Microbiological and micritic-calcisiltic dolomites have been regarded as the dominant reservoir lithology in the upper section of the 4th member of the Leikoupo Formation in Pengzhou gas field. Multiple analytical and statistical methods are applied to study the pore types, physical properties, and pore structure parameters, as well as their impact on lithofacies (especially for microbialite) and crystal size of the dolomites. Besides, four types of reservoirs and the tight zones (interlayers) are identified. The main factors controlling pore formation and distribution are hereby proposed. The results show that dolomites with various microbial textures account for nearly 3/4 of the total formation, most of which are silty-micritic and calcisiltic dolomites. In a high-frequency cycle, the dolomite textures vary upwards from algal clot, to algal spheroid (or algal arene), to algal lamina, and then to laminated silty types. In terms of grain size, a cycle is composed regularly of grained, very fine-(fine-) crystal, micritic, and silty dolomites from the bottom to the top. A dry, hot and alkaline sedimentary environment of hypersalinity prevailed during the Middle Triassic, favorable for the development of microbialites. The meter-scale shallowing-upward cycles of thin (algal) laminae, that is, from the thrombolites and stromatolites in the upper part of the subtidal zone to the intertidal algal laminae-stromatolites, and then to the thin (algal) laminae in the supratidal zone, caused the well development of zones of high porosity and permeability in the upper and middle parts of the sequence. In addition, factors including rapid seepage reflux dolomitization and meteoric freshwater dissolution, early hydrocarbon charging in the shallow burial stage, and multi-stage tectonic fracturing, all play an important role in pore development and preservation.
