Sequence and sedimentary features of the Changxing Fm organic reefs and their control on reservoir development in the Yuanba Gas Field, Sichuan Basin

In the Yuanba area, Sichuan Basin, the gas reservoirs in the Upper Permian Changxing Fm are now at the development stage. With the smooth progress of development, it is urgent to characterize the reservoir architectures accurately and summarize the controlling factors for reservoir development. In this paper, research was mainly performed on the Changxing Fm organic reefs in terms of their sequence stratigraphy, sedimentary facies, and reservoir characteristics and architectures based on core observation and thin section analysis, combined with physical property data and logging curves analysis results. It is shown that the Changing Fm can be divided into two third-order sequences and six fourth-order sequences, their electric logs are characterized by abrupt change above and below the high-frequency sequence boundary and are consistent with the sedimentary cycles controlled by high-frequency sequences. Besides, the Changxing Fm organic reefs mainly represents zonal distribution outside SQ2 platform margin, and they are vertically composed of two obvious two reef sedimentary cycles and laterally developed in asymmetric patterns (early in the east and late in the west). Finally, in general, organic reef (bank.) reservoirs are mainly composed of low-porosity and moderate-low-permeability dissolved dolomite reservoirs, and they are mostly distributed at reef caps in the upper-middle parts of the two fourth-order sequences, with the characteristics of multiple beds, thin single beds, different types of reservoirs with different thickness interbedded with each other, strong heterogeneity and double-layer reservoir architectures. It is concluded that the distribution of organic reef microfacies in this area is controlled by high-frequency sequence, which is the key controlling factor for reservoir development and spatial distribution.