Analysis on Sedimentary Facies of Member 7 in Yanchang Formation of Triassic in Ordos Basin

Abstract

The depositional stage of member 7 of Yanchang Formation in Triassic was the maximum of Mesozoic lake basin in Ordos basin,the subsidence of lake basin was the largest and the lake basin scope was extensive.it has been clarified that the source sediments were from five directions of basin circumference,the mainly source was located in the northeast and southwest area.In addition,there are three secondary source areas that was located in the west,south and northwest.The sedimentary facies of member 7 of Yanchang Formation are the delta and lacustrine facies,which can be further sub-divided into five sub-facies and ten microfacies.During the deposition of Member 73 of the Yanchang Formation,the lacustrine area was maximum,the turbidite of half-deep lake and deep lake facies were not developed,but the dark and high resistance mudstone were developed,which were main high quality source rock of Mesozoic,the depositional area of half-deep lake and deep lake facies were clearly reduced during the deposition of Member 72 of the Yanchang Formation,the turbidite were relatively developed;the lacustrine area and the depositional area of half-deep lake and deep lake facies continually reduced during the deposition of Member 71 of the Yanchang Formation,the turbidite was extensively developed,which was the main reservoir of Member 7.