Characterization and origin of micropores in tight gas grainstones of the Lower Triassic Feixianguan Formation in the Jiannan gas field, Sichuan Basin

Abstract

Economic importance of micropores with diameter less than 10 μm in petroleum reservoirs has attracted more and more attention in hydrocarbon exploration because of a significant influence on evaluation and production of hydrocarbons. The Lower Triassic Feixianguan Formation of the Jiannan gas field in the Sichuan basin has been gas productive over the last 40 years, producing a cumulative total of nearly two billion cubic meters of hydrocarbon gases from tight grainstone reservoirs. However, a complex problem is that there are nearly no visible pores in thin sections with a petrographic microscope, leading to the poorly understanding origin and diagenetic evolution of the pore network in the Feixianguan limestones. The studied reservoirs consist mainly of grainstones and have helium porosity less than 4% with an average of 1.3%. Observation from approximately 410 thin sections demonstrated that all primary interparticle macropores (diameter >10 μm) were totally destroyed by compaction and cementation, and minor amounts of secondary intraparticle macropores (macroporosity ≤0.5%) were visible only in five samples, occurring mainly in the upper parts of depositional cycles. The greater helium porosity than thin-section macroporosity suggests the porosity consists dominantly of micropores. Mercury injection capillary pressure, scanning-electron microscopy, and micro-CT analyses revealed further that micropores are mainly hosted within grains, and contribute more than 95% to total pore systems. The development of micropores in the Feixianguan grainstones is closely associated with the freshwater-involved mineralogical stabilization of aragonitic grains due to subaerial exposure shortly after deposition. This stabilization involves the dissolution of aragonite and re-precipitation of microrhombic calcites, accompanied by the conversion of micropores from between aragonite needles to between calcite microcrystals. The Feixianguan grainstones in the Jiannan gas field are a typical micropore-dominated reservoir for gas storage and production, which provides an excellent analog for other tight marine limestone reservoirs worldwide.