Stable Gas Hydrates Beneath a BSR: Implications for Resource Inventories and Shallow Hydrocarbon Fluid Flow

Abstract:

Bottom simulating reflectors (BSRs) are the primary indicators of the presence of gas hydrate systems and are generally considered to approximate the base of the gas hydrate stability zone. Here we use a combination of well-log, pressure-core, geochemical and high-resolution 3D seismic data, acquired in deepwater NW Borneo, to report the presence of gas hydrates both above and below a BSR at the top of a thrust-related anticline. This complex gas hydrate system overlies a conventional hydrocarbon reservoir. Hydrates beneath the BSR are interpreted to have a thermogenic origin because they contain significant quantities of C₂₊ hydrocarbons. The base of the hydrate stability coincides at the top of the anticline with a sudden decrease in resistivity in four adjacent wells. Away from the anticline top, in an environment dominated by mass-transport deposits, geochemical data from cores indicate a significant reduction in C₂₊ hydrocarbons. This change in gas composition is thought to reflect variations in hydrocarbon migration effectiveness and mechanisms. We demonstrate that, where thermogenic gases are efficiently transported to shallow parts of basins, hydrate stability zones could be much thicker than suggested by the depths of BSRs. This means that the carbon stored in thermogenic hydrate systems may be underestimated.