Characteristics and Controlling Factors of Source Rocks in Yanchang Formation Sequence Framework,Ordos Basin

Zhou Xiang; He Sheng; Chen Zhaoyou; Liu Ping; Wang Furong

doi:10.3799/dqkx.2016.087

Abstract：The Triassic Yanchang Formation,the most important oil and gas horizon in Ordos Basin,is a typical lithologic reser-voir characterized by mass oil-bearing and complex hydrocarbon accumulation,and characteristics of its source rocks are the main controlling factors of lithologic reservoir distribution.Based on outcrops and well log data,this paper divides the Yangchang Formation into three sequences including SQ1,SQ2 and SQ3 upwards to research the characteristics and controlling factors of source rocks within sequence framework.The results show that the primary source rock occurred in expansive system tract,and is characterized by Chang7 source rock belonging to semi-deep lake and deep lake underfilled type deposits during the lake maximum flood period;meanwhile,the sapropel-type kerogen source rocks with a higher organic matter abundance are dominant due to the lack of terrigenous clastic supply.There are also source rocks in highstand system tract such as Chang9 and Chang6 to Chang 4+5 belonging to the deposition of lake-shore,shallow lake facies,in which terrigenous clastic supply makes humic type source rock dominant also with a relative high organic matter abundance.The crude oil of Yanchang Forma-tion in the study area is primarily from Chang7 within expansive systems tract during maximum flooding period of Late Triassic according to crude oil correlation,which implies that contributions of source rocks in different system tracts to Yanchang reser-voir vary.The vertical and lateral change of accommodation under sequence stratigraphic sedimentary dynamics setting not only decides the distribution of sedimentary facies types,but also directly controls the abundance and type of organic materials with-in system tracts.The difference of source rock occurrences between system tracts in sequence framework facilitates prediction and evaluation of distribution laws of source rocks in the sedimentary setting of basin.