Petroleum charging history of Neogene reservoir in the Baiyun Sag, Pearl River Mouth Basin, South China sea

摘要

Recent years, numerous condensate and oil pools are found in the northern and eastern parts of Baiyun Sag in the Pearl River Mouth Basin (PRMB) South China Sea. However, their origin and filling history have not been determined yet. Firstly, this study investigated the oils' geochemical characteristics and determine the origin of the oils in different regions of the Baiyun Sag. Oil-oil correlation demonstrates that oils in the Panyu Low Uplift (PLU) and the Eastern Structural Belts (ESB) of the sag belong to two different oil groups. The condensates in the PLU were generated by the coal bearing source bed of the Eocene Enping (E(2)e) Formation in the Main Sub-sag (MSS), while ESB oils originated from the shallow lacustrine/deltaic source rocks in the E(2)e Formation. Secondly, based on fluid inclusion analysis and 1-D basin simulation, oil filling history of two wells from PLU and ESB were clarified, respectively. One episode of oil charging occurred between 6 Ma and 0 Ma in the Neogene Zhujiang Formation (N(1)zh) of Well P30 in the PLU. The charging of the condensate in the N(1)zh reservoirs at Well H34 mainly occurred during 6.5 Ma and 4 Ma, which is 5Ma-10 Ma later than that of the light oil in the ESB. However, according to the simulation of the hydrocarbon generation history, both light oils and condensates in the ESB were generated during the mature stage of the source rocks at about 20 Ma to 8 Ma. Thus, the difference between the charging times in the ESB implies that a special accumulation mechanism may occur in the ESB. Moreover, charging of condensates in the PLU and ESB occurred at late Miocene to Pliocene, which has a good correlation with the Dongsha Event. This characteristics provides an evidence for that Dongsha Event may play a critical role in the formation of effective migration conduit in the Baiyun Sag.