PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013 SGP-TR-198 RIFT ZONES AS A CASE STUDY FOR ADVANCING GEOTHERMAL OCCURRENCE MODELS Daniel King1,3 and Elisabet Metcalfe2,3 1AAAS Science & Technology Policy Fellow 2SRA International, Inc. 3Geothermal Technologies Office, U.S. Department of Energy 1000 Independence Ave. SW Washington, DC, 20585, USA e-mail: [email protected] or fully characterized. If geothermal electricity ABSTRACT generation is to increase significantly in the U.S., new blind hydrothermal resources must be identified To search efficiently for blind geothermal systems, and characterized. Unfortunately, geologic structure general geographic regions must first be identified and formation data are often sparse or incomplete, based upon gross characteristics which together introducing a high level of risk to a geothermal imply favorable heat flow, fluid flow, and industry seeking blind systems. To increase the permeability. Geothermal occurrence models seek to success rate of costly exploratory drilling, upfront strategically identify those promising locations to investment in exploration and resource focus exploration efforts and investment. In so doing, characterization is necessary. An essential such models can increase the expected success rate of component of early stage exploration is a geothermal exploratory drilling, reduce risk, and attract occurrence model (e.g., Walker et al., 2005) to investment. Among the most promising tectonic provide the framework for the set of characteristics to settings for blind geothermal systems are rift zones. be sought during exploration. A broader overview of Rift zones occur where lithospheric plates are thinned the technology pathways necessary to achieve cost by tectonic extension and convection at zones of reduction through improved exploration technologies upwelling hot material. The continued supply of is detailed in a strategic roadmap by the Geothermal magma to this separation zone increases heat flow Technologies Office, U.S. Department of Energy and thermal energy at shallower depths than in other describes (Phillips et al., 2013). tectonic settings. Given the inherent qualities of rift zones, the frequency of geothermal anomalies should A geothermal occurrence model describes a set of be relatively high, making them attractive targets geophysical, geochemical, tectonic, structural, and when searching for blind hydrothermal resources. geological features that are associated with a Geothermal occurrence models specify geothermal resource, or a "geothermal play". (A characteristics (e.g., rock type, stress field, fault play is a repeating set of prospects with common geometry, hydrology, local volcanism, etc.) that characteristics, and a “play fairway” is the indicate where to find a blind resource within a rift geographic area over which those prospects are zone. In this paper, we review literature contributions thought to extend.) These occurrence models can to the development of geothermal occurrence models provide a pathway toward integrating a variety of and explore rift zones as a case study for their observations into a quantitative assessment of application. We identify the key data and analytical resource, or play, potential. Figure 1 is a conceptual tools that are necessary to advance these models to flow chart of an occurrence model. The inputs are effectively and efficiently inform regional-scale data regarding the geothermal potential; the model resource assessment. itself is a combination of descriptive conceptual models and quantitative spatial association analysis; and the outputs are exploration products that help INTRODUCTION quantify the resource potential and/or risk profile of Most of the utility grade resources in the U.S. with geothermal development. obvious surface manifestations are either developed Figure 1: Conceptual flow chart of the implementation of a geothermal occurrence model. Evaluating risk and uncertainty with incomplete and efforts for hydrothermal systems are closely coupled sparse data is a major challenge faced by the with exploration for potential sites for enhanced geothermal industry. Robust occurrence models, and geothermal systems (EGS). their application to decision points and exploration products, can accurately assess the risk profile of a For EGS, heat is necessary, but the criteria related to given play or prospect and rank the relative fluid and permeability are somewhat relaxed. importance of missing data. Additionally, this Permeability is created through reservoir stimulation, approach is growing more relevant with the and the water or other working fluid does not development and expansion of the National necessarily need to be derived from the engineered Geothermal Data System reservoir. Ideal prospects for EGS may be a subset of (http://geothermaldata.org/). Continued refinement sub-optimal hydrothermal sites that have a favorable of geothermal occurrence models will be essential for stress state and rock mechanical properties that are leveraging these data to discover new blind well-suited for reservoir stimulation. hydrothermal resources. CHARACTERISTICS OF GEOTHERMAL In this paper, we review the established RESOURCES characteristics of geothermal systems. We also review several strategies implemented in geothermal In the following sections, we review characteristics of a variety of geological settings with geothermal (and other resource) exploration scenarios to address resources associated with active extensional the problem of imprecise or non-explicit parameters tectonics. In Table 1, the general characteristics are and values. We will focus on reviewing occurrence models for extensional rift systems with the goal of summarized within the screening protocol developing a framework for resource occurrence framework presented by Walker et al. (2005). The broadest scale characteristics are at the regional scale, models that is adaptable to diverse settings and which includes traits that indicate geothermal upcoming advances in exploration technology. Some potential without identify specific regions for detailed characteristics are applicable in multiple tectonic exploration. The prospect scale characteristics help settings, but each individual geothermal play has its own occurrence model that accounts for why heat, to focus exploration to more specific targets. Finally, fluid, and permeability are present in that setting. the project scale traits inform the decision of specifically where to drill exploration wells. This Understanding existing and new models for paper is not intended to be an exhaustive review of geothermal reservoirs is key to unlocking additional geothermal occurrence characteristics, but these resource potential. sections should provide an overview of components This paper focuses on hydrothermal system of a generalized occurrence model and of key occurrence models, with the goal of synthesizing components that might constitute region-specific occurrence models. Our focus is on information that could be useful in prospecting for structural/geological controls for geothermal systems, blind hydrothermal systems. However, exploration which is reflective of the emphasis of much of the critical to locating an electricity-grade geothermal published work on geothermal occurrence models. system (Hinz et al. 2011). Rift Zones as Geothermal Targets East African Rift Zone The East African Rift is a system of classic grabens Basin and Range about 40-80 km wide, and is an example of an The superposition of the regional effects of extension intracontinental divergence zone where rift tectonism and volcanism/intrusion with complex localized is accompanied by intense volcanism from the late structural settings make geothermal prospecting in Tertiary to present. The central section of the East the Basin and Range both promising and difficult. African Rift System corresponds to the Kenya Though large scale structures such as grabens and Dome, an area of crustal uplift and thinning. As the their associated moderate to steeply-dipping normal dome formed, it stretched and fractured the outer faults may act as a primary structural control on brittle crust into horst and graben structures and upwelling geothermal fluids, not every extensional associated normal faults typical of rift valleys. structure hosts an electricity-grade geothermal Volcanism in the rift system is consistent with a large resource. Localized controls, those which must be magma chamber at relatively shallow depths identified at the prospect level include step-overs, (Alexander and Ussher 2011). Geothermal fields are relay ramps, fault intersections and normal numerous, and are characterized by high terminations or tip-lines, which host most of the temperatures (~300 C) and shallow heat sources (~6 geothermal systems in the Great Basin (Faulds et al. km), and indicated by geothermal surface 2012). Multiple overlapping fault strands and manifestations such as fumaroles, steam jets, intersections between normal faults increase the steaming and solfatara (Alexander and Ussher 2011, fracture density and thus enhance permeability and Simiyu 2012). Geothermal manifestations are concentrate geothermal fluids (Blackwell et al. 2012, typically located along the margins of calderas, Faulds et al. 2012, MacLachlan et al. 2011, Jolie et within summit craters, within trenches, and flank al. 2012). Further, any zone accommodating greater eruption centers (Alexander