ABSTRACT: Nunn, et al.

Jeffrey Nunn1,2 and William Daugherty1,3
1Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803
2Presently located at Chevron Energy Technology Company, Houston, Texas
3Presently located at BHP Billiton, Houston, Texas

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

Advanced Studies of Petroleum Systems (GRBCC, Room 320ABC)
Monday, September 21, 2015, 4:35 pm

Well data are used 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, pore water salinity is computed from digital gamma ray, deep resistivity, and density porosity well logs. In addition, a correction for hydrocarbons in the pore space was applied. Pore water salinity estimates from logs were calibrated against core data and well head salinity samples. 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 approximately 8000 ft true vertical depth subsea. A middle hydrostatically pressured zone with hypersaline pore waters which has been documented in other Gulf of Mexico fields was not observed here. Movement of pore fluids in the study area are driven by: (1) downdip migration of dense brine fluids from salt structures; and (2) updip 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 clay dewatering during deep burial whereas shallower sands with higher salinities than adjacent shales suggest downdip migration of saline fluids.