Based on the studies of carbon isotopes and biomarkers, only one set of effective source rock was identified in Mesozoic in the southern Ordos Basin, that is, dark shale with big thickness deposited in lacustrine environment in the Upper Triassic Yanchang Formation. The source rocks mainly distributed in Qingyang, Zhengning, Zhiluo, Wuqi, Yanchi and Huanxian County. The dark shale in the seventh member of Yanchang Formation was the most favorable source rock in the study area, providing oil and gas for not only itself but also the Fuxian, Yan'an and Zhiluo Formations of Jurassic. With weak structural fluctuation and gentle stratigraphic dip angle ( usually below 1 °) , hydrocarbon had migrated for short distance before accumulating in low-porosity and low-permeability reservoirs. Sand bodies were poorly connected and few fractures worked as vertical migration pathways. Hydrocarbon mainly accumulated in lacustrine depressions or neighboring areas. The results showed that the favorable places for prospecting in the southern Ordos Basin should be the lithologic traps close to the source rocks in Yanchang Formation.%通过碳同位素分析以及生物标志化合物对比,确定鄂尔多斯盆地南部中生界只存在一套有效烃源岩,即延长组长4+5-长8厚度较大的深灰色、灰黑色湖泊相泥页岩,其主体分布在庆阳、正宁、直罗、吴旗、盐池及环县地区的范围内,又以长7沉积期沉积的暗色泥岩为研究区最有利的烃源岩,提供了包括自身储集层以及侏罗系富县组、延安组、直罗组储集层的油气.鄂尔多斯盆地南部中生界地层平缓,陕北斜坡倾角一般小于1°,储层低孔低渗、非均质性比较严重,砂体连通性差,缺乏裂缝系统作为垂向运移通道,其油气运移的距离较短,油气主要聚集在湖相生油坳陷或邻区.因此,勘探重点应放在距离延长组烃源岩较近的岩性圈闭等.
展开▼
