Testing CO₂ charge scenario in the petroleum reservoirs in offshore Sarawak, Malaysia: A simplified migration modeling approach

<p>In petroleum exploration, it is crucial to understand the origin of CO₂ in petroleum reservoirs to avoid CO₂-rich reservoirs. The gas isotopic signature is a clue to its origin; however, it is not uncommon that such geochemical data are lacking or limited. This is the case with offshore Sarawak, Malaysia, where the origin of CO₂ cannot be determined as mantle- or carbonate-origin because of lack of helium isotope data. In this study, the CO₂ origin in the area was examined using a simplified migration modeling approach that considered the CO₂ source areas for mantle- and carbonate-origin scenarios and migration pathways after basin geometry. There were remarkable differences between the CO₂-charged areas, as indicated by the two scenarios. The mantle-origin scenario, which assumed that CO₂ was sourced from an area of crustal thinning, suggested that CO₂ was mainly charged to the North Luconica, West Luconia, and Baram Delta provinces, as well as the areas along the West Balingian and West Baram Lines. The carbonate-origin scenario, assuming CO₂ sourced from the deeply buried thick-shelf carbonates, implied high CO₂ concentrations in Central Luconia, Balingian, southeastern North Luconia, the western margin of the Baram Delta, and the southeastern margin of the Tatau provinces. The modeling results from the carbonate-origin scenario suggest that the deeply buried thick-shelf carbonates alone cannot explain the entire CO₂ distribution in the study area. In contrast, the mantle-origin scenario showed better consistency between the modeling results and measured CO₂ concentrations in the reservoirs. The scenario can be an additional or even alternative explanation to the CO₂ distribution offshore Sarawak. Although this simplified methodology cannot provide conclusive results, it is an effective way to assess CO₂ charge scenarios in sedimentary basins where gas isotope data are lacking or limited.</p>