Abstract

In the ARII (Atlantic Richfield Indonesia Inc.) Offshore Northwest Java contract area, the Oligocene Talang Akar depositional environments have previously been broadly defined as deltaic and marine. An improved interpretation leads to the determination of four environments: 1) continental, 2) delta complex, 3) shore-zone, and 4) shelf. Data from wells, seismic reflection sections and biostratigraphic/paleoenvironmental studies were used to develop the new interpretation.

Each depositional environment is characterized by specific lithofacies and seismic facies which can be related to hydrocarbon potential. The shore-zone, a newly defined environment, was previously identified as either a delta complex or a fully marine setting. The geometry and type of potential reser voir rocks in the shore-zone differ from those of the delta complex. The exploration implications of this interpretation lead to a reevaluation of the hydrocarbon play types which were previously based on the delta model.

Both the delta complex and shore-zone environments have good hydrocarbon potential in terms of source and reservoir rocks. Hydrocarbons accumulate either by migrating from source rocks to surrounding channel and bar sands or updip to the structural and stratigraphic traps.

The depositional history of the Talang Akar formation has been sub-divided into four stages. Each stage corresponds to a specific geologic time as inferred from biostratigraphic zonation. Sedimentation rates, topography and subsidence rates are the three major factors controlling this continuous marine transgressive sequence. The sedimentation rates were primarily control led by the local topographic relief. The rates varied from zero in the non-depositional plains to more than 20 cm/1000 yr in the depressions. Growth faulting enhanced the rate of sediment deposition.