A New Technique to Determine Porosity and Deep Resistivity from Old Gamma Rayand Neutron Count Logs

摘要

Multivariate correlations developed via radial-basis function(RBF) network analysis of a modern suite of logs are used toestimate crossplot density-neutron porosity and pseudo-deepresistivity from 1950’s vintage well logs. Historically,porosity has been estimated with the logarithm of the neutroncount rate standardized with a 2% porosity tight spot and amaximum porosity of ~40% in a shale stringer. Despite afrequently observed visual correlation of the gamma ray trackwith the neutron count rate, the gamma ray log information isnot included in conventional porosity estimates.The visual comparison of the openhole gamma rayand the neutron porosity to the shape of the crossplot porosityfrom a new well, Foster 4, is similar to the pattern seenbetween the old gamma ray and the neutron-count rate logs.The new log data were investigated with various RBFnetworks to correlate the gamma ray and neutron-count ratewith the crossplot porosity. An optimal RBF network wasthen used to estimate the crossplot porosity of the Penrosesand for 22 wells in the Reed Sanderson Unit where onlycased-hole gamma ray and neutron-count rate logs areavailable. An equation converts the modern neutron log to theold style neutron count rate while providing a normalizedinput to the network training. The “goodness” of the RBFnetwork predictions was validated via exclusion testing.A similar procedure was used to estimate a pseudodeepresistivity in old wells. The study indicated that acarefully trained RBF network could identify the highresistivity intervals (shale stringers) with only gamma ray andneutron count rate used as the network inputs. Themethodology presented should prove useful to others facedwith characterizing old fields.