Geologic Patterns of Internal Architecture in Reef-shoal Complexes along Kaijiang-Liangping Trough in Sichuan Basin during Late Permian Changxingian

Xu, Anna; Wang, Zecheng; Zhai, Xiufen; Yin, Jifeng; Gu, Zhidong; Li, Qiufen; Bao, Dongmei

During late Permian Changxingian, the depositional pattern of Sichuan Basin is notably controlled by Kaijiang-Liangping trough. A series of reef-shoal complexes developed along the carbonate platform margin, with strip-like, oblique-arranged, or bead-like distribution patterns. The reef-shoal complexes have complicated internal structures, and their scale, accretion pattern, and distribution of reservoir rocks are significantly controlled by the ancient landform of the "Kaijiang-Liangping trough". Based on the observation and sedimentary description of outcrops and cores, analysis of drilling profiles, depositional interpretation of geophysical logging data and seismic reflection modes, and information from cast slices and SEM analysis, the ancient landform of the "Kaijiang-Liangping trough" during the Changshingian can be divided into two major categories, fault-controlled steep slope and ramp. And the corresponding internal geologic patterns of reef-shoal development, which are fault-controlled steep slope type and ramp type, are proposed in this article.The former type, fault-controlled steep type, is mainly developed close to present reverse faults or high and steep ancient tectonic breaks. Reefs, mounds, grain shoals, and deposition of inter-shoal marine facies are developed, with organic reefs being the main body. Vertically, multicycle of grain shoal, mound, and organic reef/bioherm developed in the profile, with very thick reefs. Vertical accretion is common, while lateral accretion is normally absent. The shape of the complexes is ridge-like, with great thickness but limited width. The seismic reflection mode is the ridge type, with drapes on the top and onlaps on both sides, and interior chaotic or blank reflections. Favorable reservoirs are mainly reefal dolostones and dolostones of back-reef shallow shoals, which usually appear at the top of each complex cycle. The ramp type is mainly developed in the hinge zone or local highlands of large gentle ancient slopes. Reefs, mounds, grain shoals, and inter-shoal marine facies of deposition are also developed, but the grain shoals are the main body. Multicycle of grain shoal, mound, and organic reef/bioherm are also developed vertically, but the thickness of reefs is much smaller, and there is much more lateral accretion than vertical accretion. The shape is wider and more gentle, with relatively a small total thickness. In the seismic reflection modes, its shape is like very gentle and low hills, without any drapes or onlaps on the top or flanks, or internal chaotic reflections, and parallel seismic reflection configuration is common. The best reservoirs are dolostones of grain shoal facies, usually in the mid-upper parts of each complex cycle.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013