Authigenic minerals development and differential diagenetic evolution of lacustrine shale: implications on formation of reservoirs and diagenetic effects of long-term CO2 sequestration

Determination of the mineral transformation mechanism in shale reservoirs and understanding the differential diagenetic evolution paths of various lamina combinations are of key significance to clarify the spatial development of reservoirs and predict future sealing ablitily，storage capacity for sequestered CO2. In this study, 7 wells with a maturity range of 0.6%-1.62% from Qingshankou Formation in the Songliao Basin were selected. Thin section observation combined with QEMSCAN were used to divide the types of lamina combinations. Clay mineral X-ray diffraction analysis methods, field emission scanning electron microscope, energy spectrum analysis were used to analyze the mineral textures, pore type, diagenesis path. The result show that four laminae combinations are identified of shale samples. The diagenetic development of Qingshankou Formation shale can be divided into early and middle diagenesis. Early diagenetic mineral phases include calcite cement Ⅰ, framboidal and microcrystaline pyrite, ferroan and non-ferroan dolomite. Middle diagenetic mineral phases include illitization of smectite, chlorite formed by chloritization of smectite and alteration of K-feldspar, authigenic albite and quartz,calcite cement Ⅱ，ankerite. Mechanical compaction is obvious in all laminar combination in the low mature stage. The felsic+clay, biological shell+clay, calcareous + clay laminae combination all include clay mineral transformation from the early oil generation stage and mainly develops clay mineral intercrystalline pores and interlayer crack.The main dissolved mineral particles are different in various laminar combinations in the oil window. Cementation is the main diagenesis in the biological shell+clay, calcareous + clay and carbonate lamina combination after peak oil generation stage, which has a plugging effect on pores. In addtion, the lamina combination rich in feldspar and clay minerals is beneficial to the long-term storage of CO2, while the lamina combination rich in carbonate minerals interacts is beneficial to the storage of medium and long-term carbon dioxide.