Hydrocarbon Prospectivity of East Central Luconia Carbonates, Sarawak

Since the 1970’s, carbonate reservoirs were the main reservoir and contained the largest portion of the reserves in Central Luconia. During the third Petroleum Sharing Contract (PSC) round in 1995, less than 30% of the identified carbonate prospects had been drilled, and a similar situation remains today because of perceived risks. But previous historical carbonate exploration risks in the reefs located in Central Luconia Carbonates (CLC) have been challenged recently with new ideas proven successful with discoveries. Historically, the risks incorrectly downgraded the play, and can be summarized, viz.:<br>i. Carbonate reef pinnacles were thought to be small and contain limited hydrocarbon reserves;<br>ii. Most carbonates were over pressured and would have small gas columns;<br>iii. Many reefs contained high CO2 with significant H2S concentrations; and<br>iv. Thief sands breached the reservoir seal and increased exploration risk.<br>However, for the last few years several gas discoveries made, such as PC4, F2, F38, NC4 and Kanowit fields (Figure 1), contradicts the above reasons for downgrading the carbonate plays. Specifically, the 2006 well, called PC4.1, discovered more than 640 meters of gas column which is the longest single gas column penetrated to date in east Malaysia (Figure 2). The well was drilled under normal pressure conditions thus allowing a larger gas column (Figure 2). Also, low CO2 and minimal H2S contamination dispel previous contamination risks. The crestal seal breach via a trangressive lag deposit, or sand thief, exists in shallow carbonates areas in wells such as Tiong Mas-1, F39-1 (Tiong Gajah) and B16.1. This thief is present basin ward, but marine ward reefs were drowned earlier with thicker shales, so their seal integrity has not breached. These findings are significant. Challenging previous conceived risks has allowed the CLC exploration play to be high graded. In order to map the maximum gas column potential in the carbonates, future geological/geophysical understanding needs to be enhanced regarding thief sand distribution patterns both aerially and stratigraphically, as well as overpressure distribution and hydrocarbon seal capacity. Future 3D seismic will help mitigate the latter uncertainties, delineate with more clarity the reef pinnacles as well as biostromes and platform edges, and possibly dolomitization or recrystallization and dissolution enhancement of porosity in the carbonates. Basically, 3D will increase the possibility of finding more large reserve reefs with TCF potential and potentially thick oil rims. The main gas supply for Malaysia Liquid Natural Gas (MLNG) plants in Sarawak is produced from the CLC reservoirs. At present, only 45 gas fields have been discovered in CLC; but more than 100 identified prospects and leads at present remain to be drilled.