Influence of clay and shale smear in fault zone on the potential sealing of hydrocarbon in block B8/32, Pattani Basin, Gulf of Thailand

Abstract

Fault seal study is aimed at identifying and evaluating influence of clay and shale smear in a fault zone in the hydrocarbon-bearing reservoirs, Benchamas-A field of Block B8/32, Pattani basin in the Gulf of Thailand. Six alternated sequences of 1,230 ft (378 m)-thick sands and shales from 9 drill holes, were selected for this study. Gamma-ray log interpretation indicates that these interbeds are mostly fluvial in origin, and two clay-dominated horizons (av. 220 ft or 68 m) suggest a lacustrine environment. Subsurface data from selected reservoir sand horizons, in a descending order, as U-C6, U-B2, U-B3, U-B4, U-B5, and U-B6 with thickness varying from 14 to 75 ft (4 to 18 m), were used for constructing top depth structure (reservoir sand horizon) maps, net-sand (isopach) maps, and a fault-plane section (or Allan fault diagram). Based on the top depth structure map, the N-trending fault in Benchamas-A field shows a normal sense of movement with high-dip angles to the east and a total vertical displacement of 325 ft (100 m). The net-sand maps indicate large volumes of reservoir sand located in the upthrown eastern side of the fault plane. To analyze the fault seal 3 majors consecutive stages are involved. The first stage is to investigate trapping potential using fault-plane-section analysis. Result from fault-plane-section, a total of 8 sand horizons for both upthrown and downthrown sides of a fault plane, have potentials for fault sealing. They are U-C6, U-B2, U-B3, and U-B5 sand horizons in the upthrown side and the U-B6, U-B3, U-B4, and U-B6 sand horizons in the downthrown side. However, 4 sand horizons including the U-B4 and U-B6 of the upthrown side and the U-B2 and U-B5 of the downthrown side, have leakage potentials where each horizon is juxtaposed against one another across the fault. The second stage is to evaluate fault seals using the shale gouge ratio (SGR) based on calculated shale volumes, bed thickness, and fault throws. The SGR values of40 to 42% are calculated for the U-B4 sand horizon juxtaposed against the U-B2 sand horizon and 62 to 86% for the U-B6 and the U-B5 horizons. These SGR values are obviously greater than the threshold SGR values (15-20%) for static hydrocarbon sealing. The third step is to confirm seal capacity by using pore-pressure profile. The result shows that the across-fault pressure difference varies from 5 to 23 psi (0.3 to 1.5 bar). Such values of pressure difference suggest good sealing capacity in the sand offset areas. In conclusion, the studied fault in Benchamas-A field can be sealed by clay and shale smears on the fault plane. Furthermore, the methodology and the result in this research can be applied to predict the fault seal capacity in the other areas of similar lithologies and structures.