Migration evidence and dynamic enrichment model of shale gas: Take the deep shale gas in the southern Sichuan Basin as an example

Shale gas exhibits migration phenomena, but the impact of shale gas migration on shale gas enrichment has not been figured out. To determine the migration evidences and the enrichment model of deep shale gas, this paper takes the deep shale gas reservoir of Lower Silurian Longmaxi Formation in the southern Sichuan Basin as an example to clarify the shale gas migration channels and capacities, based on the natural gas geochemical characteristics of shale gas reservoir, the features of calcite vein inclusion in bed parallel low-angle fractures and the reservoir pore structures. Then, the dynamic enrichment models of Longmaxi Formation shale gas are classified by analyzing typical gas reservoirs. And the following research results are obtained. First, the migration direction indicated by the fluid potential field of the shale gas reservoir is consistent with that indicated by the gas component, which proves the existence of shale gas migration. In addition, the fluid inclusions in bed parallel fractures reveal two migration periods of the Longmaxi Formation shale gas in the study area, which happened in the peak hydrocarbons generation stage and the later uplift stage, respectively. Second, in the present-day Longmaxi Formation shale reservoir in the study area, there are pores available for shale gas migration. The pores in the Longmaxi Formation shale are primarily the pores for Fick diffusion and secondarily for slip flow and Darcy flow. Third, shale gas migration has significant effects on shale gas enrichment. There are five new dynamic enrichment models of shale gas in the study area, i.e., high uplift transition zone enrichment model, fault loss and anticline transition zone enrichment model, fault loss and syncline core enrichment model, high position enrichment model, and weak migration fault loss model. In conclusion, shale gas migration occurred in the geological history, and the multi-stage migration phenomenon determines the current enrichment degree of shale gas and the selection of enrichment areas. © 2024 Natural Gas Industry Journal Agency. All rights reserved.