When unconventional shale plays were in their infancy, there were a lot of individuals who believed all shales were alike – that is, homogeneous source rocks feeding nearby conventional reservoirs.
The thinking in general was that you could crack into a shale wherever and, voila, the hydrocarbons would spew forth.
But after a number of disappointing wells and other problems, the shale players came to realize this ain’t necessarily so.
Unconventional reservoirs are typically poorly exposed at the surface. The lack of outcrops is a major hindrance to studying and understanding the actual puzzle presented by shale beds.
The unconventional and highly productive Cretaceous Eagle Ford in south Texas is a unique exception.
In addition to the ordinary roadside outcrops, which are affected by weathering and limited scale compared to the subsurface, there are other exposures that are beauts.
Outcrops in Lozier Canyon in western Val Verde and eastern Terrell counties in west Texas offer an opportunity to examine this unconventional carbonate mudstone reservoir at various scales across a sizeable area.
Natural occurring Eagle Ford outcrops appear in various canyon systems where the entire succession can be viewed.
Lozier is said to be the most famous.
“Lozier had been talked about in publications in the 1950s, but it was kind of off limits,” said AAPG member Art Donovan, senior geoscience adviser for unconventional resources at BP in Houston.
“The outcrops are on private property, and we started to have contact with the landowners early in 2010,” he said. “We got access in the fall and began doing studies with various graduate students on those outcrops.”
This is nirvana for any field geologist.
“We found not just the one outcrop that you could see from the highway but many outcrops over about a 50-square-mile area,” Donovan said.
“Tens of cutbank outcrops provide panoramic views of the entire Eagle Ford in exposures hundreds of feet high and thousands of feet long.”
The methodology that the study team used for the Lozier Canyon outcrops was to describe and document the vertical and lateral variability of the Eagle Ford.
“Some localities display laterally extensive exposures of individual bedding planes and key stratal surfaces to allow for the three-dimensional view of sedimentary structures, biogenic debris beds and trace fossils,” Donovan noted.
“Our work to date reveals a vertically heterogeneous facies and TOC succession with variability at the bed-, bedset-, parasequence-, sequence- and sequence-set scale,” he said. “Lateral variations in the thickness, lithofacies and bedding continuity within the various defined members and sequences (can be) observed.”
The Eagle Ford outcrops show vertical heterogeneities and zonation aplenty – and Donovan pointed out that there seems to be more continuity laterally.
“The trick is you gotta stay in zone and realize how much the vertical variability is,” he emphasized.
“There’s actually only one specific part of the Eagle Ford – and that’s the lower member – that’s the primary unconventional reservoir,” he said. “There’s been a lot of trial and error in the subsurface, and people had to learn the hard ways of where they wanted to be in the Eagle Ford.
“The lower member (Lozier Canyon member) is where people usually land the laterals, and it’s that lower member of the lower Eagle Ford that works real well.
“The upper member of the lower Eagle Ford is loaded with bentonite,” Donovan said, “and if you land a fresh water frac in that, it’s not pretty.
“With the gamma ray profile we put there, I think our management was pleasantly surprised with how similar the outcrops were to the subsurface,” he noted. “Clearly, the Eagle Ford was thinner, but a lot of the basic stratigraphy in the subsurface is present in the outcrop.”
While this was a unique opportunity to study an organic rich mudstone, there was a real bonus in that it was the Eagle Ford, which currently is one of the hottest plays in the United States.
The study’s impact likely will extend much further.
“I think it’s a good analog for a lot of similar type of reservoirs, like the basal parts of the Utica and the Haynesville,” Donovan emphasized. “So this has implications far beyond the Eagle Ford.”
At the end of the day, these outcrops provide a rare three-dimensional perspective to document, understand and predict vertical and lateral heterogeneities that exist at a variety of scales within unconventional mudstones.
“They’re some of the best outcrops of mudrocks in the world,” Donovan emphasized. “We go out and teach unconventional reservoir courses there.”
AAPG member Art Donovan will present the paper “A 3-D Outcrop Perspective of an Unconventional Carbonate Mudstone Reservoir” at 1:30 p.m. Tuesday, Aug. 13, as part of the Unconventional Resources Technology Conference in Denver.
The inaugural URTeC will be held Aug. 12-14.
Donovan’s talk will be part of a session titled “Core Evaluation and Depositional Environment.”
His co-authors are T. Scott Staerker and Aris Pramudito, both with BP in Houston, and Rand Gardner and Michael Pope, both with Texas A&M University, College Station, Texas. All are AAPG members.