Oil & Gas Geology ›› 2020, Vol. 41 ›› Issue (1): 201-208.doi: 10.11743/ogg20200118

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Origin of Qianjiang Formation dolostone in Qianjiang Sag, Jianghan Basin

Shiqiang Wu1(), Fengling Chen1, Zaixing Jiang2, Xiangxin Kong2, Chen Chen2, Wenjing Guan1   

  1. 1. Research Institute of Exploration and Development of Jianghan Oilfield Branch Company, SINOPEC, Wuhan, Hubei 430223, China
    2. School of Energy Resources, China University of Geosciences(Beijing), Beijing 100083, China
  • Received:2019-08-01 Online:2020-02-01 Published:2020-01-19
  • Supported by:
    国家科技重大专项(2017ZX05049-005)

Abstract:

The Paleogene Qianjiang Formation of Qianjiang Sag in central Jianghan Basin is characterized by 193 cyclothems composed of alternating organic-rich dolomites and evaporites. However, the origin of dolomites in the Qianjiang Formation has always been controversial, limiting the further exploration of unconventional resources. The main objective of the study lies in the origin of inter-salt dolomite-rich lithofacies in the formation through the analyses of sedimentary characteristics of inter-salt lithofacies and minerals. Three main lithofacies combinations are identified based on the observation of sedimentary features (in terms of core and thin section) and mineral composition, that is, the thin-bedded-to-lamellar dolomicrite, the lamellar marlstone bearing dolomite, and the lamellar dolomicrite interbedded with glauberite. The first two combinations show obvious laminated characteristics with alternating organic and inorganic lamellars, indicating deposition in a relatively deep and reducing environment. The calcite and dolomite crystals within these two combinations have pores resulted from organic degradation, and the dolomite crystals coexist with organic films and pyrite crystals, both of which just indicate that the origin of the two combinations may have something to do with microbial activities. Planktons and phytobenthos serve to control the precipitation of calcite and dolomite minerals respectively. Therefore, the two lithofacies combinations feature a higher content of organic matters. The third combination contains more evaporitic minerals and terrigenous clasts, indicating deposition in relatively shallower waters. The relatively higher salinity and large amount of terrigenous material inputs are not conducive to the preservation of organic matters. In addition, the formation of dolomite minerals in saline lakes is controlled by both microbial activities and evaporation environment, and the origin plays an important role in sedimentary characteristics of lithofacies and accumulation of organic matters.

Key words: calcite, glauberite, microbe, dolomite, Qianjiang Sag, Jianghan Basin

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