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2 Geotherma 2 Resources Counci 2, TRAAlSACTIONS VoZ 4, Septernber 198 A PRELIMINARY EVALUATION OF GEOTHERMAL POTENTIAL OF SAN JUAN BASIN, NEW MEXICO, USING BOTTOM HOLE TEMPERATURES FROM O'IL AND GAS WnLS Lokesh Chaturvedi and Jeffrey K Lory New Mexico State University Las Cruces, NM 883 ABSTRACT Bottom hole temperatures from existing well logs of 12,3 oil and gas wells in San Juan Basin, New Mexico were collected and analyzed to evaluate the geothermal potential of this region The data was used to calculate an approximate geothermal gradient at each well location by assuming a surface temperature of 15OC; The average gradient for the basin, computed from the entire data is 27OC/km The highest concentration of high computed gradients (>5OoC/km) 3s in the northwest corner of the state, near Shiprock Several other locations scattered throughout the basin appear to have promising geothermal potential INTRODUCTION The San Juan Basin is a relatively stable and less deformed geological region in the Northwest New Mexico It is situated in the eastern part of the Colorado@Plateau Physiographic province and is surrounded by several uplifts, arches, monoclines and thrusts Figure 1 shows the'boundary of the San Juan Basin in r'elation to other regional geologic features Because of the structural stability, lack of geothermal manifestation on the surface and relatively low heat flow values calculated from limited observations, the geothermal potential of the San Juan Basin area has generally been regarded as low or even non-existent 38 31' 36' 35' 34 Figure 1 Location of San Juan Basin (from Armstrong and Mamet, 1977) 21

3 Chaturvedi et az A systematic evaluation of the geothermal potential of the San Juan Basin has been undertaken to fill the existing gap This summary reports the results of the first stage of this study San Juan Basin contains several producing horizons of oil and gas A large number of kploratory and producing wells have, therefore, been drilled in this area during the past seventy years Well logs and bottom hole temperatures recorded during logging are available for most of the wells drilled Bottom hole temperature data was collected for 12,3 of these wells and was analyzed to determine the areas of high geothermal potential GEOLOGIC SETTING The central part of the basin is cwered by continental sediments of Late Cretaceous or Tertiary age These are surrounded by marine and continental formations of Cretaceous age, Paleozoic and Mesozoic formations are found along the rim of the basin The total thickness of the sedimentary rocks in the center of the basin ranges from 1, to 15, feet, The central part of the basin is broadly downwarped Defiance and Zuni uplifts flank the basin to the west and south respectively, Naciemiento uplift to the east, Gallina-Arthuleta arch to the northeast and the Hogback monocline to the north, flank the San Juan Basin Oil and gas is located in domal, faulted and stratigraphic traps in the basin BOTTOM HOLE TEMPERATURES There has been some concern about using bottom hole temperature readings to estimate the correct formation temperature at depth and to use these values to estimate the approximate thermal gradients However, the results obtained by Jaeger (1961), Schoeppel, et a2 (1966), Tanner (1976), and Hodge, et a2 (1979) have shown that the bottom hole temperatures can be used as reliable data to estimate the regional variations in temperature gradients We have used this data to outline the areas of anomalous geothermal gradients and, therefore, of high geothermal potential in the San Juan Basin The temperature gradient for each location was calculated by using the bottom hole temperature at a given depth obtained from the well log, minus the estimated surface temperature, divided by the well depth The mean air temperature for the Northwest New Mexico is reported to be 12OC However, in order to keep the computed gradient values on the conservative side, a value of 15OC was used for the surface temperature The calculated gradients from 12,3 wells, ranging in depth from 113 meters to 23 meters, yielded a low value of 1 8 O C / k m and a high value of llooc/km More than 95% of the wells recorded have a total depth greater than 5 meters The data from shallower vd1~ ves used in the analysis, I LOCATIONS (TO THE NEAREST OUARTER TOWNSHIP) OF (111 AND G4S WELLS WHERE BOTTOM HOLE 1 tmpfratures WFHE RECORDED ~s"m~-----~-p"'l'' %-A -3Z-Y-4 ~namnacm IN tjorlhwcotern NEW MEXICO LFWCSII CIIIUYEW nu, Figure 2 22

4 Chaturvedi et a2 especially since several of these have an obviously anomalous recorded temperature These wells can be easily used to confirm the accuracy of the data The average gradient computed from the entire data is approximately 27OC/km Figure 2 shows the locations (to the nearest quarter township) of the wells where the bottom hole temperature data was recorded Figure 3 shows the locations of all wells with computed gradients more than 45"C/h This value was arbitrarily chosen to represent more than 15 times the average computed gradient Figure 4 shows the locations of wells with computed, gradients more than,5oc/ian Seven wells show computed gradients which are over 9 C/km CONCLUSIONS Use of bottom hole temperatures to locate the areas of geothermal potential is an inexpensive and time-saving technique which appears to work for reconnaisance of large areas It is especially useful in areas where a large number of well log records are available In the Northwest New Mexico, the area around Shiprock appears to have the maximum geothermal potential Several relatively shallow wells in this area (3 to 6 meter deep) have a recorded bottom hole temperature ranging from 32OC to 55 C There are a number of outcrops of igneous intrusives of Tertiary age in this area besides the famous Shiprock It is conceivable that the anomalous recorded temperatures are related to this igneous activity The center of the basin, 1 to 2 miles south and southeast of Farmington-Aztec area, shows a number of computed anomalous gradient locations Similarly, a number of wells in the Rio Arriba and Sandoval counties and some in the McKinley County show significantly anomalous temperatures (Figs 3 and 4) Field checking of these wells, correlation with detailed geologic setting and the geohydrologic assessment of these areas will form the contents of future studies ACKNOWLEDGMENTS This study was conducted with funds supplied by the Department of Energy through New Mexico Energy Institute at New Mexico State University, Las Cruces, New Mexico, under the Western States Cooperative Direct Heat Geothermal Program of DOE 4 -* :, 4 mu" CR 5nr cn 1: b * LOCATIONS OF OIL AND GAS WELLS WITH COMPUTED GRADIENTS > 45WKM IN NORTHWESTERN NEW MEXICO br ,* e 1,11c SCILC LOlLIM CMIIUIVLOI I9 Figure 3 23

5 A\ \ \ e e * A A e a e 9 a A e * %AM JUAN ca YC El EO e a II n I A NOR'THWESTERN NEW MEXICO LOCATIONS OF OIL AND GAS WELLS WITH COMPUTED GRADIENTS > 5 C/KM LEG NO A CIWM CIKU o IO-BOWKU 8-9-CfWU >SOCfKY LOKESH CHATURVEDI 198 Figure 4 REFERENCES Schoeppel, R J, and Gillaranz, S, 1966, Use of well log temperatures to evaluate regional Armstrong, A K, and Mamet, B L, 1977, geothermal gradients, Jour Petroleum Biostratigraphy and paleogeography of the Technology, June 1966, p Mississippian system in northern New Mexico Tanner, W F, 1976, Geothermal exploration from and adjacent San Juan mountains of south- deep-well data, Trans Gulf Coast Assn of western Colorado, N M Geol SOC Guide- Geol Societies, Vol XXVI, p book, 28th Field, Conf, San Juan Basin 111, p Hodge, D S, et at, 1979, Preliminary Geothermal investigations in New York State, Trans Geoth Res Council, Vol, 111, p , Jaeger, J C, 1961, The effect of the drilling fluid on temperatures measured in bore holes, Jour Geophys Research, Vol 66, p