EVOLUTION OF PRESSURE AND GAS MIGRATION DURING UPLIFT AND DENUDATION OF HIGH THERMAL MATURITY FORELAND BASIN SHALE GAS ACCUMULATIONS

Shales rich in organic matter can be buried to depths of 7 to 10 km in foreland basins by burial and thrust loading, reaching high levels of thermal maturity (vitrinite reflectance, R_o>3 %) with maximum burial temperature (T) > 250^oC, generating large volumes of methane-rich natural gas. Aqueous and methane-rich fluid inclusions document paleo-pressures consistent with lithostatic load (150 - 250 MPa) at maximum burial. At the pressure (P) and T of deepest burial methane-rich gases have densities of ~ 330 kg/m³. As deeply buried gas-rich shales are exhumed, the extremely low permeabilities (~10^-23 m²) of gas shales cause pore P to evolve under undrained conditions, approaching a limit of unconnected, isolated pores containing fluids of constant density. Pore P changes as a function of Skempton’s coefficient, the ratio of the compressibility of the rock and the pore fluid. During cooling the high compressibility and isochore slope (dP/dT = 0.54 MPa/^oC) of methane-rich gases cause the P of gas saturated pores to exceed lithostatic load, driving cyclic gas expulsion as pore P repeatedly exceeds and then falls below the fracture P (70 - 80 % lithostatic load) causing late-stage gas migration.

Present day accumulations of thermally mature shale gas are commonly at depths of 2 to 4 km with T of 60 to 120 ^oC and maximum P at the fracture limit of about 40 to 80 MPa. Using typical reservoir conditions T = 80 ^oC and depth, 2.5 km, methane-rich gas density can range from ~ 218 kg/ m³ at fracture P to ~ 144 kg/m³ at hydrostatic P or lower in under-pressured reservoirs. During uplift gas loss from the initial charge at maximum burial could be 34 % if the system remains at pressures near the fracture P, or 56 % if the pore pressure equilibrates to hydrostatic P. Using the large estimates of recoverable shale gas resources in basins (2 to 10 x 10¹² m³) at least an equivalent volume of gas migrated from the known area of shale gas accumulation during uplift. Late-stage migration of large volumes of thermally mature gases from deeply buried shales can charge shallower accumulations and mix with previously generated gases in conventional and unconventional accumulations during post-generation stages of basin evolution.