China Petroleum Exploration ›› 2022, Vol. 27 ›› Issue (4): 75-90.DOI: 10.3969/j.issn.1672-7703.2022.04.006

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Development characteristics and genesis of thin layered dolomite of the Middle Permian Qixia Formation in the platform in Weiyuan-Gaoshiti area, Sichuan Basin

Yang Wenjie1,2,Tan Xiucheng1,2,Li Minglong1,2, Xia Jiwen3,Long Hui3,Ni Jia3,Li Jun3,Xiao Di1,2, Zhang Zhaokun1,2   

  1. 1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University); 2 Southwest Petroleum University Division of CNPC Key Laboratory of Carbonate Reservoirs; 3 Branch of Shunan Gas Field, PetroChina Southwest Oil & Gasfield Company
  • Online:2022-07-15 Published:2022-07-15

Abstract: The re-examination of core and well drilling data showed that the large-area beach facies thin layered dolomite was developed in the Middle Permian Qixia Formation in Weiyuan-Gaoshiti area in Sichuan Basin, and commercial gas flows were obtained from the dolomite reservoir in several wells. Therefore, it is of great significance to clarify the genesis and distribution law of dolomite for gas exploration of Qixia Formation in the near future. The petrology and distribution characteristics of dolomite are studied by using core, wireline logging and mud logging data. The results indicate that the thin layered dolomite is divided into two types of porphyritic dolomite and laminated dolomite in terms of the macroscopic occurrence, with fine-meso crystalline dominated of dolomite grains, and distinct phantoms of the residual bioclastic grains. The main protolith is sparry bioclastic limestone with microbial bond stucco fabric. Significantly controlled by the sedimentary cycle, the dolomite has a thin single layer thickness of 1-4 m, and cumulative thickness of 3-16 m, showing the characteristics of multi-stage superposition in vertical and mainly developed in the middle-upper part of the sedimentary cycle. The geochemical experiment results show that the dolomitization fluids of Qixia Formation were dominated by penecontemporaneous seawater, and only part of the vuggy dolomite was transformed by deep hydrothermal fluids to a certain extent. By analyzing the coupling relationship between the paleogeographic pattern of uplift and depression during the deposition period and the distribution of wells encountering the thin-layered dolomite, it is found that the thin-layered dolomite was generally developed in the slope break between uplift and depression in the platform, and new understanding is proposed that the superimposing and migrating microbial mound beach bodies and the restricted seawater environment promoted the infiltration reflux dolomitization. The multi-stage superimposed microbial mound beach bodies in the platform laid a good material foundation for the development of dolomite reservoirs. In addition, the penecontemporaneous karstification was conducive to the improvement of porosity and permeability of mound beach body and provided a dominant pathway for the migration of dolomitization fluids. The results confirm that the thin layered dolomite in the platform has a large-area discontinuous zonal distribution pattern in the circum subsag, which provides an important theoretical support for expanding the exploration field of the intra platform dolomite reservoir of Qixia Formation.

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