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GEOREF RECORD

Findings from the Eagle Ford outcrops of West Texas and implications to the subsurface of south Texas

Arthur D. Donovan, T. Scott Staerker, Rand Gardner, Michael C. Pope, Aris Pramudito and Matthew Wehner
Findings from the Eagle Ford outcrops of West Texas and implications to the subsurface of south Texas (in The Eagle Ford Shale; a renaissance in U. S. oil production, John A. Breyer (editor))
AAPG Memoir (2016) 110: 301-336

Abstract

The Eagle Ford Group crops out in a series of spectacular cut-bank exposures within Lozier Canyon region in Terrell County (west Texas). These outcrops provide an unparalleled opportunity to examine the Eagle Ford Group and gain valuable insights into explaining and predicting the vertical and lateral variability, as well as the thickness changes that can occur regionally within an unconventional source rock play. In the subsurface of south Texas, the Eagle Ford Group is typically divided into an organicrich Lower Eagle Formation and a carbonate-rich Upper Eagle Ford Formation. Both formations are petrophysically distinct, especially on gamma ray (GR) and sonic logs. When geochemically analyzed, the basal portion of the Upper Eagle Ford Formation also contains a unique positive carbon isotope delta (super 13) C excursion interpreted as the Ocean Anoxic Event 2 (OAE2). The peak of this isotope excursion is the assigned proxy for the base of the Turonian Stage. Within the Eagle Ford outcrops of west Texas a vertical succession of five informal lithostratigraphic units, referred to as units A to E from the base up, are fairly obvious. Unit A consists of interbedded grainstones and carbonate mudstones. Unit B is dominated by organic-rich black carbonate mudstones. Unit C consists of packstone beds interbedded with light gray carbonate mudstones. Unit D consists of bioturbated marls, while Unit E consists of grainstones interbedded with carbonate mudstones and bentonites. By incorporating petrophysical and geochemical data, the Lower and Upper Eagle Formations from the subsurface of south Texas can also be defined in the Eagle Ford outcrops of west Texas. Our work suggests that outcrop units A and B represent the Lower Eagle Ford Formation, while outcrop units C, D, and E represent the Upper Eagle Ford Formation. Similar to the subsurface of south Texas, a distinct positive carbon isotope delta (super 13) C excursion also occurs in the basal portions of the Upper Eagle Ford Formation (unit C) in outcrop. More detailed analysis of the outcrop and subsurface data from the Eagle Ford Group in west Texas indicates that the five informal lithostratigraphic units can be further divided into a vertical succession of 16 subunits. This more detailed vertical facies succession was used to define four genetically related depositional sequences each with distinctive geochemical and petrophysical characteristics which make them particularly suitable for regional subsurface mapping. For nomenclature simplicity, these four sequences are herein termed the lower and upper (allo-) members of the Lower Eagle Ford Formation and the lower and upper (allo-) members of the Upper Eagle Ford Formation. The lower member of the Lower Eagle Ford Formation is an organic-rich, high-resistivity, uranium-poor mudstone-dominated sequence. A distinctive clay-rich, low-resistivity zone also marks its base. This sequence appears to be the primary unconventional reservoir interval in the subsurface of south Texas. The upper member of the Lower Eagle Ford Formation can be characterized as a uranium- and bentonite-rich, mudstone-dominated sequence. The lower member of the Upper Eagle Ford Formation is a uranium-poor interbedded mudstone and limestone succession characterized by an overall (low) blocky GR pattern, the presence of a distinctive positive carbon isotope delta (super 13) C excursion, and a clay-rich, low-resistivity zone at its base. The upper member of the Upper Eagle Formation is a bentonite-bearing, low-TOC interval that is more bioturbated toward its base and interbedded toward its top. It is characterized by the presence of a high GR, low resistivity, and low velocity mudstone at its interpreted maximum flooding surface. Regional correlations of the four defined Eagle Ford depositional sequences (allomembers) reveal that the unconformities at the base of each of the four sequences, as well as the one at the base of the overlying Austin Chalk, modify the thickness and distribution of underlying strata. Thus any attempt to explain and predict the distribution and thickness variations of any of the four sequences (allomembers), especially the organic-rich lower member of the Lower Eagle Ford Formation, is highly dependent on the recognition and regional mapping of these unconformities.


ISSN: 0271-8529
Serial Title: AAPG Memoir
Serial Volume: 110
Title: Findings from the Eagle Ford outcrops of West Texas and implications to the subsurface of south Texas
Title: The Eagle Ford Shale; a renaissance in U. S. oil production
Affiliation: BP Exploration, Houston, TX, United States
Affiliation: Marathon Oil, Houston, TX, United States
Pages: 301-336
Published: 2016
Text Language: English
Publisher: American Association of Petroleum Geologists, Tulsa, OK, United States
References: 36
Accession Number: 2018-018654
Categories: Economic geology, geology of energy sourcesStratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. charts, sects., sketch maps
N29°49'60" - N30°34'60", W102°34'60" - W101°40'00"
Secondary Affiliation: Texas A&M University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 2018
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