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GEOLOGIC MODEL of the BACA GEOTHERMAL RESERVOIR, VALLES CALDERA, NEW MEXICO Dennis L. Nielson and Jeffrey B. Hulen Earth Science

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GEOLOGIC MODEL of the BACA GEOTHERMAL RESERVOIR, VALLES CALDERA, NEW MEXICO Dennis L. Nielson and Jeffrey B. Hulen Earth Science Proceedings Ninth Workshop Geothermal Reservoir Engineering Stanford University, Stanford, California, December 1983 SGP-TR-74 GEOLOGIC MODEL OF THE BACA GEOTHERMAL RESERVOIR, VALLES CALDERA, NEW MEXICO Dennis L. Nielson and Jeffrey B. Hulen Earth Science Laboratory, University of Utah Research Institute 420 Chipeta Way, Suite 120 Salt Lake City, UT 84108 ABSTRACT such as the Madera Limestone, are regional groundwater aquifers outside the caldera. Logging of drill cuttings from Union Oil Company's Baca geothermal project has Previous workers have reported that the Baca resulted in a detailed geologic picture of field has a permeability-thickness product of the Baca geothermal reservoir. The reservoir about 6,000 md-ft, which is low compared with is located in a complex geologic environment other producing geothermal areas. A model with both fracture and stratigraphic permea- for the development of the Redondo Creek bility, which have been modified (largely resurgent dome, within which the Baca field reduced) by hydrothermal alteration. The is located, explains these low permeabilities geothermal system is principally hosted by by emphasizing the nature of strain and ash-flow tuffs and volcaniclastic rocks asso- resulting fracture permeability in the host ciated with the development of the Valles and lithologies. The model also helps emphasize To1 edo cal deras and precedi ng expl os ive rhyo- the importance of inherited structures in the litic eruptions. The cooling history of formation of the dome and suggests that these these rocks has produced non-welded tuffs, faults may be the principal conduits for with considerable intergranular permeabi 1 ity geothermal fluids from depth in the system. and extensive hydrothermal alteration, Another imp1 ication of model i ng dome develop- through densely welded ash-fl ow tuffs with ment is that the magmatic heat source for the 1 ittle intrinsic permeabi 1 ity. Lithologic geothermal system lies beneath present drill- units beneath the ash-fl ow tuff sequence are ing depths. commonly altered and metamorphosed but con- tain few hot-water entries, although some, 0 1 2 3 4 5 MILES '\\ ,_-' '\ -------e-- -..___/' 0 1 2 3 4 5 KILOMETERS '. .-__----- 11111111111 Figure 1. Index map showing location of the Valles-Toledo caldera complex. -145- The stratigraphic and structural hydrothermal horizons and perhaps ai r-fall pumice beds fluid paths at Baca can be characterized as (Hulen and Nielson, 1982). Structural follows: 1) those sealed by alteration, 2) permeability is developed along high-angle those which do not produce fluids but do normal faults disrupting the felsic tuff accept fluids, and 3) those which produce sequence as well as underlying units. We geothermal f 1 uids. believe these faults to be the principal paths along which thermal fluids ascend. I NTRUOUCTION Once within the felsic tuff sequence, how- ever, the fluids appear to be diverted in The cal dera environment represents a complex part into intra-tuff stratigraphic aquifers interaction of volcanic, structural, and (Fig. 2), creating an intricate circulation often, hydrothermal processes. As a result system requiring much more caution in tar- calderas are often targets for geothermal geting from the surface than a fault-con- expl oration and development. From the trolled system alone. standpoint of the reservoir engineer, such geothermal systems would be hosted by rocks The Otowi and Tshirege Members of the that display a complex interplay of strati- Bandelier Tuff (Fig. 2), initially deposited graphic permeability, structural permeability as thick, hot (600-7OO0C) ash flows, have (both faults, fractures and joints), and undergone profound compaction, welding, changing permeability which results from the devitrification (a process by which silica process of hydrothermal alteration and new minerals and feldspar nucleate from the glass fracture generation. The purpose of this initially present) and widespread granophyric paper is to present a geologic model of the crystallization. As a result, both members, Baca geothermal reservoir which is situated generally exceeding 300 m in thickness in the within a small portion of the Valles caldera Redondo Creek wells, are typically dense and in New Mexico (Fig. 1.). The geologic his- impermahle except where faulted or fractured tory of the Valles caldera is presented in or where they include thin permeable Smith and Bailey (1968). The data we present strata. The Otowi and Tshirege, in fact, may is largely based on our studies of subsurface serve as local caprocks above geothermal samples from Union Oil Company's Baca project fluids circulating in the Lower Tuffs and area. Additional results of our work have underlying rocks. been published previously (Hulen and Nielson, 1982, 1983; Nielson and Hulen, in press). Several productive thermal fluid entries have been encountered in the Paliza Canyon Forma- STRAT I GRAPH Y tion, a sequence of Pliocene intermediate- composition flows , tuffs, subvol canic intru- The Precambrian to Quaternary stratigraphic sives (?) and volaniclastic rocks beneath the sequence penetrated by deep Union wells in "Lower Tuffs". The Paliza Canyon has not the Redondo Creek area of the Valles caldera been sufficiently investigated to state with has been characterized in the reports cited certainty that the entries are strati- immediately above and to which the reader is graphically or structurally controlled. guided for more detailed description of Furthermore, one of the most productive individual lithologies. This paper will Paliza Canyon entries occurs at the bottom of restrict discussion of these rocks mostly to well 8-24, where the nature of permeability features related to structural or strati- control cannot be ascertained. graphic pe rmeabi 1 ity. Stra t igraph ic geothermal t a rgets in s ub- The Baca geothermal reservoir is hosted Paliza Canyon rocks included friable arkose principally by a sequence of Pleistocene of the Tertiary Santa Fe Sandstone/Abiquiu felsic ash-flow tuffs and associated volcani- Tuff (Fig. 2), red-bed arkose of the Permian clastic sediments locally exceeding 1800 m in Aho Formati on, sol ution voids/ karstic terrain thickness within the Valles caldera. The two in limestone of the Pennsylvanian Madera members of the Bandelier Tuff, the Otowi and Formation and gruss developed on deep overlying Tshirege, form the middle portion Precambrian granitic rocks. No thermal fluid of this sequence iFig. 2). The Tshirege is entries were penetrated beneath the Paliza overlain by the Upper Tuffs" (Nielson and Canyon Formation. Lost-circulation zones, Hulen, in press), dominantly non-welded to however, were very commn (Molloy and poorly welded felsic ash-flow tuffs; the Laughtin, 1982). This implies locally ade- Otowi rests unconformably on complexly quate permeability in these rocks, but either interstratified, variably welded felsic ash- no connection with major thermal fluid flow tuffs and sediments of limited local conduits or perhaps pervasive underpressuring extent grouped by Nielson and Hulen (in of the reservoir at depth. press) as the "Lower Tuffs". STRUCTURE Permeability (and former permeability) in the felsic tuff sequence at Redondo Creek is both Nielson and Hulen (in press) have modeled the structural and stratigraphic. Stratigraphic development of the Redondo dome following the permeability is controlled by thin, intra- philosophy developed by Johnson (1970) for tuff sandstones , discrete, non-welded tuff the emplacement of the Henry Mountains -146- A in, ,do Creek Tuff nantly Pm . itotion Figure 2. Geologic cross-section through the Redondo Creek project area, showing possible thermal fluid migration channels. -147- laccolith in Utah. We did not intend to is able to intersect the principal fracture model rigorously the formation of the Redondo zones which carry the geothermal fluids. In Dome, but rather to use theoretical concepts addition, in the stress field above the of dome formation to help explain the neutral plane, it is probable that stimu- permeabilities found in the Baca project lation will open fractures which are asso- area. ciated with extension along the top of the domed structure rather than the reactivated Most workers agree that resurgent domes are structures which we feel may be the more formed after caldera collapse by upward important geothermal conduits. pressure of still molten magma. This upward pressure results in the formation of a One conclusion from our structural work at structural dome, generally in the center of the Baca project area has been that resurgent the caldera. As a result of the doming domes are inherently impermeable due to their process, the affected strata are subjected to mode of structural development. Observations differential stresses. In the upper portions of other resurgent dome features tend to of the dome, the rocks are under tension. support this conclusion. In most This results in the development of faulting hydrothermally active caldera systems (such along the crest of the dome that forms most as Yellowstone), hot spring and fumarolic of the fracturing that is often identified as activity are spatially associated with the an apical graben of the dome. Approximately margins of resurgent domes, where these dome half way down through the dome, a surface structures are thought to intersect the ring called the neutral plane separates the upper fracture system of the caldera. Very little tensional environment from a lower region hot spring or fumarolic activity is present characterized
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