Study On The Oil And Gas Resource And Accumulations Of The Paleogene Wenchang And Enping Formations In The Huizhou Sag, Pearl River Mouth Basin

The Huizhou Sag lies in Zhu I depression, which is in the east of the Pear River Mouth Basin. The recent explorations show that the Paleogene Wenchang-Enping formations is a promising target of oil and gas in Huizhou Sag. However, since the bed of interest is deeply buried and scarcely drilled, the rules of the Wenchang and Enping formations accumulatios in Huizhou sag were studied scarcly by previous researches. In this paper, with the latest data, the source rock burial evolution, thermal evolution, hydrocarbon generation simulation and the conditions of hydrocarbon accumulations were analyzed to study the potential of hydrocarbon generation and discuss the accumulation of the Wenchang and Enping formations.Using the balance cross-section technique, the structural frame of the deposition of Wenchang formation was recovered, and Huilu low uplift was found to be uplifted after the deposition of Wenchang formation. With the palaeontological and geochemical datas, we got to know that the Huizhou sag was a whole lacustrine basin when the Wenchang formation SQ2 sequence were deposited, and Huilu low uplift and Huixi half-graben were mid-deep and deep lake facies deposition. On the basis of new understanging of Wenchang and Enping formations’sendimentary facies, the source rock distribution was re-recongnized. The source rock of Wenchang formation were mainly developed in SQ2 sequence, there also contain source rock in the SQ5 and SQ6 sequence of Enping formation. The original organic carbon recovery coefficients of Wenchang-Enping formations were established by thermal decomposition modeling.The geologic evolution of Huizhou sag is consistent with the regional structural evolution through the analysis of structural and geologic evolution. From the deposition of Eocene sequence, the geologic evolution is devided into four stages, the rapid-subsidence stage of early-mid Eocene, the late Eocene-early Oligocene, the continuous-subsidence stage during late Oligocene-mid Miocene and the tectonic rapid-subsidence stage from late Miocene to present day. The source rock laid in high location of Huizhou sag became mature in the continuous-subsidence stage and generated hydrocarbon in structural rapid-subsidence stage. While in the central of sag, the source rock became mature at the end of rapid-subsidence in late Eocene-early Oligocene stage and generated hydrocarbon in continuous-subsidence of late Oligocene-mid Miocene.With the basin modeling technique, the amount of hydrocarbon generating was calculated from the Paleocene Wenchang-Enping formations in Huizhou sag. The most important source rock is Wenchang formation SQ2 sequence, whose hydrocarbon generation amounts account for 74.095%. The dominant hydrocarbon generation stage was Upper Zhujiang-Wanshan, and the secongdary stage was Zhuhai- Lower Zhujiang.There exist excellent source-reservoir-seal associations within Wenchang- Enping formations in the Huizhou sag. The Zhuhai - Lower Zhujiang (30-18.5Ma) and late Miocene-present day (10.2Ma-present) are two critical periods for accumulations of Wenchang and Enping formations. The patterns of Wenchang and Enping formation accumulations are defined into four types. With the combination of the generation, structure and trap conditions, the favourable traps were delineated, which is useful to the next exploration.