Abstract

Total Organic Carbon Trends Within the Eagle Ford of South Texas: Sub-mesoscale Vortices and the Eddy Ocean Hypothesis

L. E. Waite¹, W. W. Hay², and P. R. Clarke¹
¹Pioneer Natural Resources Co., Irving, Texas 77039
²Department of Geological Sciences, University of Colorado at Bolder 2045 Windcliff Drive, Estes Parke, Colorado 80517

Total organic carbon (TOC) is a critical component of hydrocarbon source rocks within unconventional shale plays. Regions of higher TOC often show a strong positive correlation to well performance and hydrocarbon reserves and therefore prediction of TOC is important to increased economic success. Areas of high TOC deposition in the modern ocean are regulated by global atmospheric and oceanic circulation patterns that foster high oxygen levels throughout the global water mass. The present-day icehouse climate with polar ice caps generates cold, dense water masses and thermal “fronts” that effectively limit the development of anoxia to intermediate waters at lower latitudes. Regions of high TOC in today’s ocean are mainly limited to anoxic basins and to regions of local upwelling and enhanced productivity along the western margins of the continents. However a few seasonally stable cyclonically rotating, mesoscale (10 – 100 km diameter) eddies provide a local mechanism to pump nutrient-rich water bottom waters upward, promoting phytoplankton blooms at the surface.

Theoretical considerations of an ice-free Cretaceous world suggest atmospheric-oceanic conditions were far more conducive to the formation and prevalence of mesoscale eddies, some of which are stable, throughout the world ocean. Computer models of the Late Cretaceous North Atlantic supports the hypothesis of a so-called “eddy ocean,” with numerous mesoscale to sub-mesoscale upwelling vortices indicated along the southern margin of the Western Interior Seaway of North America. Mapped distribution patterns of TOC within the Eagle Ford shale of south Texas are consistent with deposition by multiple sub-mesoscale eddies. An eddy-dominated ocean replete with sub-mesoscale vortices may explain local producing trends within the Eagle Ford and has implications for regional reserve estimates throughout the play. The eddy ocean hypothesis may have application to other ice-free ocean systems throughout Phanerozoic time.

AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013