AAPG Bulletin, Vol.104, No.4, 939-959, 2020
Influence of tectonic exhumation on porosity of Wufeng-Longmaxi shale in the Fuling gas field of the eastern Sichuan Basin, China
Thermal cracking of organic matter (OM) can enhance gas-shale porosity. However, thermally matured shales are generally characterized by low porosity with high OM content in tectonic exhumation settings. The Wufeng-Longmaxi shale in the Fuling gas field, China, was investigated via bulk analysis and scanning electron microscopy to reveal the influence of tectonic exhumation on the porosity of OM-rich shale. Thermal maturity and organic and mineralogical compositions are regionally homogeneous in the Fuling gas field. Shale porosity is positively correlated with total organic carbon (TOC) and clay mineral content in the gentle and open anticlines, respectively. The OM-hosted pores were preserved with spherical morphologies because of overpressure-associated overburden stress unloading in the gentle anticlines, whereas OM-hosted pores in the open anticlines are characterized by high aspect ratios and are regularly aligned perpendicular to lateral compression stress. A simple simulation confirmed that tectonic folding and compaction during exhumation caused the loss of porosity in the inner arcs of the open anticlines. Only pressure shadows formed by assemblages of clay mineral platelets and mechanically competent grains inhibited tectonic compaction of some OM-hosted pores in the open anticlines. Tectonic compaction of porosity generally increased the mean value of water saturation in the open anticlines. The extremely high water saturation (>60%) in the high-TOC interval (>2 wt. %) was generally associated with natural fractures, suggesting the occurrence of gas leakage.