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Spatial Variations in Pore Water Salinity: Implications for Fluid Flow Pathways and Reservoir Compartmentalization in a Deepwater Gulf of Mexico Field

Abstract

In this study, we used well data to estimate the spatial distribution of pore water salinity in a deepwater salt withdrawal minibasin located on the upper slope of the Gulf of Mexico. Using a dual conductance model (Revil et al. 1998), we computed pore water salinity from digital gamma ray, deep resistivity and density porosity well logs. In addition, a correction for hydrocarbons in the pore space was applied (Waxman and Smits, 1968). Pore water salinity estimates from logs were calibrated against core data and well head salinity samples. Two-dimensional seismic data was used to correlate salinity distribution to salt structures and faults. Within the study area two hydrologic zones were identified: 1) a shallow hydrostatically pressured zone with near sea water salinity (35 g/L) and 2) a deeper, overpressured zone with variable pore water salinities ranging from 80 g/L to more than 200 g/L. The boundary between the two zones is around 7500 ft SSTVD. A middle hydrostatically pressured zone with hypersaline pore waters which has been documented in other Gulf of Mexico fields (e.g. Bruno and Hanor, 2003) was not observed here. Movement of pore fluids in the study area are driven by: 1) down dip migration of dense brine fluids from salt structures; and 2) up dip brine migration along fault planes and salt structures into shallower sediments driven by overpressure. Vertical compartmentalization of reservoirs was evident by the difference in pore fluid salinity between sands and adjacent shales. Sands that exhibited fresher pore waters than adjacent shales were interpreted to be the result of sediment dewatering during overpressure generation whereas shallower sands with higher salinities than adjacent shales suggest down dip migration of saline fluids from salt structures.