--> The Heist of the Ngimbang “vault” of the Madura Strait

AAPG Asia Pacific Technical Symposium

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The Heist of the Ngimbang “vault” of the Madura Strait

Abstract

The light isotopic signatures of the multiple shallow dry gas accumulations found across the Madura Strait, Offshore East Java, have led to the general assumption and acknowledgement by the industry of a bacterial degradation origin, with those accumulations being commonly qualified as biogenic gases. However, we consider this interpretation has a weak foundation. Firstly, it relies on the simple observation that the shallow depth of the Mundu and Paciran reservoirs in which those gas accumulations are discovered, correspond to temperature conditions lower than the assumed pasteurization temperature threshold where methanogenic microbes can theoretically be present. Secondly, it assumes that bacterial gas possess an unequivocal geochemical signature (a combination of gas bulk composition and measureable isotopic fractionation) that cannot be the product of thermogenic gas migration mechanisms, a matter which remains heavily debated by the scientific community. As such, there is currently no direct evidence that points towards a unique biogenic origin of those accumulations. To determine which source system is more likely providing the gas in accumulations found across the Madura Strait, we reviewed the regional subsurface setting with respect to processes controlling the activation and preservation of biogenic and thermogenic systems, along with gas migration mechanisms. We considered previous available assessments, and factored in depositional environments and facies, active thermogenic system, burial rate and geothermal gradient, total organic content, temperature and pressure evolution along with methane solubility and exsolution. We conclude that both bacterial degradation and isotopic fractionation during migration could be currently considered as realistic scenarios. If the prevailing system happens to occur via the thermogenic activation of the deep Ngimbang source, it would require a complex plumbing system to link the source to the shallow accumulations, and provide the potential for various deep plays to be present.