The Distribution Controlling Factors of Reef and Shoal, and Their Favorable Plays Analysis: Changxing (Permian)- Feixianguan Formations (Triassic), Sichuan Basin

Reef and shoal gas reservoirs in the Permian-Triassic Changxing-Feixianguan formations, Sichuan Basin, are typical represent ative of the marine carbonatite gas reservoirs in china. Breakthroughs in exploration make it necessary to know the distribution controlling factors of reef and shoal. Based on the comparative analysis of various well data and 3D seismic, the impact of various factors is studied on the distribution of reef and oolitic shoal, such as the geometry of the Kaijiang-Liangping trough, Late Permian regional palaeogeomorphologic framework, tectonic extension, system tracts of the Longtan-Feixianguan formations. The following conclusions were drawn. (1) At the end of Late Permian, the palaeogeomorphologic framework of the Sichuan Basin was "three depressions separated by three uplifts" which played a key role in controlling the distribution of reefs and ool itic shoals. (2) Transgressive systems tract (TST) deposits dominated the Longtan-early Changxing period, mid-late Changxin g period has a highstand system tract (HST) deposits feature. Highstand system tract (HST) deposits dominated Feixianguan period. The change of system tracts made it possible to develop multistage reefs near the platform margin. Oolitic shoals may occur on the progradation slope margin in the Kaijiang-Liangping trough, while the aggradational platform margin is the most favorable development zone of Oolitic shoals. (3) The unevenness of extension caused the differences in the development of platform margin palaeogeomorphologic, which influenced the development time, continuity and their superimposed relationship of reef and oolitic shoal. (4) The methods, based on 3D seismic, are studied for characterizing the bioherms distribution along the platform margin. The distribution and shape of Bioherms along the platform margin can occur as curved belt, moniliform and atoll, or occur as echelon belt oblique to the platform margin. Favorable Plays are analyzed based on the study results.