How bedding-parallel fractures affect fluid activity in a relatively closed lacustrine shales system: Evidence from calcite veins

Abstract

The redistribution of materials and elements in rocks, the transport of hydrocarbon, the metamorphic reaction of lithosphere rheology, and the sequestration of greenhouse gases all depend on fluid activity. In a relatively closed lacustrine shales system that has experienced strong physical and chemical compaction, there has been not fully understood how fluid activity channels are generated due to few records of fluid activity. To investigate these processes, we present the petrography, isotope geochemistry, cathodoluminescence, fluid inclusions, electron probe microanalysis, and rare earth elements of bedding-parallel calcite veins (BPCVs) found in the lacustrine organic-rich shales of the Chang 7₃ sub-member of the Yanchang Formation in Ordos Basin. Our investigations show that the opening of bedding-parallel fractures (BPFs) in a relatively closed lacustrine shales system can significantly promote fluid flow and fluid-rock interactions. Three distinct stages of BPCVs can been identified: Cal-1, Cal-2, and Cal-3. Cal-1 was the result of active fracturing of host rocks under the crystallization force in the process of in-situ recrystallization after pressure dissolution of inorganic micrite carbonate minerals. The δ¹³C values of Cal-2 is consistent with the values of organic carbon generated by thermal decarboxylation, indicating that the BPFs may be opened under the hydrocarbon generation overpressure and promote hydrocarbon expulsion. Cal-3 is associated with the micro-thrust fault structure, δEu value is a positive anomaly, which is significantly different from host rocks, reflecting that the BPFs opened under the horizontal tectonic compression and then acted as flow channels for organic fluid and deep hydrothermal fluid. This study emphasizes that the opening of BPFs in a relatively closed lacustrine shales system has obvious multi-stage characteristics, which significantly promotes fluid flow and material exchange, affects the diagenetic evolution and hydrocarbon expulsion, and is an important response to the multi-scale and episodic fluid activity in sedimentary basins.