Development of gas hydrates in a deep-water anticline based on attribute analysis from three-dimensional seismic data

Abstract

Bottom simulating reflectors (BSRs) on two-dimensional (2D) and 3D seismic data commonly mark the contact between a gas hydrate zone and underlying free gas. They are common features across anticlines in the Northwest Borneo Trough. Amplitude analysis of one anticline covered by 3D seismic data has shown that its BSR is associated with a class III type AVO (amplitude variation with offset) anomaly. The positive AVO anomaly and opposite polarity sign to the water bottom is consistent with a gas hydrate over gas interface at the BSR. Bright spots under the BSR with a positive AVO anomaly are interpreted to represent free gas in more permeable units forming a column as much as 250 m deep, trapped by impermeable gas hydrate. Gas is mainly present on the backlimb of the anticline. The depth of the BSR from the water bottom varies spatially and indicates geothermal gradients ranging from 4.8 to 6.4 °C/100 m, unusually high for a deep-water setting. The temperatures are probably indicative of hot fluid transport into the crests of anticlines, as suggested by previous shallow coring surveys. A double BSR is present in some places and interpreted as residual gas hydrate following an upward migration of the base of the gas hydrate stability zone; this feature corresponds with recent deposition of a mass transport flow. BSR events are regionally located within the anticline cores, indicating that they have channeled warm to hot, thermogenically generated hydrocarbon-charged fluids from a large volume of sediment toward the crest of the anticline. Fluid migration is focused along permeable sedimentary units in the backlimb of the folds, up thrust faults at depth, and along vertical fluid pipes and crestal normal faults shallower in the section.