OS5B-5:IDENTIFICATION OF BOTTOM SIMULATING REFLECTORS (BSRs) WITHIN THE ORANGE BASIN, SOUTH AFRICA

    Bottom Simulating Reflectors (BSRs) were identified and mapped to determine the potential for the presence of gas hydrates in an area covering approximately 72,000km2, within the Orange Basin off the west coast of South Africa. The development of the Orange Basin is related to the breakup of the super continent Gondwana, and underlies the Atlantic Ocean along the southwest African coast between latitudes of about 27 and 35 degrees south. BSRs were interpreted within the Late Cretaceous (~88-67Ma) from 2D multichannel seismic in time, as well as the relative acoustic impedance seismic attribute and well data. 

    BSRs were recognised up to 200km seaward of the South African shoreline in water depths ranging between 100 and 450m. The BSRs were seen between 0.3s (~400m) and 1.1s (~800m) two-way-time which corresponds with previous work (Ben-Avraham, 2002) of water depths in excess of 400m. Furthermore, the BSRs were found to be generally continuous along the seismic sections and to exhibit a decrease in p-wave velocity and acoustic impedance. The primary purpose to identify BSRs is to define the base of the gas hydrate stability zone (GHSZ). The thickness of the GHSZ varies between 0.15s and 0.57s (two-way-time), and the calculated thickness varies from approximately 200m to 500m in the study area. Average interval velocities vary between 1500m/s and 2500m/s. The zone is thickest in the central to southern parts of the Orange basin, and thins towards the present-day shoreline and to the north of the basin. The estimated geothermal gradient for approximately 23 wells in the study area is 3.180C/100m [i.e. 31.80C /km higher than the average worldwide geothermal gradient of 25-300C/km for continental crust. The difference is likely due to an inaccurate estimate of the geothermal gradient as suggested by the sometimes large variance in the calculated values from the data. However should these values be realistic, the higher than normal value could be attributed to volcanics in the basin. 

    The BSRs in the study area are often accompanied by vertical faults, fluid escape and sediment collapse features within the sub-surface, as well as carbonate mounds, mud volcanoes and pockmarks at the seafloor. 

    Since BSR delineation is only a first step in hydrate evaluation, future work aims to identify evidence of gas sources and gas-hydrate-bearing units that are of sufficient reservoir quality to be viable future resource development targets.