Datapages, Inc.Print this page

Role of Petrographic Analysis in Wireline Log Interpretations

William L. Harmon

Log analysis interpretations of potential reservoirs are based on numerous assumptions that may not always be valid. Log responses are directly related to mineralogic and porosity characteristics. If sedimentologic and diagenetic attributes are misinterpreted, the productive capabilities of a formation may be underestimated or overestimated. Petrographic analyses using thin-section petrography, scanning electron microscopy, and x-ray diffraction techniques provide specific information on mineralogy and porosity characteristics. Using this information, the geologist, engineer, and petrophysicist can better evaluate wireline log signatures and resulting log analysis calculations.

Wireline logs typically affected by rock characteristics include the spontaneous potential (SP), gamma-ray, density, acoustic, and neutron logs. Petrographic studies reveal textural, mineralogic, and porosity data, which are then compared to wireline log signatures. Although the SP log responds to the permeability characteristics of a formation, the SP response may be suppressed by a decrease in grain size, addition of detrital and/or authigenic clays, and introduction of carbonate cements. The gamma-ray log records the natural radioactivity of a formation. This log is commonly used to establish depositional environments and shale volume corrections for density and sonic porosity calculations. Many clay minerals do not contain radioactive elements, whereas many nonclay minerals do con ain radioactive elements. Therefore, interpreted clay mineral content based on gamma-ray log responses may be misleading. However, porosity can be calculated from density and acoustic logs by establishing matrix density and traveltime values, respectively. To measure porosity accurately, these values must account for all minerals in the formation. Neutron logs respond to the total amount of hydrogen in a formation, irrespective of the element's distribution. Due to potentially high amounts of irreducible water, neutron logs measure the total porosity of a formation, but at least part of this total porosity will be ineffective porosity in terms of fluid-flow characteristics.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.