Application of 3D Basin Modeling to Address Hydrocarbon Charge Risk in a Frontier Area, Offshore Sarawak Basin, Malaysia

Analysis of offset wells in a frontier exploratory area located in offshore Sarawak identified hydrocarbon generation, migration timing with respect to trap formation, and migration fairways into the drilled prospects as the key uncertainties in exploration. A semi constrained 3D basin modeling study was undertaken based on regional 2D depth maps and calibration from offset wells to address the uncertainties. Three source rock units viz. Rift, Cycle-I and Cycle-II, from older to younger sequence, below a major regional unconformity were modeled, constraining the boundary conditions with regional geological understanding. Four geological scenarios were run, to account for uncertainties in mapping the top of basement, and also in fixing the duration of erosion for the <br>unconformity, which has a direct impact on timing of deposition of the regional top seal. Source rock maturity and transformation ratio based on modeling suggest that Cycle-I source is in optimum window in the study area for charging the identified traps. The older Rift source is expended by around 20 Ma, with possibility of charging only the deeper Cycle-I traps. Cycle-II source over major part of the study area is in immature to early oil stage. It can be an effective source in the southern and western parts of the study area where maturity is adequate due to deeper depth of burial. The distribution of hydrocarbon accumulation modeled is validated by two of the offset wells with hydrocarbon discovery. Compositional kinetics indicates dominantly vapor phase for the<br>accumulation in discovery well location, consistent with samples recovered through MDT. Modeling results suggest significant hydrocarbon generation and expulsion in the study area. However, the total hydrocarbon accumulated is relatively small compared to the quantity expelled, as the peak migration timing predates the major unconformity over which the regional top seal section was deposited. Reducing the duration of erosion and early deposition of top seal has increased the volume accumulated, but the parameter remains a key uncertainty due to limited well control. In the absence of direct evidence from well data, the present study has improved the understanding of the frontier area and aided in preparation of play fairway map to risk leads/prospects. The current base model can be refined by incorporating additional data from future exploratory activities. The present approach is a very useful tool in screening new exploration blocks and their ranking.