NATURAL GAS COMPOSITION IN A GEOLOGICAL ENVIRONMENT AND THE IMPLICATIONS FOR THE PROCESSES OF GENERATION AND PRESERVATION

Abstract

Gases recovered from their location of formation (for example, gas recovered from drill cuttings) are enriched in C2+ components relative to gases recovered from reservoir production or testing streams. Because reservoir gases have been derived through a series of processes including generation from a source with a specific chemical character, primary and secondary migration, accumulation and (selective) preservation, the compound distribution within these reservoir gases is less likely to be representative of the generation process than in situ gases such as cuttings gas. Thus, rather than comparing compound distributions within reservoir or pipeline gas data, to test whether laboratory catalysis experiments for the generation of gas are consistent with geological systems, it is better to compare laboratory results of the distribution of gas compounds with gas data for which the opportunity for a wide range of different processes can be precluded. The mismatch between compound distributions within cuttings gas recovered from fine grained rocks and those recovered from laboratory experiments disproves the hypothesis of gas generation catalysis in geological systems, whereas the distribution of compounds within cuttings gas is consistent with simple, non-catalytic cracking. The generation of natural gas in geological systems has been inferred, in some publications, to be a universally catalytic process on the basis of observations made during laboratory experiments. However, this extrapolation has been predicated on the premise that the distribution of compounds within natural gas (% methane in C1-C4) in reservoirs is different from that observed from non-catalytic or simple cracking experiments, and that reservoir gas precisely represents natural gas composition as it is generated within source rocks. The extrapolation of the inferred kinetic stability of low molecular weight hydrocarbons such as propane determined during high temperature (>=530°C), low pressure (<600 mm Hg), water-free experiments to geological time scales yields lower reaction rates than those actually observed for geological systems: the inferred catalytically mediated kinetics yield reaction rates that are too high. These observations, coupled with the proof that transition metal catalysis is not operating in geological environments, suggest that the overall kinetic controls under geological conditions are different from those in the dry, quartz, laboratory experiments.