AbstractsEarth & Environmental Science

This study utilizes three cores provided by Devon Energy from LaVaca and Dewitt Counties, TX, which were analyzed with a focus on diagenetic fabrics within the Lower Eagle Ford. A better understanding of paragenesis within the organic-rich Lower Eagle Ford was developed with the use of XRF and XRD measurements, thin section samples, stable isotope analysis, and CL analysis. The majority of diagenetic alteration occurred early at shallow depths within anoxic/suboxic environments and slightly sulfate-reducing conditions from diagenetic fluids with varying isotopic composition. Diagenetic expression variation is most influenced by the depositional facies in which alteration occurs. Within the study area, four identified depositional facies (A-D), and five diagenetic facies (1-5) were identified within the Lower Eagle Ford study area based on recrystallization, crystalline growth pattern, and elemental composition. Four of these facies were determined to significantly alter the character of the rock through recrystallization. The majority of recrystallized forms within the Lower Eagle Ford are expressions of pervasive growth recrystallization occurring within foraminifera-rich layers. Elemental analyses of these pervasive forms indicates that foraminifera-rich beds that underwent recrystallization contain abundant clays (indicated by relatively high Al) that can be correlated to the diagenetic expression. Early diagenesis preserved foraminifera tests, but multiple phases of diagenesis are apparent in these recrystallized forms where clays served as the conduit for diagenetic fluids. Stable isotope values ranging from ?13 C (-7 to+2 ?) and ?18 O ranges of (-5.25 to -2.25 ?), depleted from organic carbon in samples, indicate a range of isotopic composition with possible mixed marine influence and varying degrees of water-rock interaction. Other diagenetic facies that occurred predominantly within the Pepper Formation exhibit dense recrystallized calcite seams identified as calcite beef, and cone-in-cone structures. Crystalline structures within these digenetic facies exhibit cone-in-cone and fibrous form where clays and organic matter occur adjacent to seams and at rims of calcite blades. Clay positioning could indicate clay was mobile during recrystallization. Stable isotopes for calcite beef and cone-in-cone structures range from ?13 C of (-3 to 1 ?) and ?18 O of (-4.40 to -3.35 ?). These values indicate a different diagenetic history from pervasive recrystallized forms. The cone-in-cone and calcite beef diagenetic fabrics may have formed through a physical diagenetic process occurring either during early or late diagenesis including displacive recrystallization, or crack-seal/ parallel expansion seams. Advisors/Committee Members: Sonnenberg, Stephen A. (advisor), French, Marsha (committee member), Hull, David (committee member).