Fluid escape from reservoirs: implications from cold seeps, fractures and injected sands - Part II. The fluids involved

Petroleum fluids escape from hydrocarbon reservoirs through permeable networks of fractures, injected sands and byseepage through low permeability host rocks. Carbon stable isotope analysis of carbonate cement associated with such structures shows that the escaping petroleum fluid is intimately involved in precipitating carbonate cement. Within fractures and injected sands, oxidation of chained hydrocarbons supplies bicarbonate to the co-existing aqueous solution from which carbonate precipitates (δ13C around − 20 V PDB). δ13C values within carbonate crusts associated with seeps are lower (as low as − 50 ℵ VPDB) suggesting a component of carbon derived from methane within these structures. This suggests a separation within the escaping petroleum fluids, with chained hydrocarbons remaining trapped within sand injectites and fractures, whereas more buoyant fluids (methane) continue to escape through low permeability host rocks. The abundance of fluorescing (chained) hydrocarbon inclusions within cement associated with fractures and injected sands versus the scarce occurrence of fluorescing inclusions within carbonate crusts associated with seeps is in agreement with results from stable isotope analysis. Oxygen stable isotope ratios indicate that carbonate cement within fractures and sand injectites precipitates at temperatures between 30 and 50 °C in the subsurface, whereas carbonate cement associated with seeps at the seafloor precipitates at temperatures around 0 °C.