Source rock deposition controlled by tectonic subsidence and climate in the western Pearl River Mouth Basin,China: Evidence from organic and inorganic geochemistry |
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| Institution: | 1. China University of Petroleum (Beijing), 18 Fuxue Road, Changping District, Beijing 102249, China;2. China State Key Laboratory of Shale Oil and Shale Gas Resources and Effective Development, Sinopec Petroleum Exploration and Production Research Institute, 31 Xueyuan Road, Haidian District, Beijing 100083, China;3. Sinopec Jianghan Oilfield Company, 18 Daxueyuan Road, Gaoxin District, Wuhan, Hubei 430223, China;1. College of Earth Sciences, Jilin University, 130061 Changchun, China;2. Key- Laboratory for Evolution of Past Life and Environment in Northeast Asia, Ministry of Education, Jilin University, 130061 Changchun, China;3. Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia;4. Department of Geology, Federal University Lokoja, P. M., B 1154, Nigeria;5. Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology, Qingdao, 266590, China;1. School of Geosciences, China University of Petroleum, Changjiang west road 66, Tsingtao 266580, China;2. College of Earth Sciences, Jinlin University, Jianshe Street 2199, Changchun 130061, China;3. Key laboratory for Oil Shale and Paragenetic Energy Minerals, Jilin Province, Changchun 130061, China;4. Department of applied Geosciences and Geophysics, Montanuniversität Leoben, Peter-Tuuner-Str. 5, Leoben A-8700, Austria;5. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;6. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China |
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| Abstract: | Interest in factors controlling lacustrine source rock deposition has increased over the last few decades because this type of deposits contain significant petroleum resources. Generally, tectonic subsidence and climate are the two root causes as they control the accommodation potential, water column properties and sources of organic matter. In this study, coupling organic geochemical and elemental geochemical data, two potential source rocks, i.e., the Eocene Wenchang Formation (E2w) and Oligocene Enping Formation (E3e) were investigated. Two models were finally raised to explain deposition of the two set of source rocks according to their paleoclimatic and tectonic properties. The source rock potential shows a strong heterogeneity. The second member of the Eocene Wenchang Formation (E2w2) is characterized by high organic matter content and oil-prone kerogen type. In contrast, the first member of the Eocene Wenchang Formation (E2w1) and the Oligocene Enping formation (E3e) are characterized by low organic matter content and gas-prone kerogen type. The primary productivity and depositional environment exhibit notable differences between the two potential source rocks horizons and show an obvious variation from the depocenter to the slope and can be best explained by the coevolution of tectonic subsidence and climate. During the E2w depositional stage, low sediment supply led to mudstone deposited in deep lacustrine environment and resulted in underfilled lake basin. The low water inflow provided little terrigenous organic matter (low bicadinane, perylene and floranthene contents) and oxygen. Besides, the low area/depth ratio impeded the water circulation, thus resulted in shallow thermocline and anoxic-suboxic bottom environment (abundant dibenzothiophene and high C35/C3122S hopane ratios). Therefore abundant algae, which contributed to the high amorphous organic matter (AOM) content, can be preserved. The warm and wet climate (high Mn/Mg ratios) gave birth to autochthonous organism, such as dinoflagellates and Pavlova gyrans (abundant 4-methyl sterane). During the E3e depositional stage, the sufficient sedimentary supply resulted in expanding, shallow lacustrine and swamp environment. The higher area/depth ratio and high sediment supply made environment unstable and can be strongly influenced by external environment (broader range of Mn/Mg ratios). Enough terrigenous organic matter (TOM) was transported to the slope but little to the depocenter. The slightly hot and dry climate (low Mn/Mg ratios) led to decreasing autochthonous organism and evaporation environment. The shallow water depth and relative dry climate resulted in saline, suboxic-dysoxic acid bottom environment. The co-variation of organic and inorganic indexes indicates the combination is a valid method in reconstructing source rock depositional models. |
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| Keywords: | Source rock Organic matter input Sedimentary environment Source rock depositional model |
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