Research article
Organic geochemistry of a lacustrine shale across the Toarcian Oceanic Anoxic Event (Early Jurassic) from NE China

https://doi.org/10.1016/j.gloplacha.2020.103214Get rights and content

Highlights

  • A prominent negative excursion in δ13Corg values (~ 2‰) was observed in the Toarcian Beipiao lacustrine shale.

  • A shift to lower abundance of eukaryote relative to bacterial community in lacustrine system during the onset of the T-OAE.

  • Progressive accelerated hydrological cycle enhanced fresh water input and increased mixing of NE China lacustrine system.

  • The organic enrichment and good hydrocarbon generation potential resulted from enhanced delivery of riverine nutrients.

Abstract

Thirty-nine shale samples of the Lower Jurassic Beipiao Formation from drill core ZK01 in the Jinyang Basin, NE China were investigated by comprehensive organic geochemical methods in this study. A prominent negative excursion in δ13Corg values (~ 2‰) was observed in the Beipiao Formation, which is most likely associated with Toarcian Oceanic Anoxic Event (T-OAE) negative carbon isotope excursion (CIE) that is recorded globally in marine and terrestrial sediments constrained by previous studies. Elemental and Rock-Eval analyses of Beipiao shales demonstrate that the total organic carbon (TOC) ranges from 0.56 to 4.33 wt% (average of 1.40 wt%) and S2 values range from 0.12 to 6.91 mg/g (average of 1.3 mg/g) and implicate fair to good source rock potential. A majority of the Tmax values of the Beipiao shales range from 456 to 465 °C, indicating that the organic matter is within the mature oil window apart from few overmature samples with extreme high Tmax values that might have been affected by local magmatic intrusions. Elemental analysis suggests that the kerogens are mainly Type II-III. Unlike many marine organic-rich sediments deposited during OAEs, redox proxies indicate no significant upper water column oxygen-depletion during the formation of Beipiao lacustrine shale. The decreasing upward trend of organic source indicators such as tricyclic terpanes/C30 hopane values, relative distributions of regular C27 steranes to C29 steranes, and steranes/hopanes in the Beipiao Formation consistently suggest a shift to a lower abundance of eukaryotic organisms relative to bacterial community abundance during the onset of the T-OAE. Meanwhile, gradually decreasing values of gammacerane/C30 hopane in the Beipiao Formation during the negative CIE suggest decreasing salinity and increased mixing of the Jinyang paleo-lake system, potentially corresponding with the shift toward an accelerated hydrological cycle with enhanced fresh water input to this lacustrine system. Moreover, increasing C30 moretane/C30 hopane values contemporaneous with the negative CIE also suggest enhanced land erosion during putative Toarcian climatic warming, which could have increased delivery of riverine nutrients and elevated regional primary productivity, eventually resulting in massive organic matter accumulation in the Beipiao shales.

Introduction

The lacustrine environment has garnered much attention due to its significance in paleoenvironmental reconstructions and hydrocarbon exploration, particularly for continental interiors. The importance of lacustrine environments for the production and preservation of oil/gas-prone organic matter in the geological record has been recognized for many years since many oil-rich shales are known to have been deposited in these settings (Powell, 1986; Hao et al., 2011; Zou et al., 2019). The predominance of large-scale, petroleum-bearing basins in China, which were the sites of extensive and long-lived lakes, provided the rationale and impetus for the development of a continental model for petroleum generation and occurrence (Huang et al., 1984; Ding et al., 2015; Tong et al., 2018). Furthermore, lakes are sensitive recorders of continental paleoclimatic change, and they can also provide excellent high-resolution archives of past environmental conditions since sedimentation rates are usually higher than average marine settings (Castañeda and Schouten, 2011; Qiao et al., 2019).

The Mesozoic lacustrine revolution represents a series of major evolutionary events in the continental realm (Cohen, 2003; Wang et al., 2020), and a great number of lacustrine source rocks were formed in northeastern China and elsewhere during the Mesozoic. The Beipiao Formation (Fm.) is one of the most actively explored and significant hydrocarbon-producing Lower Jurassic lacustrine shale plays in northeastern China (Sun et al., 2018). Systematic characterization of its maturity and organic content of shales by organic geochemistry will aid future exploration. Apart from being characterized as a time interval with widespread lacustrine source rock, the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ~183 Ma) is also recognized as an interval with one of the most intense and geographically extensive events of marine redox change (Gill et al., 2011; Them et al., 2018) and accompanying organic‑carbon burial in the Mesozoic era (Jenkyns, 2010; Them et al., 2017a). The T-OAE is associated with a global carbon-isotope excursion (Hesselbo et al., 2000, Hesselbo et al., 2007; Suan et al., 2008; Them et al., 2017a; Izumi et al., 2018), elevated extinction rates (Harries and Little, 1999; Caruthers et al., 2013; Ait-Itto et al., 2018), potential increase in atmospheric methane concentrations (Hesselbo et al., 2000; Beerling and Brentnall, 2007; Them et al., 2017a), enhanced weathering both locally (Brazier et al., 2015; Percival et al., 2016; Fu et al., 2017) and globally (Them et al., 2017b), and an expanded area of oceanic pyrite burial potentially related to an increased geographic extent of marine euxinia (anoxic and sulfidic water column; e.g., Gill et al., 2011). Additionally, photic zone euxinia increases within some of the European epeiric seaway in early Toarcian (French et al., 2014; Ruebsam et al., 2018 and references within). In addition to marine systems, a recent study has suggested that major inland lakes are able to respond to changes in the global carbon cycle and potentially affect climate by burying large quantities of organic carbon (Xu et al., 2017). The lacustrine Beipiao shale therefore provides an excellent opportunity to evaluate environmental and ecological responses to the Toarcian carbon cycle perturbations. In order to determine vertical variations of organic facies, thermal maturity, and depositional environments of the Beipiao Fm., we used Rock-Eval, organic carbon isotopes, and biomarkers from a drill core (ZK01) in the Jinyang Basin, northeastern China in this study.

Section snippets

Jinyang Basin

The Jinyang Basin lies in the eastern part of the Yan-Liao Orogenic Belt and covers a total area of 9000 km2 (Sun et al., 2018). As one of the largest intracontinental basins in northeastern China, the Jinyang Basin is restricted by several NE-strike faults. Similarly, a NE-strike syncline controlled the deposition of the Mesozoic strata, with the steeper northern limb structure. During the Early Jurassic, a great number of lake systems existed in the continental interior in China with

Samples

The ZK01 well is located in the central depression of the Jinyang Basin, to the west of the Chaoyang City. The total thickness of the Beipiao Fm. in the core ZK01 reaches 280 m. Thirty-nine shale samples of the Beipiao Fm. were collected from interval of 297–17 m within the core. Core samples were first polished to remove any potential surface contamination such as drilling fluid residues. Samples were subsequently air dried at 60 °C for ~12 h and then crushed and homogenized into a fine powder

Elemental geochemistry and Rock-Eval

The TOC concentrations vary between 0.56 wt% and 4.33 wt% (averages 1.40 wt%). Twenty samples have moderate-to-good TOC content (TOC > 1.0 wt%; Peters and Cassa, 1994). Most samples with high TOC contents (>2.0 wt%) are located between 160 and 120 m. H/C and O/C values of the kerogens range from 0.42 to 1.02, and from 0.03 to 0.11, respectively. The free hydrocarbon contents in the Beipiao shales (S1) are generally low (averages 0.17 mg/g). The S2 values of the Beipiao shales range from 0.12 to

Hydrocarbon generation potential

Based on the source rock description criteria (Peters and Cassa, 1994), the TOC values suggest that the Beipiao shales are mostly fair to good source rocks (Fig. 6A). The Beipiao shales have a relatively low amount of free hydrocarbons (HCs; S1) and HCs generated during Rock-Eval pyrolysis (S2; mg HC/g rock), indicating the remaining HC potential for these rocks is low to fair. A cross plot of the hydrogen index (HI) versus Tmax yields a general estimation of thermal maturity level and type of

Conclusion

The Lower Jurassic lacustrine Beipiao shales recovered from drill core ZK01 in the Jinyang Basin, NE China are characterized as fair-good source rocks, within a mature oil-wet gas window based on Rock-Eval and molecular geochemical analysis. Our new δ13Corg and organic geochemistry data of the Beipiao Fm. also provide an important record of the Pl-To interval from an interior continental lake system characterized by a distinct negative CIE during the T-OAE, displaying a similar trend to those

Declaration of Competing Interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the China Geological Survey (No. DD20160167), the National Natural Science Foundation of China (No. 41790451 and 41972148) and the Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (No. K-2018-23). We appreciate the field work assistance of Fanhao Gong and Fei Xiao. We thank Andrew Caruthers and Benjamin Gill for comments on some portions of this manuscript. This paper has benefited significantly from

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