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Salton Sea Geothermal Field, Imperial Valley, California

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Salton Sea Geothermal Field, Imperial Valley, California PROCEEDINGS, Twenty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 29-31, 2001 SGP-TR-168 NEWLY-DISCOVERED, ANCIENT EXTRUSIVE RHYOLITE IN THE SALTON SEA GEOTHERMAL FIELD, IMPERIAL VALLEY, CALIFORNIA Implications for Reservoir Characterization and Duration of Volcanism in the Salton Trough 1Jeffrey B. Hulen and 2Fred S. Pulka 1 – Energy & Geoscience Institute (EGI), University of Utah 423 Wakara Way, Suite 300, Salt Lake City, UT 84108 [email protected] 801-581-8497 2 – CalEnergy Operating Company 950 West Lindsey Road, Calipatria, CA 92233 [email protected] 760-348-4011 ABSTRACT INTRODUCTION Thick (150-300 m), ancient rhyolites and Two recently completed (1998, 1999) tuffs unambiguously erupted onto a CalEnergy Operating Company (CEOC) paleosurface have been discovered beneath injection wells in the eastern part of the 1.7 km of clastic sedimentary strata in the Salton Sea geothermal field (Smith IW-2 eastern part of the Salton Sea geothermal and Vulcan IW-8; Figs. 1 and 2) penetrated field. The rhyolites are aphyric and flow- thick (150 m and 300 m, respectively) banded, and consist entirely of intercepts of felsic igneous rock identified at micropoikilitically devitrified glass. The the time of drilling by one of us (Pulka) as tuffs contain accretionary lapilli, blocky rhyolite. These newly discovered rhyolites glass shards, and sedimentary debris; they greatly exceed in thickness any previously are interpreted as phreatomagmatic. encountered at depth either in the Salton Sea field (maximum of 38 m in the bottom of Assuming an average sedimentation rate of well Elmore 1; Fig. 2) or the entire Salton 2.24 mm/yr for this part of the Salton trough trough (Reed, 1984; Robinson et al., 1976). (a figure based on occurrence of the The earlier-documented Salton Sea rhyolites petrographically distinctive, 0.76 Ma Bishop were considered to be intrusives, possibly Tuff fallout deep in the nearby State 2-14 the subsurface equivalents of five small scientific borehole), the age of the new Pleistocene rhyolite domes (the Salton rhyolite is calculated to be about 0.73 Ma. Buttes) at the southeastern margin of the A potentially valuable marker horizon, the Salton Sea (Figs. 1 and 2; Robinson et al., new rhyolite is envisioned as part of a much op. cit.). The Smith and Vulcan rhyolites larger buried dome field, perhaps analogous were also thought initially to be intrusive, to the one now exposed above the Coso but as we demonstrate in this paper, there is geothermal system about 390 km to the more than ample evidence that these volca- north. nics were erupted onto the contemporaneous surface. 1 This finding is significant for several 2) is developed above the northernmost of reasons. First, these rhyolites are buried the spreading centers, where interaction beneath about 1.7 km of Salton-trough between the North American and Pacific fluviolacustrine clastic sediments (e.g., plates changes from stretching and crustal Muffler and Doe, 1968), so the volcanics extension to dominantly right-lateral slip must be considerably older than the ~10 ka along the San Andreas transform fault zone (Herzig and Jacobs, 1994; Friedman and (SAF; Fig. 1). Obradovich, 1981) domes of the Salton Buttes. Secondly, the great thicknesses of Whereas the Gulf of California is a narrow the Smith IW-2 and Vulcan IW-8 intercepts new sea developed above fresh oceanic imply that these rhyolites could be part of a crust, its extension, the Salton trough, is much larger buried volcanic field. Thirdly, filled by up to 6 km of Pliocene to Holocene the rhyolites could constitute a useful time- deltaic fluviolacustrine clastic sediments stratigraphic marker horizon. If laterally supplied by the ancestral and modern extensive, this rhyolitic marker could enable Colorado River (Elders and Sass, 1988; van geoscientists and reservoir engineers to de Kamp, 1973; Muffler and Doe, 1968; constrain more accurately three-dimensional Merriam and Bandy, 1965). These sedi- stratigraphic and structural models currently ments serve as efficient thermal insulators being refined for the entire Salton Sea above the continental spreading centers, geothermal field. enabling the formation of large, hot (up to at least 370oC) hydrothermal systems like This paper provides the first detailed those at the Salton Sea and, to the south, description of the thick, buried rhyolites in Mexico’s Cerro Prieto geothermal field the eastern part of the Salton Sea field. Our (Reed, 1984; Fig. 1). focus is the rhyolitic interval penetrated in Smith IW-2; work in progress on the thicker There are few volcanoes exposed in the Vulcan IW-8 intercept will be deferred for a Salton trough, and all are Quaternary future account. We first establish the extru- (Herzig and Elders, 1988a, 1988b; Robinson sive nature of the rhyolite and associated et al., 1976). The age of a lone dacite dome tuff using a variety of compositional and near Cerro Prieto (Fig. 1) is estimated, based textural criteria, then discuss the implica- on paleomagnetic data, to be 100-10 ka (de tions of this origin for the physical nature Boer, 1980). Obsidian Butte, at the Salton and volcanic history of the geothermal Sea field (Fig. 2) yielded a K-Ar age of 16 reservoir, as well as for the Quaternary ka + 16 ky (Muffler and White, 1969). The volcanic evolution of the entire Salton Obsidian Butte and Red Island rhyolites trough. (Fig. 2) also produced obsidian-hydration- rind ages of 2.5-8.4 ka (Friedman and GEOLOGIC SETTING Obrado-vich, 1981); we will show that the Smith IW-2 rhyolite is likely older by two The Salton trough (Fig. 1) is the northern orders of magnitude. landward extension of the Gulf of California tectonic regime, within which oceanic crust THE RHYOLITE OF SMITH IW-2 is being formed at pull-aparts, or spreading centers, developed at the oversteps between General – These rhyolitic rocks were major, en echelon, right-lateral strike-slip penetrated in the injection well between faults (Elders and Sass, 1988; Elders et al., drilled depths of 1646 m and 1801 m (Figs. 1972). On land as in the Gulf, these 2 and 3). The rhyolites interrupt an spreading centers are the sites of intense otherwise compositionally consistent stack magmatism, volcanism, and high- of clastic sedimentary rocks (see also the temperature hydrothermal activity. The following paragraphs) in which Smith IW-2 Salton Sea geothermal system (Figs. 1 and was terminated at a depth of 2484 m (true 2 vertical depth of 2464 m). Measured banded. It is a micropoikilitically devitrified temperatures reach 360oC at total well (criteria of Anderson, 1969; and McPhie et depth, and range between 323oC and 333oC al., 1993) glass, characterized by through the rhyolitic interval. interlocking to scattered, snowflake- textured quartz patches incorporating Like all Salton Sea wells, Smith IW-2 is feldspar microlites along with bands and dominated by late Cenozoic, fluviolacustrine stringers of secondary chlorite. The turbid sandstones, siltstones, mudstones, and “snowflakes” account for 20-50% of the probably marls. Since the trough first began rock. The remainder is a microcrystalline to form about 4 m.y. ago (Herzig and (3-7 mm), massive to trachytic-textured Jacobs, 1994), the clastic source for these mosaic of quartz, K-feldspar, and chlorite, deposits has been the heavily sediment- along with accessory titanite and allanite. laden Colorado River (Fig. 1). The latter is both a primary and an alteration phase, as well as a common constituent of Mudstones dominate the upper 400 m of hydrothermal veins in the rhyolite. Smith IW-2, and help provide an effective caprock, or top seal, on the underlying, Cuttings of the tuff overlying the rhyolite liquid-dominated hydrothermal system (Fig. reveal a chaotically-textured aggregate of 3; see also Moore and Adams, 1988; and silt- and sand-sized grains (quartz, feldspar, Elders and Sass, 1988). Below the caprock, lithic fragments, and minor detrital mica) the proportion of mudstone to siltstone plus mixed with varying amounts of pumice, sandstone, averaged over 30-m intervals, is glass shards, and devitrified rhyolite relatively constant at about 1:1 (Fig. 3). fragments compositionally and texturally identical to the main rhyolite below. Glass Stratigraphy of the clastic rocks penetrated shards up to 2 mm in diameter in the tuff are by Smith IW-2 is highlighted by a gamma- entirely altered to adularia and/or chlorite. log trace (Fig. 4), on which potassium-rich These shards are blocky in outline, sparsely shales show as highs, and intervening vesicular, and broken across the ovoid sandstones and siltstones as lows. The vesicles. Pumice shards and lapilli, mostly strongest gamma response in the well, of the filamentous, or “tube” variety however, closely corresponds to the rhyolitic (McPhie et al., 1993), are also entirely interval. chloritized and/or adularized. A spontaneous-potential (SP) log also Compaction-deformed accretionary lapilli reveals the Smith rhyolite (Fig. 5). The are common throughout the tuff interval. reasons for this response remain to be These lapilli, of the massive “core-type” determined, but the SP and gamma signals (Schumacher, 1989), are crudely ovoid, combined show great promise for tracking agglutinated masses of fine ash and subsurface rhyolites in the Salton Sea field sedimentary clastic debris. Along with the in the absence of drill cuttings, or where blocky glass shards noted above, these these volcanics are thin or otherwise accretionary lapilli indicate significant difficult to recognize from borehole samples groundwater involvement in the causative alone. eruption(s) (e.g. Sheridan and Wohletz, 1983). Petrology and Petrography – The Smith IW-2 rhyolitic interval comprises two The Smith IW-2 rhyolite is not only tex- distinct compositional and textural turally but mineralogically distinct from subzones: the main rhyolite, and an overlying and underlying clastic strata (Fig.
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