| Abstract: | The NW - SE trending Cerro Prieto fault zone is part of a major regional lineament that extends into Sonora, Mexico, and has characteristics of both a wrench fault and an oceanic transform fault. The zone includes a number of separate identified faults and apparently penetrates deep into the basement and crustal rocks in the area. The zone serves as a conduit for both large and rapid heat flow. Near well M-103, where the Michoacán fault obliquely intersects a shorter NE - SW trending fault (i.e., the Pátzcuaro fault), large circulation losses during drilling indicate greater permeability and hence increased natural convective fluid flow. Temperature contour maps for the southern portion of the field suggest that a shear fault zone also exists in the vicinity of wells M-48, M-91 and M-101. This shear zone aids in rapidly distributing geothermal fluid away from the Cerro Prieto fault zone, thus enhancing recharge to the western part of the reservoir.We have studied the distribution of lithologies and temperature within the field by comparing data from well cuttings, cores, well logs and geochemical analyses. Across the earliest developed portion of the field, in particular along a 1.25 km NE - SW section from well M-9 to M-10, interesting correlations emerge that indicate a relationship among lithology, microfracturing and temperature distribution. In the upper portion of the reservoir of this section, between 1200 and 1400 m, the percentage of sandstones ranges from 20 to 55. Well logs, calcite isotope maxima, and the Na - K - Ca geothermometer indicate temperatures of 225–275°C. The isothermal high in this vicinity corresponds to the lowest total percentage of sandstones. Scanning electron microphotographs of well cores and cuttings from sandstone and shale units reveal open microfractures, mineral dissolution and mineral precipitation along microfractures and in pores between sand grains. Our working hypothesis is that these sandy shale and siltstone facies are most amenable to increased microfracturing and, in turn, such microfracturing allows for higher temperature fluid to rise to shallower depths in this part of the reservoir.Our ongoing research is aimed at achieving a coherent geological model that provides a basis for estimating reservoir capacity, and that illustrates our understanding of fluid flow along major faults, laterally through fault shear zones, and within predominantly silty and shaley deltaic clastics that have been microfractured. |
