Oil fields—A source of heat flow data

Abstract

A method of calculating terrestrial heat flow from oil fields is evaluated by reviewing its application to twelve petroliferous basins, three of them in Brazil and nine in Indonesia. In each well, terrestrial heat flow was calculated by multiplying the temperature gradient (obtained by commonly used procedures) by the effective thermal conductivity. The latter was calculated from quick thermal conductivity measurements on core samples by a transient hot-wire method and from the lithologic log. Typically, in each basin the conductivity of about 300 core specimens was measured and logs of 50–150 wells were examined. In each formation penetrated by a well, the total thickness of each rock type such as shale, sandstone, etc., was determined from the composite lithologic log. The harmonic mean of the thermal conductivity of a formation in the well was computed from these thicknesses and the average values of the conductivities of the rock types measured on cores collected from that formation. The conductivity of the well is the harmonic mean of the conductivities of the formations. The average precision of the determination of formation conductivity can be expressed as the α 95 limits divided by the average value. For twelve basins this measure of precision for formation conductivities averages 3.6% and the average precision of the conductivity of a basin is 2.0%. On the same basis, the precision of the determination of the temperature gradient in a basin averages 4.7% and that of the heat flow 5.8%. Expressing the variability of the geothermal quantities in a basin determined at discrete locations as the standard deviation divided by the average value, we get for the mean variability in the twelve basins: 6.7% for conductivity, 15.0% for the gradient and 16.4% for the heat flow. The variability must come in part from natural causes. In general, at the present level of accuracy no correlation between anomalous heat flow and the occurrence of oil in a basin was found. Values of thermal conductivity of sedimentary rocks and their variation with depth are useful in calculating paleotemperatures. The magnitude of the mean heat flows of the basins is normal (63 mW/m 2) for the Jurassic basins of the stable continental margin of Brazil and somewhat elevated in the Tertiary basins of Indonesia, north of the Sunda trench subduction zone, the values decreasing from 130 mW/m 2 in Central Sumatra to 70 mW/m 2 in eastern Kalimantan. Future improvements in the procedures for determining conductivity might possibly bring out local anomalies in heat flow that might be caused by vertical seepage of water. This would support the hydraulic theory of accumulation of oil and gas.