Integrated Fault Seal Analysis and Risk Assessment: Okan and Meren Fields, Nigeria

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

Integration of production, geochemical, seismic, well log, and structural data provides important constraints on the sealing capacity and dynamic behavior of fault juxtaposed reservoirs in Okan and Meren fields, offshore Nigeria. Correlations were found between pressure decline histories, juxtaposed fluid types, oil composition, fluid contact relationships, fault sealing/leaking condition, and estimates of the composition of the fault gouge. Fault plane sections defined reservoir juxtapositions and potential cross-fault spill points. Smear gouge ratios calculated from E-logs were used to estimate the composition of fault-gouge materials between juxtaposed reservoirs. These tools augmented interpretation of seal/nonseal character in proved reservoirs and were used to quantify fault seal risk of untested, fault-dependent closures.

In the Okan Field juxtapositions of the G-, H-, L-, M-, and O-sands were analyzed. Smear gouge ratios correlated to 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 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 column heights and potential reserves within the O-sand package.

In the 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, the 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 and the composition of displaced stratigraphy. Combination of the data from the Okan and Meren fields continues to refine the correlation between observed fault sealing behavior and inferred fault gouge composition and can be used to help assess fault seal risk of untested fault-dependent prospects within the Niger Delta.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California