Widespread development of bedding-parallel calcite veins in medium–high maturity organic-rich lacustrine shales (Upper Cretaceous Qingshankou Formation, Northern Songliao Basin, NE China): Implications for hydrocarbon generation and horizontal compression

Abstract

Bedding-parallel calcite veins (BPCVs) occur widely in Upper Cretaceous lacustrine shales of the Qingshankou Formation in the Northern Songliao Basin, China. Research on the formation processes and geological significance of BPCVs has focused on the sources of fluid, timing of formation, initiation, dilation and growth mechanisms, and on relationships with hydrocarbon generation. In this study, we addressed these issues through a comprehensive study of drill cores containing BPCVs using petrographic observations, isotope geochemistry, rare earth element (REE) analysis, fluid inclusion analysis, in situ U–Pb dating, and basin modeling. BPCVs were observed in medium–high maturity shales with high organic matter contents with heterogeneous compositions. Similarities in δ13C, δ18O, 87Sr/86Sr, and REEs contents between BPCVs and the host shale indicate that locally derived formation fluids driven by chemical gradients were involved in BPCV formation. The in situ U–Pb age dating (70 ± 8.9 Ma and 70.6 ± 9.7 Ma) constrained BPCVs formation to the late Cretaceous, corresponding to a burial depth of ∼1600 m, indicating that the BPCVs formed in the oil window. The latter is also supported by fluid inclusions-derived temperatures (∼76–109 °C). BPCV formation included vein initiation and dilation, and the microstructures, e.g., domal and pincer structures, sigmoidal calcite veins, conjugate en-échelon calcite vein arrays, as well as mechanical crystal twins of the BPCVs, all indicate that tectonic compression influenced fracture development as also indicated in other basins where beef occur. Fluid overpressure caused by hydrocarbon generation, tectonic compression, and clay mineral transformation (mainly smectite to illite conversion) was responsible for vein dilatation. Antitaxial fibrous crystals with smooth crystal boundaries and high aspect ratios, containing sinusoidal solid inclusions as well as median planes, indicate that crystallization force promoted vein growth. Thus, BPCVs can be used as a good indicator of hydrocarbon generation, and horizontal tectonic compression.