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Abstract. The sandy-clayey hydrocarbon reservoirs of the Upper Paleocene and Lower Eocene located to the north of Veracruz State, Mexico, present highly complex geological and petrophysical characteristics. These reservoirs, which consist of sandstone and shale bodies within a depth interval ranging from 500 to 2000 m, were characterized statistically by means of fractal modeling and geostatistical tools. For 14 wells within an area of study of approximately 6 km2, various geophysical well logs were initially edited and further analyzed to establish a correlation between logs and core data. The fractal modeling based on the R/S (rescaled range) methodology and the interpolation method by successive random additions were used to generate pseudo-well logs between observed wells. The application of geostatistical tools, sequential Gaussian simulation and exponential model variograms contributed to estimate the spatial distribution of petrophysical properties such as effective porosity (PHIE), permeability (K) and shale volume (VSH). From the analysis and correlation of the information generated in the present study, it can be said, from a general point of view, that the results not only are correlated with already reported information but also provide significant characterization elements that would be hardly obtained by means of conventional techniques.

In an effort to characterize the crustal structure of northwestern Mexico (and constrain the Mojave-Sonora megashear) we studied the Magsat magnetic anomalies from that area. Published anomaly maps covering this area include an extensive positive anomaly covering the southern United States, a positive anomaly over the southern half of the Baja California peninsula, and a magnetic low in between. We interpreted a magnetic profi le over these anomalies, focusing on its tectonostratigraphic terrane nature. The profi le was further constrained by crustal thicknesses from seismological studies and heat fl ow data. In our model the Cochimi terrane and the North American craton (Colorado Plateau and southern Basin and Range) are characterized by high magnetic susceptibility in agreement with the mafi c nature of their corresponding crusts. The Yuma and the Seri terranes have lower magnetic susceptibilities as expected from their felsic to basic crustal nature. The oceanic crust from the Gulf of California is modeled with a low magnetic susceptibility value due to the high heat fl ow observed at the extensional basins. Our model satisfi es the presence of a subvertical contact between crusts with contrasting magnetic signatures of the Seri terrane (comprising the Caborca subterrane) and the southwestern sector of the North American craton. The Mojave-Sonora megashear itself is below the resolution of the Magsat data. Nevertheless, our model implies that because of their different magnetic signatures, the crystalline basements from the southern United States and northern Mexico (Seri terrane) are of different nature, which does not support the continuity of the North American craton into northwestern Mexico.

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