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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

FUNAYAMA, MASAAKI, Teikoku Oil Co., Ltd., Tokyo, Japan, and JEFFREY S. HANOR, Louisiana State University, Baton Rouge, LA

Spatial variations in pore-water salinity provide useful information on the 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 of hydrocarbons. Applying these techniques in highly mature hydrocarbon-producing districts, however, can be complicated by the fact that many old SP-resistivity logs, which may be the primary source of information on pore-water salinity in a particular area, lack header information, such as temperature and Rmf, required to make salinity calculations from SP response.

In our study of the regional migration of pore water and hydrocarbons in the Wilcox Group of central Louisiana, we investigated the statistical relation between Rmf and Rm, mud type and weight, year of measurement, and geographic location for 451 wells drilled after 1957 in four selected townships and established the regional temperature gradients from approximately 300 wells throughout the region of study. The graphs and correlation equations thus generated were then used to calculate salinities in key areas where only older logs lacking complete header information were available.

Pore-water salinities in the shallow updip Wilcox systematically increase with stratigraphic depth and with distance downdip. Some of this dissolved salt has apparently been derived from dissolution of salt domes far to the north. There is no evidence for significant vertical or lateral hydrologic compartmentalization of the updip Wilcox in spite of the fact that there are numerous shaly interbeds.

Our work supports the concept that the shallow Wilcox is sufficiently continuous hydrologically to have permitted long-range migration of hydrocarbons from source beds 100 mi or more to the south.

 

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)