Coiled Tubing Drilling and Real-Time Sensing—Enabling Prospecting Drilling in the 21st Century?
Richard R. Hillis, David Giles, Simon E. van der Wielen, Aaron Baensch, James S. Cleverley, Adrian Fabris, Scott W. Halley, Brett D. Harris, Steven M. Hill, Peter A. Kanck, Anton Kepic, Soren P. Soe, Gordon Stewart, Yulia Uvarova, 2014. "Coiled Tubing Drilling and Real-Time Sensing—Enabling Prospecting Drilling in the 21st Century?", Building Exploration Capability for the 21st Century, Karen D. Kelley, Howard C. Golden
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Tier 1 mineral resource discoveries are critical to maintaining Australia’s, and indeed the world’s, mineral resource inventory without continuing decline in the grade of mined resources. Such discoveries are becoming less common because, increasingly, remaining prospective, underexplored areas are obscured by deep, barren cover. We argue that improving the rate of Tier 1 discoveries obscured by deep, barren cover requires a step change in mineral exploration techniques that may be provided by “prospecting drilling,” i.e., extensive drilling programs that map mineral systems beneath cover, enabling geophysical and geochemical vectoring toward deposits. The technological platform for prospecting drilling must include low-cost drilling due to the dense subsurface sampling required. Low-cost drilling may be provided by transferring coiled tubing drilling technology, with its continuous drill pipe on a reel, from the oil and gas sector. Key challenges to the deployment of coiled tubing drilling in mineral exploration, i.e., its rate of penetration in hard rocks, the durability of coiled tubing, and the recovery of cuttings, are being assessed and addressed by researchers of the Deep Exploration Technologies Cooperative Research Centre (DET CRC). The optimum technology platform for prospecting drilling would be coiled tubing drilling complemented by downhole and top-of-hole sensing, providing realtime petrophysics, structure/rock fabric, geochemistry, and mineralogy. The first manifestation of real-time, downhole sensing is our newly developed autonomous sonde that is deployed by the driller and logs natural gamma radiation as the dill rods are pulled. Our experimentation on real-time, top-of-hole sensing (on drill cuttings from diamond cored holes) has demonstrated cost-effective, rapid, repeatable, and accurate determination of geochemistry and mineralogy with the necessary depth fidelity. The rationale for prospecting drilling is provided by two examples: (1) a dataset of antimony from the Kalgoorlie district of Western Australia, which shows that subsampling at a 2-km spacing would map the mineral system and enable vectoring toward the contained deposits, and (2) analysis of hypogene alteration systems of iron oxide-copper-gold (IOCG) deposits in South Australia that presents the possibility of vectoring toward the deposits within such systems starting from >10 km distant. At the target cost of $50/m, coiled tubing drilling could cost effectively undertake prospecting drilling in large, covered provinces, such as the IOCG prospective Gawler craton of South Australia.
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Building Exploration Capability for the 21st Century
Earth’s near-surface mineralogy has diversified over more than 4.5 b.y. from no more than a dozen preplanetary refractory mineral species (what have been referred to as “ur-minerals” by Hazen et al., 2008) to ~5,000 species (based on the list of minerals approved by the International Mineralogical Association; http://rruff.info/ima). This dramatic diversification is a consequence of three principal physical, chemical, and biological processes: (1) element selection and concentration (primarily through planetary differentiation and fluidrock interactions); (2) an expanded range of mineral-forming environments (including temperature, pressure, redox, and activities of volatile species); and (3) the influence of the biosphere. Earth’s history can be divided into three eras and ten stages of “mineral evolution” (Table 1; Hazen et al., 2008), each of which has seen significant changes in the planet’s near-surface mineralogy, including increases in the number of mineral species; shifts in the distribution of those species; systematic changes in major, minor, and trace element and isotopic compositions of minerals; and the appearance of new mineral grain sizes, textures, and/or morphologies. Initial treatments of mineral evolution, first in Russia (e.g., Zhabin, 1979; Yushkin, 1982) and subsequently in greater detail by our group (Hazen et al., 2008, 2009, 2011, 2013a, b; Hazen and Ferry, 2010; Hazen, 2013), focused on key events in Earth history. The 10 stages we suggested are Earth’s accretion and differentiation (stages 1, 2, and 3), petrologic innovations (e.g., the stage 4 initiation of granite magmatism), modes of tectonism (stage 5 and the commencement of plate tectonics), biological transitions (origins of life, oxygenic photosynthesis, and the terrestrial biosphere in stages 6, 7, and 10, respectively), and associated environmental changes in oceans and atmosphere (stage 8 “intermediate ocean” and stage 9 “snowball/hothouse Earth” episodes). These 10 stages of mineral evolution provide a useful conceptual framework for considering Earth’s changing mineralogy through time, and episodes of metallization are often associated with specific stages of mineral evolution (Table 1). For example, the formation of complex pegmatites with Be, Li, Cs, and Sn mineralization could not have occurred prior to stage 4 granitization. Similarly, the appearance of large-scale volcanogenic sulfide deposits may postdate the initiation of modern-style subduction (stage 5). The origins and evolution of life also played central roles; for example, redox-mediated ore deposits of elements such as U, Mo, and Cu occurred only after the Great Oxidation Event (stage 7), and major Hg deposition is associated with the rise of the terrestrial biosphere (stage 10; Hazen et al., 2012).