Austin Chalk, onshore U.S. Gulf Coast
By Charlie Y.C. Zheng, University of Texas at Austin, Austin, TX, USA
The Upper Cretaceous Austin Chalk Group was deposited on a broad carbonate shelf that trends from Texas into Louisiana and contains many major oil fields (Fig. 1) (Pitman et al., 2020). It remains an active exploration target onshore Gulf Coast since the first hydrocarbon discovery in the 1920s. To date, 1.28 billion barrels of oil and 6.27 trillion cubic feet of gas have been produced from the Austin Chalk cumulatively (Amelia Resources, 2017). The Austin Chalk reservoir is traditionally known as a low-permeability reservoir charged by the underlying prolific Eagle Ford Group, producing oil and gas from naturally fractured zones that allow horizontal drilling to connect a permeable network and facilitate drainage within low-permeability matrix (Corbett et al., 1994; Pearson, 2012). Accordingly, exploration had been focused on the naturally fractured fault zones (these major fractures oriented parallel to the Lower Cretaceous shelf edge and are linked to underlying tectonic structures), such as the largest Giddings field, which has produced 565 MMBO and 4.8 TCF to date (Amelia Resources, 2017). Lately, operators noticed that “quiet chalk” is also economical as the contrasting “noisy chalk” (fractured chalk) due to the porosity contained in the matrix (over 8%) that may have allowed charging under over-pressured settings. In fact, the Austin Chalk production increase over the past years was driven by the quiet chalk focusing on the middle Austin Chalk Group (Enverus, 2021).
Although the Austin Chalk production is not comparable to the scale as the Permian Basin after the unconventional production boom, it still plays an unneglectable role in the US tight oil and gas production (Fig. 2). Austin Chalk production’s share to total US tight oil and gas production dropped from ~18% in 2000 to ~2 % in 2013 and maintained at ~2% since then (Fig. 3). However, since 2013, the success of combining horizontal drilling and multi-stage fracture stimulation in horizontal wells in the underlying Eagle Ford Group was applied to the Austin Chalk Group. The up-pick of Austin Chalk production is shown since 2015 regardless of the reduced number of producing wells during the period (Austin Chalk Revival, 2021). The peak Austin Chalk production reached at ~ 0.12 million bbl per day in 2019-2020, mostly driven by the activities in Karnes County (Fig. 3). (Also, notice the water-jumping curves in US tight oil and gas production linked to the pandemic and O&G price collapse in 2020) (Fig. 2 & 3).
The U.S. Geological Survey (USGS) recently updated the assessment on undiscovered, recoverable oil and gas in the Upper Cretaceous system of the Gulf Coast, including the Austin Chalk and the coequal Tokio and Eutaw formations. The estimated mean totals for conventional and continuous oil and gas resources are 6.9 billion barrels of oil with an F95-F5 range from 2,067 to 12,582 MMBO; 41.5 trillion cubic feet of gas with an F95-F5 range from 13,178 to 71,883 BCFG; and 1.3 billion barrels of natural gas liquids with an F95-F5 range from 423 to 2,353 MMBNGL (see Table 1 for details of each Assessment Units) (Pitman et al., 2020).
Classic sedimentologic studies on the Austin Chalk are based on trace fossil records from outcrops in Ellis County, northeastern Texas (Dawson, 1985, 1990). Different interpretations of the regional depositional profile were proposed (Dravis, 1991; Hovorda and Nance, 1994). Cooper (2020) pointed out the fluctuation in depositional environments associated with regional and local tectonics. Most recently, the Reservoir Characterization Research Laboratory (RCRL) project at the Bureau of Economic Geology is actively evaluating the Austin Chalk Group in subsurface. Loucks et al. (2020a) performed core-log calibration and defined basic lithofacies based on a type core in south Texas (Fig. 4); Loucks et al. (2021) performed a regional study characterizing the matrix of Austin Chalk; Zheng et al. (2021) defined the benthic oxygenation history during the Austin Chalk deposition through integrating sedimentologic, ichnologic, geochemical analyses; Male and Zahm (2021) investigated the productivity of wells linked to fault distribution in Karnes Trough area.
Besides the Karnes Trough area that attracted most of the attention, the potential of Austin Chalk within the Gulf Coast was extensively evaluated in recent years. And the players and the hottest exploration target changed drastically. ConocoPhillips and Marathon once leased over 200,000 acres in Louisiana. Many other large operators were also present. But the exploration in Louisiana was discouraged by the preliminary production results ConocoPhillips reported. On the other hand, EOG discovered a large natural gas resource play on its Dorado prospect in Webb County, with a total of 21 Tcf estimated net resource potential in the Austin Chalk and Eagle Ford groups combined (EOG, 2020). According to EOG, the Austin Chalk has an estimated net resource potential of 9.5 Tcf with 530 net drilling locations identified (EOG, 2020).
References[edit]
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Austin Chalk Revival, 2021. Source: https://austinchalkoilgas.com/
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Dawson, W.C., Reaser, D.F., 1990. Trace fossils and paleoenvironments of lower and middle Austin Chalk (Upper Cretaceous), north-central Texas. Transactions–Gulf Coast Association of Geological Society 40, 161–173.
Dravis, J.J., 1991. Regional depositional setting and porosity evolution of the Austin Chalk Formation, South Texas. In: Austin Chalk Exploration Symposium: Geology, Geophysics and Formation Evaluation, South Texas Geological Society, 17–23.
Enverus, 2021. Austin Chalk Exploration in the West (Presentation). In: Eagle Ford Play Fundamentals. Enverus.
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Loucks, R.G., Larson, T.E., Zheng, C.Y.C., Zahm, C.K., Ko, L.T., Sivil, J.E., Sheng, P., Ruppel, S.C., Ambrose, W.A., 2020a. Geologic characterization of the type cored section for the Upper Cretaceous Austin Chalk Group in southern Texas; A combination fractured and unconventional reservoir. AAPG Bulletin, 104(10), 2209-2245.
Loucks, R.G., Reed, R. M., Ko, L.T., Zahm, C.K., Larson, T.E., 2021. Micropetrographic characterization of a siliciclastic-rich chalk; Upper Cretaceous Austin Chalk Group along the onshore northern Gulf of Mexico, USA. Sedimentary Geology, 412, 105821.
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Pearson, K., 2012. Geologic models and evaluation of undiscovered conventional and continuous oil and gas resources¬¬¬- Upper Cretaceous Austin Chalk, U.S. Gulf Coast. Geological Survey Scientific Investigations Report 2012-5159, 26 pp.
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Zheng, C.Y.C., Loucks, R.G., Kerans, C., Buatois, L.A., Mángano, G.M., Ko, L.T., 2021. Benthic oxygenation history of South Texas during the Austin Chalk Group deposition: An integrated ichnologic, sedimentological, and geochemical study, in preparation.