Stratigraphic architecture and platform evolution of the Changxing Formation (Upper Permian) in the Yuanba Gas Field, northeastern Sichuan Basin, China

During the late Permian, the Yuanba area of the northeastern Sichuan Basin, China, was predominantly occupied by a carbonate platform in an extensional setting. On the basis of wireline log, core sample, and cutting fragment, two depositional systems are recognized in the Changxing Formation, including a homoclinal ramp and a rimmed carbonate platform system. During deposition, the platform was evolved from a homoclinal ramp through a poorly rimmed platform into a rimmed platform. High-energy deposits of shoals and reefs preferentially formed on the pre-existing palaeohighs and grew upward to develop an aggradational architecture. Two depositional (third-order) sequences, consisting of several parasequences, are recognized in the Changxing Formation, the development of which was mainly controlled by eustatic fluctuations, with minor influence of tectonism. Individual sequences were defined by conformable surfaces along the depositional dip section. The distribution of facies and parasequence composition are variable both laterally across the platform-basin profile and vertically through time due to a combination of relative sea-level changes, palaeogeographic architecture, and tectonism. The sedimentological and stratigraphic characterization will aid further future exploration and production of hydrocarbon from the Changxing Formation.