Tectonic Background And Analysis On The Favorable Belt Of Oil And Gas In The Upper Paleocene (E₁～2), Lishui Sag Of East China Sea

The East China Sea Basin (the East China Sea Shelf Basin) is a Cenozoic basin developed over the continental shelf of East China Sea, under which the Mesozoic residual basin is developed. The tectonic evolutionary history of paleo-plates in East China Sea Basin can be divided into three developing stages: the assembly stage of middle and small plates in the Pre-Jurassic period, the stage of the intra-plate mobilization and metamorphose and plate-boundary accretion in the coastal Pacific tectonic tract in the Jurassic-Cretaceous period, and the developed stage of rift basin from the late Cretaceous to Paleogene. The tectonic evolution of East China Sea can be approximately divided into four evolutionary stages: rifting age, depression age, upwelling denudation age and regional subsidence age. In the rifting age (K₂²-E₁), it formed the extensional environment like simple shear, and developed the half graben-like tectonics—fault-depression basin. Beneath the Taibei depression deposited a set of volcanic-clastic rock strata and above that were the marine facies deposits, In the depression age (E₂), the Taibei depression entered into thermal-cooling shrinkage subsidence stage, and the extensional structures proceeded in the state like tensional shear. In the upwelling denudation age (E₃), the upwelling denudation mostly occurred in the Oligocene, which is one of the largest tectonic thermal events in the Taibei depression, probably associated with the collision and convergence between the paleo-oceanic island-arc of Diaoyu Island and active continental margin. As for the Taibei depression, the age of regional rifting subsidence, which was late, was from the Tertiary to the Quaternary. When the Taibei depression entered into the whole subsidence stage, all East China Basin was transferred to inactive back-arc basin.The nature of East China Sea Basin is back-arc rift basin of the Cenozoic active continental margin. Not having a large scale of characteristics of rigid lithosphere crust, the main body of basement is accretion belt of the Mesozoic(Jurassic-Cretaceous) active continental margin. The tectonic region of East China Sea Basin can be divided into the east and the west. Eastern tectonic region is dominated by two groups of conjugated fault systems, NNE trending and NW trending, and the NNE trending faults are dominant. As the eastern boundary fault of East China Sea Basin, it connects with the Hupo fault of South Korea to be a large moderate-new strike-slip fault. Both the southern and northern blocks are intermediate tectonics of uplift and sag. The scale of basin is less, and the whole permeation is bad, but in Lishui and Jiaojiang sags which have a good prospective of oil and gas, the fault-depressions are deeper. It is believed that the Mesozoic basins developed under the Cenozoic basins of East China Sea have the nature of residual basins and they are the new deep oil-gas prospecting. According to the research of stratigraphy, lithology and organic geochemistry, Lishui sag has three sets of source rocks: the lake-mudstone of Yueguifeng formation in lower Paleocene, the marine-mudstone of Lingfeng formation in upper Paleocene and the coastal-marine coal-series strata of Mingyuefeng formation. In the Jiaojiang sag, the buried depth of the Paleocene strata is relatively shallow, and that of Mingyuefeng formation is mostly less than 2500m (Ro<0.5%) which does not reach the hydrocarbon-generating threshold, so the effective source rocks in Jiaojiang sag are mudstone of Lingfeng formation in upper Paleocene and mudstone of Yuejiafeng formation in lower Paleocene. In the Minjiang River and Fuzhou sags, the Mesozoic strata spread broad and thickly. In the Lishui and Jiaojiang sags, the Mesozoic strata distribute a relative large scale and their thickness is thinner than that of the Minjiang River and Fuzhou sags. There are definite hydrocarbon-generating abilities in mudstone of Shimentang formation, mudstone of Yushan formation, and coal-series strata of Fuzhou formation in the Mesozoic.The types and abundance of mostly hydrocarbon reflected by organic geochemistry index are better in Li Shui east hypo-depression (WZ6-l-lwell) and Jiao Jiang depression (WZ4-l-lwell), and Li Shui west hypo-depression is taken second place. Combining the maturation of organism we think the quality ofhydrocarbon in Li Shui east hypo-depression is the best, next is Jiao Jiang depression, Li Shui west hypo-depression is bad. However oil shale is the thickest in Li shui west hypo-depression, next is Li Shui east hypo-depression, Jiao Jiang depression is thinner. On the basis of resource rock, we think Yue Guifeng set is medium-better, Ling Feng set mostly is medium, Ming Yuefeng set is bad-medium. Mesozoic Shi Mengtan set, Yu Shan set and Fu Zhou set are medium-bad resource rocks, and the quality of resource rock of every set will change in different depression. To sum up, the hydrocarbon is growth in Taibei depression, which has the big-scale oil and gas resource.Through the analysis of sedimentary facies of single well and seismic phase research , it is believed that there are prodelta facies, delta-front facies, underwater fan, swamp facies, interdistributary bay> sand sheet sedimentary facies and distal-bar facies? Comparing with well data, the favorable sedimentary facies are delta-front facies, underwater fan and interdistributary bay0 The sedimentary facies of resource rock are delta-front facies and interdistributary bay* According to the tectonic characteristic, seismic profile and stratum sequence, the vertical fault of Paleocene is not develop , the continuity of sandstone is bad and mudstone is growth and thick in research area, thus the distance of the vertical and transverse movement of oil and gas is short. Also quite a few oil and gas gather in the deep depression on Paleocene, which be approved by known well data. There is no industry oil in MFY-K WZ20-1-K LF-1 well in the uplift in LiShui depression, according to the oil geological conditions , this belt should be favorable for hydrocarbon gathering, so it shows that the quantity which be moved to the uplift is limited. There is no industry oil and gas inWZ15-l-l, WZ13-1-1 and SMT-1 well, it can be known that the quantity of oil and gas is also limited on the edge of deep depression , but the industry oil may obtained from the center of the sedimentation which has good display, such as WZ26-l-land 2336-1-lwell, because the stratum is completely ,thick and the oil resource is abundance on Paleozoic in deep depression.