Oil & Gas Geology ›› 2021, Vol. 42 ›› Issue (1): 66-75.doi: 10.11743/ogg20210106

• Petroleum Geology • Previous Articles     Next Articles

Characterization and genesis of deep shale reservoirs in the first Member of the Silurian Longmaxi Formation in southern Sichuan Basin and its periphery

Hongyan Wang1,2(), Zhensheng Shi1,2, Shasha Sun1,2, Leifu Zhang1,2   

  1. 1. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
    2. National Energy Shale Gas R & D(Experiment) Center, Langfang, Hebei 065007, China
  • Received:2020-04-24 Online:2021-02-28 Published:2021-02-07

Abstract:

Deep reservoirs are among the future major targets for shale gas exploration and development in China. A study on the first Member of deep Longmaxi Formation (Long 1 Member) shales in the Sichuan Basin and its periphery was carried out by means of whole rock component X-ray diffraction (XRD), TOC determination, large thin section and argon ion (Ar+) milling SEM imaging. The results show that, compared with shallow shales, the deep shale reservoirs are characterized by relatively higher silica content, lower TOC, lower silty lamina fraction, higher porosity, and more effective pore networks. From shallow to deep, the silica content increases from 30% to 62%. Accordingly, the contents of carbonate minerals, clay minerals, and TOC decrease from 32%, 33%, and 7.1% to 14.3%, 7.8%, and 4.25% respectively. The stripe-shaped siltstone-bearing lamina developed in deep shale reservoir is significantly lower in content and thickness compared with shallow shales. Organic/inorganic-matter pores micro-fractures are widespread within the black shales. From shallow to deep, the thin section porosity increases from 1.6% to 10.8% along with significantly increasing organic and inorganic pores, as well as micro-fractures (with a proportion increase from 1% to 12%). In deep shales, the organic/inorganic-matter pores and micro-fractures are interconnected to form an effective pore network. In addition, the higher silica content and porosity, as well as effective pore network are possibly associated with the biogenic silica; the relatively lower TOC may be due to the long distance from provenance; and the low silty lamina fraction is related to deep water environment. Biogenic silica can produce and preserve a large amount of organic/inorganic-matter pores and well as micro-fractures during diagenesis. The supply of nutrient substance determines the generation of organic matter, and the distance to provenance is in negative correlation to nutrient supply. Silty lamina is mainly composed of carbonate minerals, which, however, are hard to form in deep reservoirs.

Key words: formation mechanism, deep-water shelf, reservoir characteristics, gas-bearing shale, Longmaxi Formation, Silurian, Sichuan Basin

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