鄂尔多斯盆地庆阳地区晚三叠世延长期长8沉积期物源与沉积体系研究

Provenance and depositional systems during the depositional period of interval 8 of Upper Triassic Yanchang Formation in Qingyang area,Ordos Basin

通过野外露头古流向实测,结合砂岩骨架矿物、重矿物组合特征,利用碎屑岩地球化学方法,分析了鄂尔多斯盆地庆阳地区晚三叠世延长期长8沉积期的物源。结果显示,研究区长8沉积期古水流主要为SW→NE方向,次为NW→SE、SE→NW方向;重矿物特征具有明显的分区性。长8油层组的REE配分型式为"右倾型",δEu值平均0.67;其微量元素协变图特征与盆地西南缘陇西古陆中的陇山群基本一致。推测庆阳地区长8油层组母岩主要来自盆地西南缘陇西古陆中上元古—古生界、祁连造山带花岗岩类和秦岭造山带奥陶系—二叠系,源区母岩主要为变质岩和火成岩。通过野外露头资料,结合最新钻探成果,认为长8沉积期研究区主要为扇三角洲沉积体系,发育扇三角洲前缘亚相和(半)深湖亚相浊积扇,而扇三角洲前缘水下分流河道的砂体带,构成了研究区油气聚集的有利相带。

Based on the field outcrop palaeo-current measurements and studies of the sandstone framework mineralogy and heavy mineral assemblages, we utilized clastic rock geochemical methods to define the provenance during the deposition of interval 8 of the Upper Triassic Yanchang Formation in Qingy-ang area, Ordos Basin. The resuhs show that the palaeo-current in the study area was mainly SW-NE and NW-SE and SE-NW currents were of secondary importance. Distribution of heavy minerals is also characterized by obvious zoning patterns. The REE matching pattern of the interval 8 is a ＂right-declined＂ type. The average value of δEu is 0. 67. The variation characteristics of trace elements are largely consistent with those shown by the Longshan Group of the Longxi Palaeo-land in the southwestern margin of the basin. According to the sandstone clastic components especially the lithic components and heavy mineral characteristics, the source rocks of interval $ in Qingyang area were mainly derived from the Meso-Proterozoic to Paleozoic of the Longxi Palaeo-land, the granitoids along the Qilian Orogenic Belt and the Ordovician-Permian along the Qinling Orogenic Belt. The detrital source rocks are dominated by metamorphic and igneous rocks. According to the current drilling results combined with the field outcrops, it is proved that the interval 8 of Yanchang Formation was dominated by the fan delta depositional system with fan deltaic front and （semi） deep lacustrine turbidite fan subfacies being developed. The subaqueous distributary channel sandbodies of the delta front constitute the favorable facies belt for petroleum accumulation.