中国中新生代沉积盆地箕状断陷类型、形成机理及含油性

TYPE,GENERATION MECHANISM AND OIL POTENTIAL OF HALF-GRABEN BASINS IN MESO-CENOZOIC SEDIMENTARY BASINS IN CHINA

中国大陆和大陆架自进入地台活化阶段以来形成的沉积盆地,不论位于东部、中部或西部,都有过拉张期和挤压期。箕状断陷是盆地唯一的基本构造单元,它可分为对翘型、反翘型和单翘型三大类九种亚类。它的形成是由于盆地中古隆起的抬升导致拉张作用所致,而上覆的坳陷型盆地则是侧向挤压所致。中央隆升力和侧向挤压力的形成则取决于板块俯冲倾角的大小。实践证明,当含油气盆地以箕状断陷发育阶段为主时,其发育中期即为主导成油期。

Sedimentary basins formed in the platform actived period on China's continent andshelf used to be classified into extensional,central transitional and west compressionalhydrocarbon-bearing basins by geological circles.The study of long seismic sections acrossthese basins in recent years led to the conclusion that all the basins in China underwentboth 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-depressedbasins 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 boundariesof 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 tiltedcompression,single outward-tilted,fault-depressed unidirectional-tilted subsidence,fault-depressed unidirectional tilted compression and single unidirectional-tilted subtypes.Thehalf-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 paleocontinentalkernels,and the kernels are situated at mountain roots where silica-alumina layer developewell due to the fold of paleogeosynclines,when new geosynclines or plates subductedtoward 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.