Porosity In Microbial Carbonate Reservoirs in the Middle Triassic Leikoupo Formation (Anisian Stage), Sichuan Basin, China

Abstract

Abstract Microbial carbonates developed in the Middle Triassic (Leikoupo Formation, Anisian Stage) of the western Sichuan Basin. The microbial components have been identified and include Renaclis-resembling, Rivularia lissaviensis, Carpathocodium anae, Hedstroemia moldavica, Bacinellacodium calcareus, and Paraortonella getica. These form stromatolitic, laminitic, thrombolitic, spongiostromata stones, dendrolites, and oncolitic structures. Microbial carbonate reservoirs occur in submember unit (SMU) 3–3 in the Zhongba area of the northern segment and SMU 4–3 in the middle segment of the western Sichuan Basin, both of which are of low porosity and permeability. Core descriptions and thin-section analysis show that reservoir porosity is mostly microbial coelom pores, framework pores, fenestral pores, and inter- and intraclot dissolved pores, within which the pores of ≥200 μm in diameter and throat of (40~50) μm are the most important. The SMU 4–3 microbial carbonate reservoirs are more thoroughly studied because of recent exploration activities, including the identification of three reservoir intervals. The middle reservoir interval, composed of thrombolitic and stromatolitic dolostone, hosts the reservoir of best quality. However, this high-quality interval loses effective porosity and thins to the northeast. It is proposed that extreme geological conditions, dolomitization, and burial dissolution influenced the development and distribution of the microbial carbonate reservoirs. The dolomitization process is thought to be penecontemporaneous to very early postdeposition. This early dolomitization contributed significantly to porosity of the microbial carbonate reservoirs and was likely enhanced through burial dissolution.