Petroleum geology of Yacheng 13-1, the largest gas field in Chinaʹs offshore region

Yacheng 13-1 is the largest gas field in Chinaʹs offshore region, with proven initial in place gas and condensate reserves around 98.2 billion cubic meters (bcm) and 3.74 million cubic meters (mcm), respectively. The gas and condensate in this field are trapped in a composite structure developed along the eastern flank of the Yacheng uplift, with unconformity-truncated, fault, partly dip-controlled closure. Favorable geological conditions leading to the formation of this field include (1) abundant gas supply from multiple source rocks from both the coal-bearing Yacheng Formation in the Yanan Sag of the Qiongdongnan Basin and the more mature and probably younger strata in the Yinggehai Basin; (2) large net gas pays in vertically stacked high-quality sandstone reservoirs in the Oligocene–Miocene Yacheng, Lingshui, and Sanya formations; (3) early formation of a composite structural–stratigraphic trap along the eastern flank of the Yacheng uplift, with unconformity-truncated, fault, partly dip-controlled closure; (4) relatively late in gas generation and migration, with most of the gas accumulation probably occurring in the past few million years; and (5) good preservation conditions resulting from the high-pressure top seal for the normally pressured reservoirs.