Genesis and geological significance of dolomite in shales of the first member of the cretaceous Qingshankou Formation, central depression, Songliao Basin, China

This study presents the first nano-, micro-, and macro-scale analyses of dolomite minerals of varying maturity (all references to maturity in this study pertain to the maturity of organic matter) sampled from the first member of the Cretaceous Qingshankou Formation in the central depression of the Songliao Basin, China. Mineralogical, petrological, isotope and elemental analyses were combined to elucidate the pore fluid evolution and dolomite diagenesis, as well as their implications for the shale organic matter and reservoir space. Three types of authigenic dolomites are identified. The precipitation of authigenic dolomite during the early diagenesis to the mesodiagenesis followed the sequence of anhedral dolomite in low-maturity dolomitic shale (Ro: 0.58–0.70 %) → rhombic ankerite formed overgrowths on anhedral dolomite in medium-maturity dolomitic shale (Ro: 0.78–1.29 %) → saddle-shaped ankerite in crack in high-maturity dolomitic shale (Ro: 1.34–1.70 %); this was accompanied by the transition of pore fluid from seawater to organic fluid, and subsequently to high-temperature hydrothermal fluid. During the early diagenesis to the A₁ stage of mesodiagenesis, the methanogenic archaea in seawater induced the precipitation of anhedral dolomite, which prevented interparticle pore destruction during compaction and preserved the micropores in low-maturity dolomitic shales. During the A₂ stage of mesodiagenesis, the cementation of rhombic ankerite decreased the effective porosity, and consumed organic carbon as the carbon framework in the medium-maturity dolomitic shales. In the hydrocarbon expulsion environment, the dissolution of rhombic ankerite by organic acids promoted the development of micro-nano-scale dissolution pores and increased the effective porosity, which facilitated the expulsion of hydrocarbons from the medium-maturity dolomitic shales. During the B stage of mesodiagenesis, although the high-temperature hydrothermal fluid led to the occlusion of cracks by the cementation of saddle-shaped ankerite, the heat source provided by hydrothermal fluids and/or burial promotes the pyrolysis of organic matter and the development of organic pores, which increased the effective porosity and the expulsion of hydrocarbons in the high-maturity dolomitic shales. Therefore, the low-maturity dolomitic shales exhibit higher effective porosity and higher total organic carbon (TOC) content compared to medium- and high-maturity dolomitic shales. This study expands the high-quality shale from the traditionally organic pore region in higher-maturity conditions to the inorganic pore region with the enrichment of organic matter in lower maturity conditions, which is crucial for developing the hydrocarbon exploration field of shale.