Eagle Ford Group, southwest Texas
By Justin E. Birdwell, U.S. Geological Survey, Denver, CO, USA
Background
The Eagle Ford Group (or Shale) is a major oil, natural gas, and condensate/natural gas liquids play in southwest to central Texas (Fig. 1). The Eagle Ford play extends from the Maverick Basin northeast to the Karnes Trough, with some development extending to and just beyond the San Marcos arch. The area included in the Eagle Ford is approximately 50 miles wide and 400 miles long with thicknesses varying from less than 100 feet (ft), to over 600 ft thick in some areas including the Maverick Basin, north of the Giddings Field (Burleson County), and near Dallas. The Eagle Ford Group is typically divided into upper and lower strata with the primary target being the mainly Cenomanian, organic-rich, and calcareous lower Eagle Ford. The Turonian upper Eagle Ford also contains source potential. It is calcareous in much of the play area, but contains more siliciclastic material moving southwest to northeast.
The organic matter content and quality in the Eagle Ford varies stratigraphically primarily due to variability in the depositional environment. Geographic variation may have been affected by localized depositional conditions and detrital inputs. The organic matter is mainly represented by oil-prone Type II and Type II-S kerogens (French et al., 2020 and references therein). Production trends in the Eagle Ford correspond to geologic structure, main based on depth. The distribution of fluid properties produced from Eagle Ford wells are strongly related to depth, which ranges from ~4,000 to over 14,000 ft (Fig. 2) across a roughly northwest to southeast trend. Eagle Ford oil is also widely produced in Texas and parts of Louisiana from wells in the overlying Austin Chalk.
The viability of the Eagle Ford play is dependent on horizontal drilling techniques, with laterals averaging ~6,500 ft overall but extending to nearly 15,000 ft for some wells (Fig. 2), and hydraulic fracturing using slick water and acid treatments in multiple fracturing stages. The mineralogy of the Eagle Ford facilitates this development approach, as the high carbonate content of marlstone, limestone, and other mudstones (as much as 70%) make lower Eagle Ford strata brittle and amenable to fracture propagation. Completed wells show average initial production rates (first full month) of ~600 barrels (bbl) per day for oil and 2 million standard cubic feet (scf) per day for gas, followed by steady production declines with increasing time online. Estimated ultimate recoveries are around 150,000 bbl per well for oil and between 500 and 1,500 MMscf per well for natural gas (M = 1 thousand, MM = 1 million). Initial production rates have increased over the last decade with improvements in production technology, but decline rates for production are also steeper. In the last few years, improvements in ultimate recovery have been realized through well re-stimulation or the application of secondary recovery techniques (e.g., huff-and-puff; Hill et al., 2020).
The U.S. Geological Survey (USGS) assessment in 2018 estimated mean undiscovered, technically recoverable resources of 8.5 billion barrels of oil and 66 trillion cubic feet of natural gas in continuous (unconventional) reservoirs in the Eagle Ford and associated Cenomanian-Turonian strata in east Texas and southern Louisiana distributed among seven assessment units (Whidden et al., 2018). The Eagle Ford is somewhat unique among U.S. shale plays in that production data show and assessed resources further indicate it is prolific for both oil and gas, with assessed oil resources comparable to the Bakken Formation in the Williston Basin and natural gas resources essentially equivalent to the Mancos Shale in the Piceance Basin. Condensate is also an important product in the Eagle Ford, with annualized daily production rates averaging over 210,000 barrels per day over the last five years (Texas Railroad Commission, 2021).
Production Updates
The Eagle Ford is one of the most prolific tight oil plays in the United States representing up to ~35% of total tight oil production in 2015 (U.S. Energy Information Administration (EIA), 2021, Fig. 3), though this share has dropped to between 14 and 25% over the last five years due to expansion of other plays including the Permian Basin. Currently, the majority of wells drilled between 2011 and 2021 continue to be active producers (Fig. 4).
Daily oil production has dropped from the 1.0 to 1.2 million bbls/day range that was observed from 2014 through 2019 to ~750,000 bbls/day in the first months of 2021 (Fig. 5). Natural gas and condensate production have followed similar trajectories as oil. Gas production averaged 5.1 billion scf/day in 2020 and decreased to 4.4 billion scf/day in the first quarter of 2021. Condensate production averages approximately 134,000 bpd so far in 2021. Projections indicate Eagle Ford production is likely to be ~10% lower in 2021 relative to 2020 due in part to reduced demand caused by higher than normal oil and gas inventories and a lack of drilling during the COVID-19 global pandemic (World Oil, 2020).
A major advantage shale plays like the Eagle Ford have over other oil producing regions and formations is that product quality can be targeted due to the well understood distribution of fluid properties and gas-to-oil ratios (GOR) in different areas. The widely publicized U.S. Energy Information Administration GOR map shows a steady, northwest to southeast trend for thermal maturity progression from the oil window into condensate/wet gas and dry gas windows, making targeting for particular fluid properties relatively straightforward (U.S. EIA, 2014). An analysis of the fluid properties based on production test and cumulative production data (Fig. 6) shows that most wells drilled initially produce very light oil (50-65 °API) and wet gases (gas gravities ~0.75-0.8). Initial liquids flowrates are generally higher for wells producing oils with API gravities between 30-45° and most gas produced during tests have gas gravities in the 0.6-0.75 range (Fig. 6). Production of oil and gas products sorted by GOR shows that nearly all oil is produced from wells with cumulative GOR values less than ~20,000 Mscf/bbl (the cutoff used by the USGS to differentiate oil and gas wells), as expected, though a large fraction of the gas produced from the Eagle Ford also comes from wells with GORs below this cutoff (Fig. 6). These analyses are consistent with data shown in a recently published Eagle Ford retrospective report (Mire, 2021).
New producing wells and rig counts in the Eagle Ford play reached a decadal low in 2020 (Baker Hughes, 2021; IHS MarkitTM, 2021; Fig. 7). There was a slight uptick in drilling around the end of 2020 and the first quarter of 2021 with some new production coming online according to data available from IHS MarkitTM (2021). Most drilling continues to be conducted in a few key counties (Fig. 7). Around 1000 permits were issued in 2020, which is about half the number issued in 2019, and only 385 permits had been issued as of March 2021 (Texas Railroad Commission, 2021; Fig. 5).
Other Research
A recent survey of producers by the Federal Reserve Bank of Dallas identified prices per barrel needed for new Eagle Ford drilling to be profitable, with values ranging from $26 to 63 with an average of $44 (Geman, 2021). The report noted that while the oil price as of summer 2021 has boosted interest in new drilling, producers are concerned that possible changes to oil and gas policy from the Biden Administration could impact U.S. tight oil prospects and that a possible increase in production by OPEC countries could substantially impact the global oil market.
According to Scopus, in 2020 there were 204 articles, conference papers, or book chapters published with “Eagle Ford” in their title, abstract, or keywords. As of June 2021, another 42 publications were identified in Scopus based on these criteria. Google Scholar had 871 results that included mentions of “Eagle Ford” anywhere in an article so far in 2021, which includes many cited references. AAPG Datapages listed 20 results with “Eagle Ford” in the title published between Jan. 2020 and June 2021.
References[edit]
Baker Hughes, 2021. North America Rig Count. https://rigcount.bakerhughes.com/na-rig-count; accessed June 21, 2021.
French, K.L., Birdwell, J.E. and Lewan, M.D., 2020. Trends in thermal maturity indicators for the organic sulfur-rich Eagle Ford Shale. Marine and Petroleum Geology, paper no. 104459.
Geman, B., 2021. “Texas oil activity “expanded strongly” in Q1 of 2021”, Axios, posted March 25, 2021, https://www.axios.com/texas-oil-prices-2021-f5e481d7-3e37-41a2-b299-4ec1d8e0e03b.html; accessed June 21, 2021.
Hill, A.D., et al., 2020. The Eagle Ford Shale Laboratory: A Field Study of the Stimulated Reservoir Volume, Detailed Fracture Characteristics, and EOR Potential. Unconventional Resources Technology Conference, pp. 2992-3012.
IHS Markit, 2021. Enerdeq™ Browser, US well history and production database: Englewood, Colo., IHS Markit. http://www.ihsenergy.com; accessed June 15, 2021.
Mire, K., 2021. The Eagle Ford Shale Play of Texas Thirteen Years of Development. Houston Geological Society Bulletin, January issue, p. 23-26.
Texas Railroad Commission, 2021. Eagle Ford Shale Information & Statistics. https://rrc.texas.gov/oil-and-gas/major-oil-and-gas-formations/eagle-ford-shale/; accessed June 21, 2021.
U.S. Energy Information Administration, 2014 Updates to the EIA Eagle Ford Play Maps. https://www.eia.gov/maps/pdf/eagleford122914.pdf; accessed June 23, 2021.
U.S. Energy Information Administration, 2021. Petroleum & Other Liquids – Crude oil reserves and production, Tight oil production estimates by play (June 2021). https://www.eia.gov/petroleum/data.php#crude; accessed June 21, 2021.
Whidden, K.J. et al., 2018. Assessment of Undiscovered Oil and Gas Resources in the Eagle Ford Group and Associated Cenomanian–Turonian Strata, U.S. Gulf Coast, Texas, 2018. U.S. Geological Survey Fact Sheet 2018-3033, 4 p., https://doi.org/10.3133/fs20183033.
World Oil®, 2020, “Eagle Ford shale production expected to decline 10% in 2021”, Posted Dec. 10, 2021, https://www.worldoil.com/news/2020/12/10/eagle-ford-shale-production-expected-to-decline-10-in-2021; accessed June 21, 2021.