USE OF MULTIBEAM BATHYMETRY AND BACKSCATTER TO IMPROVE SEABED GEOCHEMICAL SURVEYS: PART 2, BEST PRACTICES AND CASE STUDIES IN SEEP HUNTING FOR EXPLORATION, WITH AN EMPHASIS ON INDONESIA

Abstract

The successful sampling of seafloor oil and gas seeps can provide information on source rock, migration, maturation and charge that can impact exploration decisions. Although 3D seismic data can be interpreted for potential seafloor seeps, multibeam can be acquired over larger areas at lower cost and in a shorter turnaround time, with bathymetry, backscatter and water column imagery leading to the interpretation of potential seafloor seep targets. The use of multibeam-based exploration early in an exploration project can provide information on the petroleum systems in an area, and also areas where no evidence of a petroleum system is present at the seafloor. In this paper we present case studies of block-specific seep hunting and regional multibeam surveys. For regional surveys, multibeam can be combined with 2D seismic, USBL (ultra-short baseline) positioned piston coring, heat flow, high resolution sub-bottom profiles, marine gravity and magnetics, and core geochemistry to evaluate exploration potential. The first multibeam-for-exploration survey, with cores acquired for geochemistry, was carried out in 2000 offshore Barbados. Applying a tool used in Academia and other industry projects, USBL positioning in real time in a GIS environment was used to confidently place a core barrel on a seafloor seep target beginning in 2002. In 2003, Unocal acquired the first basin-wide multibeam survey for exploration and demonstrate that multibeam can rapidly interrogate a very large area at a relatively modest cost. Indonesia basin- and regional-scale projects demonstrated the value of multibeam-based exploration through the successful farm-out of exploration blocks with a promote (where the farm-in-ee(s) pays a higher percentage of costs than their percentage of equity). We review several examples of seep studies, with an emphasis on Indonesia; target selection philosophy, criteria, and ranking; and target types. Mud volcano and hard substrate sampling can provide information on the sub-surface without drilling.