The experience and results described in the present paper were developed over nearly two decades, with a major R&D project around 1980. The expression Geochemical Mud Logging (GML) has ill defined meaning in the geothermal industry, and ought to be specified. We refer here to GML as featuring mud and formation fluid tracer(s) and temperature as the bare essentials and with specified accuracies. Air and water logging are expected to be less demanding with regard to analysis accuracy, but are not discussed in this report. During application of GML to several drill holes with low formation permeabilities and under conditions of high temperature and high mud weight, GML as specified, revealed unexpected influx of formation brine. Such influx was a recurring feature that has been referenced to individual fractures and reflects both fracture size and permeability. As a consequence, continuous or subcontinuous sampling of mud systems appears more cost effective than trying to keep up with cumulative changes of bulk mud composition; although, the latter approach is more sensitive to extremely low rate, steady, inflow of formation fluid into the mud system. It appears, that based on this influx of formation fluid, permeability can be estimated well before mud losses are detected and/or drill strings are stuck. The main advantages of GML are: (I) the capability to assess formation temperature and permeability in nearly real time, resulting in (a) assessments of undisturbed formation and (b) having data in hand for holes lost during drilling operations and (2) being effective under conditions of very high temperatures where electcrical logs are very costly and less reliable. Estimated cost for GML is $1500 per day (1982) based on assessments of R&D operations. However, extrapolating to larger scale services and to different operating conditions is indeed difficult. GML cost is probably the only significant point of controversy with regard to GML being a viable evaluation tool.

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