Shirley P. Dutton, Robert G. Loucks, and William A. Ambrose
Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924
Factors Controlling Permeability Variation in Onshore, Deep Paleogene Wilcox Sandstones in the Northern Gulf of Mexico Basin: Targeting High-Quality Reservoirs
Onshore Gulf of Mexico Exploration I (GRBCC, Assembly Area B)
Tuesday, September 22, 2015, 10:20 am
Onshore Wilcox sandstones in the northern Gulf of Mexico Basin show a clear trend of decreasing average permeability with increasing temperature, but at any given temperature, permeability values commonly range over several orders of magnitude. Characteristics of Wilcox sandstones having permeability in the highest 10% of measured values within a given temperature interval were investigated to determine what parameters exert the greatest control on reservoir quality. The goal was to identify factors to consider in exploration for the best-quality reservoirs in Wilcox sandstones having the same provenance and burial/temperature history. The results provide insight into reservoir quality of deeply-buried Wilcox sandstones in the deepwater Gulf of Mexico.
Reservoir quality in Wilcox sandstones at temperatures ranging from 85–433°F (29–223°C) was investigated using core-analysis and thin-section data. Permeability data from samples located in Louisiana, the upper Texas coast, and the lower Texas coast were sorted by temperature and divided into 50°F (27.8°C) temperature intervals. The P10 value for each interval was calculated as the permeability value separating the highest 10% of the data from the lower 90%. Thin sections of samples having permeability values in the top 10% (P10 samples) were compared with lower-permeability samples from the same well and temperature interval.
Both depositional and diagenetic differences control permeability variation within a temperature interval. P10 sandstones have coarser grain size, better sorting, and lower volumes of detrital clay matrix, silt grains, and ductile grains, which are all parameters related to depositional processes of the sediments and hydraulic properties of the grains. Permeability is also a function of diagenetic differences in the volume of quartz, carbonate, illite, and chlorite cements. Depositional parameters can be addressed in an exploration strategy that focuses on sequence stratigraphic setting and depositional environment. Local variations in diagenesis are harder to predict; the presence of cements such as carbonate and chlorite must be assigned risk factors, because predicting their exact location and distribution is not currently possible.