D. R. Van Alstine and J. E. Butterworth, Applied Paleomagnetics, Inc.

Our 17 years of paleomagnetic work on subsurface cores from hydrocarbon reservoirs in Venezuela demonstrates the benefits of paleomagnetically oriented cores for understanding depositional trends, fracture patterns, and in situ stress, with important consequences for permeability anisotropy. Our work has focused on the Maracaibo basin, where we have paleomagnetically oriented cores from Cretaceous (La Luna/Cogollo), Eocene (Misoa), and Miocene (Santa Barbara/La Rosa) reservoirs in the Bachaquero, Ceuta, La Ceiba, Lagunillas, Mara, Sol, and Urdaneta fields, as well as in Bloques I, V, and IX along the Icotea and VLE faults. In eastern Venezuela, we have paleomagnetically oriented cores from the Boqueron, Santa Barbara, and Orocual fields.

In case histories from central Lake Maracaibo, along the Icotea and VLE faults, we illustrate how paleomagnetically oriented cores can be used to calculate structural dip and to determine sediment transport directions in cross-bedded Misoa sandstone reservoirs. In “positive flower structures” along these faults, calculating and correcting for up to 42° of structural dip reveals eastward sediment transport directions in the Misoa C sands. These eastward transport directions may reflect shedding of sediments off the upthrown side of the Icotea and VLE faults during Early Eocene sedimentation.

In case histories from central and eastern Lake Maracaibo, we illustrate how paleomagnetically oriented cores can be used to determine the orientation of face cleats in Misoa Formation coal. This application may become more important in future development of coalbed methane reservoirs in Venezuela, since maximum permeability in coalbed methane reservoirs is generally parallel to the trend of coal cleats.

In case histories from across the Maracaibo basin, we illustrate how paleomagnetically oriented cores can be used to determine the orientation of natural fractures in both carbonate (La Luna/Cogollo) and terrigenous clastic (Misoa) reservoirs. We find that partially open natural fractures generally strike within 45° of being parallel to the direction of present-day in situ stress, especially in Cogollo limestone reservoirs. Cores from all 15 Misoa reservoirs we have studied also contain natural fractures. Some Misoa cores contain numerous “deformation bands,” which are shear fractures normally considered to be permeability barriers and to compartmentalize reservoirs.

In most cores we have studied from the Maracaibo basin, the normal-polarity, present-field magnetization we use to paleomagnetically orient cores has been superimposed on an older, reversed-polarity magnetization. This reversed-polarity magnetization is present in all La Luna/ Cogollo cores, in Misoa cores (especially reddened, sideritic intervals), and also in La Rosa (Miocene) cores. The reversed-polarity magnetization probably is chemical remanent magnetization (CRM) acquired either during Oligocene uplift or during maximum burial in the past 10 million years. Although the reversed-polarity CRM precludes magnetostratigraphic applications (at least in the La Luna/Cogollo), understanding the origin and spatial distribution of this remagnetization will probably help determine the diagenetic and fluid-migration history in hydrocarbon reservoirs throughout the Maracaibo basin.