Multistage Fracturing History in the Paleocene Lacustrine Shale Oil Reservoirs of the Subei Basin, Eastern China

This study examines sealed fractures in core samples from the Paleocene organic-rich lacustrine shales of the Gaoyou Depression, Subei Basin, China, using a multi-pronged approach to elucidate the interplay between fracture propagation and fluid circulation in shaly successions. Based on petrography, geochemistry, and geochronology of fracture-filling minerals, four generations of vein emplacement (denoted Types A-D) were identified. These were attributed to successive episodes of burial diagenesis, tectonism, and uplift occurring at different phases of basin evolution. These vein generations were then inserted into the temporal-spatial frame-work derived from basin modeling, leading to a reconstruction of the fracture diagenesis history in the Funing shales. Two of the vein generations contain evidence for two phases of oil migration marked by two types of oil inclusions with different fluorescence. It has been shown that the studied lacustrine shales were deposited at ca. 60.2-58.0 Ma. The earliest vein generation, represented by antitaxial beef calcite veins sub-perpendicular to shale bedding, propagated in incompletely consolidated shales within a regional thrust regime related to the Wubao Movement. The vein nucleation during a short-lived burial following initial deposition was also evidenced by the presence of monoliquid inclusions in the veins and calcite LA-ICP-MS U-Pb age (57.7 +/- 3.7 Ma). Tectonic compressive stress together with overpressures arising from horizontal and vertical disequilibrium compaction were interpreted as being responsible for this fracturing episode. The second vein generation is bedding-parallel, oil inclusion-rich stretching calcite veins with a crack-seal texture, formed concurrently with kerogen maturation and early oil generation at moderate burial depths. The vein crack-seal fills containing yellow fluorescing oil inclusions provide evidence for episodic oil expulsion in response to cyclic rises and drops in pore pressure during the oil window. Bedding-parallel beef calcite veins characterized by antitaxial growth represent the next vein generation, emplaced during the maximum burial period (ca. 41.9 +/- 2.7 Ma). The vein fibers have tracked the horizontal displacement of the wall rocks, likely related to lateral shortening before or at the initiation of Late Eocene basin inversion. The first three vein generations were generated in a relative closed fluid system, with vein fill sourced locally. Host-rock carbonate components and/or earlier diagenetic carbonates were dissolved and then mobilized into these veins. By contrast, the last vein generation with blocky calcite and barite cementation is associated with input of externally derived hydrothermal fluids. The evidence comes from abnormally high fluid-inclusion homogenization temperature (Th) and salinity data, as well as the fact that Ba and sulfate were sourced from the underlying evaporite layers rather than local shales. We thus infer that cross-stratal faulting during the uplift related to the Sanduo Movement resulted in the opening of initial hydraulically closed fluid system. This episode of tectonically-driven hydrothermal circulation from deeper locations in the basin into the Funing shales led to the mixing of oil-bearing pore fluids with exotic basinal hot brines. This also accounts for the presence of abundant blue-green fluorescing oil inclusions and significantly elevated fluid salinity. Overall, this study emphasizes that during basin evolution, differences in tectonic settings and the diagenetic boundary conditions under which veins formed results in different fracturing mechanisms and associated fluid-flow regimes in shale sequences, with consequent effects on vein attributes and mineralization.