Characteristics and formation mechanism of Changxing Formation-Feixianguan Formation reef-shoal reservoirs in Yuanba Gasfield

Taking advantage of the successful experience in exploring and discovering the Puguang Gasfield, and targeting the Changxing Formation-Feixianguan Formation organic reef and shoal lithological trap, SINOPEC drilled Well Yuanba 1 in the Bazhong area in Northeast Sichuan in 2006, and discovered the Yuanba Gasfield with a high-production commercial gas flow of 503×103 m3/d. As a normal-pressured lithological gas reservoir with high H2S content, Yuanba Gasfield is characterized by weak tectonic deformation and deep burial, with an average depth of 6,600 m in the central part of the gas reservoir, and is the deepest marine gas field in the Sichuan Basin. Yuanba Gasfield is dominated by large-scale reef-shoal reservoirs of Changxing Formation. The formation of the reservoirs was primarily controlled by early meteoric freshwater dissolution and dolomitization, and less affected by deep-burial dissolution and tectonic movement. A comparative analysis was made on the characteristics of deep reef-shoal reservoirs in the Yuanba and Puguang gas fields so as to explore their formation mechanisms. It is concluded that the reservoir size and early pore development was controlled by early depositional-diagenetic environment. Fracture formation and dissolution were controlled by structure-fluid coupling, pore reworking and preservation is determined by fluid-rock interaction.