This paper discusses a methodology for estimating permeability from well logs, based on conventional core and log data. The work was done on a small set of wells containing a glauconitic sandstone reservoir in the Barrow Sub-basin offshore, Western Australia. Core porosity and permeability data were used to determine porosity-permeability transforms and to subdivide the "greensand" into various groups within two sub-units. Electrofacies patterns and decision functions were determined to establish a link to permeability transforms in order to identify sandstone groups and to obtain porosity and permeability from well logs where cores were not available. Estimates of permeability were found to be more accurate than using other methods. In this technique, permeability transforms were obtained not only by considering unique relationships between porosity and permeability but also grain size and sorting et al. The results of this work have been encouraging and the study is now being extended on a regional basis.
In formation evaluation, it is generally essential to obtain realistic values of permeabilities from well logs because core-based permeability data are often not available either because of bore hole conditions or due to the high cost of coring. For many years, attempts have been made to estimate permeability from well logs. Much literature has been published on the subject and a number of methods and models are being used to achieve this goal. Two categories of equations are generally available. One category contains the so-called 'standard methods' such as Wyllie and Rose (1950), Timur (1968), Coates and Dumanoir (1973) and some modified forms of these. They are based on establishing empirical relationship (formulas or models) which are functions mainly of porosity, permeability, irreducible water saturation or clay content. However, none of these is generally applicable from field to field, well to well or even zone to zone without making adjustments to constants or exponents, or introducing compensations. Moreover, the accurate determination of irreducible water saturation and clay content from well logs is not an easy task. Alternatively, there are 'statistical methods' such as are described by Nicolaysen (1991), Sinha (1994), Johnson (1994) and others. They attempt to establish a direct statistical relationship between log responses and permeability, or use a data base to relate permeability to log responses on a field wide basis. The methods utilise relationships which are implicit in the data to arrive at results. No predetermined equations are required, and results are obtained by statistical inference.
The subject reservoir of this study is an extensive, marine transgressive, and glauconite-rich formation. It was deposited during a southward-progressing marine transgression over deltaic topset sands during the Early Cretaceous. The glauconitic sandstone is mineralogically complex. The main mineral components include quartz, glauconite, siderite, dolomite, calcite, feldspar, kaolinite, pyrite, and their alternation products. Quartz, glauconite, feldspar and siderite generally are the most abundant minerals and occur together in most of the Mardie lithofacies Porosity (helium injection) and permeability were determined on 139 core plugs on 4 wells cored in this area. They indicate that porosities are medium to high (from 13.9% to 29.7%), that permeability ranges from 0.01 md to 47 md throughout the sandstone in the two main sub-units and that there is poor correlation between porosity and permeability. Traditional methods for estimating permeability from log responses are not applicable in the Mardie Greensand.
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