AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Experimental Gas Extraction by Rock Crushing: Evidence for Preservation of Methane in Core Samples from the Mudstones of the Eagle Ford Formation
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.
Accurately determining oil-in-place (OIP) and gas-in-place (GIP) is critical for evaluating shale oil and gas plays. Methane is typically stored in nano-size pores in low permeability mudstones, but many of these hydrocarbon-saturated pores may isolate from surrounding mineral matrix. A rock crushing experiment has been devised to test for the presence of gas and condensate in isolated nanopores. We utilize a gas-tight rock crushing cell that can directly introduce released gas to a gas chromatograph after crushing. 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 San Marcos Arch of South Texas.
Five core samples (depths : 4,758ft to 13,608 ft) were collected from the organic matter-rich lower Eagle Ford unit and used in our study. TOC content and Tmax values range from 1.8% to 8.5%, and 428°C to 543°C, respectively. Calculated Ro, based on Tmax, ranges from 0.5% to 2.6%. Hydrogen index (HI) ranges from 741mgHC/g TOC at Ro of 0.5% to 14 mgHC/g TOC at Ro > 1.6%. The large decrease in HI value with increasing thermal maturity results from the transformation of organic matter to oil and gas. CH2Cl2 extractable hydrocarbons show that the ratio of the sum of C8-C14 to the sum of C15-C32 increases with thermal maturity. The above geochemical observation clearly suggests that oil properties in the organic-rich lower Eagle Ford unit are closely related to thermal maturation of organic matter.
CH4/CO2 ratios of gases released during crushing are lower at low thermal maturities and higher at high maturities because more CH4-rich gas is generated at high maturity levels. CH4/CO2 ratios decrease with longer rock crushing time because of the increase in the CO2-rich adsorbed-gas contribution. Both thermal maturity and gas desorption contribute to changes in CH4/CO2 ratio of gas released from rock crushing. However, no obvious compositional fractionation occurs among C1, C2 and C3 during rock crushing. C1/C2 and C2/C3 ratios remain constant through crushing but greatly increase when the level of thermal maturity is high. Geochemical parameters (C1/C2, iC4/nC4) of gas released during rock crushing are good indicators of thermal maturation of organic-rich shales. CH4/CO2 ratio is a good indicator of free gas and adsorbed gas contributions.