Marine hydrocarbon source rocks of the Upper Permian Longtan Formation and their contribution to gas accumulation in the northeastern Sichuan Basin,southwest China |
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| Institution: | 1. School of Earth Sciences and Engineering, Xi''an Shiyou University, Xi''an 710065, China;2. Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology, Qingdao 266590, China;3. Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education, Wuhan 430074, China;4. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China;5. Exploration and Development Department, Zhejiang Oilfield Company, CNPC, Hangzhou 310023, China;6. School of Resources and Environmental Engineering, Guizhou Institute of Technology, Guiyang 550003, China;7. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;1. Exploration Department of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China;2. Exploration and Development Research Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610041, China;1. School of Energy Resources, China University of Geosciences, Beijing 100083, China;2. Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism, Ministry of Education, Beijing 100083, China;3. SINOPEC Exploration and Production Research Institute, Beijing 100083, China;4. Exploration Branch, SINOPEC, Chengdu 610041, China;1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, China;2. Unconventional Natural Gas Institute, China University of Petroleum, Beijing, 102249, China;3. Cooperative Innovation Center of Unconventional Oil and Gas, China University of Petroleum, Beijing, 102249, China;4. Oil and Gas Survey, China Geological Survey, Beijing, 100083, China;5. Research Institute of Petroleum Exploration and Development, Beijing, 100083, China;6. School of Earth Sciences and Engineering, Xi''an Shiyou University, Xi''an, 710065, China |
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| Abstract: | Identification of the main hydrocarbon source rocks of the large Puguang gas field (northeastern Sichuan Basin, southwest China) has been the subject of much discussion in recent years. A key aspect has been the lack of a comprehensive understanding of the development of hydrocarbon source rocks of the Upper Permian Longtan Formation, which had been thought to contain mainly coal seams and thick carbonate layers. In this paper, based on geological data from more than ten wells and outcrops and their related mineralogy and geochemistry, we investigated the depositional environment and main factors controlling organic matter enrichment in the Longtan Formation. We propose a model which combines information on the geological environment and biological changes over time. In the model, organic matter from prolific phytoplankton blooms was deposited in quiescent platform interior sags with rising sea-levels. During the Longtan period, the area from Bazhong to Dazhou was a platform interior sag with relatively deep water and a closed environment, which was controlled by multiple factors including syngenetic fault settling, isolation of submarine uplifts and rising sea-levels leading to water column stratification. Although the bottom water was anoxic, the phytoplankton were able to bloom in the well-lit upper euphotic zone thus giving rise to a set of sapropelic black shales and marlstones containing mostly algal organic matter with minor terrestrial contributions. As a consequence, these rocks have a high hydrocarbon generation potentials and can be classified as high-quality source rocks. The area from Bazhong to Dazhou is a center of hydrocarbon generation, being the main source of reservoired paleo-oils and presently discovered as pyrobitumen in the Puguang gas field. The identification of these source rocks is very important to guide future petroleum exploration in the northeastern Sichuan Basin. |
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| Keywords: | Sichuan Basin Upper Permian Longtan Formation Marine Hydrocarbons Petroleum Source rocks Gas accumulation |
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