The Onyx field is an undeveloped gas field located in the Poui-Teak Ridge, in the TSP acreage, offshore Columbus Basin, Trinidad & Tobago.

Fault sealing capacity is crucial in the Onyx gas field, which presents clear evidences of hydrocarbon fault leakage along a major fault. This is assessed in this particular case, in order to estimate possible hydrocarbon columns in non-drilled structures within the field. Results are compared against previously published models based on world wide data sets.

The paper approach is based on calibrating computed Shale Gauge Ratio (SGR) for a major fault in the field structure against RFT pressure data across two reservoir fault blocks. It considers Across Fault Pressure Differences (AFPD) as a function of reservoir fluid densities and its columns. An Allan diagram was built for all the major faults of the reservoir structure. In those areas where the diagram shows sand-sand juxtaposition, the relationship between SGR and AFPD is applied. A fault seal failure envelop has been derived from the data and the calibration method is described, making it possible to be applied in fields where lateral sealing faults are major controls in the reservoir trap.

This analysis highlights the likelihood for sands in adjacent fault blocks to be hydrocarbon bearing, and possible hydrocarbon water contacts have been estimated for undrilled structures. This is important for creating possible different success scenarios and for assessing risks for non-drilled blocks in the field.

This type of calibration is relevant and should be assessed on a field by field basis in order to obtain more reliable estimation of remaining potential structures in a hydrocarbon field.

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