Main Controlling Factors on the High-Temperature and High-Pressure Gas Reservoir in Miocene Huangliu Formation in Dongfang Area, Yinggehai Basin, South China Sea

Yinggehai basin, an import petroliferous Cenozoic transtentional basin in the Northwestern South China Sea, is characterized by very thick Neogene sediments, extensive development of high temperature and high pressure (HTHP), and strong mud diaper activities. The hydrocarbon accumulation in middle-deep HTHP area has always been one of the main challenges in this basin. With the recent commercial discovery of Dongfang13-1 in Upper Miocene Huangliu Formation in the central depression zone, the forming mechanism of middle-deep HTHP gas reservoir gets more and more attention.

Dongfang13-1 is a pinch-out lithologic gas reservoir. Based on the integrated analysis of petroleum system, main controlling factors of hydrocarbon accumulation in this area are summarized: (1) high-quality sandstone reservoir. The main reservoirs of Dongfang13-1 area consist mainly of fine and very fine sandstones, which belong to submarine fan deposits in the Lowstand system tract of the first member of Huangliu Formation. These large-scale overlapping sandbodies of submarine fans are distributed in the western and southwestern area of the famous Dongfang1-1 gas field with relatively large thickness (e.g., 87 meters in Well DF13-1-1) and relatively good reservoir quality. These sandbodies vary in size and sedimentary micro-facies, and dominantly controlled by the provenance supply from large rivers of Vietnam and Yingxi syn-depositional fault in the west of the basin, (2) good vertical migration pathway. The natural gas migration in Dongfang area is closely related to diapir activities of central depression zone. Episodic diapir activities lead to the generation of plenty of faults and fractures, which provide the main upward migration pathway for natural gas from deep source rocks of underlying Meishan and Sanya Formations, and (3) excellent overpressured cap rock. Higher sealing ability of cap rock is needed in HTHP hydrocarbon reservoir. The drilling results show that in Dongfang area a set of mudstone cap rock of transgressive and highstand system tracts in upper Huangliu Formation is developed with large thickness (e.g., about 200 meters), relatively wide distribution, and remarkable overpressure (e.g., the pressure coefficient is from 1.5 to 1.8).

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California