Geothermal - Exploration & Evaluation

Supporting the Mining and Geothermal Industries Since 1994

Sims Professional Services

Dr. Sims develops and evaluates geothermal exploration and development drilling programs that target optimum structurally-controlled geothermal permeability. Where necessary, he relies on a network of colleagues who have expertise in geochemistry, geophysics and other scientific disciplines.

Dr. Sims is an excellent writer and speaker. If warranted, he helps developers to communicate the property potential to investors and others.

Comprehensive 3-D model incorporating all geological, geophysical and geochemical data used for targeting drill holes (from Sims, D.B., 2013, Geology at Hudson Ranch: The Salton Sea Geothermal Field, San Marcos College, October 10, 2013)

XRD analysis conducted for every 10 foot drill cutting at the drill pad provides real-time mineralogical data required to determine maximum recorded temperature and position within the geothermal system (800 XRD analyses for this single well). Drawn by Dr. Denis Norton and XRD by Dr. Philip Neuhoff (from Sims, D.B., 2013, Geology at Hudson Ranch: The Salton Sea Geothermal Field, San Marcos College, October 10, 2013)

Trained as an economic geologist and having worked in ore deposits since 1987, Dr. Sims has studied structural control on hydrothermal fluid flow within ore deposits that formed in volcanic systems and sedimentary systems with or without a magmatic heat source. He has used geochemistry to define flow directions in ore deposits to successfully target exploration.

He has also applied this experience in exploration for non-magmatic geothermal systems in the Basin and Range Provence, magmatic geothermal systems in volcanic rocks of the Snake River Plain, and for development drilling within the sediment-hosted magmatic geothermal system of the Salton Sea geothermal field.

His extensive experience mapping structure in hydrothermal ore deposits for slope design and education and experience using structure and geochemistry for exploration for epithermal vein systems gives him an understanding of permeability structures that pure geothermal geologists are not be exposed to.

Epithermal vein model showing narrow ore-bearing veins deep in the system. These are the permeability targets of Blackwell (2012). Alteration minerals and ore minerals are zoned vertically and laterally. These provide directional indicators to the permeability targets in cold fossil vein systems and also in hot active geothermal systems. From Buchanan L. J., 1981, Precious metal deposits associated with volcanic environments in the Southwest, in Dickson W. R., Payne, W. D. (eds.), Relations of tectonics to ore deposits in the southern cordillera: Arizona Geological Society Digest, vol. 14, pp. 237-262.

Geothermal - Exploration & Property Evaluation

Economic geology ore deposit models commonly depict different deposit types in terms of proximity to a magmatic heat source and other geologic factors that are directional indicators. In many cases, such as the one shown above, the active geothermal system is part of the model. This example from First Majestic Silver was chosen because it is simple. Its ore deposit is interpreted to have formed in the green oval. This diagram was modified from Hedenquist J. W., Arribas, A., Jr., and Reynolds, J. T., 1998, Evolution of an Intrusion-Centered Hydrothermal System: Far Southeast-Lepanto Porphyry and Epithermal Cu-Au Deposits, Philippines: Economic Geology, vol. 93, p. 373–404.

Geothermal systems are commonly characterized as a permeable reservoir rock that is overlain by an impermeable cap rock. Geothermal brine is produced through a wellhead and the resource is evaluated and managed by reservoir engineers. In these ways, a geothermal reservoir is very much like a petroleum reservoir. It reasonably follows that petroleum companies are heavily involved in the geothermal industry because they are best suited to produce the resources.

However, a geothermal reservoir has no genetic relationship to a petroleum reservoir. Instead, geothermal reservoirs are analogs for many types of hydrothermal ore deposits. As with geothermal systems, some of the hydrothermal ore deposits are directly related to magmatic heat sources and others are not; some form in volcanic rocks and others form in sedimentary basins; and structure is the most important component for permeability in most settings. Because the two types of systems are so closely related, and ore forming fluids no longer flow in most hydrothermal ore deposits, economic geologists try to better understand fluid flow dynamics and fluid chemistry in ore deposits by studying active geothermal systems. Using the same rationale, understanding the geometry of permeable structures in a geothermal system can benefit greatly from study of ore deposits, which have structure permeability systems exposed at the surface and in mine workings.

Dr. Sims attended the GRC annual conference in 2012 and was interested to see this concept acknowledged in a presentation by Dr. David Blackwell (Why Basin and Range systems are hard to find: the moral of the story is they get smaller with depth!). He made a comparison of ore shoots in epithermal veins to high productivity permeability targets for geothermal wells, and pointed out that the targets are small relative to the larger thermal footprint that is at the surface. These are concepts that have been in economic geology literature for decades and it is likely that most economic geologists will agree on these general points.

Dr. Blackwell concluded that common geophysical tools provide little help in locating permeable structures and that temperature is the only reliable exploration tool. He supported his point that temperature is the only reliable tool by stating that "most epithermal deposits are found using grid drilling and sampling." The implication is that because veins are no longer hot, there are no tools for finding epithermal ore deposits. This statement exemplifies the lopsided relationship between geothermal and economic geology.

Indeed there are no thermal anomalies related to epithermal ore shoots, and temperature would be a useless exploration tool, but that does not mean that epithermal veins are found by blind luck with a grid drilling method. To the contrary, Dr. Sims once learned that the only ore deposit that was ever discovered by grid drilling was in the Illinois-Kentucky fluorspar district. Epithermal veins contain hundreds or thousands of drill holes to prove up reserves. Drill intercepts can only be used to characterize an area of influence that may be tens of feet in diameter and that diameter dictates the allowable drill spacing for declaring proven, probable or inferred ore. This looks like a grid pattern sometimes, but it is done only after discovery drill holes intercept specific well-reasoned targets.

The well-reasoned methods used for discovery of epithermal veins have been developed over hundreds of years. As an economic geologist explorationist, your job is to learn to read the rocks to know whether you are a hundred yards or a hundred feet from a deposit and how to determine which direction to go to find it. The tools of the trade are structure, chemistry and mineralogy. Structure provides permeability and chemistry and mineralogy record paleo fluid properties and fluid temperature. Changes in these variables along the flow path define flow direction and fossil up-flow and out-flow zones have been defined using these tools in many ore deposits.

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