Sampling and Interpretation of Drill Cuttings from Geothermal Wells

Abstract

Drill cuttings from geothermal and mineral exploration boreholes, by contrast with those from most petroleum wells, commonly are derived highly fractured and faulted, hydrothermally altered igneous and metamorphic rock sequences, and are likely to be severely contaminated. Characterization of a subsurface resource from cuttings thus requires not only especially careful sample collection, preparation, storage and examination, but also a thorough knowledge of drilling technology, local geology and the full range of potential borehole contaminants. Accurate identification of lithology from cuttings is critical for recognition and correlation of rock types likely to selectively host the desired commodity. However, many of the rocks encountered in geothermal and mineral exploration boreholes (such as gneisses and granitic rocks) can resemble one another closely as cuttings even though dissimilar in outcrop or core. In such cases, the actual rock type(s) in a cuttings sample generally can be determined by comparison with simulated cuttings of representative surface rocks, and with various geophysical and other well logs. Many other clues in cuttings, such as diagnostic metamorphic mineralogy, or sedimentary rounding and sorting, may help identify subsurface lithologies. Faults and fractures commonly are the dominant physical controls on geothermal and mineral resources. Faults occasionally can be recognized directly in cuttings by the presence of slickensiding, gouge, or other crushed material. More commonly, however, the ''gouge'' observed in cuttings actually is pseudo-gouge created beneath a bit during drilling. Since most faults and all fractures produce no direct evidence apparent in cuttings, they are best recognized indirectly, either by commonly associated hydrothermal alteration, or by responses on appropriate geophysical well logs. Hydrothermal alteration, useful for locating and defining a geothermal or mineral resource, is far more difficult to recognize and interpret in cuttings than in core or outcrop. Alteration textures and paragenetic relationships can be obscured or obliterated as cuttings are produced. Less resistant alteration (and rock-forming) minerals can be disaggregated during drilling and lost from cuttings during sampling or washing. Relict and contemporary alteration can be indistinguishable, and a wide variety of borehole contaminants can closely resemble natural alteration products encountered during drilling. These contaminants also can produce confusing geochemical signatures.