Oil-Bearing Basins Offshore China: A Paleolimnological Perspective

Based on integration of tectonic, sedimentological, micropaleontological and organicgeochemical data, this paper is concerned with the geological and geographic background of theTertiary oil-bearing basins offshore China from paleolimnological aspect and the accumulationand preservation of sedimentary organic matter and the hydrocarbon potential in the lacustrinesediments. The formation, development and evolution of the Paleogene paleolakes in theoil-bearing basins offshore China are comprehensively studied,and two types of paleolakes aredistinguished. The characteristics of the mudstones, microfossils and sedimentary organic matterfrom the sediments in the two kinds of the paleolakes are analyzed. The development periods ofthe paleolakes favourable to the accumulation and preservation of sedimentary organic matter aredetermined according to the relative abundances of the phytoplanktonic fossils and amorphousorganic matter (AOM). The distribution of excellent source-rocks in the oil-bearing basinsoffshore China is discussed. In order to investigate the distribution of excellent source-rocks, thePaleogene organic facies in the Bozhong Sag and adjacent areas are analyzed by combination ofpaleolimnological and organic geochemical analysis methods.Great changes in the topography of China occurred during the Mesozoic and Cenozoic. Themainland of China inclined to the west slightly before the Neogene, whereas that inclined to theeast intensely during the Neogene. During the transition period of the Paleogene, a lot of largelakes, including Bohai Bay paleolakes and Pearl River Mouth paleolakes, were formed in the eastof China due to extensional rifts. Both of Bohai Bay paleolakes and Pearl River Mouth paleolakesunderwent different evolution. The former is polycycle lake basin, while the latter is monocyclelakebasin.The Bohai Bay paleolakes are closed saline-water lakes, whereas the Pearl River Mouthpaleolakes are open fresh-water lakes. There are obvious differences in the characteristics offossils and geochemical composition of the sediments between Bohai Bay paleolakes and PearlRiver Mouth paleolakes. The calcareous fossils such as ostracod are sparse in the sediments fromthe fresh-water lakes. The fresh water green algae, such as Pediastrum and Botryococcus, aredominant in the phytoplanktonic fossil assemblages. The calcareous fossils such as ostracod andgastropod are common in the sediments from the solt water lakes. The diversity ofphytoplanktonic fossil is relatively high and the phytoplanktonic fossil assemblages arecharacterized by the higher percentage value of the fresh water dinoflagellate cysts. The differencebetween the open fresh-water lakes and closed saline-water lakes are recognized through O and Cisotopes.δ¹³C values of the calcareous fossil tests from the fresh-water lakes are depleted,whereas those from the saline-water lakes are rich. Moreover, the change ofδ¹³C values of fossiltests from the fresh-water lakes is not related to those ofδ¹⁸O, while that ofδ¹³C values offossil tests from the saline-water lakes is closely related to tbose ofδ¹⁸O.The algal-rich lacustrine mudstones are the immediate evidence for the high productivityduring the mature period of the Paleogene lakes development in the oil-bearing basins offshoreChina. The algal-rich lacustrine mudstones could be formed in both of the saline-water lakes andthe fresh-water lakes. The periods of high productivity of Pearl River Mouth Basin, BeibuwanBasin and Bohai bay Basin are the Eocene to the early Oligocene. The periods of high productivity of the paleolakes in Pearl River Mouth Basin, Beibuwan Basin and Bohai Bay Basinare the Eocene to the early Oligocene. During this age, the climate of the east of China waswarmer and humid. The paleolakes came up to an advanced level of lake maturity with respect tonutrient input and phytoplanktonic productivity, even reaching phytoplankton blooming.Consequantly, the organic matter rich in hydrogen were produced in a great quantity and depositedin paleolake bottom, provided good organic sources for the lacustrine sediments.AOM, derived from the plankton and bacterium, dominates in the sedimentary organic matterassemblages of the Wenchang Formation in Pearl River Mouth Basin, the 2nd Member ofLiushagang Formation to Weizhou Formation in the Beibuwan basin, Shahejie Formation andMiddle to Lower Member of Dongying Formation in Bohai Bay. The high abundance of AOMreflects the anoxic lake bottom condition. The high abundance values of phytoplanktonic fossilsand AOM could be regard as a proxy for good oil source rocks. Huilu Sag, Enping Sag, XijiangSag and Wenchang C Sag in Pearl River Mouth Basin, Weixinan Sag, Wushi Sag and MaichenSag in Beibuwan Basin, and Bozhong Sag, the southern part of Liaodong Gulf, Huanghekou Sagand Shanan Sag in Bohai Bay Basin show the characteristics of rich oil kitchen sag.The evaluation of source-rock potential by organic facies analysis was conducted in BozhongSag. Analysis results show the 3rd Member of Shahejie Formation and the Lower Member ofDongying Formation contain good oil source beds, particularly the latter is the most importantsource rocks in Bozhong Sag.