Geochemical Fingerprints of the Eagle Ford Sequences: Learning and Implications from the Outcrops in Terrell and Val Verde Counties, Texas

Utilizing spectral gamma ray, X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Rock-Eval Pyrolysis, Leco TOC data, and biomarker analysis, distinct geochemical fingerprints could be defined for the vertical facies succession of Eagle Ford (Boquillas) Formation in outcrops in West Texas. In turn, the distinct geochemical fingerprints for each of the Eagle Ford sequences defined in outcrop can also be utilized to help explain and predict the thickness and distribution patterns of Eagle Ford unconventional mudstone reservoirs in the subsurface.

Spectral gamma ray offers an additional portrayal of stacking pattern in the Eagle Ford vertical facies succession, herein defined as Facies A - E, from the base up. Spectral radioactive components and their cyclicity can be utilized to interpret depositional conditions (extrabasinal vs. intrabasinal inputs, sedimentation rate, bottom-water oxygen level, and surface water productivity). Uranium enrichments are correlated to maximum flooding surfaces. Thorium values can determine the presence of heavy minerals, for example in bentonite layers. Potassium readings amplify upward indicates increasing of illite - smectite content in Langtry member (facies D and E).

In addition to spectral gamma data, we also performed XRF and XRD analyses to capture mineralogy variations and additional elemental data distribution across Eagle Ford section in this locality. Mineralogy was modeled from correlations between XRF elements and XRD common mineralogy. Having high resolution mineralogy and elemental data facilitates more detailed characterization of depositional sequences interpreted from measured section and spectral gamma log.

Rock-Eval pyrolysis and Total Organic Carbon (TOC) results also confirmed the interpretation of four distinctive depositional sequences and their correlative surfaces. Higher hydrogen index and enrichment of TOC corresponds to submarine hiatal breaks in lower section of the outcrop, facies A, B and C. Biomarkers assisted on determining organic matter type and interpreting paleoenviromental conditions during Eagle Ford time. Integrating inorganic and organic geochemical data to sequence stratigraphic interpretation is crucial to understand the controls on reservoir quality variations within mudstone succession. This technique was also applied in the subsurface realm to superior our regional reservoir quality prediction capability using integrated regional sequence stratigraphic interpretation.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California