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Stratigraphic Play in Active Margin Basin: Fluvio-Deltaic Reservoir Distribution in Ciputat Half Graben, Northwest Java Basin

Abstract

Ciputat Sub-basin is a north-west half graben within North West Java Basin. Rifting has begun during Late Eocene until Late Oligocene, formed by subduction activities. Syn-Rift in Ciputat Sub-basin divided into two phases. Syn-Rift Phase-1 begun with the deposition of the volcanic clastic and carbonate sediment. In the Syn-Rift Phase-2, fluvio-deltaic clastic sediment deposited unconformable above Syn-Rift phase-1 sediment. The fluvio-deltaic sediment is composed by clastic rock and proven as stratigraphic reservoir in the Fields around Ciputat Low. Knowledge about the clastic reservoir distribution and sedimentation will assist to determine hydrocarbon resources and prospect area for exploration and development activities. Stratigraphic Interpretation generated using 3D Seismic data is difficult to detect clastic reservoir distribution, due to thin reservoir (below tuning thickness). However, nearly 90% of wells drilled in the Ciputat Low discover hydrocarbons in this reservoir despite trapped stratigraphically. Utilization of Frequency-derived Attributes performed to illustrate the geologic layering in thin layers. Geologic frequency is simply the number of reflecting layers within unit thickness of rock, higher geologic frequency means more layering (Brown, 2010). In this circumstances, interpretation of reservoir distribution conducted by applying the Average Instantaneous Frequency. The attribute shows sedimentation pattern of clastic reservoir validated by existences production wells inside it. Low frequency thus correlates with better production from clastic reservoir. It can be considered thicker sand-shale layering have lower frequency than thinner sand shale layer, even the thickness of these layers are below tuning thickness. The clastic reservoir described by Average Instantaneous Frequency showing distribution pattern that follows axial fluvio-deltaic system along the half graben. Sediment provenance of this sub-basin comes from the Sunda Microplate in North-West. According to the well data showing the presence of conglomerate, sediment provenance can also come from the local highs around. Development and exploration activities in this area can be focused to recognize stratigraphic trap of clastic reservoir.