Pulsed Neutron Capture Log Interpretation In Laminated Formations: A Dual-Exponential-Decay Model

Abstract

Summary Industry standard shaly sand equations for pulsed neutron capture (PNC) logs are premised upon homogeneous formations. Many interlaminated sands and shales, however, may not appear as homogeneous mixtures to PNC logs, despite the fact that the laminations may be very small (3 or 4 in.) and well below the vertical resolution of the PNC device. This can result in serious interpretation errors leading to bypassed pay. Log response equations and interpretation techniques for inhomogeneous formations are derived in this report and compared with a large sampling of observed data in river-dominated delta fringe sands of the Louisiana Gulf Coast. The excellent agreement warrants consideration of the new PNC log equation over the current industry standard equation as the primary method of evaluation in this type sandstone. The response of the natural gamma ray log to shaly sandstones is also reviewed. Possible misapplication of the industry standard PNC equation in deriving gamma ray/shale volume relations for inhomogeneous laminated formations is illustrated. The resulting impact on log interpretation is examined, and more appropriate relations are discussed.