Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (2): 353-364.doi: 10.11743/ogg20220209

• Petroleum Geology • Previous Articles     Next Articles

Reservoir characteristics and evolution mechanisms of the Upper Ordovician Wufeng-Lower Silurian Longmaxi shale， Sichuan Basin

Ruyue Wang1,2,3(), Zongquan Hu1,2, Shengxiang Long1, Wei Du1, Jing Wu4, Zhonghu Wu5, Haikuan Nie1, Pengwei Wang1, Chuanxiang Sun1, Jianhua Zhao6   

  1. 1.Petroleum Exploration and Production Research Institute，SINOPEC，Beijing 100083，China
    2.State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development，Beijing 102206，China
    3.Key Laboratory of Shale Oil and Gas Exploration & Production，SINOPEC，Beijing 102206，China
    4.College of Earth Science and Engineering，Shandong University of Science and Technology，Qingdao，Shandong 266500，China
    5.College of Civil Engineering，Guizhou University，Guiyang，Guizhou 550025，China
    6.School of Geosciences，China University of Petroleum （East China），Qingdao，Shandong 266580，China
  • Received:2020-12-05 Revised:2021-12-26 Online:2022-04-01 Published:2022-03-11

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Abstract:

The characterization of the Upper Ordovician Wufeng - Lower Silurian Longmaxi shale in terms of mineralogy， organic geochemistry， storage space， physical properties and gas-bearing potential， is applied to discuss the evolutionary mechanisms and their influence on shale gas exploration and development， proposing a shale reservoir evolution mechanism of “framework development as controlled by biogenetic silica-calcite， pore generation as promoted by co-evolution， and pore preservation as controlled by pressure evolution”. The results show that： （1） lithofacies along with the type and occurrence of organic matters （organic pore carrier） plays an important role in controlling the development of organic pores. The intergranular pores （with a size of greater than 3-5 μm） filled with organic matters in the siliceous shales are highly developed， the most conducive to the development of organic macropores. The development of organic pores in argillaceous shales is affected by the TOC content and the structure of organo-clay complexes， and the organic pores are characterized by a wide range of distribution， large pore size on average， and small total number.（2） The rigid framework composed of bio-quartz， microbial dolomite and pyrite formed in the early of contemporaneous-early diagenetic stage contributes to the preservation of original pores. In the early stage of the middle diagenesis， the production and consumption of organic acids， unstable mineral dissolution， clay mineral transformation and oil generation from kerogens are synchronized， serving to provide spaces favorable for the charging and retention of liquid hydrocarbons during the oil generation period. From the late mesogenetic stage to the late diagenetic stage， gas generation， organic pore formation and pressure increase from cracking of kerogen and retained hydrocarbon jointly promote the development of organic pores and micro-fractures.（3） The compaction mitigation by overpressure allows the maintenance of organic pore morphology and shale physical properties. Its influence on highly stress-sensitive argillaceous shale at upper interval is obvious， while on the other hand， unapparent on highly brittle shale intervals at the bottom. With the tectonic modification and pressure relief intensity getting enhanced from the basinal center to edges， the physical properties of siliceous shale at the bottom largely remain the same， while the physical properties of the upper argillaceous shale become poor resulting in an enhanced sealing capacity.

Key words: lithofacies, formation pressure, reservoir evolution, shale gas, Wufeng Formation, Longmaxi Formation, Ordovician, Silurian, Sichuan Basin

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