PROCEEDINGS, Thirteenth Workshop on Geothermal Reservoir Engineering Stanford Universily, Stanford, California, January 19-21, 1988 SGP-TR-113 Evolution Of The Thermal Cap In Two Wells From The Salton Sea Geothermal System, California Joseph N. Moore and Michael C. Adams University of Utah Research Institute Salt Lake City, Utah ABSTRACT terized the upper parts of the geothermal field. Furthermore, because there is The Salton Sea geothermal system is evidence that conditions within the geother- overlain by a thermal cap of low permeability mal system have changed with time (Skinner rocks that restricts the upward movement of et a1 ., 1967; Huang, 1977; Andes and the high-temperature reservoir brines. McKi bben, 1987), the present borehole Petrographic and fluid inclusion data from temperatures and fluids may not accurately two wells show that the thermal cap in the reflect the conditions associated with the southern part of the field consists of an alteration of the shallow sediments. upper layer of lacustrine and evaporite Alternatively, this data can be obtained deposits with low initial permeabilities and from fluid inclusions contained within the a lower layer of deltaic sandstones. The authigenic minerals. In this paper we sandstones were incorporated into the thermal describe the results of fluid inclusion and cap as downward percol ating fluids deposited petrographic studies of two boreholes anhydrite and calcite in the pore space of drilled in the southern part of the field the rocks, reducing their permeabilities. (Fig. 1). The samples were provided by During development of the thermal cap, base- Unocal . metal sulfides, potassium feldspar and quartz veins were deposited by brines from higher temperature portions of the system. LITHOLOGIC AND MINERALOGIC RELATIONSHIPS INTRODUCTION The rocks within the Salton Sea The Salton Sea geothermal system is geothermal system can be divided into an located near the center of the sediment upper sequence of lacustrine claystone and filled Salton Trough of southern California evaporite deposits and a lower sequence of and Mexico. Mineral assemblages and textures deltaic sandstones, siltstones and shales formed in response to geothermal activity can (Randall, 1974). Figure 2 details the be categorized as diagenetic or metamorphic 1 ithologies and mineral assemblages found in (McKibben and Elders, 1985). Diagenetic the upper parts of the wells studied by us. processes occur at temperatures of less than These lithologic columns are based on an about 25OoC and have resulted in the recrys- examination of dri11 chip samples col 1ected tallization of the sheet silicates, and the at 6 or 9 m (20 or 30 foot) intervals. deposition of anhydrite, carbonates, and The primary and secondary minerals we sulfides in the pore space of the sediments. observed (Fig. 2) are similar to those found The reduction in the permeabilities of the in other parts of the field (e. g. Muffler shallow sediments caused by the deposition of and White, 1969; McDowell and Elders, 1979, these pore filling minerals has led to the 1980, 1983). With increasing depth, the development of a thick thermal cap, or zone secondary assembl ages are characterized by: of conductive heating, over the reservoir 1) anhydrite, calcite, interlayered il- (Younker et a1 ., 1982). Metamorphic proces- lite/smectite, and interlayered chlorite/- ses occurring in the reservojr., where smectite (interlayered illite/smectite zone: temperatures range from 250° to 365OC, have 0-222 m, Well A; 0-241 m, Well B); 2) led to the development of hornfelsic textures chlorite, illite, interlayered chlorite- and mineral assemblages typical of the / smect ite , cal cite, an hydri te , potass ium greenschist facies (Muffler and White, 1969). feldspar, quartz, sphene, and base-metal Fluids encountered at the depths where these sulfides (chlorite-calcite zone: 222-616 m, metamorphic processes are occurring have Well A; 241-789 m, Well B); and 3) epidote, salinities of up to 25 weight percent (Hel- calcite, chlorite, quartz, potassium feldsp- geson, 1968). ar, albitic plagioclase, anhydrite, illite, In contrast to the deep portions of the and sphene (chlorite-epidote-calcite zone: Salton Sea geothermal system, little data 616-1646 m, we1 1 A; 789-1292 m, We1 1 B). has been presented on the temperatures and Traces of pyrite and hematite occur through- salinites of the brines that have charac- out these zones. -107- FLUID INCLUSION DATA described by Somner et a1 . (1985). Methane, ethane, propane and carbon dioxide were Fluid inclusions in anhydrite, sphaler- detected in primary inclusions in sphalerite ite and vein quartz were studied. In the and quartz. Inclusions in anhydrite con- lacustrine deposits, anhydrite occurs as tained mainly carbon dioxide and methane. rosettes in the claystones and as aggregates Anhydrite suitable for fluid inclusion associated with calcite in the evaporite measurements typically occurs as cleavage beds. In the deltaic deposits, anhydrite and fragments up to 2 mn across. Typically calcite cement the sandstones above 387 m in several generations of secondary or pseudo- well A and 351 m in well B. At greater secondary inclusions are present. Primary depths, anhydrite-rich zones are restricted inclusions containing a small (<1micron), to a few narrow intervals. unidentified birefringent daughter mineral Sphalerite occurs in significant were observed in samples from 168-177 m in amounts (up to several percent) as a cement well B. These inclusions form three dimen- in the sandstones between 341 and 387 m in sional arrays that define growth zones in well A and from 277 to 360 m and 424 to 433 m the cleavage fragments. in well B. In places, the sphalerite The homogenization temperatures of contains inclusions of anhydrite, suggesting fluid inclusions in anhydrite are plotted that it has replaced preexisting anhydrite against depth in Figure 3A. The data show cement. Rocks between depths of 300 and 600 that irrespective of the origin of the m also commonly contain secondary potassium inclusions in these samples, homogenization feldspar after detrital plagioclase. The temperatures vary little within each depth significance of these mineral assemblages is interval, and that the two wells record discussed bel ow. simi 1 ar thermal profiles. No pressure Quartz veins containing minor barite, correction has been appl ied to these data chlorite, hematite, and traces of galena, because of the shallow depths. sphalerite, calcite and anhydrite occur Ice melting temperatures of fluid between depths of 305 and 378 m in well B inclusions in anhydrite (Fig. 38) range from (Fig. 2). Barite, in particular, is an -21.4O to -4.7OC. Liquid was observed in uncommon mineral in the Salton Sea geothermal some of the larger inclusions at tempera- system. In these veins, it is found as tures near -5OOC (first melting), indicating embayed crystal s surrounded by quartz. that the inclusion fluids are enriched in The petrographic relationships suggest CaCl . For reference, melting point that the deposition of anhydrite began prior deprgssions ranging from 4.7O to 20.8OC to both sphalerite cementation and quartz correspond to salinities of 7.4 to 23.2 veining. Thus, fluid inclusions in anhydrite equi Val ent weight percent NaCl (Potter et can potentially provide information on the a1 ., 1978). temperatures and compositions of the brines Fluid inclusions in sphalerite suitable since the early evolution of the thermal for freezing and heating measurements were system. As discussed below, inclusions in found in a few crystals from two depth sphalerite and vein quartz provide informa- intervals in well B. These inclusions are tion associated with the episodic upwelling primary, forming small isolated groups or of brines from deeper parts of the system. three-dimensional arrays that para1 le1 color Most of the fluid inclusion data from the banding. The inclusions yielded homogeniza- Salton Sea geothermal system is of this tion temperatures ranging from 179O to 223OC latter type (Huang, 1977; Freckman, 1978; (Fig. 3A), and ice melting temperatures Andes and McKibben, 1987; Roedder and Howard, (Fig. 38) ranging from -12.0 to -lO.O°C 1987). (16.0 to 14.0 equivalent weight percent All the fluid inclusions examined in NaCl ) . this study were liquid-rich and contained a Data were obtained on primary (refer to small vapor bubble that occupied about 20 Figs. 3A and B) and secondary inclusions of percent of the inclusion volume at room vein quartz. The primary inclusions occur temperature. Most had maximum dimensions in in three dimensional arrays that define the range of 2 to 10 microns. No evidence of growth zones. Homogenization temperatures a separate gas phase was observed in these of these inclusions range from 194O to 242OC inclusions and, with the exception of (Fig. 3A). Because of their small size (1 anhydrite from 168-177 m in'well B, none of to 3 microns) only a few freezing point the inclusions found contained any solid depressions were measured. The ice melting phases. In order to assure the validity of temperatures ranged from -13.9O to -9.2OC our data, fluid inclusions in anhydrite from (17.8 to 13.1 equivalent weight percent three depth intervals were systematically NaCl ) . overheated to determine their stretching Secondary incl usions have homogenization characteristics. These experiments demon- temperatures ranging from 189O to 21OoC and strated that stretching caused by 1 aboratory ice melting temperatures that vary from heating is not likely to have affected the -16.5O to -0.7OC (20.0 to 1.2 equivalent results reported in this study. weight percent NaC1). The high-sal inity In addition, the gas contents of in- inclusions have freezing point depressions clusions in ten samples were analyzed by similar to the brines trapped in anhydrite at mass spectrometry using the techniques these depths and thus may represent locally -108- derived fluids. The low-sal inity fluids may deltaic rocks must have resulted from the represent condensate mixed with a small influx of brines derived from higher- amount of the higher salinity brines.