Characteristics of the Highest Permeability Wilcox Sandstones within Temperature Slices, Gulf of Mexico, USA
Lower Tertiary Wilcox sandstones show a clear trend of decreasing average permeability with increasing temperature. At any given temperature, however, permeability values commonly range over four or more orders of magnitude. We investigated the characteristics of Wilcox sandstones that had permeability in the top 10% (P10) within a temperature slice to determine what parameters other than temperature are important in controlling permeability. The goal was to identify factors to consider in exploration for the best-quality reservoirs in a given temperature interval. Core-analysis permeability data from 9,860 Wilcox samples from Louisiana, upper Texas, and lower Texas were sorted by temperature and divided into 50-°F-temperature slices. P10, P50, and P90 geometric-mean permeability values were calculated for each temperature slice within each study area. Characteristics of samples having permeability in the top 10% of values (P10) were compared in thin section to low permeability samples from that same well and temperature interval. P10 permeability samples (>38 md) from Beauregard Parish, Louisiana at 250°–300°F are coarser grained, better sorted, and contain fewer ductile grains than low-permeability sandstones from the same well. P10 permeability samples (>0.2 md) from Brazoria County, Texas at 400°–450°F have lower volumes of detrital clay matrix and fewer ductile grains compared with low-permeability samples. These parameters are all related to depositional processes of the sediments and hydraulic properties of the grains. In contrast, diagenetic differences distinguish high- and low-permeability Wilcox sandstones in other areas. P10 permeability (>6 md) samples from Montgomery County, Texas (250°–300°F) contain lower volumes of carbonate and fibrous illite cement and have coarser grain size. P10 permeability (>1 md) samples from Duval County, Texas (350°–400°F) contain higher volumes of chlorite cement and lower volumes of quartz cement. Chlorite coats partly inhibited quartz cementation and preserved more primary pores in the higher permeability sandstones. This study distinguished the parameters that have the largest impact on permeability among sandstones that share the same burial and thermal histories. Parameters related to depositional processes can be addressed in an exploration strategy that focuses on sequence stratigraphic setting and depositional environment. Local variations in diagenesis are harder to predict.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014