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ABSTRACT: Shale Microfacies-Eagle Ford Group (Cenomanian-Turonian) North-Central Texas Outcrops and Subsurface Equivalents

DAWSON, WILLIAM C. , Texaco, Inc., Houston, TX

The Eagle Ford Group (Late Cretaceous) records a second order transgression and contains a major (third-order) condensed interval (cycle 2.5 of the UZA-2 supercycle). Eagle Ford strata are overlain disconformably by the Austin Chalk. The Eagle Ford/Austin contact represents the Turonian/Coniacian (89 ma) boundary. The Cenomanian/Turonian boundary (92 ma) occurs within the Eagle Ford Group. Regionally, the Eagle Ford Group consists of two major depositional units: a lower transgressive and partly condensed unit and an upper highstand unit. These two depositional units have distinctive sedimentologic and geochemical characteristics. Eagle Ford strata exhibit considerable lithologic variability. Six major argillaceous microfacies compose Eagle Ford strata: 1) fissile shales; 2) phosphatic shales; 3) bentonitic shales; 4) fossiliferous (foraminiferal) shales; 5) silty (quartzose) carbonaceous shales; and 6) sapropelic shales and claystones. Eagle Ford shales are composed predominantly of smectite. Microfacies 1, 2, and 3 comprise the condensed Eagle Ford interval. Microfacies 4 and 5 represent highstand Eagle Ford strata, and Microfacies 1 and 6 comprise the transgressive part of the Eagle Ford. These six shale microfacies are thinly interstratified with bentonites, carbonaceous siltstones and limestones (foraminiferal, inoceramid, and peloidal grainstones). Fossils (especially fish teeth, pelecypod, and ammonites) are abundant locally in Eagle Ford strata. Although Eagle Ford shales are recognized as having source rock potential, this potential is not evenly distributed. Lower (transgressive) Eagle Ford shales exhibit higher generation potentials and are more oil-prone than the overlying shales. Eagle Ford strata provide sedimentologic and geochemical analogs for other condensed intervals.


AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas