Abstract

To date eight gas/condensate fields have been discovered through exploration and appraisal drilling by Carigali Triton Operating Company Sdn. Bhd., Operator of Block A-18, JDA on behalf of its Shareholders Petronas Carigali (JDA) Sdn. Bhd., Triton Oil Company of Thailand (JDA) Ltd., and Triton Oil Company of Thailand Inc., Contractors to the Malaysia-Thailand Joint Authority (MT JA). Sequences 0-II of these fields include stacked shaly sand' reservoirs containing low contrast pay. The suppressed resistivities of these reservoirs are caused by cm-dm clay distributions, which vary between facies, making the identification and quantification of reserves difficult.

An integrated petrophysical approach has been adopted to overcome these difficulties. Borehole image-log interpreted facies are treated as classes of clay distribution, and the log responses of each facies are calibrated to core data, acquired separately for each facies. Saturation-height functions are used with wireline formation pressures to project initial conditions capillary pressure saturations into all cored intervals, providing foot by foot log independent gas volumes. For each facies, the log data and Waxman Smits equation are calibrated to these gas volumes through the saturation exponent (n *) The role of n* as a core-log matching parameter is analogous to apparent fluid density in a core-log porosity evaluation. In addition, gamma ray is used to optimize n* for each facies. Results are supported by the fluid mobility seen on limited magnetic resonance logs, and all other available data.

The technique provides a simple, reliable and inexpensive means to evaluate the hydrocarbon potential of low contrast pay. The determination of formation water resistivity (Rw) is circumvented and data acquisition costs are minimal as the required data has often been acquired for other purposes. In this case study its application will result in a gas reserves up-grade for CTOC, its Shareholders and the Malaysia-Thailand Joint Authority.

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