COMBINED STRUCTURAL ANALYSIS OF CORE AND IMAGE LOG OF TGH MB 76-31 EAST OF MOUNT BAKER, WASHINGTON STATE

Abstract

Despite active volcanism, few geothermal energy resources have been developed in the Cascades Range. Temperature Gradient Hole (TGH) MB 76-31 was drilled to approximately 440 m measured depth to probe for zones where fractures provide fluid conduits that transport deep volcanic heat to shallow depths that could support baseload, carbon neutral electrical generation. These zones were predicted by a Play Fairway assessment (PFA) of resource potential along a zone 11 km west-southwest of the summit of Mount Baker Volcano, Washington State. Rock core, temperature logs, and an acoustic image log were obtained. By comparison to outcrops, the core is interpreted as the Chilliwack Group, comprised of partially metamorphosed basaltic to andesitic volcanics, but due to similar physical and mineral composition may represent the Nooksack Formation. Mapping of core reveals complex, steeply dipping networks of fractures and brecciation along slickensided strike slip faults; clay alteration is common in many of these structures. Most fractures are thoroughly healed by layers of chlorite and calcite, whereas chlorites and vermiculite line open fractures. Fracture porosity is primarily hosted by very dense fractures a few centimeters or less in length. These small fractures are not clearly evident or interpretable in image logs, leading to under-estimated fracture density and secondary porosity, although the image log provides good insights into frequency and attitude of fractures that fully transect the core. The combination of complex, non-planar fracture zones containing many short fractures and healing promote misinterpretation of natural fracture attitude and density in the image log. The equilibrated measured temperature reaches a maximum temperature 32°C at 408 m measured depth along a conductive gradient of 64ºC/km and calculated heat flow of 145 mW/m2 which is more than twice the regional average of 30ºC/km. The presence of vermiculite and several chlorite minerals lining fractures is consistent with the conductive temperature gradient measured in the well below a shallow isothermal zone, although, several fractures are open or only partially healed and resulted in fluid entries into the well. Together, the temperature gradient and vermiculite formation in the fractures indicate local influence of the Mt Baker magmatic system at the Little Park Creek TGH site and that TGH MB 76-31 reaches the upper edge of a caprock above a much deeper hydrothermal system. Do you conclude that this site will/will not support electrical generation? Should have a sentence here describing this since you start your abstract with the idea that you’re testing viability.