A Petrophysical Model for Shale Reservoirs to Distinguish Macro Porosity, Micro Porosity, and TOC

A petrophysical model is presented whereby a number of shale reservoir components, both solids and porosity, can be identified. Input data are standard open-hole logs to include density, neutron, gamma ray, and resistivity.

The starting point is a standard shaley formation analysis to identify total and effective porosity, shale volume, and fluid saturations in the effective porosity fraction of the rock. A second task is to determine TOC content. The procedures used in this model are those of Passey et al, 1990, and include calibration to core measurements, if available.

Using the data from the initial analyses, detailed examination of the shale fraction of the rock can then be undertaken. First, density and neutron log responses are reduced to the shale only fraction, by subtracting contribution from matrix, effective porosity, and TOC. Then, by using a density/neutron cross plot of the shale only fraction, points are chosen by the interpreter recognizing silt, clay species #1, and clay species #2. Once they have been chosen, it is possible to recognize the volumetric contribution of silt and the two shale components. The properties of the two clay points, bulk density and neutron porosity, are used to define porosity of the clays, which is water filled. The cross plot porosity of the shale contains both clay porosity and free shale porosity, and is greater than or equal to clay porosity. If this relationship is not honored (i.e. clay porosity greater than shale porosity), adjustments need to be made with respect to the log choices of silt, clay 1, and clay 2. Finally, free porosity in the shale is available by subtracting clay porosity from cross plot shale porosity.

A volumetric balance of the porosity components - free shale porosity, TOC, clay porosity, and effective porosity - is compared with total cross plot porosity, to ensure the model accounts for all porosity elements correctly. Mismatches can be rectified by adjusting input parameters of silt, clay species #1 and clay species #1.

Comparisons are now possible among macro porosity (effective porosity), micro porosity (free shale porosity), and TOC.

Examples are presented from a number of shale reservoirs, both oil and gas bearing.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California