Effects of pressure on gas generation and pore evolution in thermally matured calcareous mudrock: Insights from gold-tube pyrolysis of the Eagle Ford Shale using miniature core plugs

Abstract

Together with an increased interest in deep petroleum and shale gas exploration over the last decade, the effects of pressure on gas generation and pore evolution in organic-rich shales have received renewed attention. In this study, miniature core plugs drilled from a thermally matured Eagle Ford Shale sample were pyrolyzed in sealed gold tubes to investigate the effects of pressure on gas generation coupled with pore evolution in the presence of a relatively intact rock fabric. The gold-tube-capsuled samples were isothermally heated at a temperature of 425 °C for 36 h under different confining pressure conditions (10, 25, 35, 50, 68, 80, and 120 MPa), with maturation being induced from the original oil window to the gas window.Experimental results show that increasing the confining pressure from 10 to 120 MPa has initially retards and then promotes the generation of low molecular hydrocarbons (C1–C5 alkanes), whereas gas dryness (C1/C1–5, vol%) increases continuously from 51% to 63% in this pressure range. The inflection pressures that correspond to the maximum retardation of the generation of individual hydrocarbon compounds are shifted to higher values with increasing the molecule carbon number, e.g., 35 MPa for methane, 50 MPa for ethane to butane, and 68 MPa for pentane. Similar pressure effects have also been found for liquid hydrocarbons (C6–14 hydrocarbons and C14+ compounds), with a single inflection pressure of 68 MPa. These observations indicate that high pressure inhibits residual oil cracking but simultaneously favors generation of methane-rich gas. A mechanism for the interaction of retained hydrocarbons and porous matured kerogen (and/or pyrobitumen) is thus suggested to explain the enhanced methane generation. The extent of such interactions is highly dependent on petroleum expulsion efficiency from an intact rock fabric.BET specific surface area and pore volume measured by N2 adsorption decrease significantly for samples matured at 10–35 MPa and then increase at confining pressures above 50 MPa. The simultaneous variations in pore development and methane generation with increasing confining pressure indicate that the observed effect of confining pressure on the evolution of the pore structure of shales depends on both oil retention and gas generation. Our experimental results provide some new insights into gas generation and pore evolution in shale systems, and the retardation effect of pressure on hydrocarbon generation and expulsion as well as pore evolution may provide a mechanism for the Type II kerogen-dominated oil-prone shales to become potential shale gas reservoirs.