The Eagle Ford Group (also called the Eagle Ford Shale) is a sedimentary rock formation deposited during the Cenomanian and Turonian ages of the Late Cretaceous over much of the modern-day state of Texas. The Eagle Ford is predominantly composed of organic matter-rich fossiliferous marine shales and marls with interbedded thin limestones. It derives its name from outcrops on the banks of the West Fork of the Trinity River near the old community of Eagle Ford, which is now a neighborhood within the city of Dallas. The Eagle Ford outcrop belt trends from the Oklahoma-Texas border southward to San Antonio, westward to the Rio Grande, Big Bend National Park, and the Quitman Mountains of West Texas.Denne, R. A., Breyer, J. A., Callender, A. D., Hinote, R. E., Kariminia, M., Kosanke, T. H., Kita, Z., Lees, J. A., Rowe, H., Spaw, J. M., and Tur, N. (2016). Biostratigraphic and geochemical constraints on the stratigraphy and depositional environments of the Eagle Ford and Woodbine Groups of Texas: "in" Breyer, J. A. (ed.), The Eagle Ford Shale: A renaissance in U.S. oil production, AAPG Memoir 110, pp. 1–86. It also occurs in the subsurface of East Texas and South Texas, where it is the source rock for oil found in the Woodbine, Austin Chalk, and the Buda Limestone, and is produced unconventionally in South Texas and the "Eaglebine" play of East Texas.Hentz, T. F., Ambrose, W. A., and Smith, D. C. (2014). Eaglebine play of the southwestern East Texas basin: Stratigraphic and depositional framework of the Upper Cretaceous (Cenomanian-Turonian) Woodbine and Eagle Ford Groups: AAPG Bulletin, v. 98, p. 2551-2580.
The Eagle Ford was one of the most actively drilled targets for unconventional oil and gas in the United States in 2010, but its output had dropped sharply by 2015.Jennifer Hiller, "Hard Times Hit Eagle Ford," "San Antonio Express-News," January 3, 2016, pp. 1, A20 By the summer of 2016, Eagle Ford spending had dropped by two-thirds from $30 billion in 2014 to $10 billion, according to an analysis from the research firm Wood Mackenzie. This strike has been the hardest hit of any oil fields in the world. As of 2016, the spending was, however, expected to increase to $11.6 billion in 2017. A full recovery was not expected any time soon.Jennifer Hiller, "Spending in Eagle Ford has dropped by 67 percent", "San Antonio Express-News," July 22, 2016, pp. 1, A6
Fossils are relatively common in Eagle Ford rocks. Fossilized Plesiosaurs, mosasaurs, Fish, shark teeth, crustaceans, sea urchins, feather stars, ammonites, oysters, clams, and other gastropod shells have all been found there.Jacobs, L.L., Polcyn, M.J., Winkler, D.A., Myers, T.S., Kennedy, J.G., and Wagner, J.B. (2013) Late Cretaceous strata and vertebrate fossils of North Texas, "in" Hunt, B.B., and Catlos, E.J., eds., Late Cretaceous to Quaternary Strata and Fossils of Texas: Field Excursions Celebrating 125 Years of GSA and Texas Geology, GSA South-Central Section Meeting, Austin, Texas, April 2013: Geological Society of America Field Guide 30, p. 1–13, doi:10.1130/2013.0030(01)Moreman, W. L. (1942) Paleontology of the Eagle Ford Group of north and central Texas: Journal of Paleontology, v. 16, p. 192–220.Donovan, A. D., T. S. Staerker, A. Pramudito, W. Li, M. J. Corbett, C. M. Lowery, A. M. Romero, and R. D. Gardner (2012) The Eagle Ford outcrops of West Texas: Understanding heterogeneities within unconventional mudstone reservoirs: GCAGS Journal, v. 1, p. 162–185.
Depositional environment
The Eagle Ford rocks were created by the remains of sea life that dropped to the floor of an inland sea (or epeiric sea) that covered much of modern-day Texas. The Texas shelf during the Cenomanian-Turonian was bounded by the Ouachita Uplift to the north, the Sabine Uplift to the East, relict reef margins of the Stuart City Formation and the Sligo Formation to the southeast, and the Western Interior Seaway to the west. The East Texas and South Texas regions were divided by an extension of the Llano Uplift known as the San Marcos Arch. Primary basins active during Eagle Ford deposition were the East Texas and Brazos Basins in East Texas and the Maverick Basin in South Texas.Hentz, T. F., and S. C. Ruppel (2010) Regional lithostratigraphy of the Eagle Ford Shale: Maverick Basin to East Texas Basin: Transactions of the Gulf Coast Association of Geological Societies, v. 60, p. 225–337.
The bottom waters of the Eagle Ford sea were starved of oxygen when most of the Eagle Ford material dropped to the sea floor, and this is related to the global Oceanic Anoxic Event 2 (OAE2), or Cenomanian-Turonian boundary event, although the Texas shelf became that way nearly two million years prior to OAE2. The low-oxygen conditions helped preserve the organic matter that ultimately generated the hydrocarbons associated with the Eagle Ford in the subsurface. Evidence for anoxia include the high amounts of organic matter, the lack of fossils or trace fossils of the kind of creatures that live on the sea floor, and enrichment in the redox proxies molybdenum and vanadium.
After the significant drop in sea level (marine regression) associated with deposition of the Woodbine during the Early Cenomanian, the sea level began to rise (marine transgression), allowing for the deposition of Lower Eagle Ford organic-rich marls in South Texas and limestones of the Terrell Member of the Boquillas Formation in West Texas starting at about 96 million years ago.Gardner, R. D., M. C. Pope, M. P. Wehner, and A. D. Donovan (2013) "Comparative stratigraphy of the Eagle Ford Group in Lozier Canyon and Antonio Creek, Terrell County, Texas": GCAGS Journal, v. 2, p. 42-52.Wehner, M., R. Gardner, M. M. Tice, M. C. Pope, A. D. Donovan, and T. S. Staerker (2015) Anoxic, storm dominated inner carbonate ramp deposition of Lower Eagle Ford Formation, west Texas: Unconventional Resources Technology Conference, San Antonio, Texas, USA 20–22 July 2015, doi:10.15530/urtec-2015-2154667 The rise in sea level eventually drowned the East Texas Woodbine river deltas, initiating Eagle Ford deposition in East Texas.Denne, R. A., and Breyer, J. A. (2016) Regional depositional episodes of the Cenomanian-Turonian Eagle Ford and Woodbine groups of Texas: "in" Breyer, J. A. (ed.), The Eagle Ford Shale: A renaissance in U.S. oil production, AAPG Memoir 110, p. 87-135. The initial deposits, known as the Six Flags Limestone in Dallas and the Bluebonnet Limestone in Waco, are calcarenites predominantly composed of disaggregated prisms of "Inoceramus" clams and planktonic foraminifera tests.Norton, G. H. (1965) Surface geology of Dallas County, "in" The geology of Dallas County: Dallas Geological Society, Dallas, Texas, p. 40–125.Silver, B. A. (1963) The Bluebonnet Member, Lake Waco Formation (Upper Cretaceous), Central Texas-A lagoonal deposit: Baylor Geologic Studies Bulletin 4, Waco, Texas, 46 p.
Following deposition of the calcarenites, a river delta began to prograde from the Ouachita Uplift to the northern East Texas Basin. Although the sandstones and siltstones from this delta, known as the Templeton Member, were originally placed within the Woodbine Formation,Bergquist, H. R. (1949) Geology of the Woodbine Formation of Cooke, Grayson, and Fannin Counties, Texas: USGS, Oil and Gas Investigation, Preliminary Map 98. the ammonites found within them indicate that they are better associated with the Eagle Ford. In areas unaffected by the Templeton Delta, depositional rates were low, producing a condensed section composed of organic-rich, calcareous marls, limestones, and volcanic ash beds in both South Texas and West Texas. The microfossils found within the marls are predominantly coccoliths and planktonic foraminifera, whereas the limestones contain abundant radiolaria and calcispheres (calcareous cysts produced by some dinoflagellates). Inoceramus fragments and fish bones are also found in these deposits.
During the Late Cenomanian the Sabine Uplift along the modern-day Texas/Louisiana border became active, causing erosion of Eagle Ford and Woodbine sedimentsHalbouty, M. T., and J. J. Halbouty (1982) Relationships between East Texas field region and Sabine uplift in Texas: AAPG Bulletin, v. 66, p. 1042–1054. and deposition within the Harris Delta complex.Turner, J. R., and S. J. Conger (1981) Environment of deposition and reservoir properties of the Woodbine Sandstone at Kurten field, Brazos County, Texas: Transactions of the Gulf Coast Association of Geological Societies, v. 31, p. 215–249. Clay from this delta reached as far south as DeWitt County, Texas.
Towards the end of the Late Cenomanian, the bottom waters of the Texas shelf and the Western Interior Seaway became oxygenated,Eldrett, J. S., D. Minisini, and S. C. Bergman (2014) Decoupling of the carbon cycle during Ocean Anoxic Event 2: Geology, v. 42, p. 567–570. which may be related to the sea-level maximum associated with the Cenomanian-Turonian boundary event. Evidence for this oxygenation event, known as the "Benthonic Zone,"Eicher, D. L., and P. Worstell (1970) Cenomanian and Turonian foraminifera from the Great Plains, United States: Micropaleontology, v. 16, p. 269–324. include an increase in the abundance of benthic organism fossils and bioturbation, a decrease in redox proxies uranium, molybdenum, and vanadium, and a reduction in organic matter.Denne, R. A., R. E. Hinote, J. A. Breyer, T. H. Kosanke, J. A. Lees, N. Engelhardt-Moore, J. M. Spaw, and N. Tur (2014) The Cenomanian-Turonian Eagle Ford Group of South Texas: insights on timing and paleoceanographic conditions from geochemistry and micropaleontologic analyses: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 413, p. 2-28.Boling, K. S. and Dworkin S. I. (2015) Origin of organic matter in the Eagle Ford Formation: Interpretation, v. 3, p. SH27–SH39. This oxygenation event marks the boundary between the Lower and Upper Eagle Ford in West Texas and the subsurface of South Texas. In general, Upper Eagle Ford rocks deposited during the Cenomanian-Turonian boundary event (OAE2) contain much less organic matter than Lower Eagle Ford rocks, which is the reverse of organic matter trends seen in the global ocean. An unconformity occurs throughout East Texas at this level, possibly due to a lack of sediments reaching the basin during the sea-level maximum.