EXPLORATION OF STRUCTURALLY CONTROLLED GEOTHERMAL SYSTEMS – SYSTEMATIC AND INTEGRATED WORKFLOW FROM FIELD WORK TO QUANTITATIVE 3D MAPPING, MODELING, AND DRILL TARGETING

There is a growing body of research showing that structurally controlled geothermal systems in extensional or transtensional settings are associated with specific structures, such as pull-aparts in strike-slip faults, accommodation zones, step-overs in range-front faults, terminations of major normal faults, intersections of major faults, or displacement transfer zones in contrast to central segments of major normal faults with maximum displacement. The structural framework of each of these principal structural settings is innately complex, and the permeable and productive areas are commonly spatially discrete. Examination of developed systems in the Great Basin region shows that only one productive fault per system may exist at economic depths of <2 km within a locally abundant region of faults and fractures.

Because each geothermal system is hosted within a local stratigraphic and structural framework and has discrete geometric permeable pathways, successful development of these systems relies on accurately defining these permeable up-flow zones in 3D space for precise well targeting. However, there is no single tool that can both define the detailed structural framework of a geothermal area and also define the individual parts of faults that currently host the geothermal fluids. Through recent and ongoing detailed studies of over 20 systems in the Great Basin region, we have developed a systematic workflow that integrates multiple geologic and geophysical data sets. The workflow is centered on constructing an accurate and detailed 3D geologic model while using the same conventional geologic reasoning as is used in constructing 2D geologic maps and cross-sections. The 3D geologic model is used as the framework for quantitative 3D structural analysis, evaluate discrete geothermal indicator data sets relative to the entire 3D model, define a conceptual fluid-flow model, and select well targets. The construction of data-based 3D geologic models and subsequent modeling in structurally controlled geothermal systems has the potential to mirror the successes that petroleum industry achieved with building detailed 3D geologic models from 3D seismic data sets and incorporating directional drilling.