| dc.description.abstract | Petrophysical, geochemical, and isotopic characteristics were used to define unconformity-bounded formations and members within the Woodbine and Eagle Ford Groups in the shallow subsurface along the northwestern margin of the East Texas Basin (ETB). These unconformity-bounded strata were mapped in the deeper subsurface portions of the ETB to define plays within these groups throughout the basin.
The K600sb marks the base of the Woodbine Group. This unconformity displays angular discordance that truncates the Buda Formation to the north and the west, controlling the limits of Buda Formation play fairways. The K615ts subdivides the Woodbine Group into the more sandstone-prone Dexter Formation (below) from the more mudstone-prone Lewisville Formation (above).
In the Dallas area, the K630sb marks the base of the Eagle Ford Group and occurs at the base of the “Tarrant Beds”. This surface shows angular discordance and is geochemically distinct, based on X-Ray Fluorescence (XRF) data. The change to fossiliferous marine strata above is significant because the ammonite fauna of the Eagle Ford Group also occurs in the Cretaceous Western Interior Seaway, marking the time that the seaway first established. The K650sb subdivides the Eagle Ford Group into the more organic-, uranium, and carbonate-rich Lower Eagle Ford Formation (LEF) and organic- and uranium-poor, and argillaceous-rich Upper Eagle Ford Formation (UEF). This boundary controls the northern and eastern limits of the LEF unconventional reservoir play by truncation and geochemical data reveals a transition from anoxic sea-floor conditions (below) to oxic sea-floor conditions (above).
The interval between the K650sb and K670sb is the Lower Member of UEF (LM:UEF). Geochemical work indicates that the positive (δ13C) carbonate isotope excursion associated with the Cenomanian/Turonian Boundary Event (CTBE), which is also commonly termed the Ocean Anoxic Event 2 (OAE2), also coincides with this member. Regional correlations indicate that the major siliciclastic depocenter (delta system) within the LM:LEF is coeval to the classic Harris Delta System from the southern portions of the ETB. Paleogeographic maps of the LM:UEF in this study suggest that the sandstone-play fairway associated with the Harris Delta System is more regionally extensive than previously reported. | |
