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Influence of Kerogen and Oil Cracking Processes on Gas in Place in Unconventional Shale Reservoirs

Abstract

Kerogen cracking to gas (primary cracking) and/or oil cracking to gas (secondary cracking) are the two thermogenic processes involved in gas generation from source rocks. In conventional systems, oil expulsion from oil-prone source rocks has been traditionally considered an efficient process (∼60 to 90% efficiency; Cooles et al. 1986). In such systems, secondary cracking has generally been expected to have a negligible effect on the ultimate gas generation yield. In unconventional shale gas systems, however, the volumes of gas in place suggest that oil expulsion is not as high as estimates for conventional systems imply. In these systems, secondary cracking may play an important, if not dominant role in the ultimate gas potential of the shale reservoir. Detailed geochemical evaluation of the molecular and isotopic properties of the products generated during maturation is fundamental for proper assessment of the impact of cracking processes on the economics of unconventional plays. A series of kerogen and oil cracking experiments were designed to quantitatively assess the effect of primary and secondary cracking on volumes of gas generated. The selected sample set consisted of a series of produced oils generated from type I, type II, and type IIS kerogen-rich source rocks, as well as kerogen extracts from these units. Close-system pyrolysis was used in the artificial maturation of the oils and kerogens. Results from this study revealed similarities in the cracking behavior of oils generated from the same kerogen type and significant differences in the oil to gas transformation process between oils derived from type I, type II, and type IIS kerogen. Key among the differences was variation in the timing of oil-to-gas breakdown of oils generated from different kerogen types. Although the cracking rates observed during breakdown of oils derived from the same kerogen type were very similar, the rate and timing of methane formation was distinct between the samples. Additionally, gas generated by the breakdown of oils derived from the same kerogen type exhibit measurable differences in their compound specific carbon isotopic compositions. This is a carry-over from the original isotopic composition of the oil. Gas generated from kerogen cracking was chemically distinct from gas derived from oil cracking experiments.