TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractRapid petrophysical evaluation is critical in such expensive venues as the deepwater Gulf of Mexico. Recently, a realtime petrophysical analysis program was introduced based on the combination of elemental concentrations from spectroscopy and standard triple-combo logging 1 .The application consists of a spectroscopy-based quantitative lithology consisting of total clay, sand, carbonate and pyrite 2 . Elemental concentrations are used to correct neutron and density for matrix effects 3 to directly compute total porosity and to correct for lithology and thereby enhance neutrondensity crossover in the case of gas. Intrinsic permeability is computed from the lithology and total porosity using the k-Lambda approach 4 . Water salinity is input and water saturation is computed using the Waxman-Smits-Thomas equations and porosity and lithology 5,6 . Finally, irreducible water saturation is computed from the Coates-Timur 7,8 equation using the calculated porosity and permeability.

Summary A recently introduced rapid petrophysical wireline evaluation has been tested and demonstrated to provide accurate petrophysical answers in a deepwater Gulf of Mexico well. This new interpretation scheme uses the combination of elemental concentrations from nuclear capture spectroscopy logs and standard neutron, density, resistivity, or "triple-combo" log measurements to get a more accurate description of the lithology and matrix parameters. Of particular value in the interpretation are the straightforward computation of matrix density from elemental concentrations and the resulting accurate porosity. Accurate clay and porosity values combine to provide a good estimate of permeability. These are all integrated with resistivity logs to estimate water saturation and irreducible water saturation to complete the petrophysical interpretation (Herron et al. 2002). In this study, the rapid petrophysical evaluations of porosity, permeability, and irreducible water saturation are demonstrated to agree well with core data. Initial disagreement between the interpreted lithology and core data jeopardized future use of the new interpretation, but on further investigation, the discrepancies were proved to be caused by the core measurements, not the wireline data. Therefore, part of the study focuses on the reanalysis and evaluation of core samples.