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Palaeomagnetic Properties of Hydrocarbon Reservoirs |
Department of Geosciences, University of Houston, Houston, TX, USA
To evaluate the influence of fluid hydrocarbons on the magnetic properties of sedimentary rocks, two cores (V1 and V2) have been examined from a producing oil field reservoir (Vacuum Field) in the Permian Basin of southeastern New Mexico. The cores sample back-reef dolomitic mudstones-packstones of the Guadalupian San Andres Formation that were deposited on the northern shelf margin of the Delaware Basin. Rock magnetic and palaeomagnetic analyses show that low natural remanance magnetization (NRM) intensities and negative susceptibility values are better correlated with petrophysical properties (i.e. relatively high porosity and permeability) than with lithological patterns (core V1). These petrophysical properties are well correlated with zones of oil production (flow zones) identified in core V1. Samples from zones of lower porosity and permeability, non-productive horizons (no-flow zones), are characterized by more variable magnetic properties, but almost uniformly exhibit higher values of NRM and susceptibility.
The palaeomagnetic results indicate that these rocks were subjected to at least two identifiable magnetization events. The weakness of the magnetic signal in many samples allowed this observation to be made in only some instances. A high blocking temperature (300–500°C), low inclination (0–10°) component is believed to record a magnetization event early in the history of the unit (Late Permian). A low blocking temperature (<300°C), moderately steep (30–40°) inclination component may have been acquired in the medial Jurassic, marking a diagenetic event possibly associated with the migration of hydrocarbons or formation waters that preceded the introduction of hydrocarbons into the reservoir.
