Abstract:

Sedimentary basins formed in the platform actived period on China's continent and shelf used to be classified into extensional,central transitional and west compressionalhydrocarbon-bearing basins by geological circles.The study of long seismic sections across these basins in recent years led to the conclusion that all the basins in China underwent both tensional and compressional movements.The hydrocarbon-bearing basins in EastChina did not only undergo tensional movement,but also compressional movement.Theearly half graben-like basins resulted from tensile stress,but the overlying fault-depressed basins reflect compression.In the west part of China,the compressional basins did not onlyundergo compression.The early half graben-like basins are also the result of tensile stress.The difference of the west part to the east part is that the normal faults on the boundaries of the half graben-like basins were replaced by overthrusts resulted from intensive com-pression in late stage,the overlying compressional basins were the result of compression.According to their evolution characteristics,assemblage pattern and relation to overly-ing strata,these half graben basins can be divided into inward-tilted,outward-tilted andunidirectional tilted types which can be subdivided into nine subtypes,namely fault-depressed inward-tilted subsidence,fault-depressed inward-tilted compression,singleinward-tilted,fault-depressed outward-tilted subsidence,fault-depressed outwards tilted compression,single outward-tilted,fault-depressed unidirectional-tilted subsidence,fault-depressed unidirectional tilted compression and single unidirectional-tilted subtypes.The half-graben basins are formed due to the rise of paleo-uplifts within the basins,thus faulting the weakest parts of resistant to tensile strength,The rise of paleo-uplifts can beexplained as follows:since the paleo-uplifts are usually distributed at paleocontinental kernels,and the kernels are situated at mountain roots where silica-alumina layer developewell due to the fold of paleogeosynclines,when new geosynclines or plates subducted toward paleoplatforms or paleocontinents to make lithosphere above soft mantle regionaltensility and less crowded,then the light silica-alumina mountain roots float upward forspheric balance of soft mantle,resulting the rise of paleo-uplifts gradually.As the subduc-tion angle increased during the late stage,the plates compressed or collided towards paleo-continents,thus forming the depressed basins overlying the half graben-basins.It has beenproved by prospecting practice that the mid stage of the formation of half graben-likebasins is usually the main oil-forming period.So the half graben like basins and its synde-posit mid-stage formations should be taken as the main targets for oil prospecting.