The tembakau gas accumulation, tenggol arch, offshore peninsular malaysia: Petroleum system, gas composition and gas migration

Abstract

The Tembakau gas discovery is a mix of biogenic and thermogenic gases, with condensate wetness index increasing with depth in the reservoir. Surprisingly though, even the dry gas in the upper reservoirs appears to have an affinity with oil and Type II kerogenous rock of probably Lower Tertiary age. The temperature regime in the gas-bearing reservoirs is relatively elevated, with a temperature gradient of 4.55 ⁰C/100m. Given that the potential source rocks in the area surrounding Tembakau were found to be immature for hydrocarbon generation, the discovered hydrocarbons might have reached the accumulation by long distance migration. A deeper source below the Palaeozoic unconformity can be excluded in view of mud-log and gas composition data. Therefore we propose in this study a model that addresses the possibility of aquifer degassing in the Tembakau accumulation, perhaps as a result of a combined recent pressure drop and/or temperature hike in the order of 30 ⁰C, which could have prompted rapid degassing of the aquifers. The Tembakau gas might therefore be a secondary gas cap, derived from gas saturated aquifers (“water washing”) connected to the Sotong and South Angsi fields at the western edge of the Malay Basin, and having reached the Tembakau area after a tortuous migration pathway, whilst being subjected to some degree of biodegradation.