鄂尔多斯盆地南部前侏罗纪古地貌对延安组下部油藏的控制作用

Pre-Jurassic palaeogeomorphic control on the hydrocarbon accumulation in the Lower Yan'an Formation in southern Ordos Basin

三叠纪末的印支运动使鄂尔多斯盆地整体抬升,上三叠统延长组顶部遭受到强烈风化及河流侵蚀等地质作用,形成水系广布、沟壑纵横、丘陵起伏的古地貌景观。通过对下侏罗统延安组延10油层组和富县组地层等厚图及延长组顶面起伏图的编绘,基本恢复了前侏罗纪古地貌形态,并将其划分为河谷、斜坡、阶地、高地、平原和潜丘6种古地貌单元。该种古地貌景观严格控制了下侏罗统富县组及延安组下部的沉积相类型、相带展布及压实披盖构造的形成。古河谷的下切作用不仅使延安组下部与油源层的接触距离缩短,又为油气运移提供了良好的通道,如遇到有利的储集体与背斜、鼻隆等构造,即可有机配置、富集成藏;同时地层水特征表明斜坡带保存条件最好。指出前侏罗纪古地貌斜坡带上的坡嘴、阶地及潜丘是勘探延安组下部油藏的有利地带。

The Indosinian Movement at the end of the Triassic uplifted the whole Ordos Basin. The top of the Yanchang Formation suffered a strong weathering and fluvial erosion which formed a palaeogeo- morphic landscape with widely distributed streams, ditches and pools alternated, uplands and lowlands. Based on the compilation of stratigraphic isopach maps of the interval 10 of Yah＇an and Fuxian formations and the top surface structure contour map of the Yanchang Formation, the pre-Jurassic palaeogeomorphic framework was divided into valley, slope, terrace land, upland, plain and burial hill. It had a great control on the sedimentary facies types, sedimentary facies belts distribution, and the structure formation of the Fuxian Formation and the Lower Yah＇an Formation of Lower Jurassic. The incision of the palaeo-valley not only decreased the distance between the Lower Yah＇an Formation and the hydrocarbon source intervals, but also provided hydrocarbon migration pathways. If there existed profitable reservoirs combining with anticline and nose-uplifting structures, they can entrap hydrocarbons and form hydrocarbon reservoir. The formation water characteristics show that the slope area has the best preservation conditions, and the slope mouth, terrace land and burial hill on the preJurassic palaeogeomorphic slope are the profitable areas for the exploration of the Lower Yan＇an Formation.