Origin of Authigenic Minerals in Shale and Their Influence on the Reservoir Quality: A Case Study From the Cretaceous Qingshankou Formation, Songliao Basin, China

ABSTRACT Origin of authigenic minerals in shale and their impact on reservoir quality remains poorly understood. In this study, the integrated analytical methods are used to determine authigenic minerals and their origin, elucidate diagenesis and the impact of diagenetic evolution on the development of pores from the continental shale of the Upper Cretaceous Qingshankou Formation in Songliao Basin, China. Research results show that the major authigenic minerals in the shale are clay minerals (including illite‐smectite, illite and chlorite). The second most abundant authigenic minerals are carbonate minerals (including calcite, ankerite, calcite veins) and albite, with a small amount of pyrite and quartz. Formation of illite and illite‐smectite was closely related to the transformation of smectite and dissolution of K‐feldspar. Chlorite, albite, calcite and ankerite precipitated during feldspar dissolution and transformation of smectite to illite‐smectite and illite. This process was facilitated by magmatic hydrothermal fluids and organic fluids. Pyrite precipitation was associated with sulphide deposition. Compaction occurred from the early diagenesis to the A1 stage of mesodiagenesis reduced some pores, while precipitation of pyrite promoted the development of intercrystalline pores in pyrite. Cementation of authigenic minerals occurred in a semi‐ to closed environment during the A2 stage of mesodiagenesis, which further reduced the porosity and permeability of the shale. However, with the increase in burial and formation temperature, and assisted by magmatic hydrothermal and organic fluid activity during this period, part of the K‐feldspar, albite and clay minerals dissolved. This dissolution resulted in the development of microscale and nanoscale inorganic pores, which serve as effective storage spaces for low‐molecular‐weight natural gas and condensate hydrocarbons. Precipitation of clay minerals during Stage B of mesodiagenesis lowered the reservoir permeability. While, deep burial and higher temperature, as well as overpressure in the shale, increased the solubility of ions in pore water, promoting the formation of nanoscale dissolution pores in K‐feldspar, albite and clay minerals. Both the inorganic and organic pores constitute storage space within the shale, but the inorganic pores formed in authigenic minerals during the diagenetic evolution are very important and their contribution to improving reservoir quality cannot be ignored.