| 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 |
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| 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 and attempted to correlate these wireline 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 |