Heat flow and surface hydrocarbons on the Brunei continental margin

Abstract Simultaneous heat flow and geochemical gravity coring data from 186 sites on the Brunei margin reveal abundant thermogenic hydrocarbons in the landward half of our study area, where the mean heat flow is 83.7 ± 66.5 mW/m2. Seaward, the mean heat flow is 59.0 ± 22.6 mW/m2, and surface thermogenic hydrocarbons are largely absent. In accord with active accretionary complexes, the low-heat-flow zone coincides with the Palawan (northwest Borneo, Nansha) Trough paleosubduction zone. The high-heat-flow zone of hydrothermal convection and hydrocarbon seepage coincides with the landward, land-derived Baram delta sediments, constituting a pseudo–accretionary prism. The transition from oil to gas production with increasing geothermal gradient, observed in well data, appears to be reflected in our surface data. Equality of Brunei and China margin heat flow predicts a common thermotectonic origin that predates by less than or equal to 5 m.y., the oldest (32 Ma) magnetic lineations in the South China Sea Basin. Thermal effects of prior active subduction, if any, have dissipated, and Brunei margin heat flow has rebounded to theoretical passive-margin values. A single megaseep exhibits maximum heat flow (604 mW/m2) coincident with anomalous thermogenic hydrocarbons. Vertical fluid flow at 1.7 cm/yr (0.67 in./yr) (5.5 × 10−10 m/s; 1.80 × 10−11 ft/s) from 6 km (3.7 mi) depth, implying greater than 30 times focusing of flow, can account for this heat flow and provide hydrocarbon transport from potential sources. A 42 times higher flow rate via bubble ascent or continuous gas-phase flow can also account for our data. Simple models of fluid flow around fault-bounded sediment troughs reproduce the observed heat flow. These models predict that measurements confined to trough interiors, where heat flow is uniform, seriously underestimate mean regional heat flow (23–80%) and thermal maturation; whereas heat flow at all geochemical coring sites yields reliable means. Megaseep data reveal systematic changes in thermogenic hydrocarbons and heat flow with distance from the seep axis. A simple diffusion model represents these changes in terms of bulk near-surface processes. A simple thermogenic model also simulates gas data; however, thermal-maturation parameters indicate no causal connection between megaseep heat flow and thermogenesis. Invariant parameters, less affected by migration, fractionation, mixing, and biodegradation, remain anomalous more than 250 m (800 ft) from the megaseep axis, encompassing all four high-heat-flow sites. This constitutes a significantly greater aperture for identifying seeps in coring data compared with headspace gases, found anomalous at one site only. Like heat flow, invariant parameters that are extreme at the megaseep may particularly reflect more active seepage, where hydrocarbons are less altered and more closely reflect their sources. Regional data covering 10,000 km2 (3600 mi2) largely reflect the same near-surface processes occurring within 500 m (1600 ft) of the megaseep. Consequently, distances from regional seeps and paleoheat flow can be inferred.