Petroleum Exploration and Development >

2017 , Vol. 44 >Issue 1: 69 - 78

DOI: https://doi.org/10.11698/PED.2017.01.08

PETROLEUM EXPLORATION

Mechanisms of shale gas generation and accumulation in the Ordovician Wufeng-Longmaxi Formation, Sichuan Basin, SW China

  • BORJIGIN Tenger ,
  • SHEN Baojian ,
  • YU Lingjie ,
  • YANG Yunfeng ,
  • ZHANG Wentao ,
  • TAO Cheng ,
  • XI Binbin ,
  • ZHANG Qingzhen ,
  • BAO Fang ,
  • QIN Jianzhong
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  • 1. State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Wuxi 214000, China;
    2. Wuxi Research Institute of Petroleum Geology, Sinopec Petroleum Exploration & Production Research Institute, Wuxi 214000, China;

Received date: 2016-02-23

  Revised date: 2016-11-03

  Online published: 2016-12-30

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Abstract

The source rock quality, organic pore structure, occurrence state and sealing mechanisms of shale gas in the Ordovician Wufeng-Longmaxi Formation (O3w-S1l), Fuling region, Sichuan Basin were studied using ultra-microscopic organic maceral identification, FIB-SEM, high temperature/pressure isothermal adsorption and isotopic age dating of noble gas. The results show that: (1) O3w-S1l organic-rich shale was mainly formed in a sedimentary environment with high productivity in surface water and hypoxia in bottom water, it can be divided into two sections according to TOC, of which the lower section (TOC≥3%) is mainly composed of graptolite, phytoplankton, acritarch, bacteria and solid bitumen, among them, graptolite is the main contributor to TOC, but the shale gas is mainly derived from phytoplankton, acritarch and other hydrogen-rich organic matter, as well as the pyrolysis of liquid hydrocarbons produced by this kind of organic matter. (2) Organic pores, as principal reservoir space for shale gas, exist in hydrogen-rich organic matter and solid bitumen. The graptolites and plenty of other organic matter stacking distribution in lamina provide more reservoir space for shale gas, and effective pathways of connected pores for fluid flow. (3) Shale gas in Fuling region is in supercritical state and dominated by free gas; the match of formation time of closed shale gas system and gas-generation peak, as well as slight alteration degree of sealing conditions in the later stage, are key factors controlling the retention and accumulation of shale gas in the regions with high thermal maturity and complex structural areas; adsorption, capillary sealing and slow diffusion of shale are the main microscopic mechanisms for the retention and accumulation of shale gas. It thus can be seen that the generation and accumulation of marine shale gas with high thermal maturity in complex structure areas is controlled jointly by anoxic depositional environment, excellent hydrocarbon rock quality, superior reservoir space and favorable sealing conditions.

Key words： Sichuan Basin; Fuling gas field; shale gas; hydrocarbon-forming organisms; reservoir space; occurrence state; sealing mechanism

Cite this article

BORJIGIN Tenger , SHEN Baojian , YU Lingjie , YANG Yunfeng , ZHANG Wentao , TAO Cheng , XI Binbin , ZHANG Qingzhen , BAO Fang , QIN Jianzhong . Mechanisms of shale gas generation and accumulation in the Ordovician Wufeng-Longmaxi Formation, Sichuan Basin, SW China[J]. Petroleum Exploration and Development, 2017 , 44(1) : 69 -78 . DOI: 10.11698/PED.2017.01.08

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