The Fuling Shale Gas Field — A highly productive Silurian gas shale with high thermal maturity and complex evolution history, southeastern Sichuan Basin, China

Abstract

The organic-rich Lower Silurian Longmaxi and Upper Ordovician Wufeng Shale (LSLUOWS) is one of the most important marine shale gas plays in southern China, with relatively high thermal maturity and complex structural evolution. The delineation of the Jiaoshiba shale gas play has been highly successful for the LSLUOWS in the Fuling area of southeastern Sichuan Basin. The drilling targets the basal part of the LSLUOWS, where the approximately 38–45 m of organic-rich (total organic carbon of greater than 2%) section corresponds to a maturity range around 2.2%–3.0% Ro, with high brittle mineral content (55%–65%). The produced shale gas displays a clear reversal in the stable carbon isotopes. The shale gas play zone is between the overlain mudstone in the Middle-Upper Longmaxi Formation (Fm) and the underlain Upper Ordovician tight limestone in the Jiancaogou Fm. The shale gas reservoir is overpressured, with an average pressure coefficient of 1.55 (relative to hydrostatic pressure). From December 2013 to late 2014, results of production tests determined relatively stable pressure and production curves. Fifty prolific gas wells have been completed to date in the Jiaoshiba Shale Gas Field in Fuling. Preliminary study revealed several characteristics of this structurally complex shale play that are distinctly different from those in the United States, including an anticlinal structure with mild deformation, abundant free gas, and very short gas migration through microfractures, the natural fracture network formed by the two fault systems, and the slip parallel to the layer in the basal LSLUOWS. Movement along strike-slip faults has gone through up into the Quaternary, which is considered to be a significant factor in the establishment and preservation of the overpressured region in the LSLUOWS.