Controlling role of sedimentary environment on the distribution of organic-rich shale: A case study of the Chang7 member of the triassic Yanchang formation, Ordos basin

The thickness and distribution of organic-rich shale are important parts of forming conditions appraising about shale gas and shale oil. The distribution of shale is controlled by sedimentary environments. This study determined the sedimentary environment of dark shale of the Chang 7 Member by field profile and cores. Vertical overlaying features of dark shale were confirmed by field profile and wells. Sedimentary structures and micro-bedding structures were delineated by cores and sections. Differences of total organic content and organic types between dark shale in different sedimentary environments were revealed by testing data and logging interpretation. Dark shale of the Chang 7 member distributes in the subaqueous distributary bay of delta front and semi-deep to deep lacustrine. In semi-deep to deep lacustrine, dark shale has great continuous thickness and sandstones have thin thickness. However, in delta front, dark shale has thin continuous thickness, shale and sandstone interbeddings have similar thickness or thin shale layer and thick sandstone layer. In semi-deep to deep lacustrine, dark shale always has lamellation, horizontal lamination and sandy lamination, whereas lamellation does not develop very well in delta front shales. Organic types of delta front are mainly type II2 and III, and type I and type II1 to semi-deep to deep lacustrine shales, respectively. TOC of semi-deep to deep lacustrine shale are higher than that of delta front shale. Based on these comparisons, we concluded that shales which distribute in semi-deep to deep lacustrine are the more favorable prospecting target for shale gas/oil in the Yanchang Formation. ©, 2015, Science Press. All right reserved.