Characteristics and formation mechanism of oil shale in the upper member of the Dalazi Formation in the Songjiang basin, Jilin

Abstract：The upper member of the Early Cretaceous Dalazi Formation in the Yanbian area, Jilin, is characterized by occurrence of oil shale. The oil shale in the upper member of the Dalazi Formation in the Songjiang basin is commonly grayish-brown or grayish-black and paper-sheeted and compact massive. The organic matter in oil shale is of sapropelic type and humus-saproptlic type with an oil-bearing rate of 3.8% to 6.2%. There are 7 beds of oil shale in the upper member of the Dalazi Formation. Of these, beds 2 and 7 are of commercial value; they are not persistent in attitude and their cumulative thicknesses increase from east to west and from south to north. The compact massive oil shale is developed in the Yangmutun area in the axial region of the Songjiang syncline and the Laopianpo-Dongshan area in the north limb of the syncline. The sheeted oil shale occurs in the Yangmutu-North Songjiang area in the south limb of the syncline and Shenglitun area. The upper member of the Dalazi Formation may be divided into two third-order sequences. Beds 2 and 7 are both of semi-deep lake-deep lake origin and formed in the highstand systems tract of sequences I and II respectively. The persistently layered lake water, algal blooming, semi-deep lake-deep lake, protracted, stagnant, anoxic, reducing environment and effective accumulation and preservation of organic material-all these were favorable for the formation of oil shale. However, repeated subaqueous fan deposition by gravity flows was disadvantageous for the formation of oil shale.