Eagle Ford Group

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. 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.

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.

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.

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. The rise in sea level eventually drowned the East Texas Woodbine river deltas, initiating Eagle Ford deposition in East Texas. 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.

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, 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 sediments and deposition within the Harris Delta complex. 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, 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," 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. 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.

The sea-level began to drop after the Early Turonian sea-level maximum; this is most obvious at outcrops near Langtry, Texas, where water depths became shallower than 100 ft (30 m). This limestone-rich unit is known as the Langtry Member of the Boquillas Formation. It contains very little organic matter, and abundant sea urchin fossils. The Kamp Ranch Limestone is found above the unconformity in the Dallas area. It is similar to the older Six Flags and Bluebonnet Limestones, as it is predominantly composed of disaggregated prisms of Inoceramus clams and has ripple marks indicative of shallow-water deposition. As the sea level continued to fall during the Late Turonian, deltaic sediments originating from the Ouachita Uplift prograded into the northern East Texas region. These sandstones are known as the Sub-Clarksville Delta in the subsurface and the Bells Sandstone in outcrop. In the South Texas subsurface, the age equivalent unit to the Langtry Member is more calcareous than the underlying Upper Eagle Ford rocks, making them difficult to distinguish from the limestones of the overlying Austin Chalk, although an unconformity is found between the Eagle Ford and the Austin Chalk in both South Texas and East Texas.