鄂尔多斯盆地热演化史及其对油气的控制作用

Thermal history and its controls on oil and gas of the Ordos Basin

鄂尔多斯盆地为大型含油气盆地,是我国重要的能源基地。盆地现今地温梯度为2.8℃/100m,属于中温型盆地。根据现今地层测温及地温梯度,编制了盆地3个重要烃源岩发育层位延长组长7段、石炭二叠系、奥陶系顶面现今地温分布图,分析了3个界面现今地温分布特征。根据收集及实测的奥陶系沥青反射率资料,编制了奥陶系顶面镜质体反射率等值线平面分布图。盆地不同层位的镜质体反射率值随着深度增加或层位变老,镜质体反射率值逐渐增高,在各地层分界面处不存在镜质体反射率(Ro)明显的错断。各构造单元镜质体反射率(Ro)与深度关系曲线图差异主要表现为曲线斜率的差异及曲线接近地表起始值的差异,这两项差异反映了地温梯度及后期剥蚀厚度的差异。不同层位的镜质体反射率的高值区均位于盆地南部的庆阳—富县—延安—吴旗一带,盆地南部不同层位镜质体反射率高值区的存在表明盆地南部热演化程度高,存在热异常。盆地热演化史恢复表明早白垩世鄂尔多斯盆地发生过一期构造热事件,地层在早白垩世末达到最大古地温。早白垩世以后,鄂尔多斯盆地整体大幅度抬升剥蚀,地温梯度减小,地层冷却降温。现今地层温度远小于在早白垩世末达到的最大古地温,油气的生成主要受古地温控制。早白垩世的构造热事件控制了上、下古生界、中生界3套主要烃源岩油气的主要生成、成藏期。

The Ordos basin is a large oil and gas basin of great energy resources. The present geothermal gradient in the basin is 2. 80 ℃/100 m, making it the medium temperature type basin. Based on today＇s formation temperature and geothermal gradient, we draw present top surface geothermal maps in three important hydrocarbon source rocks layers, which include the Yanchang 7-segment, Permo-Carboniferous and Ordovician, and analyze present geothermal distribution of three stratigraphie interfaces. Using collected and measured Ordovician bitumen reflectance data, the reflectance contour map of the Ordovician top surface is drawn. According to the contour map, the vitrinite reflectance values increase gradually with deeper depth or older layer age in different layers, and there is no obvious dislocation at the interface of all formations. Moreover, the difference of the vitrinite reflectance-depth graph in each structural unit mainly reflects the differences in the slope of the curve and the initial value of the surface, which is associated usually with the geothermal gradient and difference of late erosion thickness. High values of vitrinite reflectance in different layers are consistently located in the southern basin （Qingyang - Fuxian - Yan＇an - Wuqi area） ; the presence of different layers in the high vitrinite reflectance areas indicates high thermal maturity in the southern basin with thermal anomaly. The thermal history shows that the Early Cretaceous Ordos basin occurred in a tectonic event strata and reached the maximum paleotemperature in the early Cretaceous period. After Early Cretaceous, the Ordos basin as a whole has experienced intensive uplift and erosion whereas the geothermal gradient and formation temperature decreased. Today, the formation temperature is much less than the maximum paleotemperature, which the strata had reached in the early Cretaceous; and oil and gas generated are mainly controlled by paleogeotemperature. The early Cretaceous tectonic-thermal event controlled the main generation ＆ accumulation of main hydrocarbon source rocks in three sets, namely the upper and lower Paleozoic and Mesozoic.