鄂尔多斯盆地南部晚三叠世沉降与沉积中心研究

从古地貌恢复的角度出发,依据残留厚度和补偿厚度印模法原理,恢复了鄂尔多斯盆地晚三叠世早期古地层厚度,从而确定了沉降中心;选取粒度矩法平均值这一参数,对同井位同层位的粒度值求平均值,将所得值投放平面图,获得粒度均值平面等值线图,湖盆中细粒碎屑物质的聚集区反映了古水流的汇合区,指示沉积中心。研究结果表明,鄂尔多斯盆地晚三叠世早期沉降中心位于马家滩-白豹-宜君一线,西南部镇原-崇信一线存在局部沉降区,盆地的地层厚度变化较大,说明晚三叠世差异沉降作用明显;而沉积中心主要位于正宁-合水-姬塬-吴起-志丹-太白-黄陵-黄龙一线围成的区域,由西北向东南延展,并以北西-南东为轴向两侧迁移,长10至长7沉积期湖岸线的变化,反映了受构造沉降速率控制,湖盆的沉积中心不断扩大,长7沉积期湖盆面积最大。

CENTER OF LATE TRIASSIC SUBSIDENCE AND DEPOCENTER FOR THE SOUTHERN ORDOS BASIN

From the angle of recovering palaeogeomorphology, according to the theory of impression way of remnant thickness and bucking thickness, the thickness of early Late Triassic paleo-formation is recovered and depocenter is confirmed. In terms of the average value of the grade moment method, the grades of same well location and stratigraphic position are averaged out,then projecting these data in a plane figure to obtain the plane isogram of grade mean value.The gathering area of fine-grained clastics of the lake basin reflexed the confluence of palaeocurrents and instructed the depocenters.The results indicate that the depocencer of the Ordos Basin in early Late Triassic lies in the Southwest Majiatan-Baibao-Yijun and Zhenyuan-Chongxin areas. The formation thickness of the basin changed greatly, which shows that in early Late Triassic the differential sagging is obvious and depocenter lies majorly in the range surrounding by Zhengning, Heshui, Jiyuan, Wuqi, Zhidan, Taibai, Huangling and Huanglong, stretching from northwest to southeast. From C10 to C7 of the Yanchang Formation lake strandlines extended along the northwest-southeast axis, and the area of the lake basin had been increasing until the depositional stage of C7 was over. The depocenter of the lake basin changed regularly and the area of the lake basin for every depositional stage changed obviously because of control of tectonic subsidence rate. Moreover, tectonic subsidence rate was biggest during the depositional stage of C7. The ranges of centers of subsidence and depocenters are different. Their migration and coincidence are of importance to the study of the abnormal high value of thermal evolution of source rocks.