Fluid evolution and gas enrichment in the Wufeng-Longmaxi shale reservoirs of the eastern Sichuan Basin, SW China

Abstract

Abundant calcite and quartz veins have been developed in the Wufeng-Longmaxi (O3w-S1l) shale reservoirs, which are direct records of fluid flow events in the southern Jiaoshiba area, Sichuan Basin. To investigate their origin and timing, multiple in-situ analytical approaches were conducted on selected calcite and quartz veins. Three stages of calcite veins and two stages of quartz veins are identified in the studied region as: stage 1 calcite veins (Cal-1 veins), stage 2 quartz veins (Qtz-1 veins), stage 3 calcite veins (Cal-2 veins), stage 4 quartz veins (Qtz-2 veins), and stage 5 calcite veins (Cal-3). The Cal-1, Qtz-1, Cal-2, Qtz-2, and Cal-3 veins were formed at 177–166 Ma, 167–145 Ma, 156–145 Ma, 84–83 Ma, and 65 Ma, respectively, as indicated by combining with fluid inclusions data in both the calcite and quartz veins and burial-thermal history reconstruction. Intra-layer diagenetic fluid in the O3w-S1l shale was responsible for the formation of Cal-1, Qtz-1, Cal-2, and Qtz-2 veins. The Cal-3 veins were only identified in the O3w-S1l shale reservoirs of the Zilichang zone, and the vein-forming fluid was derived from extraneous fluids under an oxidative environment. In addition, methane inclusions in the calcite and quartz veins show three episodes of natural gas expulsion in the southern Jiaoshiba area. Due to the effective sealing capabilities of shale, the three episodes of gas expulsion were transported for a short distance within the O3w-S1l shale reservoirs in the Piangqiao zone. The last episode of gas expulsion occurred in the Zilichang zone during the Late Yanshanian uplift was corresponded to the formation of Cal-3 veins. Extraneous fluid can infiltrate the reservoir through the local fault-fracture systems in the Zilichang zone, indicating the destruction of sealing capability. Our methane inclusion data analysis indicates the loss of shale gas in the Zilichang zone, which was potentially associated with the fluid flow responsible for the formation of Cal-3 vein in the O3w-S1l shale reservoirs. Fluid evolution in different gas-bearing shale reservoirs reveals preservation conditions during the process and has an implication for the differential enrichment of shale gas.