Geochemical Investigation of the Role of Kerogen in the Retention of Hydrocarbons by Organic-rich Shales

Modern production technology has made it economically feasible to exploit source rocks as unconventional hydrocarbon resources. Because the resource magnitude depends on hydrocarbon retention in these rocks, understanding and quantifying their hydrocarbon retention capacity is critical for profitable exploitation. Although hydrocarbon retention in organic-rich shales is strongly influenced by fluid/rock interaction, it is not entirely clear whether this is due to hydrocarbon/kerogen or hydrocarbon/mineral interactions. The objective of this research was to determine the relative importance of hydrocarbon/kerogen vs. hydrocarbon/mineral interactions in the retention phenomenon. One approach to this question is through measuring heats of adsorption (ΔHa) for the interactions of hydrocarbons with kerogen versus whole-rock. Previous research demonstrated that ΔHa values for gases on solid adsorbents can be measured by gas-solid chromatography. No study has used this method to quantify hydrocarbon adsorption on kerogen. In this study, a simple system was investigated. The interaction of ethane, propane, n-butane, and i-butane with kerogen isolated from a thermally-immature Green River shale was determined using the chromatographic method. Additionally, ΔHa values were determined for the interaction of ethane and propane with the whole shale. ΔHa values for ethane, propane and n-butane on the kerogen were determined to be -1.76, -2.23, and -4.13 kcal/mol, respectively. The ΔHa values for ethane and propane on the whole shale were quite similar, measuring -1.40 and -2.32 kcal/mol. The ΔHa value for i-butane/kerogen interaction measured only -1.14 kcal/mol. All ΔHa values are relatively low, suggesting there is no site-specific chemical interaction between the hydrocarbon components and the adsorbents. ΔHa values increase with the size of the n-alkanes, and are similar for interactions with the kerogen as with the whole shale. The ΔHa value measured for i-butane on the kerogen is lower than the ΔHa value determined for n-butane on the same adsorbent, which can be attributed to the larger molecular diameter of i-butane. These observations suggest that the interactions are controlled primarily by dispersion forces between the organic molecules and the kerogen structure. For this system, the hydrocarbon adsorption potential of the shale is influenced predominantly by adsorption on the kerogen, and not on the matrix minerals.