The Eagle Ford Shale – A Geologic Review
BHP Billiton, 1360 Post Oak Blvd., Houston, TX 77056
The Cretaceous Cenomanian-aged Eagle Ford Shale trend has emerged as one of the most prolific plays in North America. Since the first horizontal wells were drilled in Hawkville Field in the fall of 2008, a tremendous amount of geological data has been collected from the Eagle Ford Shale source rock. These data divide into a few basic categories that were used to evaluate the economic potential of the Eagle Ford Shale and can be used to assess the potential of other regional source rocks.
Comparing the Eagle Ford Shale to other global Cretaceous source rocks and their basin settings helped fast track the unconventional play along the northern edge of the Gulf Coast Basin. The Eagle Ford source rock that now produces hydrocarbons from the Rio Grande River into the East Texas Basin is an organic rich marlstone bounded by the unconformities at the top of the Buda Limestone and Base Austin Chalk. Regional mapping indicates the Eagle Ford Shale can be divided into 4 sub-basins: East Texas, San Marcos Platform, Hawkville Basin and the Maverick Basin areas. These sub basins are controlled by basement features including the Sabine Arch, San Marcos Arch and the Chittum Arch.
Understanding the Eagle Ford Shale’s source rock potential includes determining TOC content, using Rock Eval Pyrolysis data and indentifying kerogen types and maturity. A method to high-grade areas within the Eagle Ford Shale trend was developed by combining the source rock database and a quick look log cross-plot method used to identify organic rich intervals within the Eagle Ford. Mapping these parameters and knowing their distribution within the Eagle Ford was important in the early days of leasing. Hydrocarbon production from the Eagle Ford Shale (down-dip dry gas to up-dip oil) is actively controlled by the kerogen type, thermal maturity of the kerogen, and reservoir pressure. The change from dry gas to wet gas to oil is a continuum that is regionally predictable using a hydrocarbon phase diagram (Temperature vs. Pressure).
Rock typing using both core and petrophysical data is very important in predicting reservoir parameters and their impact on drilling parameters, completion techniques and production characteristics of the Eagle Ford Shale. The Eagle Ford is a marlstone throughout most of the trend. Mapping the relationship of the percentages of quartz, carbonate and clay has also helped in high grading areas in the trend. For example in East Texas, XRD data indicates that the Eagle Ford has higher clay content than the marlstone dominated areas to the Southwest.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012