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Eagle Ford Geology Assessment and Activity Update


The Eagle Ford is located in South-Central Texas extending from the Maverick Basin, through the Central Texas Platform, across the San Marcos Arch and ending in the East Texas Basin. Overall the Eagle Ford is about 50 miles wide and over 400 miles long extending beneath 15 counties. The Eagle Ford continues to be one of the top plays despite low oil prices. The image below shows the current rigs and permit activity in the last 30 days. The highest area of interest is in the eastern section of the play.

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The active rig count of 96 (8/27/15) in the Eagle Ford places it in second amongst the most popular shale plays in the US.

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Here is a typical production curve in the Eagle Ford.

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Keeping this production curve in mind we can take a look at the total production from the Eagle Ford color coded by year. Interestingly March through May 2015 shows the only consecutive monthly decline in production. During this period production fell from 77,690 MBOE 6:1 to 71,116 MBOE 6:1. Each year the production increases compensate for the declines from older wells, however, production in 2015 appears to be insufficient to continue the production increases. We’ll be watching next months production figures (and updates to previous months) closely.

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The Drillinginfo Eagle Ford Play Assessment is an ongoing dynamic study of the Geology of the play area that is released every four months. Currently the assessment contains 6,567 wells in the project, and of these wells 2,582 contain stratigraphic picks. Below is a table of all the picks and the counts. (Our next release will be in October and will include additional wells and picks.)

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Eagle Ford Geology

The Eagle Ford formed in the Late Cretaceous from 82 to 97 million years ago. At this time there was a marine transgression, a point in time where the sea level rises relative to land. The transgression led to the formation of the Western Interior Seaway, a shallow ocean which extended from the Arctic Ocean to the Gulf of Mexico.

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The earth overall was much warmer during the cretaceous than it is today, causing the temperatures between the poles and the equator to be mild, resulting in a decrease in oceanic currents. As ocean currents slowed and stagnated areas of anoxic zones formed creating a favorable environment for organic shales to be deposited. The Eagle Ford formed in the southern portion of the Western Interior Seaway.

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The Austin Chalk, the first unit on the above cross section, is stratigraphically below the Anachaho and above the Eagle Ford, and it is primary composed of planktonic coccoliths, planktonic foraminifera, and cacipheres (Ruppel et al., 2013). The Eagle Ford is split up into the upper and lower Eagle Ford. The Eagle Ford is stratigraphically below the Austin Chalk and above the Buda Limestone. The Upper Eagle Ford is composed of calcareous mud rock and the lower Eagle Ford is composed of mud rock (Hentz and Ruppel, 2010). The Eagle Ford marks the base of the Austin Chalk, this boundary can sometimes be hard to distinguish and people have been known to count some of the lower Eagle Ford as part of the Austin Chalk because of the amount of calcareous material. The Buda Limestone is recognizable on gamma ray logs because of its blocky signature. It lies below the Eagle Ford and is above the Del Rio Shale. The Del Rio Shale separates the Buda from the Georgetown limestone. It has a high gamma ray and a low resistivity signature, it is not the place to go drilling for oil. The stratigraphy can be seen in below in a type log.

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In the cross section view we can pull in multiple wells at a time which creates a great appearance for viewing the subsurface. The geologic units thin moving from the Maverick Basin (west) to the San Marcos Arch (east). The cross section in the upper right shows the relationship of the formations to ground level elevation. The large cross section beneath is flattened on the Buda Limestone (referencing the depths to the top of the Buda Limestone). This cross section shows the relative thickness of the other units. The Lower Eagle Ford, which is stratigraphically on top of the Buda Limestone, appears brown and changes in thickness going from west to east.

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I hope you enjoyed this brief peak into the geology of the Eagle Ford. In a future post we will look at some of the structure and isopach maps that can be generated from this stratigraphic information.

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Tiffany Guiltinan

Tiffany Guiltinan is a geologist at DrillingInfo. She works with the DI Geology team modeling unconventional oil plays. After obtaining her degree in geology from Cal State University Long Beach she worked in the oil industry in California focusing mainly in the Huntington Beach and Sunset Beach oil fields.

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