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Abstract: Characterizing Fault Seals and Their Effects on Hydrocarbon Distribution and Reservoir Performance

EISENBERG, RICHARD A., Chevron USA Production Company; ROBERT BRENNEMAN, Western Australian Petroleum Ltd.; ADEPOJU ADEOGBA, Chevron Nigeria Ltd.; U. K. ACHARAYA, CALTEX Petroleum Inc.

A variety of strategies can be developed for characterizing the static and dynamic behavior of fault seals affecting the distribution and production of hydrocarbons. The strategies employed depend largely on the scale of interest and on the types of available data. Seismic data sets can sometimes be used to directly image fault trapped hydrocarbon accumulations. More typically, seismic data combined with horizon picks and fluid contacts derived from well data is used to characterize fault and horizon geometries, reservoir juxtapositions, and potential cross-fault communication pathways. The composition of the fault damage zone between these juxtaposed reservoirs can then be estimated using E-logs. When adequate production data is available, history matching and full field simulation can be used to quantify the relationship between fault gouge composition and fault-zone transmissibility. Analysis of current and original fluid contact relationships, pressure decline histories, and, fluid and gas geochemical signatures within the structural framework forms the basis of our seal characterization methodology in developed reservoirs and has been used to quantify fault seal risk of untested fault-dependent closures. Our seal characterization methodology has been applied in the Okan and Meren fields, offshore Nigeria, and has become an important component of depletion planning and reservoir management.

In the Okan and Meren fields, relationships between fault gouge composition, original fluid contacts, pressure decline histories and reservoir fluid and gas geochemical signatures within fault juxtaposed reservoirs were used to empirically calibrate sealing potential. In these fields, bottom hole pressure measurements taken over thirty years of production provide valuable constraints on the static and sometimes dynamic nature of fault seals. Coincident pressure decline histories of reservoirs juxtaposed across synthetic and antithetic faults are characterized by relatively sandy fault gouges (gouge ratios >3.0). Where data exists, fluid and gas geochemical fingerprints of these reservoirs are identical and further support cross-fault communication. In contrast, specific cross sealed reservoir juxtapositions have different original fluid contacts, distinct pressure decline histories and geochemical signatures, and are characterized by shaley fault gouges (gouge ratios < 2.0). Faults in these fields can not be characterized as sealing or non-sealing but individual reservoir juxtapositions can be. The cross-sealing and/or cross-leaking nature of compartment boundaries is related to fault displacement variation and the composition of displaced stratigraphy. Fault geometry and the relationships between intersecting faults was found to play an equally important role on the distribution of fault trapped hydrocarbons. Combination of the data from the Okan and Meren fields refines the correlation between observed fault sealing behavior and inferred fault gouge composition and is used to help quantify fault seal risk of untested fault dependent prospects within the Niger Delta.

In our fault characterization study of the Meren field an attempt was made to estimate the transmissibility of faults through reservoir simulation. The entire field, consisting of 49 reservoirs, was considered in order to account for the inter-reservoir communication. Connections across faults between reservoirs were accounted for by using a non-neighbor connection option within the reservoir simulator Through the process of history matching the production data available over a 26- year period, fault transmissibilities were estimated. These derived fault transmissibilities, normalized to juxtaposed area, were then cross plotted against calculated fault gouge ratios. The semilog relationship observed between normalized transmissibility and fault gouge composition can be used to constrain the conditions of dynamic cross fault leakage and help optimize exploitation and development strategies.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah