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Division and Characteristics of Shale Parasequences in the Upper Fourth Member of the Shahejie Formation, Dongying Depression, Bohai Bay Basin, China

Jing Wu *1, 2, 3, Zaixing Jiang4, Chao Liang5
1. Petroleum Exploration and Production Research Institute of SINOPEC, Beijing 100083, China 2. State key laboratory of shale oil and gas enrichment mechanisms of SINOPEC, Beijing 100083, China 3. Sinopec Key Laboratory of shale Gas/Oil Exploration & Production, Beijing 100083, China 4. Faculty of Energy Resources, China University of Geosciences, Beijing 100083, China 5. School of Geosciences, China University of Petroleum, Qingdao 266000, China

ABSTRACT: Shale parasequence analysis is an important part of sequence stratigraphy sudies. This paper proposed a systematic research method for analyzing shale parasequences including their delineation, division, characteristics and origins. The division method is established on the basis of lithofacies. Multi-method analysis and mutual verification were implemented by using auxiliary indicators (such as mineral compositions, geochemical indicators and wavelet values). A typical shale parasequence comprises a lower interval of deepening water-depth and an upper interval of shallowing water-depth [e.g., a shale parasequence including a high-total organic carbon (TOC) shale-low-TOC limy shale]. Abrupt increases in pyrite content, TOC value, relative hydrocarbon generation potential [(S1+S2)/TOC], and wavelet values are indicative of parasequence boundaries. The proposed research method was applied to study the upper fourth member of the Shahejie Formation in the Dongying Depression, Bohai Bay Basin. Results show that there were seven types of parasequences developed. A singular and a dual structured parasequences were identified. Three factors controlling the development of the shale parasequences were identified including relative lake level change, terrestrial input and transgression. The development of high-TOC (>2%) shale parasequences was mainly controlled by biological and chemical sedimentation. The low-TOC (<2%) shale parasequences were mainly deposited by chemical sedimentation. The diversities of shale parasequences were caused by four major controlling factors including climate, relative lake level change, terrestrial input and emergency (e.g., transgression).  

Keywords:shale, parasequence, Dongying depression, upper fourth member of Shahejie formation

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