Matrix Characteristics Defined By Porosity Computations Available to Purchase

Accurate porosity determination is necessary for effective well evaluation. Several logging services are available for the computation of this necessary rock parameter; however, the interpretation of each is subject to inaccuracies caused by improper knowledge of the matrix or fluid characteristics. Since the various porosity devices, such as sonic, formation density, and neutron, are affected by matrix variations, combinations of logs from these tools frequently supply the data necessary for matrix identification. When the matrix is composed of a single, but unknown, mineral, the mineral may be identified by a comparison of data from two appropriate porosity devices. In horizons containing a mixture of two known matrix minerals, this comparison may be used to determine the percentage of each. Once the rock matrix is identified, a reliable value of porosity can be computed. Although the matrix may usually be identified through the use of any two porosity tools, better resolution is obtained with a proper selection. For example, in the sand and shale sequences of the Gulf Coast, the sonic and formation density logs are most appropriate. In predominantly carbonate areas, the neutron and formation density logs are generally recommended. Field examples show the relative efficiencies of the various combinations for different rock sequences.