Sedimentary basins contain much of the earth's exploitable metalliferous mineral resources and all of its hydrocarbon reserves. These accumulations result from either vertical and/or lateral movement of fluids from source rocks to reservoirs. An idea of the scale of fluid movement associated with resource emplacement can be obtained from studies of the occurrence of sparingly soluble minerals such as barite (BaSO4).
The occurrence of large accumulations of barite provides a ready indicator of fluid mixing processes in the sub-surface. Although cementation of sandstones by barite during diagenesis may be relatively rare, it provides evidence for the movement of appreciable quantities of pore fluids. Barite has a solubility product at 25C of 10-10 moles l-1, such that its occurrence in significant quantities, i.e. as a pervasive cement, implies that mixing of two chemically-distinct fluids, one relatively Ba-rich and SO4-poor; the other Ba-poor and SO4-rich, must be invoked. The incompatibility of Ba and SO4 in the same fluid means that effectively all the Ba must be sourced in one fluid and all the SO4 in the other.
Published data on the occurrence of pervasive barite (BaSO4) cement associated with faulting in two sandstone reservoir systems, one containing hydrocarbon gas in the southern North Sea (the Amethyst Field), the other bearing metalliferous minerals in the onshore UK Cheshire Basin (Alderley Edge), have been used to attempt to constrain the amounts of fluid involved in resource emplacement. Mass balance calculations using plausible estimates of source fluid composition suggest fluid volumes of at least 3-40 km3 were responsible for barite precipitation. These fluid volumes could have transported a substantial proportion of the metals and hydrocarbons observed in the two reservoir systems. The lower estimates of the volumes of groundwater calculated are at least an order of magnitude greater than those released during 'single-event' historic large magnitude normal-fault earthquakes documented in the literature. The upper estimated volumes are similar to those calculated for the time-integrated interactions of rare gases with sedimentary pore fluids.
Given suitable source rocks, it is almost inevitable that economically-significant constituents (heavy metals, oil, gas) will accompany this migrating fluid, either as a single, or multi-phase fluid. Quantifying the amounts and mechanisms of fluid movement in the deep sub-surface will lead to a better understanding of the accumulations of economic resources.
For more information, please contact Richard Metcalfe.