Gold Fool's: Detecting, Quantifying and Accounting for the Effects of Pyrite on Modern Logs

摘要

Pyrite is found in reservoir rocks throughout the world and is well known for its effects on logs. In 1976 Clavier et al reported a comprehensive study of these effects on most of the conventional logs then in service. In the quarter century since then a number of new logging measurements have arrived. These include resistivity tools that exploit very high frequencies, the photoelectric measurement which is now a standard output of the density tool, a new generation of neutron tools, including the so-called geochemical tools and NMR. Some of these measurements are very sensitive to pyrite whilst others are in theory completely unaffected by it. It is therefore a good time to consider how pyrite effects' these and conversely how they can assist in its recognition and quantification. Of the commonly encountered sedimentary minerals that occur at above trace levels, pyrite is distinguished by having: 1. the highest electrical conductivity. 2. the highest density 3. a high iron content and therefore a high average atomic number. Its effects on resistivity tools are particularly important, because at concentrations above a few per cent it can reduce the formation resistivity to the point where it is practically impossible to distinguish water and hydrocarbon bearing rocks. Moreover, all other things being equal the resistivity falls with increasing frequency so that LWD tools are expected to be particularly severely effected by it. Pyrite consists of nearly 50% by weight of iron, which strongly absorbs neutrons and lower energy electrons, it therefore drastically increases the so-called photoelectric factor and the neutron porosity. These effects are both a help to identifying pyrite and a hindrance to conventional interpretation. Under the right conditions the photo-electric factor can be used to get an approximate pyrite concentration, although the measurement suffers from low accuracy and under some circumstances hole conditions preclude its use. Neutron porosity is also influenced by the increase in density that results from introducing pyrite and this can be exploited to obtain an accurate estimate of matrix density and pyrite content. Neutron tools that exploit epithermal interactions are superior in this respect. In this paper these effects will be illustrated and discussed with reference to real modern logs obtained in pyrite containing sandstones.