Formation and preservation of pores in deep limestone reservoirs: A case study of Upper Permian Changxing Formation, central Sichuan Basin, SW China

Abstract

In recent years, drilling data from wells Pengshen 10, Heshen 9, Tongshen 17 and Zhengyang 1 in the Sichuan Basin have confirmed the presence of a set of porous reef-beach limestone reservoirs in the Upper Permian Changxing Formation, which breaks the traditional view that deep carbonate oil and gas are only distributed in porous dolomite reservoirs and karst fracture-cavity limestone reservoirs. Through core and thin section observations, geochemical analysis, and well-seismic based reservoir identification and tracking, the study on formation mechanism of pores in deep reef-beach limestone reservoirs is carried out, this study provides insights in four aspects. (1) Porous reef-beach limestone reservoirs are developed in the Changxing Formation in deep-buried layers. The reservoir space is composed of intergranular pores, framework pores, intrafossil pores, moldic pores and dissolution pores, which are formed in depositional and epigenetic environments. (2) The intermittently distributed porous reef-beach complexes are surrounded by relatively dense micrite limestone, which leads to the formation of local abnormal high-pressure inside the reef-beach complexes with the temperature increased. (3) The floor of the Changxing Formation reservoir is composed with interbedded tight mudstone and limestone of the Upper Permian Wujiaping Formation, and the roof is the tight micrite limestone interbedded with mudstone of the first member of Lower Triassic Feixianguan Formation. Under the clamping of dense roof and floor, the abnormal high-pressure in the Changxing Formation is formed. Abnormal high-pressure (overpressured compartment) is the key to maintain the pores formed in the depositional and epigenetic environments in deep-buried layers. (4) Based on the identification of roof, floor and reef-beach complexes, the favorable reef-beach limestone reservoir distribution area of 10.3×10⁴ km² is predicted by well-seismic integration. These insights lay the theoretical foundation for the development of deep porous limestone reservoirs, expand the new field of exploration of deep-buried limestone reservoirs in the Sichuan Basin.