Characteristics and diagenesis of platform margin reef-shoal reservoirs of Upper Permian Changxing Formation around Kaijiang-Liangping Trough, eastern Sichuan Basin

In this paper, the authors utilized cores and thin sections observations, SEM, cathodoluminescence and physical property analysis to study characteristics and diagenesis of the reservoir within Upper Permian Changxing Formation, and preliminarily established diagenetic evolution sequence. The interval is mainly featured by platform margin reef-shoal around KaijiangLiangping trough in eastern Sichuan Basin. Reservoir characteristics reveal that the Changxing reservoirs are developed well. Reservoir rocks are composed of"piebald"bioclastic dolomite and reef dolomite dominate, whereas reservoir spaces mainly comprise intergranular pores, dissolved intergranular pores and dissolved intragranular pores. The reservoir porosity is mainly between 2% and 6%, and the permeability is between 0.1 mD and 1 mD. Four types of reservoirs were found, i.e., "low-porosity and high-permeability" type, "medium-porosity and medium-permeability" type, transitional type and "high-porosity and lowpermeability" type. The diagenetic evolution indicates that Changxing Formation experienced cementation, compaction, dolomitization, dissolution and tectonic fracturing. A decreasing trend in porosity due to the superimposed modification by multiplephase diagenesis was observed. Cementation and compaction occupied most of primary pores and reduced the porosity after the deposition. Karstification and dolomitization improved the porosity by transforming the textures of pores. Tectonic fractures further optimized the reservoir. Research shows that primary intergranular pores and eogenetic secondary pores formed by eogenetic karstification and hydrothermal dolomitization not only serve as the prototype of reservoir space but also generally determine the final appearance of the reservoir.