The Tertiary Wilcox sands in the Gulf Coast region have long been a favored go-to target for oil and gas prospectors.
The Wilcox Trend extends from south Texas across central Louisiana to the Mississippi border.
Like numerous other geologic formations, the allure of the Wilcox has waxed and waned owing to fluctuating commodity prices, among other issues.
Insufficient porosities in the Middle/Lower Wilcox and problematical petrophysical evaluations, combined with low prices essentially eliminated the Wilcox as an exploration target through most of the 1980s and throughout the 1990s, according to AAPG member Tim Rynott, owner of Durango-based Ridge Resources.
As hydrocarbon prices recovered and technology applications like hydraulic fracturing became commonplace, the Wilcox beckoned anew, attracting principally the mom-and-pop shops and the small to mid-size independents.
USGS Assessment
In 2007, the U.S. Geological Survey released a resource assessment of undiscovered technically recoverable oil and gas potential of Tertiary strata underlying the onshore areas and state waters of the northern Gulf of Mexico coastal region. Assessment Units (AU) were determined using a geology-based methodology.
USGS research geologist Catherine Enomoto provided a succinct overview of the Wilcox AUs included in the 2007 USGS effort:
“Wilcox Stable Shelf Oil and Gas AU reservoirs are sandstones that were deposited in fluvial, deltaic and near-shore marine shelf environments as a result of mostly highstand and transgressive systems tracts,” she said.
She noted there has been considerable exploration and development of the Wilcox Group in this AU area, where reservoir intervals are thin, having temperature and pressure judged to be “normal.”
The USGS estimated the mean undiscovered resource potential of the Wilcox Group here to be 54 MMBO, 472 BCFG and 15 MMBNGL. Some of the productive fields within the extent of this AU were in east-central Louisiana.
In the Wilcox Expanded Fault Zone Gas and Oil AU, the reservoirs are sandstones deposited in deltaic to submarine fan environments, as a result of highstand and lowstand systems tracts, according to Enomoto.
“Deposition was contemporaneous with significant growth faulting controlled by salt tectonics and underlying, older shelf margins,” she said. “Units exhibit expansion adjacent to the extensional faults.”
Reservoir intervals in this AU are thin to thick, having normal to high pressure, and they frequently exhibit an abnormal temperature profile. E&P activity in the Wilcox Group of this AU ranges all the way from mature to frontier.
The estimated mean undiscovered resource potential here was pegged at 52 MMBO, 2,498 BCFG and 75 MMBNGL.
Central Louisiana is the location for some of the productive fields in the AU.
“Within the Wilcox Slope and Basin Floor Gas AU, the reservoirs are sandstones deposited in distal deltaic, continental slope and basin floor environments that were part of distal highstand and lowstand systems tracts,” Enomoto said.
She noted the reservoir intervals range from thin to moderate, with minor to no unit expansion as a result of growth faulting.
With regard to development activity, the Wilcox Group is frontier given that there are no fields in this area in Louisiana. This AU is an inadequately explored area harboring the largest mean undiscovered resource potential, which was estimated by the USGS to be 26,398 BCFG and 423 MMBNGL.
2-D Seismic Data, Well Data
Subsequent to the 2007 USGS effort, the agency acquired 2-D seismic data and well data that augment previous interpretations and studies, according to Enomoto.
“The new seismic and well data should reduce some of the risks considered in the 2007 assessment,” she said, “and will help to refine the range of possible input for the resource assessment forms during the next assessment.”
Enomoto recently completed a study describing some of the well log and reflection seismic data characteristics of the slope and basin floor reservoirs with gas discovery potential that may lay in the area between the Wilcox Group producing trend onshore Louisiana and the Davy Jones 1 discovery well in the South Marsh Island area offshore Louisiana.
“Well logs and 2-D seismic data were used to interpret the depth and morphology of potential Paleocene and lower Eocene Wilcox Group Slope and Basin Floor reservoirs in south-central Louisiana,” she emphasized.
These may occur in the poorly explored area previously estimated by the USGS to contain the largest mean undiscovered conventional resource potential of the three AUs, according to Enomoto.
“It’s difficult to determine whether the sediments preserved in the Wilcox group were deposited on a continental slope or basin floor based on the available data,” she commented. “The interpretation of the newly acquired seismic data suggests that the lower Tertiary strata have been folded and faulted.
“Therefore,” she added, “the Wilcox Group likely has anticlinal closures and fault traps that may have accumulated hydrocarbons.”