| Title | Use of Wireline Logs at Cerro Prieto in the Identification of the Distribution of Hydrothermally Altered Zones and Dike Locations and their Correlation with Reservoir Temperatures |
|---|---|
| Authors | D. T. Seamount Jr., W. A. Elders |
| Year | 1981 |
| Conference | DOE-CFE Symposium on Cerro Prieto |
| Keywords | |
| Abstract | Previous research has shown that zones of hydrothermal minerals at Cerro Prieto can be used as an indication of temperature distribution and patterns of fluid flow. Progressive hydrothermal alteration also has a marked effect on the physical properties of rocks. Therefore, we have studied downhole electrical and gamma-gamma density logs from nine wells and attempted to correlate these wireline log parameters with petrologic, temperature, and petrophysical data. In this paper wells M-43, T-366, and M-l07 are discussed in detail as typical cases. Log data for shales show good correlation with four zones of hydrothermal alteration previously recognized on the basis of characteristic mineral assemblages and temperatures. These zones are the unaltered montmorillonite zone «150°C), the illite zone (150°C-230°C to 245°C), the chlorite zone (235°C-300°C, equivalent to the calc-silicate I zone in sands), and the feldspar zone (>300 oC, equivalent to the calc-silicate II zone in sands). Comparison of laboratory measurements with gamma-gamma density logs shows good agreement, suggesting that this tool is fairly accurate in this environment. Logs of seven of the wells studied show that certain sand horizons from the montmorillonite zone have high deep induction resistivity (Rsd > 15 ohm-m) and low or reversed SP deflection, as deep as 1.7 km below sea level. This suggests deep recharge of fresh water into the reservoir. The illite zone is normally indicated by an abrupt rise in gamma-gamma density (aB) , much greater than that due to normal compaction. The chlorite zone is characterized by increase in the deep induction resistivities of both sands and shales (Rsd and Rsh) , and a gradual increase in density of both (aBsd and DBsh). The feldspar zone, the main steam-producing zone, corresponds with an increase in shale resistivity (Rsh) from approximately 6 ohm-m to >15 ohm-m. An altered diabase dike was recognized in well NL-l by its low GR count, negative SP deflection and DB > 2.7 gmtcc. The ratio Rsd/Rsh, that is the ratio of the deep induction sand resistivity to the deep induction shale resistivity on logs, computed over 50 ft (15 m) intervals, shows a general decrease from 1.0 to 4.0 in the illite zone to 3000C, equivalente a la zona II de silicatos de calcio en arenas). La comparacion de datos petrofisicos con datos de densidad gama-gama muestra una buena concordancia, sugieriendo que el registro de densidad gama-gama es una herramienta razonablemente precisa en este ambiente. Los datos de registros de siete de los pozos indican que algunos estratos de arena de la zona de montmorillonita tienen altas resistividades «15 ohm-m) y desviaciones de autopotencial bajas 0 inversas a profundidades bien por debajo del nivel del mar (en el pozo T-366, 5500 pies [1.7 km] bajo el nivel del mar). Esto indica recarga profunda de agua dulce en el yacimiento. La zona de illite esta normalmente indicada, tanto en las arenas como en las lutitas, por un abrupto aumento en la densidad gama-gama (OB) de aproxi= madamente 0.3 g/cm3. Este aumento es mucho mas grande que el debido a una compactacion normal. La zona de feldespatos, donde se encuentra la zona principal de produccion de vapor, corresponde con un aumento de la resistividad de las lutitas (Rsh) de aproximadamente 6 ohm-m, a mas de 15 ohm-m. En el pozo NL-l se identifica un dique de diabasa alterada en base al bajo valor de rayos gama natural, a la desviacion negativa de potencial espontaneo, y a una densidad gama-gama mayor de 2.7 g/cm3• La razon Rsd/Rsh, 0 sea 1& razon de las resistividades de induccion profunda de las arenas (Rsd) y de las lutitas (Rsh) en los registros, calculada & intervalos de 50 pies (15 m), muestra una disminucion general, de 1.0 a 4.0 en la zona de illita, a menos de 0.3 en la zona de feldespatos. El grafico de interrrelacion de densidad gama-gama (O~) versus resistividad de induccion profunda (R) permite identificar facilmente las cuatro zonas mineralogicas a base de datos de lutitas y, por 10 menos dos zonas mineralogicas a partir de datos de areniscas. |