Bulletin of the Geological Society of Malaysia, Volume 32, Nov. 1992, pp. 239 – 246
Sarawak Shell Berhad, Locked Bag 1, 98100 Lutong, Sarawak.
Abstract: The SW Luconia area, offshore Sarawak, is prone to CO2 and N2 contamination. The level of CO2 contamination is extremely high (>60%), especially in the carbonate, when compared to other areas in the Sarawak Basin. This is further evident from the recent exploration well, J32-1, drilled by SSB in the area. The well discovered five separate gas-bearing reservoirs in Neogene sands and limestones. Analysis of the gas from all the reservoirs indicate contamination of CO2 (2-76%) and N2 (1-12%). A high concentration of CO2 was observed in the limestones and an even higher level was recorded in the overlying reservoir sands. Although these contaminants (CO2 &: N2) are generally thought to be basement derived, the vertical gas distribution trends of J32-1 identify the sudden influx of CO2 in these reservoirs as late in situ inputs. It is possible that the gas was released from low temperature reactions of the carbonates with formation waters and from the oxidation of methane in oxygenated waters. This is possible as fluid samples from the reservoirs suggest freshwater conditions and the region experiences a high heat flow. The invasion of meteoric water may have occurred during the Pliocene major uplift when Sundaland emerged in the South China Sea. The distribution trend of N2 suggests contribution from basement. Another possible origin is from humic organic matter which is in abundance in Oligocene deposits. However, the probable inputs from surface via dissolved atmospheric N2 and from humic acids of peat swamps cannot be ruled out. The presence of the contaminants in the well is apparently in general agreement with the overall CO2 and N2 contamination trend seen in the area.