Development characteristics, distribution patterns and favorable exploration zones of Permian reef shoals in Sichuan Basin

The exploration of carbonate rocks around the Permian Kaijiang-Liangping trough in the Sichuan Basin has achieved great success, but exploration has entered a bottleneck period in the past decade. In order to further deepen the understanding of Permian reef shoals and achieve new breakthroughs in large-scale exploration, based on drilling, logging, seismic, and geological data, a systematic study has been carried out on the Permian tectonic-sedimentary evolution characteristics and the distribution patterns of Permian reef shoals, and the favorable zones for further reef shoals exploration have been identified. Research shows that: (1) The shoal in the Qixia Formation tableland is controlled by the late Caledonian ladder-like annular erosion, featuring a ring-like distribution in the early, middle, and late stages. The Jiulong Mountain–Longgang region in northern Sichuan and the Yibin–Neijiang region in southern Sichuan have good conditions for the inner shoal formation of the ring-like tableland in the late stage of the second member of the Qixia Formation, with a low degree of exploration, needing further exploration in the next step. (2) The second stage of the Maokou Formation is the key period for the transformation of the sedimentary pattern of the Permian system. The sedimentation of the lower sub-member of the Maokou Formation was controlled by the residual landform of the Caledonian paleo-uplift, thus, the inner shoal is distributed in a ring belt. The ancient landform of the area to the upper sub-member of the Maokou Formation was affected by the uplift of the Emei mantle plume, so, the favorable sedimentary facies transformed into a "strip-like" distribution pattern trending northwest-southeast. The platform margin shoal of the upper sub-member of the Maokou Formation in the north slope region of Moxi is a major successor area of the Maokou Formation. (3) The platform margin reef shoals of the Changxing Formation migrated into the Kaijiang-Liangping trough to form multiple rows of reef shoals, which have superior source and reservoir distribution conditions, being a potential exploration area of the Permian system.