Abstract

A basin-wide petroleum systems approach to exploration has revealed a new exploration trend in the previously explored Sandakan Basin of the Philippines. Geochemical, biostratigraphic, and geophysical data acquisition and analyses in this large (>12,000 square km), thick (15 km) Tertiary deltaic complex are the basis for this exploration concept, which we hope to test with a well in 1997.

Geochemical analyses of well samples reveal a previously unrecognized mid-Miocene mixed oil/gasprone marginal marine mudstone facies. A recently acquired basin-wide seismic survey shows for the first time, numerous, widespread hydrocarbon indicators. Their areal distribution suggests that the marginal marine source facies along with migration-bounding listric normal faults, limit significant hydrocarbon entrapment to the distal, unexplored part of the delta complex.

Field and subsurface geological studies show that extensive Miocene uplift, erosion, and redistribution of Eocene-Oligocene Crocker Formation siliciclastic sediments account for most of the Sandakan Basin fill. Stacked 5-30 m thick sandstone reservoirs are expected to have 20%-25% porosity and 200-500 mD permeability at target depths. Large mapped structures include normal faults, shale diapirs and ridges, and distal toe-of-slope compressional folds and thrusts. These structures probably formed in response to rapid sedimentation rates (estimated to be 1m per thousand years).