THERMAL RESULTS of the SANTIAM PASS 77-24 DRILL HOLE David D. Blackwell Dept. of Geological Sciences Southern Methodist Universi
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THERMAL RESULTS OF THE SANTIAM PASS 77-24 DRILL HOLE David D. Blackwell Dept. of Geological Sciences Southern Methodist University Dallas, TX 75275 a Report to Oregon Department of Geology and Mineral Industries 800 NE Oregon St, #28 Portland, Oregon 97232 April, 1992 ABSTRACT The thermal results from the 929 m deep Santiam Pass 77-24 well are described. The well in its completed state is characterized by downflow of cold groundwater in the bore that prevents accurate detern1ination of the true rock thermal conditions below 160m. The average thermal conductivity of the 400 to 927 m interval is 1.66+0.07 W/m/K. The calculated gradients for the 410-718, 718-920, 912.0-928.0 and 919.0-928.0 m depth intervals are 16, 52, 115.8±0.9, and 103.6+2.3 °C/km respectively. The gradients in the first two intervals are based on the BHT data. The heat flow values for the bottom three intervals of the well are 86, 175 and 204 mW/n12 respectively. Including the uncertainty these values bracket the regional average for the High Cascade Range of 105+5 mW/m2 (Blackwell et al., 1990). The increase of heat flow with depth is due to the presence of regional groundwater flow whose effect on the thermal field din1inishes as a function of depth due to decreases in permeability as the rocks become progressively more altered. The hot springs and the other thermal features along the McKenzie River are probably related to upward focusing of the gravity driven regional groundwater flow along the faults bounding the west side of the Cascade Graben. The focusing is related to the fault bounded termination of the higher (relative) permeability rocks of the graben against the lower (relative) permeability rocks of the Western Cascade Range. The heat flow at the bottom of the well is equal to or higher than the true heat flow and examples from other Cascade Range wells are illustrated to demonstrate the basis for this conclusion. The results do demonstrate that the heat flow along the Cascade Range axis, even in an area between active stratovolcanoes, is equal to or higher than that along the west edge of the High Cascade Range as described by Blackwell et al. (1982, 1990). INTRODUCTION The Santian1 Pass 77-24 well was drilled 929 meters deep at 1464 meters elevation on the axis of the Oregon Cascade Range at Santiam Pass as a geothermal observation hole. The location and the thermal data in the region are shown in Figure 1. The Santiam Pass 77-24 site was chosen to assess the geothermal conditions along the axis of High Cascade Range away from major volcanic centers. The scientific goals of this project were to obtain temperature gradient, heat flow, hydrologic, and lithologic data that could not be obtained from nearby shallow (100 to 400 m deep) drill holes. The preliminary results have already been summarized (Hill et al., 1991a). The upper 140 meters of the hole were rotary drilled by Woytek Drilling and cased with 10 ?. r.m T n c;;:nrf::~r.e r.::~c;;:1nP" in the f::~ll of 1QRQ. D1~monci core ciril11np- w~c;;: clone hv ··-~-- --·- ---- -·- · --------- -------o ------- ---- -- -- -- · - ----------· ---- ---------o ··--- -·---- -..~ Tonto Drilling Services under the direction of Oxbow Power Corporation and DOGAMI during August and Septen1ber of 1990. The hole was drilled from 140 meters to T.D. using HQ (10.2 em O.D.) diamond core rods, with > 99.5% core recovery. Drilling operations commenced on August 10, 1990, and proceeded at an average daily rate of 28 meters per day with no significant delays. Total drilling costs, including rotary drilling of the upper 140 meters, were approximately $224,000; core drilling costs averaged about $250/meter, and rotary costs were about $200/meter. The hole was conditioned with heavy drilling mud and completed on Septetnber 14, 1990 with 4.8 em I.D. water-filled black pipe to total depth. A caliper log was run prior to completion, natural gamma-ray and temperature logs were run on September 19, 1990, a second temperature log was run on September 27, 1990, and a final temperature log was run on September 3, 1991. The equilibrated bottom hole temperature is 25.06°C at a depth of 928.4 m, with apparent temperature gradients of 50- 600C/km from 700-900 meters and between 103 and 116°C/km below 910 meters. 122° 15' 121° 45' IfTFJ l • \ •"61 I \Q SP 77-24 C. BL o ( I l t } (.ww ( .2 <. •67 -19 Jac b. -5 *'r b. \ - ' \ • \ J 53 l -92 -38 b. l \ I 50 NS. I •80 -18 ws* b. ( t ss;I I 122° 15' 121° 45' Figure 1. Location map of t~e Santiam Pass area. Geothermal gradient (°C/km, upper value) and heat flow values (mW/m , lower value) are shown. Holes in Holocene rocks are marked by open triangles. Major volcanic centers are shown as stars; Cascade Range crest is dashed line. Major west-edge faults of Cascade graben are shown. Rocks older than Quaternary shown by the caret pattern (from Black et al., 1987; Priest et al., 1988). Stippled pattern marks heat flow anomalies. Abbreviations are TFJ, Three Finger Jack; BL, Blue Lake; MW, Mt. Washington; BC, Belknap Crater; NS, MS, SS; North, Middle and South Sister; SM, Sand Mountain. Modified from Blackwell et al. (1990) Figure 5. Santiam pass site is shown. 2 REGIONAL SETIING The regional setting in central Oregon of the well is illustrated in Figure 1. The geology of the area is shown in very simplified form in Figure 1 (from Priest et al., 1988; Black et al., 1987). The western part of the area is underlain by Pliocene and older volcanic rocks, whereas the eastern part of the area is covered by Quaternary volcanic rocks. The well site is along the topographic axis of the Cascade Range between the Three Sisters and Mt. Jefferson stratovolcanoes. The nearest large Quaternary centers are the Three Finger Jack and Mount Washington volcanoes. The area was the site of extensive basaltic volcanism about 3000 years ago and the Sand Mountain chain of centers (SM on Figure 1) dates from that episode of volcanism. Most of the area of young rocks west of the crest and north of 44 o 10'N is covered by these 3000 year old basalt flows (Taylor, 1968). A series of down-to-the-east normal faults representing the west side of the Cascade graben divides the two areas. The regional geologic evolution of the central Cascade Range has been discussed by Priest (1990). There are extensive thermal data available in the area to the west along the McKenzie River drainage (Blackwell et al., 1982, 1990, Cox et al., 1981). The thermal conditions of the area there have been discussed by Blackwell et al. (1990). Locations of drill holes are shown in Figure 1 along with geothermal gradients and heat flow values. This area has a complex thermal pattern. Heat flow in 150m holes is zero to negative on the east side, increases to quite high values in the center of the map ( ~ 100 mW/n12), and then decreases west of the map area to values of less than 60 mW/m2. This same pattern is observed elsewhere in the Cascade Range (Blackwell et al., 1982, 1990). Temperature-depth curves for several of the shallow wells in the area are shown in Figure 2. These wells have characteristic temperature-depth curves based on their topographic and geologic setting. The wells in the Holocene basalt (SA3, SA10, SA13, SA14, SA17) have isothern1al to negative slopes on the temperature-depth plots. These types of temperature depth curves are due to the rapid flow, both vertically and horizontally, of ground water in the extremely porous basalt. The series of holes with low but positive gradients (SA5, SA 7, 3 ~ II).... I (.) 0') Q) r-..- 0 (l)ro<{CO ZN 1-::J(i),; Q)- 0 /(J)/ '-::s .... J +' _.,... U")- 0 _.,... <{CO '- _.,... / (/)- Q) z~ -r6 (/)co c.. / /~- / -- E _.,... _.,... _.,... _.. / ,..,..; ~ _.,... -:.--<""'- -- _.,... II) ' M-/<{CO' ~t:: - (/)- --- f' -- - vo 0 zr-.. M -co -- -;:)~ --<(X) <- <{CO cnc.o (/)Ln --<{CO (I)M (/)c;;-- ZN cn- -;:)- z~ z- (/)co z~ -;:)- -;:)- -;:)-cnc.o (/)- (I)N 00 It) 0 It) 0 It) 0 N It) "' 0.... N.... It).... SJ9l9Vi '4+dea Figure 2. Temperature depth plots for Sunedco 150m deep holes. Temperatures were measured every 5 m. Numbers are hole identifications. 4 SA12, and SA16) were drilled in Quaternary volcanic rocks that make up the middle part of the Cascade graben fill. The gradients in these wells are somewhat below the regional gradient of 65 °C/km. However, in one hole (EWEB-CL or EWEB-1, Blackwell et al., 1982, Figure 6) the geothermal gradients range from 26°C/km (0 to 560 m) to 112°C/km (15-200 m). The shape of the temperature-depth curve clearly indicates lateral flow of warn1 water in a confined aquifer through an area otherwise characterized by a lower-than-background temperature and heat flow related to groundwater recharge. A 150 m well 4 km to the northeast has a gradient of 100°C/km, the highest gradient along the boundary province not near a known geothermal anomaly. Consequently, in the northern part of the map (stippled area on Figure 1) there is probably lateral flow of warm water at relatively shallow depth. The gradients for \vells SA5 a.11d SA.ll are near the expected regional v~lue of 65 o C/km.