Lithology Prediction and Storage/Transport Properties Evaluation in Clastic Sedimentary Rocks Using Seismic Velocities

VERNIK, LEV, and AMOS NUR, Stanford University, Stanford, CA

An extensive database of petrophysical measurements at high effective pressure on a variety of rock types encountered in clastic sedimentary sequences was used to derive new models that describe relations among physical parameters of these rocks. Based on optical microscopy, SEM, and experimental data four major petrophysical groups are considered: (1) clean arenites (clay content C < 3% vol.), (2) altered arenites and arkoses (3% < C < 15%), (3) wackes (35% > C > 15%), and (4) shale (C > 35%). Each of these petrophysical groups is characterized by its distinct mineralogy, texture, and the structural position of clay. The clay minerals in the second group are mostly the product of the instable silicate and rock fragment diagenetic breakdown which results in a sharp r duction of elastic moduli of these grain-supported rocks. Alternatively, shales and to a lesser degree wackes are matric-supported lithologies, which accounts for their lower elastic moduli at comparable porosity than in the first two groups considered. The compressional velocity-porosity relation for each of these groups is found to be linear with high correlation coefficients allowing remarkably accurate porosity estimates and lithology prediction from sonic logs, compared to Willie et al.'s time average equation or the linear regression by Han et al., both of which neglect mineralogical and textural factors. On the porosity-velocity plots, each petrophysical group has similar but shifted (relative to each other) linear trends of incipient lithification and diagenesis. The incipient li hification (consolidation) in both sandstones and shales is primarily brought about via the formation of small, contact-localized bonds due to the chemical processes at the grain boundary and is characterized by sharp increases in the elastic (primarily, shear) moduli. This results in dramatic velocity increase while porosity reduction at this stage is very small. Further diagenesis as traced on porosity-velocity plots is characterized by much gentler linear trends reflecting a considerable porosity reduction caused by the temperature-induced processes of dissolution and precipitation. There is a significant difference in porosity-permeability relation between grain-supported and matrix-supported lithologies, which can be accounted for by the fundamental differences in texture and porosi y structure of these two rock types. This difference is petrophysically substantiated by the irreducible water saturation measurements and the effective porosity calculation.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)