Sedimentary pattern and exploration significance of Permian reefs and shoals around the Wusheng-Shizhu intra-platform depression, eastern Sichuan Basin, China

摘要

There exists the Wusheng-Shizhu and Chongqing-Qijiang intra-platform depressions in the eastern Sichuan Basin during the Late Permian to the Early Triassic. The clear understanding of the sedimentary pattern and evolution of reef-shoal around the depression-margin has a practical significance for guiding the exploration and development of the Permian–Triassic reef and shoal gas fields. Based on observation, the systematic sampling and laboratory analysis of typical outcrops of the Changxing Formation (i.e. Juandong, Juhuaba, Lengshuixi, and Taiyuan) are located in the eastern Sichuan Basin. Combined with drilling data and previous studies, the study area is dominated by a ramp sedimentary environment, except for the Kaijiang-Liangping Trough in the Early Changxingian. The main sediments are micrite-containing chert nodules and micrite. With the regression that took place during the Middle-Late Changxingian, there is a distinct sedimentary differentiation that began to appear. There is a “two depressions–one platform–one trough” sedimentary pattern from south to north in the study area in the Changxingian. In essence, there exists the Chongqing-Qijiang and the Wusheng-Shizhu intra-platform depressions (two depressions), platform-flat (one platform) between the Wusheng-Shizhu intra-platform depression and the Kaijiang-Liangping Trough, and the Kaijiang-Liangping Trough (one trough). The reef-shoal of the northern depression-margin distributes between the 2nd Member and lower part of the 3rd Member. By comparison, the reef-shoal of the southern depression-margin distributes in the upper part of the 3rd Member; this indicates that the deposition of reef-shoal shows “earlier in the north and later in the south.” It is considered that the sedimentary palaeogeomorphology of the Changxingian is characterized by “higher in the north-lower in the south.” The clear understanding of the sedimentary pattern and evolution of reef-shoal around depression-margin reveal a new field for exploration and development of the Permian Changxing reef-shoal gas fields in the eastern Sichuan Basin.