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Pore Water Salinity as a Tool for Evaluating Reservoir Continuity and Fluid Migration Pathways in the Wilcox Group of Central Louisiana
Masaaki Funayama (1,2), Jeffrey S. Hanor (1)
Spatial variations in pore water salinity derived from SP logs can provide useful information on the degree of hydrologic continuity and compartmentalization of sedimentary sequences and can thus aid in evaluating both local and regional reservoir continuity and in determining possible migration pathways for hydrocarbons. Applying these techniques in highly mature hydrocarbon-producing districts, such as the Wilcox, however, is complicated by the fact that many of the older SP logs lack header information, such as bottom hole temperatures, BHT, and values for Rmf, required to make salinity calculations from SP response.
We have determined the statistical relation between Rmf and Rm, mud weight, and year of measurement for wells having complete header information in the Wilcox Group of central Louisiana. We have also determined BHT-depth relations in this region from nearly 700 paired measurements. The correlation equations thus generated can be used to calculate formation water salinities in key areas of the Wilcox where only older logs lacking complete header information are available.
Formation water salinities in the shallow, updip Wilcox in central Louisiana vary from less than 35 g/L to over 100 g/L. Salinities generally increase with stratigraphic depth and with distance downdip to the south, until the Wilcox starts to become overpressured. Evidence for lateral compartmentalization exists within the overpressured downdip Wilcox, where there is an abrupt reduction in salinity to the south across a series of growth faults. There is no evidence for significant regional vertical or lateral hydrologic compartmentalization in the shallow updip Wilcox in spite of the presence of numerous shaly interbeds. Some of the saline water in the shallow Wilcox may have been derived from dissolution of salt domes 100 km or more to the north. Our work supports the concept that much of the shallow Wilcox is sufficiently continuous hydrologically to have permitted large-scale vertical and lateral fluid migration and solute transport.
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