Proven Deepwater Play and Exploration Potential in Qiongdongnan Basin, North South China Sea

Sun, Zhipeng; Guo, Minggang; Yao, Zhe

Qiongdongnan Basin is the deepwater exploration frontier with few wells in the north South China Sea. The passive margin basin underwent Paleogene rift episode and Neogene post-rift episode, with the development of shelf-slope sedimentary system since Middle Miocene.

One gas discovery was made in the west of the Central Canyon, proving that an effective petroleum system exists in Lingshui Subbasin.. The successful play type comprises late Miocene thick turbidite channel reservoirs, sealed by a hemipelagic mudstones caprock. The most prospective traps in the canyon are anticlines, three-way dipping anticlines against the canyon wall, associated with individual turbidite lobes.

Through the Carbon isotope analysis of gas, the effective source rocks were lower Oligocene coaly mudstones, common to the several gas discoveries in the shallow water. The organic matters were rich in Type III kerogen. The Eocene lacustrine mudstones were not penetrated and only predicted from seismic data. The hydrocarbon migration was primarily vertical through mini-faults pathways into the canyon. There was overpressure in the deep Lingshui Subbasin in lower and upper Oligocene, favorable to the hydrocarbon charging. The hydrocarbon generation and dynamic analysis indicated that the charging time were mainly 3 million years B.C

The other deepwater plays in upper Oligocene and Lower Miocene remain potential in Baodao Subbasin and Changchang Subbasin. Turbidite channel complex and fans existed in these subbasins, with the provenances of north uplift or south uplift. Though relatively thin layers of sandstones with limited area were penetrated, many sandbodies and reservoirs were interpreted and distributed mainly in the southern slope. The traps are primarily faulted-anticlines and structural-lithological traps. Developed faulting in late Oligocene and early Miocene controlled the trapping formation, and could be the hydrocarbon migration pathway, though there was some risk for sealing for some later faults.

One main challenge for exploration is the limited dimension of separated turbidite lobes in the Central Canyon, which need more wells to meet to economically joint development of the existing discovery. Another challenge is the reservoir quality of the new play of upper Oligocene and lower Miocene. Reservoir distribution and quality would also affect the lateral hydrocarbon migration and accumulation.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013