Abstract: Fault Seal Analysis of Okan and Meren Fields, Nigeria

Richard A. Eisenberg, Robert J. Brenneman, Adepoju A. Adeogba

The sealing capacity and the dynamic seal behavior of faults between juxtaposed reservoirs were analyzed for Okan and Meren fields, offshore Nigeria. In both fields correlations were found between reservoir performance, juxtaposed fluid types, oil geochemistry, interpreted fluid contact relationships, fault sealing/leaking condition, and calculated smear gouge ratios. Integration of these data has been invaluable in quantifying fault seal risk and may effect depletion strategies for fault-juxtaposed reservoirs within these fields.

Fault plane sections defined reservoir juxtapositions and aided visualization of potential cross-fault spill points. Smear gouge ratios calculated from E-logs were used to estimate the composition of fault-gouge materials between the juxtaposed reservoirs. These tools augmented interpretation of seal/nonseal character based on fluid contact relationships in proved reservoirs and, in addition, were used to quantify fault seal risk of untested fault-dependent closures in Okan.

In Okan Field juxtapositions occur between the G-, H-, L-, M-, and O-sands. Smear gouge ratios correlated to observed fluid contact relationships and pressure decline histories within these juxtaposed reservoirs empirically calibrate sealing potential. The results of these analyses were then used to interpret production-induced fault seal breakdown within the G-sands and also to risk seal integrity of fault-dependent closures within the untested O-sands in an adjacent, upthrown fault block. Within this fault block the presence of potential fault intersection leak points and large areas of sand/sand juxtaposition with high smear gouge ratios (low sealing potential) limits potential reserves within the O-sand package.

In Meren Field the E- and G-sands are juxtaposed, on different pressure decline, geochemically distinct, and are characterized by low smear gouge ratios. In contrast, specific G- and H-sands, juxtaposed across the same fault, contain similar OOWCs and are characterized by high smear gouge ratios. The cross-sealing and/or cross-leaking nature of compartment boundaries at Meren is related to fault displacement variation and the composition of displaced stratigraphy.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France