Stability of natural gas in the deep subsurface

Abstract

Natural gas is becoming increasingly important as a fuel because of its widespread occurrence and because it has a less significant environmental impact than oil. Many of the known gas accumulations were discovered by accident during exploration for oil, but with increasing demand for gas, successful exploration will require a clearer understanding of the factors that control gas distribution and gas composition. Natural gas is generated by three main processes. In oxygen-deficient, sulfate-free, shallow (few thousand feet) environments bacteria generate biogenic gas that is essentially pure methane with no higher hydrocarbons ({open_quotes}dry gas{close_quotes}). Gas is also formed from organic matter ({open_quotes}kerogen{close_quotes}), either as the initial product from the thermal breakdown of Type III, woody kerogens, or as the final hydrocarbon product from all kerogen types. In addition, gas can be formed by the thermal cracking of crude oil in the deep subsurface. The generation of gas from kerogen requires higher temperatures than the generation of oil. Also, the cracking of oil to gas requires high temperatures, so that there is a general trend from oil to gas with increasing depth. This produces a well-defined {open_quotes}floor for oil{close_quotes}, below which crude oil is not thermally stable. The possibility of a {open_quotes}floor for gas{close_quotes} is less well documented and understanding the limits on natural gas occurrence was one of the main objectives of this research.