Professor Richard Worden, University of Liverpool - Carbonate oil and gas reservoirs that contain hydrogen sulphide (H2S): a dangerous gas with a surprising benefit

Conventional oil and gas fields sit in either carbonate or sandstone reservoir rocks. It is typically assumed that the inorganic, crystalline minerals in carbonate and sandstone reservoir rocks have negligible chemical interaction with organic fluids in oil and gas fields. However, anhydrite (calcium sulphate) is fairly common in both sandstone and carbonate reservoirs. The sulphate in anhydrite has been shown to react with chemically-reduced hydrocarbons in oil and gas fields in a process known as thermochemical sulphate reduction. This is a less dramatic version of the type of explosive redox reaction that happens in gunpowder. Thermochemical sulphate reduction leads to large-scale conversion of anhydrite to calcite in the reservoir as well as creating the highly dangerous and corrosive gas, hydrogen sulphide (H2S). Discoveries with elevated H2S are a great disappointment to oil and gas exploration companies; note that H2S can represent up to 30 or 40% of the gas in some thermochemical sulphate reduction–affected accumulations. H2S degrades the economic value of a discovery due to the unusual need for expensive high-grade steel in all pipework and processing equipment, the cost of H2S-removal from the produced fluids and the environmental premium that must be dealt with. Sandstone reservoirs seldom have high concentrations of H2S since iron-bearing clay minerals naturally mop up H2S resulting in growth of pyrite in the reservoir. Carbonate reservoirs are the ones that are at risk of high H2S concentrations and thermochemical sulphate reduction has been the focus of much work in reservoirs in the SE states of the USA, the Western Canada Sedimentary Basin, the Arabian Basin, and the Sichuan and Tarim Basins in China. Recent research has focused on the conditions that lead to anhydrite-petroleum thermochemical sulphate reduction. Much work has also been focussed on the long term geological fate of reactive H2S in Cambrian reservoirs in the Tarim Basin with attention directed at the coincidence of (what are effectively) lead-zinc mineral deposits within H2S-rich carbonate fields. There has also been new work addressing the natural loss of H2S by back-reaction with remaining oil compounds leading to an array of newly generated organo-sulphur compounds, such as sulphur-bearing diamondoid compounds. Finally, recent work has found that H2S-rich oil and gas fields tend have higher porosity and permeability than equivalent H2S-free fields with the suggestion that reservoir quality is actually enhanced by the generation of H2S in reservoirs.