POROSITY EVALUATION IN THE LOWER EAGLE FORD FORMATION, ATASCOSA COUNTY, SOUTH TEXAS
Matrix density values from Dry & Dean Stark extracted conditions were used to determine true porosity at various sample points throughout the section. True porosity was calculated using the following formula: Φ = ρmatrix - ρbulk / ρmatrix - ρfluid
The calculated true porosity values were compared to the density log values and 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 material is confirmed by SEM and SEM-EDS 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 impact significantly oil and gas prospection applied to other lithostratigraphic units.