni could easily be enriched and should have high EF values. This conclusion was obtained based on marine shale samples. Later this has also been successfully used in lacustrine80−82 and carbonate settings.83,84 Thus, it seems that these elements would behave with eligible differences in their availability in marine and lacustrine systems. In this study, the EF values of U, V, and Ni in all the black rock series samples reflect their enriched status (Table S3, Figure 6b), and the EF values of the shale samples and micritic limestone samples are higher than those of the marl samples, implying that the sedimentary environments were relatively reducing. On the basis of the results of a study of representative mudstone samples, a series of standards for assessing the ratios of trace elements with respect to organic matter preservation were established.78 Typical indexes include the ratios of U/Th and Ni/Co. For example, a U/Th value greater than 1.25 in sediments indicates anoxic conditions (dissolved oxygen < 0.2 mL/L), a value between 0.75 and 1.25 indicates suboxic conditions (dissolved oxygen, 0.2−1.0 mL/L), and a value less than 0.75 indicates oxidizing conditions (dissolved oxygen > 1 mL/L). For Ni and Co, a ratio of Ni/Co of less than 5 represents an oxidizing environment, a value of 5−7 reflects suboxic conditions, and a ratio of greater than 7 represents an anoxic environment.78 For the black rock series in this study (Table S5, Figure 9), the ratio of U/Th is 0.28−1.05 (average, 0.59), generally indicating an oxidizing−suboxic sedimentary environment, while the ratio of Ni/Co is 4.03−11.69 (average, H


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Figure 10. Correlation of Al-normalized redox-sensitive trace elements and TOC of samples from the Shengli River outcrop. Data from Table S5. Red squares indicate samples with possible hydrothermal influences.

and V reside mainly in authigenic phases rather than organic phases.40 In this study, four trace elements were used to identify the redox conditions of the black rock series by analyzing the covariation between Al-normalized TEs and TOC contents, including two “strong euxinic affinity” elements (U and V) and two “weak euxinic affinity” elements (Ni and Cr) (Table S5). Covariation of Al-normalized elements and TOC contents was presented in Figure 10, excluding the samples which are possibly influenced by hydrothermal fluids (samples with Fe/Ti > 20 or Al/(Al + Fe) < 0.4. The standard according to Boström et al. and Liu et al.,86,87 and the result of study samples comes from Yang et al.17), which mainly appears in shale samples with TOC > 6.0 and in micritic limestone samples. Results show moderate (Ni and Co) or strong (U, V) covariations, suggesting that the depositional environment in the study area was most likely to be suboxic. This conclusion is consistent with the analyses of element ratio parameters and EF values of redox-sensitive trace elements above. In summary, the samples from the black rock series in this study formed in a weakly oxidizing to anoxic transitional environment that was primarily suboxic. Of the different rock types, the sedimentary environment of the marl was oxidizing, that of the shale was mainly suboxic, and that of the micritic limestone was anoxic. 5.1.3. Sedimentation Rate. In addition to high primary productivity and favorable preservational conditions as presented above, the sedimentation rate and burial are also important factors in determining the type and abundance of organic matter,1,88,89 although the mechanism is complex and may be affected by the other two factors.1,68 For example, under normal oxidizing conditions, the organic sedimentation rate is

5.67), suggestive of a suboxic to sightly anoxic environment. Therefore, in general, the samples from the black rock series in this study were formed in a weakly oxidizing to mainly suboxic sedimentary environment. This is consistent with the presence of zoobenthos remains in the samples.16 Note that the ratios of U/Th and Ni/Co in the micritic limestone samples are the same as those of the anoxic standard materials, implying that the anoxic conditions might appear occasionally. In addition, according to Jones and Manning,78 the distinguishing value of the oxic−suboxic boundary is more effective than that of the suboxic−anoxic boundary. As a result, the conclusions should be made in combination with other parameters. This is the methodology used in our study. In addition, the redox conditions can also be determined by the correlation between Al-normalized redox-sensitive trace elements (TEs) and TOC contents. According to Algeo and Maynard,85 the behavior of some redox-sensitive trace elements follows two patterns: (1) some elements (such as Mo, U, V, Pb, and Zn) exhibit strong covariation with TOC in oxic or suboxic conditions but exhibit weak covariation with TOC in euxinic environments, which are called “strong euxinic affinity”; and (2) some elements (such as Cu, Cr, Ni, and Co) exhibit moderate to strong covariation with TOC in all conditions, which are called “weak euxinic affinity”. Anoxic conditions are similar to euxinic conditions in terms of dissolved oxygen contents (dissolve oxygen ≈ 0) but have no H2S occuring in the water column, which is different from the euxinic conditions. In redox condition analyses, the euxinic conditions could be identified by extremely higher EF values of elements U and V than Ni and Cu. While under nonsulfidic anoxic conditions, these elements have similar EF values because U I


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Figure 11. Correlation of geochemical parameters indicative of paleoproductivity, preservational conditions, and sedimentation rate and TOC of samples from the Shengli River outcrop.

form of aluminum silicates (clay minerals) in the sedimentary rocks, whereas silicon is present in both aluminum silicates and quartz. Titanium occurs in aluminum silicates and can be present in silt-sized heavy mineral grains such as titanite and ilmenite.53,64,92,93 Therefore, higher ratios of Si/Al or Ti/Al in sediments can reflect a higher proportion of sandy and silty mineral grains in terrigenous materials, which can imply a higher sedimentation rate.92−94 These parameters have been successfully applied to evaluate the sedimentation rate of shales and marls, but seems not well suited for micritic limestones. This is because the sedimentation rate of micritic limestone mainly depends on chemical precipitation of carbonate rather than terrigenous input.95 Therefore, the Si/Al or Ti/Al values cannot represent the sedimentation rate of the micritic limestone well indeed. The parameters relating to the sedimentation rate of the black rock series in the Shengli River area are summarized in Figure 9 and Table S5. There are clear variations in the SiO2/

usually positively correlated with TOC because rapid burial can decrease the residence time of organic matter in the degradation zone of aerobic bacteria and thus reduce the oxidative decomposition of organic matter.72,88,89 For comparison, in the anoxic environment (i.e., when the oxygen content in water approaches zero), decomposition by biologic oxidation is almost non-existent, no correlation is observed between the TOC and sedimentation rate.89 For environments with a high sedimentation rate, because of the dilution of organic matter by sedimentary material, the organic content of the sediment is negatively correlated with the sedimentation rate.90,91 Also, given the same or a similar sedimentation rate, preservation is better under anoxic conditions than in an oxidizing environment.68 Previous studies have suggested that there is a positive correlation between sedimentation rate and the grain size of terrigenous material, which can be used to estimate the relative sedimentation rate.92,93 In general, aluminum is present in the J


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correlations between TOC and paleoproductivity, preservational environment, and sedimentation rate, indicating that all three factors had some effects, although there is variation between rock types because of differences in the controlling factors according to lithology. There is no obvious correlation between the ratios of P/Al and Ba/Al indicative of paleoproductivity and TOC content for either the shale or micritic limestone samples (R2 = 0.250 for P/Al-TOC and R2 = 0.180 for Ba/Al-TOC, respectively). In contrast, relatively modest correlations are apparent between indexes such as the Mn/Ca (controlled by paleoproductivity and redox conditions) versus TOC and U/Th (simply controlled by redox conditions) versus TOC. Correlation coefficient R2 reaches 0.625 and 0.690 in the Mn/Ca-TOC and U/Th-TOC diagrams, respectively. These values would stand for relatively high correlation compared with the generally accepted criteria in the primary reference,78 although the values appear to be not so high mathematically. The SiO2/Al2O3 and TiO2/Al2O3 ratios, which reflect the sedimentation rate, are also not correlated with TOC content, indicating that the sedimentation rate had little effect on the amount of organic matter.90,91 Therefore, it can be deduced that, in the Shengli River area, for the shales and micritic limestones that preservational conditions were the main factor controlling the accumulation of organic matter and that productivity had a secondary effect. For the marl samples, weakly positive correlations of the ratios of P/Al and Ba/Al (which reflect paleoproductivity) with TOC content were observed, implying that productivity was one of the factors controlling the amount of organic matter. In contrast, high correlations were observed between indexes such as the Mn/Ca ratio and U/Th ratio and TOC content, which indicates that favorable conditions for organic preservation was an important controlling factor. In addition, the SiO2/Al2O3 and TiO2/Al2O3 ratios, which reflect the sedimentation rate, have very poor correlation with TOC content (R2 = 0.470 for SiO2/Al2O3-TOC and R2 = 0.0464 for TiO2/Al2O3-TOC), demonstrating that the sedimentation rate had little effect on the organic matter content of the marl samples. To summarize, the characteristics of the shale samples are similar to those of the micritic limestone samples. The effect of productivity on organic matter abundance and type was limited, and the effect of the rapid sedimentation rate on organic matter was negligible. In contrast, the marl samples were deposited in a low-productivity, low-sedimentation-rate, relatively oxidizing sedimentary environment. Thus, although all three factors influenced the type and abundance of organic matter, with the most important factor remains the preservational environment. Therefore, organic matter preservation was the main mechanism controlling the amount of organic matter in the Shengli River black rock series, independent of lithology. This is consistent with the sedimentary environment of a lagoon in a residual bay area,25,26 where the suboxic−anoxic conditions for the preservation of organic matter are favorable.1,71 5.3. General Process of Organic Matter Accumulation. Based on the above discussion, we propose that the organic matter accumulation in residual bay environments is mainly controlled by preservational conditions, while the other two factors productivity and sedimentation rate have relatively less effects. In addition to the case of this study, the suboxic−anoxic conditions are also regarded as the main factor oragnic-matter accumulation in other settings, such as the transgressive carbonate platform setting in Yangtze Block during the Late

Al2O3 and TiO2/Al2O3 ratios for all samples. The ratios for the marl samples are much lower than those for the shale samples. This indicates that the marls were formed at a lower sedimentation rate compared with shales. For samples of each lithology, the SiO2/Al2O3 and TiO2/Al2O3 ratios decrease slightly with increasing burial depth, which implies that there was a tendency for fine-grained/muddy components to precipitate at the bottom of the sediments. REE contents and patterns can also reflect the sedimentation rate of terrigenous material. In general, as REEs are absorbed by clay minerals during deposition, as sedimentation rate increases the REE content of sediments decreases because the contact time between clay minerals and REEs is reduced.49,50 Therefore, variations in the ΣREE content (ΣREE) can represent variations in the sedimentation rate.49,50 Like the ratios of Si/Al or Ti/Al discussed above, the ΣREE content of shale and marl samples mainly depends on the amount of terrigenous input, while the sedimentation rate of micritic limestone samples mainly depends on chemical precipitation. Thus, the ΣREE content of micritic limestone samples cannot indicate sedimentation rate under this condition either.95 ΣREE values of the black rock series samples from the Shengli River area are higher for marl and lower for shale (Table S4 and Figure 7). This corresponds with the slower sedimentation rate of the marl samples than shale samples and is consistent with the results obtained from the ratios of SiO2/Al2O3 and TiO2/ Al2O3. The REE composition of sediments varies with the conditions of electrovalency and adsorption during the processes of transport and precipitation.49,50 Such fractionation of LREEs and HREEs and variations in the REE distribution pattern result from the different amounts of time spent in contact with the sedimentary water.49,50,96 A high sedimentation rate and short residence time can result in a weak fractionation of LREEs and HREEs and, therefore, result in the (La/Yb)shale value close to 1 (the subscript “shale” in this study indicates normalization to PAAS).96 Since the (La/Yb)n ratio just depends on the residence time rather than the amount of REEs in the water column, it can be used across all three lithologies in this study. This was confirmed in many studies of carbonates and shales, e.g., the Ordos Basin in China96 and the Wharton Basin in the Indian Ocean.97 The degree of fractionation of REEs of the black rock series samples is lower than that of standard shales because the data plot below the horizontal line that corresponds to the standard shales (Figure 8). In Figure 8, the curves representing REE distribution patterns are flat and have similar shapes, indicating only small variations in the rate of sedimentation. With respect to rock type, the values of (La/Yb)n for the marl samples are 1.06−1.19 (average, 1.15) and are higher than the corresponding values for both the shale samples (0.91−1.17; average, 1.00) and the micritic limestone sample (1.01). This suggests that the sedimentation rate of the marl samples was lower than that of the other two rock types. The difference between shales and micritic limestones do not appear statistically significant because the variation of chemical compositions of the samples is slight. 5.2. Mechanism of Organic Matter Accumulation. The mechanism of organic matter accumulation is assessed by examining the correlation of organic matter type and abundance (represented by TOC content) with paleoproductivity, preservational environment, and sedimentation rate (Table S5 and Figure 11). There are generally positive K


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Figure 12. General process of organic matter accumulation in the Lower Cretaceous black rock series from the Shengli River outcrop.

Ediacaran−Early Cambrian98 and the restricted marine basin setting in Colorado during the Cretaceous.99 Thus, it seems that the preservational conditions can be the dominate factor of organic matter accumulation in marine to continental transitional settings. Under the influence of this factor as well as the other two factors, the Lower Cretaceous black rock series in the Shengli River area contains abundant organic matter (TOC = 1.74− 7.71%), with the Type II/III organic matter (Table S6). Specifically, the organic matter abundance of the marls (samples SLR-01−SLR-04) is relatively low (TOC = 1.74− 3.77%) and of Type II−III kerogen, but more close to Type III, which implies a poor preservation. Si and Al are enriched, demonstrating that there was relatively abundant terrigenous material in a drainage basin. The organic matter abundance of the shales (samples SLR05−SLR-10 and SLR-13−SLR-28) is relatively high (TOC = 4.12−7.71%) and is classified as Type II/III kerogen. Abundant benthic algae were present in the shale.16 The preservational condition was better than that of the marls, and thus the shales have a better organic matter type more favorable for oil generation with a similar shale matrix of carbonates. This, combined with the discovery of biogenic calcareous detritus,16 suggests that this rock was deposited in a semienclosed lagoon under transgressive conditions. The organic matter abundance of the micritic limestones (samples SLR-11 and SLR-12) is relatively high (TOC = 5.61− 6.46%). The organic matter is Type II/III, and benthic algae were present.16 These samples are rich in Ca and Mg with low CIA value (70.07), indicating that the sedimentary environment was an evaporitic lagoon with low weathering. In addition, according to previous studies on the stratigraphy,100 the micritic limestones were found to be intercalated with gypsum and salt rocks, which could further indicate the arid climate and evaporitic conditions. On the basis of these results, a sketch map of general process of organic matter accumulation in the study area was established (Figure 12).

6. CONCLUSIONS The Lower Cretaceous black rock series in the Shengli River area was formed in a residual bay. The sequence is, in general, a potential hydrocarbon source rock that has a high abundance of organic matter. The organic matter type is II/III, and the oxygenation state of the water column during sediment deposition was weakly oxidizing to anoxic. The organic matter accumulation mechanism and associated properties of the three different lithologies present (marls, shales, and micritic limestones) are different. The marls were deposited within a drainage basin near the coast under warm and humid conditions, which is characterized by a relatively low paleoproductivity, a weakly oxidizing sedimentary environment, and a slow sedimentation rate. The shales were deposited in a semienclosed lagoon under transgressive conditions with a paleoproductivity higher than that of the marl samples. The preservational environment was suboxic with a high sedimentation rate. The micritic limestones were formed in a high-salinity and suboxic−anoxic sedimentary environment. The paleoproductivity and sedimentation rate were the same as those of the shale. The amount and type of organic matter in black rock series in residual bay environments is controlled mainly by a preservational environment, while paleoproductivity and sedimentation rate play relatively less significant roles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b02248. (Table S1) Bulk geochemistry of the Lower Cretaceous black rock series, (Table S2) concentrations and enrichment factors of major element oxides, (Table S3) abundance and enrichment factors of trace elements, (Table S4) abundance and parameters of rare earth elements, (Table 5) geochemical parameters, and (Table S6) summarized characteristics and controlling factors (PDF) L


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-25-83686016. ORCID

Jian Cao: 0000-0003-4132-5939 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the editor and three anonymous reviewers for their insightful reviews, which greatly improved the article. This work was jointly funded by the National Science and Technology Major Project of China (Grant No. 2016ZX05002-006-005) and the National Natural Science Foundation of China (Grant Nos. 41322017 and 41472100).

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The Early Cretaceous Qiangtang Basin, Tibet - ACS Publications

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