Boquillas appears to have same depositional setting

Outcrops Instructive for Eagle Ford

Shale gas plays have elevated land drilling activity to the point that it’s a tad off-putting to imagine the state of the onshore industry today had they not materialized only a few years ago.

In fact, these plays have taken on added appeal lately owing to the associated quantities of high-value condensate and natural gas liquids they sometimes contain, not to mention actual shale oil plays.

For example, think Bakken shale oil in the Williston Basin and the shale gas plays with a considerable liquids component, such as the Cretaceous-age Eagle Ford shale in south Texas.

It’s no surprise that these type plays have become increasingly alluring as natural gas prices are languishing around $4/Mcf. Meanwhile, oil currently is maintaining a grip around the $80/bbl range.

This has prompted a number of companies – including EOG Resources, Devon Energy, Anadarko and committed natural gas devotee Chesapeake Energy – to pump up their attention on shale plays offering something more than gas.

Reportedly jumping into the Eagle Ford action are industry heavyweights Royal Dutch Shell, ExxonMobil, ConocoPhillips and BP.

This new twist on shale gas potential has elevated the appeal of the Eagle Ford play in particular. It boasts a confirmed oil and condensate window and extends from near the Mexican border outward to the east/northeast across several counties.

When Petrohawk Energy Corp. went looking for natural gas in the Eagle Ford, it ultimately drilled the discovery well in the now-high-profile play in 2008 in what would be christened the Hawkville Field.

The Hawkville is characterized by a downdip dry gas play in the southwest, a mid-dip gas condensate and an updip oil play, according to AAPG member Dick Stoneburner, executive vice president and COO at Petrohawk.

The Eagle Ford takes on the moniker Boquillas at the eastern county line of Val Verde County slightly northwest of the westernmost part of the main play. This area is significant given that outcrops in Val Verde and Terrell counties appear to be in the same depositional basin setting as the Maverick Basin Eagle Ford wells to the south, enabling improved understanding of the play, according to AAPG member Brian Lock, who is the Robert and Barbara Pettit professor of geology and graduate coordinator at University of Louisiana, Lafayette.

“The Boquillas formation, as it outcrops in Val Verde County, is the lateral equivalent of the productive Eagle Ford formation of southwest Texas,” said Lauren Peschier, geologist at Newfield Exploration and an AAPG member who penned her master’s thesis on this topic working with Lock.

“The outcrops display components of the formation not seen in the subsurface,” she added, “enabling a full sequence stratigraphic interpretation.”

These outcrops include certain deep road cuts along U.S. Highway 90 in Val Verde County that are particularly instructive for understanding the depositional environment of the Eagle Ford.

Visits to study the exposed geological sequence are very “in” these days.

“A lot of people didn’t even know these beautiful road cuts are there,” Peschier said. “Now they’re all wanting to go on field trips, including the big companies like Shell and ConocoPhillips.”

The Three Members

The Boquillas is approximately 200 feet thick in Val Verde County, slightly thicker in the producing counties and over 10 times thicker in northern Mexico, according to Lock. He noted it was deposited during the Cenomanian-Turonian oceanic anoxic event.

The outcropping Boquillas formation can be informally divided into three members, Lock noted:

  • The lowest member consists of unstable slope strata, including debris flows, slump folds and breccias, possible turbidites and contourites.
  • The upper member is marked by several thicker limestone beds at the base, with chondritid burrows normally associated with low oxygen environments. The main part of this member contains abundant irregular echinoids and is interpreted to represent a return to more normal oxygenated water conditions.
  • The middle member is a turn-on, economically speaking, for the industry folks.

“The middle member can be further divided into lower beds comprising limestone-shaly limestone cycles, or parasequences, that form a transgressive parasequence set with an upwards decrease in the number of carbonate beds,” Lock said.

“The middle beds in the middle member are composed almost entirely of shaly carbonates,” he noted, “while the upper beds include increasing proportions of limestone toward the top and are interpreted as a regressive parasequence set.

“Where it appears in an unweathered state, the middle member is comprised of black organic-rich strata that emit a strong petroleum odor when broken,” Lock remarked.

“Fine laminations are undisturbed by bioturbation, and every indication suggests anaerobic sub-seafloor and dysaerobic bottom water conditions during sedimentation,” Peschier added. “Planktonic forams and calcispheres are the common microfossils accompanied by small ammonites; the low-oxygen tolerant inoceramids are the only bottom-living fauna.”

The Potential

Peschier succinctly summarized some salient points of the middle member of the Boquillas:

  • TOC values up to 8.3 percent have been reported from Central Texas and 5.7 percent from some Val Verde County samples.
  • High carbonate content equals increased brittleness.
  • Porosity: pores within organics, between clay flakes, between larger carbonate grains.

“Although thermally immature in outcrop, the high TOC content of these rocks combined with significant matrix porosity indicate the excellent reservoir potential in areas where maturation has progressed, and sufficient natural and induced fractures become significant,” Lock said.

“The Type I kerogen explains the potential for shale oil production.”

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