Fault Seal Analysis Application in Carbonate Rock Sequences: Issues and Solution

Abstract

Fault seal analysis (FSA) is a crucial component of hydrocarbon exploration, production, and reservoir management. However, most FSA workflow and theories are developed for normal fault systems that commonly apply to siliciclastic rock. In the case of the carbonate rock, a simple algorithm of FSA using shale gouge ratio (SGR) analysis may generate some errors in sealing and leaking faults. The FSA method is primarily controlled by shale volume distributions rather than by throw, as demonstrated by sealing faults behavior with small throws. This result suggested that good quality sand reservoir distributions become the primary control for the fault sealing capacity. There are three approaches to determining the clay volume: (1) the gamma-ray clay volume calculation, (2) dual parameter (gamma-ray and NPHI-RHOB) clay volume calculation, and (3) multimineral analysis results (probabilistic approach) clay volume calculation. However, carbonate rock sequences have low concentrations of radioactive material that give low gamma-ray readings and SGR values. Therefore, the shale volume value must be calculated using modified petrophysical approaches for better sealing capacity distributions. A detailed FSA study was carried out using integrated research of fieldwork and shallow drilling core data. This field study was in the Rajamandala Carbonate Complex in the Padalarang area. The result of the study will become the focus of this paper. The evaluation involves five core data from shallow drilling at various levels of the Cikamuning normal fault supported by field mapping, thin sections, and XRD. The tentative results suggested that fault gouge in the carbonate rock sequences is complex and varies significantly in composition, thickness, and diagenetic. Further detailed works need to be done, particularly with deep core data.