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Geochemical Controls on Gas Adsorption and Preservation in Organic-Rich Shale Systems

Zhang, Tongwei; Milliken, Kitty L.; Ruppel, Stephen C.; Sun, Xun

Nano-size pores typically host methane in low permeability mudstones, and the presence of pores that are filled with oil and gases but isolated from the pore system within the surrounding mineral matrix is highly possible. A rock crushing experiment has been devised to test for the presence of gas and condensate in isolated nanopores. We have tested this method on mudstones of the Upper Cretaceous Eagle Ford Formation, an emerging oil/gas shale play in the Maverick Basin and the adjacent western San Marcos Arch in South Texas. Our results show that both thermal maturity and gas desorption contribute to changes in CH4/CO2 ratio of gas released during rock crushing. Gas geochemical parameters (C1/C2, iC4/nC4) of gas released in rock crushing are good indicators of thermal maturation of organic-rich shales. CH4/CO2 ratio is proposed to identify the relative contribution of free gas versus adsorbed gas; CH4/CO2 ratios decrease with longer rock crushing time because of an increase in the CO2-rich adsorbed-gas contribution. CH4-rich free gas and CO2-rich adsorbed gas are identified as two major gas storage components in organic-rich shalesand gas extraction in rock crushing appears to be an efficacious means to determine their relative abundance.

A series of CH4 bulk adsorption experiments on organic-rich shales, isolated kerogens, and clay-mineral dominated rocks were conducted under dry conditions. Our results suggests that physisorption is a dominant process for CH4 sorption both on organic-rich shales and clay minerals, as indicated by a linear correlation between BET surface area and CH4 sorption capacity.. Organic matter is a primary control on gas adsorption in shale-gas systems: generally, the higher the TOC content, the greater the gas-sorption capacity. CH4 sorption on clay minerals is mainly determined by the difference in type of clay minerals, and the larger the BET surface area of clay minerals, the greater CH4 sorption capacity. The affinity of CH4 molecules on organic-rich shales is,however, much stronger than on the most common clay minerals. Thecontrol of rock properties (organic matter content, type, maturity and clay minerals) on CH4 adsorption can be quantified with the heat of adsorption and standard entropy, as determined from adsorption isotherms. Those thermodynamic parameters can be used for estimating Langmuir pressure and CH4 sorption capacity under reservoir temperature and pressure condition.

 

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013