鄂尔多斯盆地三叠系延长组长7段物源区分析

随着鄂尔多斯盆地长7段油藏勘探的不断突破,对其物源方向和母岩区性质的分析尤显重要.三叠世延长期鄂尔多斯盆地周缘存在多个古陆,包括北缘的阴山古陆、西北缘的阿拉善古陆、西南缘的陇西古陆及南缘的祁连-秦岭古陆等,它们都是盆地碎屑沉积物的主要供给源.通过碎屑组分、岩屑组分、重矿物、微量元素和稀土元素分析等研究,认为研究区长7段存在东北、西北、西及西南、南部4个方向的物源.轻、重矿物组合综合分析认为,盆地北部母岩主要为太古宙—古生代中—深变质岩和中基性火成岩,西南及南部母岩主要为元古宙—古生代浅变质岩、碳酸盐岩和碎屑岩.微量及稀土元素分析表明母岩以大陆上地壳的长英质岩石为主,并混有一定数量的含长石较高的中—基性岩浆岩如安山岩、碱性玄武岩,母岩中太古代古老地层占有相当大的比重.

Provenance analysis of Chang 7 Member of Triassic Yanchang Formation in Ordos Basin

Provenance analysis is the basis and prerequisite of depositional system and palaeogeography, and also provides to some extent the significant reference for forecasting and evaluation of sand body reservoirs. The analysis of source rock features and provenances is becoming more and more important in the exploration of Chang 7 reservoirs. During the period of Triassic, the ancient lands around Ordos Basin, including Yinshan Old Land on the north, Alxa Old Land on the northwest, Longxi Old Land on the southwest and Qilian-Qinlin Old Land on the south, were the main sources of the detrital sediments in the basin. With the help of the methods based on analysis of detrital minerals, cuttings, heavy minerals, trace elements and rare earth elements, four source directions can be found: northeast, northwest, west-southwest and south. The comprehensive analysis of light and heavy mineral combination characteristics shows that the source rocks of the northern study area are Archean to Palaeozoic mesometamorphic to hypometamorphic rocks and intermediate to basic igneous rocks, and those of the south and southwest areas are Proterozoic and Palaeozoic epimetamorphic rocks, carbonate and clastic rocks. The analysis of trace elements and rare earth elements indicates that the source rocks are mainly felsic rocks derived from the upper crust, and also a quantitative of mixed rock of meta-intermediate-basic igneous rocks with high feldspar such as andesite and alkaline basalt. Archeozoic strata possess a considerable proportion in the source rocks.