Porosity evaluation in the Lower Eagle Ford Group, Atascosa County, south Texas

Finger, Luke Joseph
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The Eagle Ford Group represents a self-sourced petroleum system in south Texas; its fine grained sediments were interpreted as being deposited in a calm environment, thus promoting a homogeneous lithofacies and low porosity. However, the occurrence of a relatively thin (twenty foot thick), high porosity zone in the Lower Eagle Ford Group located in Central Atascosa County, Texas, calls for local peculiar depositional conditions and/or diagenetic transformations that enhanced reservoir properties. Using one full diameter core, thin sections, petrophysical logs, X-ray diffraction, and scanning election microscopy, we aim to assess the accuracy of the density log readings in the high porosity zone. In particular, we intend to test if mineralogy, organic material, clay content, or tool calibration error may have influenced the density porosity readings, which average almost 17%. Matrix density values from GRI crushed grain analysis were used to determine true porosity at various sample points throughout the section using the following formula: Φ = ρmatrix - ρbulk / ρmatrix - ρfluid. The calculated true porosity values were compared to the density log values: they systematically display a negative mean offset of 4.5%, suggesting that the density tool provides overly optimistic readings compared to the calculated true porosity. On the other hand, the studied interval displays a substantial amount of quartz and kerogen, causing the matrix density to be lower (2.63g/cc) than the limestone matrix (2.71g/cc), on which the density tool was calibrated. The presence of light organic material is confirmed by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Focused Ion Beam (FIB-SEM) imaging. Determining accurate porosity values in this zone is economically significant as several exploration and production companies target it for hydrocarbon extraction, whereas refining the influence of microfacies on the density tool will significantly impact oil and gas prospection applied to other lithostratigraphic units.

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