Characteristics of lacustrine organic-rich shale: A case study of the Chang 7 member, Triassic Yanchang Formation, Ordos Basin, China

This paper presents a case study of lacustrine organic-rich shale from the Yanchang Formation, Ordos Basin, China. Sedimentary and geochemical analyses were performed on shale acquired from the Chang 7 member through its outcrop, core and thin section data indicate that depositional environments have a great impact on the distribution and quality of shale reservoirs. Shale in the Chang 7 member was interpreted to have been deposited mainly in delta-front and deep-lake environments. Delta-front shale is 2–10 m thick (average ∼5 m), laterally discontinuous and commonly interbedded with equally thick layers of sandstone. In contrast, deep-lake shale is 5–74 m thick (average ∼21 m), laterally continuous and locally interbedded with thin layers of sandstone. Delta-front shale was mostly massive without obvious sedimentary structures while deep-lake shale was characterized by abundant horizontal laminations. More importantly, delta-front shale primarily contains Type Ⅱ1 and Ⅲ kerogen, with TOC commonly less than 2%. In comparison, deep-lake shale primarily contains Type Ⅰ and Ⅱ1 kerogen, with TOC mostly higher than 2%. Based on the analysis and comparison between shales deposited in two types of environments, it is suggested that deep-lake shales in the Chang 7 member were favorable exploration target for hydrocarbons.