Migration evolution of lake basin and sedimentary response in deposition period of lower assemblage in Yanchang Formation,Ordos Basin,China

According to a large number of data on field outcrops,core observation and laboratory test,this paper makes a systematic study on the sedimentary microfacies,mineral and lithological features,grain-size characteristics and stratal configuration of Member Chang 9and Member Chang 10of the lower assemblage in Triassic Yanchang Formation.The results show as follows.(1)During the sedimentary period of the lower assemblage in Yanchang Formation,the depocenter of the lake basin was not fixed,transforming from an earlydischarge areato acollection areagradually and made a large-scale migration westward.(2)Because of the depocenter migration,the southeast and northwest sedimentary system of the basin showed the sedimentary pattern with asymmetric development successively.In the early stage(Chang 10),western(northwest)and northern fluvial sedimentary system was strong,southern provenance system was not developed basically and in the late stage(Chang 9),the southern(southwestern)provenance system began to develop.(3)The KunlunQinling Ocean in the south of the basin appeared the collision and closure as a form ofscissorsfrom east to west.It was the depocenter migration evolution and the source of that the sedimentary system developed asymmetrically.(4) Controlled by depocenter migration evolution,the favorable exploration direction of Member Chang 9and Member Chang 10,will be different.In the sedimentary period of Member Chang 9,the exploration prospect of the west system is the best and in the sedimentary period of Member Chang 10,the channel sand body matches the oil shale of Member Chang 9.Member Chang 10can form reservoir-forming combinations ofupper generation and lower storagein the longitudinal direction and should be attended to.