Carbon dioxide distributions vary in Malay Basin. Structural relief and trap seal effectiveness are the main controls over the distribution of carbon dioxide percentage (Mansor Ahmad, Petronas, personal communication). These conceptual observations could be explained by three principals, viz. CO2 segregation in reservoir, cap rock sealing capacity and hydrocarbon fill-spill. Structural closures with vertical relief of more than 150 meters will accumulate relatively high concentration of carbon dioxide. Closures with vertical relief or hydrocarbon column less than 150 meters will accumulate relatively low concentrations of carbon dioxide if top and lateral are effective. Stratigraphic plays conventionally trap low CO2 accumulation due to their subtle relief and effective seal.<br>To understand the CO2 distribution, ten fields in Malay Basin were selected randomly. These fields are Noring, Jerneh, Bunga Raya, Jambu, Angsi, Bujang, Resak, Beranang, Inas and Ledang. To calibrate CO2 distribution, five fields were selected; namely Jerneh, Bunga Raya, Angsi, Resak and Beranang. These fields have a wide range in CO2 concentration and vertical structural closures ranging from 50-220 meters. The Jerneh structure is a 4-way closure with average relief of 150 meters. Group D and E gas<br>reservoirs in this field encountered low CO2 content, ranging from 0.98 - 7.0%. The CO2 percentages in this field support the theory of CO2 segregation and spill model. These results support the theory of effective seal, which is confirmed by the Equivalent Grain Size (EGS) values ranging from 10.14 - 10.99 phi. Bunga Raya structure is 4-way closure on the northern part and a 3-way closure fault dependant on the southern part. The vertical relief ranges between 30-80 meters. The CO2 content is high (45% to 55%) with an EGS value of 8.15 phi due to silty top seal. From the EGS values, the structures expected are mixed type traps in the northern part with possible oil potential down dip or a possible capillary limited trap. Angsi is low relief and faulted anticline trending NW-SE. The vertical relief is approximately 70 meters in Group I, which increases to about 120 meters deeper at Group K. Structural and stratigraphic traps types are encountered in this field. The field encountered low concentrations of CO2 due to low structural relief, which is supported in tandem with high EGS values ranging between 8.50 - 9.20 phi. CO2 content is low in Group I channel sand due to stratigraphic trap with low hydrocarbon column. Resak is a 3-way fault dependent closure. The CO2 content varies vertically and aerially in every reservoir due to differences in top seal capacity. Thicker and cleaner overlying shale provide effective seal in Group I20.1 reservoir. I30.1 and I50.1 reservoirs contain low CO2 with the EGS value of 8.70 - 9.0 phi. Different situations exist in the I80 and J Group reservoirs, where the CO2 content is high with less values of EGS, ranging from 7.70 - 8.50phi. Beranang structure, located to the south of Resak structure, is a downthrown normal fault dependant closure with relief about 55 meters. Each structure has no relationship due to the different pressure system. This structure encountered high CO2 with EGS value ranging from 4.0 - 5.9 phi. Understanding the parameters that could possibly control CO2 distribution in a basin and top seal capacity will give an explorationist an interpretational tool to explore for low CO2 hydrocarbon prospects. The vertical relief or hydrocarbon column limit varies from basin to basin due to difference in cap rock effectiveness. Different lithologies will have different capillary entry pressures that need different buoyancy pressures before it breaches under similar condition. The EGS method helps in predicting the seal capacity, which is an important element in identifying the low CO2 plays.


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