Caprock Type and Sealing Mechanism of Quaternary Ultra Shallow Large Gas Reservoir in Deep Water Area of Qiongdongnan Basin, China

Caprock plays an important role in determining whether hydrocarbon accumulation is available. Previous studies of caprock mainly focus on the sealing ability of medium-deep consolidated rock, while less attention is paid to the shallow unconsolidated caprock, and the evaluation of caprock of shallow buried formations in deep water area is rare. Based on the research results of seismic interpretation, uniaxial stress test, logging evaluation, and cone penetration test in the deep water area of Qiongdongnan Basin, the development type and sealing ability of the caprock for the gas reservoir of the Ledong Formation in the L36 area are studied in this paper. The results show that: (1) There are three types of caprock (deep-sea mud, mass transport deposits, and hydrate-bearing strata) in the deep water area of the Ledong Formation, which could all have the effective sealing effect. (2) Based on the seismic and geological interpretation and experimental results, it is clear that the sealing capacity increases in the order of deep-sea mud, mass transport deposits, and hydrate-bearing strata; and the deep-sea mud can seal 59 m hydrocarbon at 200 m below seafloor, which is the threshold of sealing ability for the main target layer in the study area. (3) The capillary sealing and hydraulic sealing plays a dominant role in the sealing mechanism of shallow buried caprock in the deep water area, under low temperature and high pressure environment, the saturated salt water sealing mechanism plays a characteristic role in increasing sealing capacity. The above results have been verified by drilling in the past two years, which not only provides a theoretical basis for the follow-up exploration and development of the Ledong Formation in Qiongdongnan Basin, but also can provide a useful reference for the natural gas exploration in other shallow buried areas in the deep water area.