Using Shale Gouge Ratio (SGR) to Predict Fault Seal Competency: From Example from Haraz-Canar Field, Muglad Basin, Sudan

Amar A. Ahmed and Yasir M. Ghorashi
Exploration Dept., GNPOC, Khartoum, Sudan

Most of the traps in the area of study are tilted fault blocks with high dependency on the lateral seal across the bounding fault. Lateral seal is provided by the lacustrine shaly deposits of the Aradeiba formation. Fault seal analysis as applied to oil exploration has been implemented as a tool to quantify the trap risk of fault bounded structures. This fault-seal study is performed using the Shale Gouge Ratio (SGR) as a predictive algorithm for sealing capacity of the fault rock. Shale Gouge Ratio, (SGR), is an estimate of the proportion of shaly material in the fault zone. The basic assumption in the SGR algorithm is that the fault zone composition is governed by the bulk composition of the wall rocks that have slipped past that point on the fault. Estimating fault zone composition and thus its seal competency depends upon the geologic input to the model, in particular, volumetric shale fraction (V-shale) derived from the gamma-ray log, to be tied to seismic horizon data, and the precision of the three dimensional mapping of reservoir geometry in the vicinity of the fault. The shale gouge ratio, along Haraz-Canar area faults, seems to depend on Aradeiba shale thickness and amount of displacement along the fault plane. The study suggested that an SGR value between 20 and 30% represented a threshold value between non-sealing and sealing faults, in an appraisal context. The calculated fault properties were found compatible when calibrated with several examples of good and failed seal.

Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery