Hydrocarbon Generation and Inorganic Modeling of Carbon Dioxide Generation and Expulsion in The Malay Basin, Peninsular Malaysia

The presence of carbon dioxide in the Malay Basin has often raised queries regarding its origin and distribution. Accumulations in the Malay Basin have been shown to originate from both organic and inorganic sources (Figure 1). Organic sources comprise the decomposition of organic matter with increasing as well as the cracking of hydrocarbon products at high temperatures. Inorganic sources include the thermal breakdown of calcareous shales and limestones, as well as the diagenetic reactions in siliciclastic rocks whereby carbonate minerals such as siderite, dolomite and calcite in clastic sediments react with silicates at temperatures greater than 320°C to generate carbon dioxide. In addition, carbon dioxide contents in both associated and non-associated gases in the Malay Basin can vary up to a maximum of 90%. Our studies indicate that it is erroneous to generalise that carbon dioxide occurrences increase with increasing depth. Another important observation is that low carbon dioxide percentages (less than 20%) do not necessarily indicate an organic origin. However, in most cases where the carbon dioxide contents are greater than 40%, more often than not, they are of inorganic origin. Modeling of the Malay Basin’s hydrocarbon and inorganic carbon dioxide generation was performed using personalised kerogen kinetic parameters and carbonate decompositional kinetic parameters of actual Malay Basin’s samples. The former were determined for the Bergading Deep-1 Group E coals, Beranang 6F- 18.1 Group I fluviodeltaic coaly shales and Bunga Raya-1 Group K lacustrine shales (Figure 2). For the assessment of carbon dioxide generation from carbonates, the decompositional kinetic parameters were determined for the Bunga Raya-1 Group M calcareous shales and limestones. These new kinetics data provide a better control on the results of the carbon dioxide generation modeling as they are specific to the Malay Basin samples. Additionally, predictions of carbon dioxide generation were also determined from modeling<br>the diagenetic reactions within the penetrated sediments using the method of Cathles & Schoell (PGCE 2006). Three locations were selected for the carbon dioxide kinetics modeling, namely Bujang Deep, Angsi and Bunga Raya. Using the newly-acquired kinetic parameters, we were able, for the first time, to determine the timing of inorganic carbon dioxide generation and expulsion as well as its most likely origin in these areas. To ascertain the trapping feasibility of the generated carbon dioxide, the resulting timings were compared with the thermal subsidence and basin inversion of the Malay Basin which occurred between 21 Ma to 6 Ma, with peak trap formation at around 16 Ma. Based on the carbonate decompositional kinetics modeling, the carbon dioxide observed in Bujang Deep should have a mixed origin due to expulsion from the following: both the Group M calcareous shales and limestones at 21 Ma and 20 Ma, respectively (Figure 3), and from the Group K siliciclastic reactions at 14 Ma. The origins were validated by actual measured data whereby the Bujang δ13CCO2 fall within -3 to -6o/oo isotope values, indicating an inorganic origin. There are also carbon dioxide samples with isotope values of - 11.4 and -12.2o/oo, suggesting a mixed origin. The Dulang, Semangkok and Tangga fields located within the middle part of the Malay Basin also exhibit high carbon dioxide occurrences (Figure 1). In the Angsi area, the kinetics modeling indicated that the carbon dioxide encountered by the well should have a strong inorganic influence due to the thermal breakdown of the Group M limestones (Figure 4). Modeling indicated the timing of expulsion to be around 14 Ma. Carbon dioxide contributions may also be expected from the Group M calcareous shale but, since it was generated much earlier than the trap formation at 24 Ma, it is presumably lost. With the bottom temperature of the section being only at 200°C, the diagenetic reactions have not yet started. Traditionally, the carbon dioxide contents of Angsi-1 of less than 20% would be thought of as suggestive of an organic origin. However, the δ13CCO2 values range of between of -5 and -7o/oo tell a different story; these carbon dioxide gases are actually of inorganic origin. Kinetic modeling results corroborate with this indication, thus validating the model. The Bunga Raya kinetics modeling results suggest an inorganic origin for the carbon dioxide observed in the Bunga Raya-1 well, by virtue of being sourced from the Group M limestone at 2 Ma (Figure 5). Again, diagenetic reactions did not contribute to the carbon dioxide accumulation in Bunga Raya.