Faults play a key factor in controlling the accumulation and migration of hydrocarbons. Faults can create barriers for fluid flow, but faults can also act as lateral and vertical migration paths for fluid. However, no fault is perfectly sealing or leaking along its entire length throughout its tectonic history. All faults will leak when the formation pressure in the reservoir exceeds the capillary entry pressure of the fault seal or top seal. One of the main mechanisms for generating fault seal in clastic sequences is by incorporation of clay and shale, into fault zones. One of the methodologies to evaluate the possibility of fault seal is using shale gouge ratio. The Shale Gouge Ratio (SGR) is a measure of percentage shale within a part of the sequence which has moved past a point on the fault surface. Data from wells drilled in the Greater Pratu Tao area indicate that SGR ranges from 0.42 to 0.89 and differential pressure ranges from 5 to 76 psi. In order to validate the prediction of hydrocarbon column, SGR calculations need to be calibrated using lithology, reservoir pressures, and fluid parameters of proven hydrocarbons. The SGR calibration is consistent with observed hydrocarbon columns data in the area. From the cross plot of SGR against differential pressure, a minimum SGR of about 18% is capable of sustaining minimum pressure difference and capable of establishing significant seal. The seal-failure envelope, constructed from cross plot, provides a method to estimate the maximum hydrocarbon column length that can be supported by the fault. This technique can be applied to both the volumetric evaluation of undrilled blocks and the geological risk analysis of exploration projects.
Author : D Suryanto, C Supamittra