Seeking the 'Goldilocks Window' of the Austin Chalk

Sourced in part by the Eagle Ford Group, the Austin Chalk has been a hot spot for operators on and off for a century. The rise of horizontal drilling and hydraulic fracturing brought a renewed interest in the formation, which has produced new discoveries in Texas and Louisiana.

Yet in parts of the Austin Chalk, extracting oil and gas can be extremely tricky. In this highly fractured reservoir, hydrocarbons are commonly present in nanopores and micropores, requiring extensive knowledge of the rocks for sweet spots to be identified.

However, knowledge of such rocks is scarce, as many operators have relied mostly on seismic data in the past to successfully drill for the Chalk. To further exploit the reservoir at deeper depths, a major exercise in characterizing the geology is required.

Several years ago, the Carbonate Reservoir Characterization Research Laboratory at the Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin began a project to analyze approximately 40 cores from the Austin Chalk – the first group to do so.

Its goal is to make information available to extend the success of this unique play, as the Austin Chalk is not only sourced by the Eagle Ford below, but self-sourcing as well.

“Overall, until one knows the geologic characteristics, they cannot understand the data they collect from seismic, wireline-logs, production volumes, etc.,” said AAPG Member Robert Loucks, a senior research scientist at the BEG. “A solid understanding of the rock characteristics is necessary to understand how to explore for and exploit the Austin Chalk.”

History of the Chalk

The Austin Chalk extends from south Texas into central Louisiana and is a naturally fractured reservoir. It has been a significant oil and gas producer since the 1920s, with historical hotspots in the Pearsall and Giddings fields in Texas.

Image Caption

Outcrop of the Eagle Ford and Austin Chalk contact off Kiest Blvd. a fifth of a mile east of Patriot Parkway in Dallas County. Photo by Joshua Doubek.

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Sourced in part by the Eagle Ford Group, the Austin Chalk has been a hot spot for operators on and off for a century. The rise of horizontal drilling and hydraulic fracturing brought a renewed interest in the formation, which has produced new discoveries in Texas and Louisiana.

Yet in parts of the Austin Chalk, extracting oil and gas can be extremely tricky. In this highly fractured reservoir, hydrocarbons are commonly present in nanopores and micropores, requiring extensive knowledge of the rocks for sweet spots to be identified.

However, knowledge of such rocks is scarce, as many operators have relied mostly on seismic data in the past to successfully drill for the Chalk. To further exploit the reservoir at deeper depths, a major exercise in characterizing the geology is required.

Several years ago, the Carbonate Reservoir Characterization Research Laboratory at the Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin began a project to analyze approximately 40 cores from the Austin Chalk – the first group to do so.

Its goal is to make information available to extend the success of this unique play, as the Austin Chalk is not only sourced by the Eagle Ford below, but self-sourcing as well.

“Overall, until one knows the geologic characteristics, they cannot understand the data they collect from seismic, wireline-logs, production volumes, etc.,” said AAPG Member Robert Loucks, a senior research scientist at the BEG. “A solid understanding of the rock characteristics is necessary to understand how to explore for and exploit the Austin Chalk.”

History of the Chalk

The Austin Chalk extends from south Texas into central Louisiana and is a naturally fractured reservoir. It has been a significant oil and gas producer since the 1920s, with historical hotspots in the Pearsall and Giddings fields in Texas.

“The Austin Chalk is important because some of the most productive wells in the Lower 48 have been drilled in this interval,” said Michelle Thoms, senior geology associate at RS Energy Group, a part of Enverus. “Operators are most actively targeting the Austin Chalk in Karnes County, Texas, and are achieving comparable, if not more competitive returns than the Lower Eagle Ford.”

Historically, the Austin Chalk was a vertical target. As horizonal drilling ramped up, the Chalk piqued the attention of operators, Thoms explained. “Due to the geologic complexity, success has been more localized compared to the Eagle Ford and results have been mixed throughout the basin,” she said. “However, wells in Karnes and Washington counties continue to see very competitive economics and are a component of operators’ remaining inventory.”

Low porosity rocks are a new area of focus.

Knowledge of the Chalk was initially sought to provide information about the Eagle Ford, rather than the opposite, explained AAPG Member Chris Zahm, a fractured reservoir specialist and research scientist associate at the BEG.

“The Austin Chalk still holds significant resources of oil, gas and condensate. It is different from the Eagle Ford because it is more of a low porosity conventional reservoir or fractured reservoir, rather than a resource play,” he said. “Challenges remain on where to find the ‘Goldilocks Window’ of stored oil within the low-porosity Austin Chalk that can be stimulated without creating pathways for water to encroach the newly formed hydraulic fractures. If understanding the combined elements can be determined, the Austin Chalk has significant volumes of hydrocarbons that can be produced.”

The existing well log suites available for the Chalk are dated. The density of high-quality logs used to characterize key factors, such as water saturation, are not present, Zahm said. Because the Chalk has low margins owing to the drilling depths, sometimes in higher pressure and temperatures, high-quality log suites were often not acquired.

While 3-D seismic coverage of south Texas is good, emerging areas such as southern Louisiana require more for producers to make intelligent decisions for drilling, Zahm said.

Core data is a key data source.

“There is critical information on the reservoir quality and potential for hydrocarbon development within the existing core catalog, but the data is sparse with respect to the key areas where producers are looking to push the play,” he added. “’Discovery’ is an interesting term for the Austin Chalk. It has been producing for decades. The key question is whether the technology is in place for intelligent extraction.”

Knowing the Rocks

To help bridge the gaps in the geologic characterization of the Austin Chalk, the RCRL is currently studying cores and outcrops, describing approximately 40 cores that are distributed from the Texas-Mexico border through central Louisiana,” Loucks said.

The group is looking to measure and understand rock strength and rock mechanics to predict fracture distribution and magnitude; study the variability and distribution of the clay-rich material in the reservoir and how it affects natural and induced fractures; and determine the percentage of total organic matter to identify those areas that are self-sourcing.

“We have developed a database of parameters including porosity, core descriptions, and total organic matter and we have interpreted it,” Loucks said. “The Austin Chalk is composed of very fine carbonate grains and clay minerals. So small, that you can only see them well with a scanning electron microscope at the nanometer to micrometer levels. The Austin Chalk pores are all under a micron; this is important to know to understand the reservoir. Initial production comes out of the natural fractures, but you need to look at the drawdown curve. The later production comes from nanopores after the drawdown. People need to appreciate the life of this reservoir.”

The RCRL’s data repository of cores is needed to keep the Austin Chalk active. While seismic data for new areas in Louisiana is becoming available, knowledge of the rocks is still needed, especially in deeper depths when resolution becomes more challenging.

“Without knowing the rocks, you can’t know what you’re looking for and it’s difficult to get the right seismic parameters,” Zahm explained.

Because the Austin Chalk sits just above the Eagle Ford, it can be difficult to see the transitional stratigraphy between the two. Furthermore, because producers are looking at low-porosity rocks, it is imperative to understand how well they will respond to stress when flooded with water and sand.

For the Austin Chalk, it is crucial that clay content be less than 15 percent – a feature that cannot be seen in seismic data.

“That’s why it’s so important to have core data,” Zahm said. “Low clay percentage rocks respond to a higher intensity of hydraulic and natural fracturing. But if you frac a rock too much, you can have excess water with the oil. It’s expensive to dispose of that water, and Louisiana doesn’t have the infrastructure to deal with co-produced water. Really, we need that ‘Goldilocks Window’ to economically produce the oil.”

Because the Austin Chalk can also be self-sourcing as a result of its high total organic matter, Austin oil located in micropores is unable to migrate until borehole pressure is drawn down. Therefore, frac’ing techniques must be developed alongside a good understanding of the rocks, Zahm explained.

Both Zahm and Loucks have high hopes for the long-producing reservoir now that its rocks are better understood.

“Louisiana is a hot area with mixed successes recently,” Loucks said. “I think our project adds something to the Austin Chalk trend that engineers and management often forget but need to know.”

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