FORMATION FEATURES OF MIDDLE-LARGE SCALE LITHOLOGIC OIL AND GAS FIELDS IN THE SLOPES OF MAIN TERRESTRIAL OIL BASINS OF CHINA

Various kinds of terrestrial oil basins are evolved in China,and the formation features of middle-large scale lithologic oil and gas fields in the slopes of the basins are different.The structural partition in the extensional faulted Meso-Cenozoic basins is strong.And in the sags with high-very high hydrocarbon abundance,large types of radial distributed fan deltas and underwater alluvial fans are often superposed in slope areas,and middle-large scale lithologic complex oil and gas reservoirs are formed under the control of faults,formation overlap and pinchout,igneous rocks,and mud diapir.Middle-large scale hydrocarbon accumulating units are evolved in the huge Late Cretaceous Songliao Down-warped Basin which is formed due to heat decreasing,and the basin is of high-very high hydrocarbon abundance.By the controlling of regio-nal faults and bulges from bottom,middle-large scale lithologic-faulted oil reservoirs are formed in the slope areas in which middle-large scale of superposed delta sandstone bodies and tilted and inversed structures are evolved.Large hydrocarbon accumulating units are formed in the steady craton basins such as the Late Paleozoic Ordos Basin and the Mesozoic Shanganning Basin with middle scale of hydrocarbon abundance.The large-middle types of alluvial-delta sandstone bodies at the northeast parts of the basins are always the hydrocarbon accumulating systems receiving hydrocarbon from the hydrocarbon-generating center at the southwest parts of the basins.Under the violent deformation the of Yanshannian-Ⅳmovement,large-middle types of lithologic oil fields are formed in sandstone bodies which distribute in the north-south direction and pinchout eastward.Because the large-middle types of hydrocarbon accumulating units are formed in main sags,the large Mesozoic forland basins such as the Junggar Basin,the West Sichuan Basin and the Tarim Basin are of middle scale hydrocarbon abundance,and a series of middle-small types of skirt-edged fans by radial distribution are formed in the slop areas.And under the background of big existed bulges,formation overlaps and pinchouts as well as faults,lithologic oil fields can be formed in the fans.