Discovery of Yanting-Tongnan trough of late Permian in Sichuan basin and its significance

Abstract

Reef-bank of Sichuan (四川) basin have been explored along the platform margin belt on both sides of the Kaijiang (开江)-Liangping (梁平) trough for a long time. Correspondingly, a series of major breakthroughs have been obtained and a number of large oil and gas fields have been found. After the breakthroughs of Kaijiang-Liangping trough, how reef-bank exploration will proceed? Is there another “Kaijiang-Liangping trough” in Sichuan basin? Previous research shows that Sichuan basin underwent intracratonic rift and basement fault activities in Late Permian, developing a paleogeography framework of “three uplifts and three depressions” under the background of the regional large ramp, including Exi (鄂西)-Chengkou (城口) trough, Kaijiang-Liangping trough and Yanting (盐 亭)-Tongnan (潼南) trough. Based on sedimentology and tectonics dynamics analysis, data of drillings, outcrops and seismic, Yanting-Tongnan trough has been discovered in central Sichuan basin in our study which distributes from Mianzhu (绵竹), Jiangyou (江油) in western Sichuan basin through Nanchong (南充) to Guang’an (广安), as long as about 210 km, extending in NW-SE nearly paralleling to Kaijiang-Liangping trough, opening to the sea north-westward, and gradually pinchout to the east platform. Similar to Kaijiang-Liangping trough, another platform margin reef belt distributes along a U-shape belt around Yanting-Tongnan trough, which is expected to become another natural gas accumulation zone after the platform margin around Kaijiang-Liangping trough. The discovery of Yanting-Tongnan trough expands favorable reef exploration area from original 2×104 km2 on both sides of Kaijiang-Liangping trough to more than 6×104 km2 in Yanting-Tongnan trough platform margin belt and Kaijiang-Liangping platform margin belt, greatly enlarging the reef exploration field. But compared to Kaijiang-Liangping trough, Yanting-Tongnan trough is relatively shallower and smaller, and the scale and quality of reef-bank reservoirs in its margin belt is a little poor relatively. Therefore, much research should be done timely such as comparative study of two troughs, detailed research of formation and evolution of the trough, regularity of reef migration, meticulous depiction of reef-bank reservoir size and distribution in order to obtain major breakthroughs around the Yanting-Tongnan trough.