Computer-aided statistical analysis of the correlation between shaly sand analysis using old logs (pre-porosity): and modern logs : Pennsylvanian Hoover sand, Harper and Beaver counties, Oklahoma

Inaccurate calculation of water saturation and porosity can mean the difference in billions of cubic feet in recoverable gas hydrocarbons. In a "clean" formation of sand matrix, water, and gas, an accurate water saturation and porosity can be calculated from the petrophysical log response using the Archie equation. When shale is present, the Archie equation must be modified to compensate for the effect of clay minerals on log response. The more log suites available for a particular well, the more accurate and more complex the correction for shale becomes.

Many possibly productive shaly sand zones discovered prior to the 1960's were logged with only electrical logging tools. The lack of additional log devices severely limits the number of shaly sand corrections that can be made to the Archie equation. Therefore, several old electric log shaly sand methods are compared here to the dual water shaly sand model as determined from a nearby modern log (electrical, gamma-ray, density and neutron logs). Four old electric log analytical methods were attempted: uncorrected Archie equation (automatic compensation), Archie equation with a volume of clay correction, no-porosity log dual water model, and ratio water saturation. Each of these methods generated effective water saturations which were compared to the effective water saturation determined by the dual water model performed on a neighboring well with modern logs. The total porosity and effective porosity determined from old logs and modern logs, were also compared.

The formation chosen for this study was the Pennsylvanian (Virgilian) Hoover Sand located in the panhandle of Oklahoma. The Hoover Sand was chosen based on its lithology, structure, well spacing, and the availabílity of a mixture of modern logs and old logs. The Hoover Sand in the Mocane-Laverne field was discovered in 1955 and has been a productive gas sand for almost 40 years. The formatíon consísts of laminated silt and sand deposited via density flows or turbidity currents onto a shallow, broad shelf area that covers approximately 98,560 acres. Because the Hoover Sand is bounded above and below by shale and pinches out horízontally into a shale, the trapping mechanism is a structural/stratigraphic combination.

Thirty-two pairs of wells were evaluated in 4 foot intervals. Based on the comparisons between old log shaly sand methods and a "comprehensive" modern log shaly sand model (dual water), the uncorrected Archie equation (automatic compensation method) resulted in water saturations that closely approximated the results obtained from modern logs. These values can then be used to determine a more accurate value for recoverable hydrocarbons. The use of the volume of clay corrected Archie method determined effective water saturations which were consistently more optlmistic than the other methods. The ratio water saturation returned the poorest correlation to the effective water saturation as determined from the dual water method used on a modern log.