Influence of sedimentary environment on the properties of organic-rich shales in China: a review

The sedimentary environment is inextricably linked to the macroscopic and microscopic characteristics of shale reservoirs, which influences shale gas accumulation significantly. This study discusses how the sedimentary environment affects the organic-matter-rich shale reservoirs that have been deposited in typical marine, marine-continental transitional, and continental basins in China. The following four aspects were analyzed including shale rock type and thickness distribution, organic matter abundance and distribution, mineral composition and pore structure, and kerogen type and hydrocarbon generation potential. From continental to marine facies, the sedimentary setting of shales with high organic content generally ranges from shore-shallow lakes to deep lakes, deltas, tidal flat lagoons, shallow sea shelves, and deep or semi-deep seas. In deeper water, the clay mineral content decreases, but the brittleness index and siliceous content increase with darker shale color. Thick shales mostly were deposited in deep or semi-deep lakes, delta fronts, prodeltas, tidal flat lagoons, and deep or semi-deep seas from continental to marine basins. The primary factors influencing organic matter enrichment in deep-sea and deep-lake shales are redox conditions and high biological productivity under favorable sedimentary environments, whereas favorable factors for organic matter enrichment in transitional facies include warm-humid palaeoclimates and abundant debris inputs. Continental shale is characterized by the presence of intergranular and intragranular pores, a low pore volume and specific surface area, and a high average pore size and hydrocarbon potential. The kerogen types are complex in continental shales, with type I in deep lake shales and type III in lakeshore shales. Transitional shales occur mostly in coal-bearing strata with type III organic content, medium pore sizes, and hydrocarbon generation potential. The high specific surface area and pore volume, small pore size, and high brittle mineral content of marine shale facilitate the production of dissolution pores. Marine shales are mainly kerogen type I-II1 with relatively high maturity and low hydrocarbon production potential. By constructing an intrinsic link between the sedimentary environment and reservoir parameters, a sedimentary model of organic-rich shale under different depositional context should be summarized in the future, which can provide a foundation to analyze the geological circumstances of shale gas accumulation.