PALEOMAGNETIC AND DIAGENETIC STUDIES OF RESERVOIR ROCKS IN CORES

Recent integrated paleomagnetic and diagenetic studies of shale and limestone hydrocarbon reservoirs in cores at the University of Oklahoma have focused on both assessing the use of the viscous remanent magnetization (VRM) method to orient cores and determining the timing and origin of diagenetic events. A study of several oriented cores of the Mississippian Barnett Shale in the Fort Worth Basin revealed a VRM and chemical remanent magnetization (CRM) with shallow inclinations and streaked, SE to S directed declinations. The VRM method was used to orient the CRM data for two cores and the results were similar to the results from the oriented cores. The streak of directions could represent a mixing trend between two or more CRMs that were acquired during the Pennsylvanian to late Permian-Triassic by fluids sourced from the Ouachita front and/or by burial diagenetic processes.

The Mississippian Limestone in Oklahoma is a major petroleum exploration target in northern Oklahoma and southern Kansas, and diagenetic events are a significant factor in controlling reservoir quality. Thermal demagnetization of specimens from several vertical unoriented cores found a low temperature VRM and a CRM (240 - 500¢ªC) that is interpreted to reside in magnetite. The VRM orienting method was not successful because it resulted in a 300° streaked distribution of declinations with shallow inclinations. The inclinations of the CRM (mean = -2.53°, std. dev. = 9.58°) suggest that the CRM was acquired in the Permian to Triassic. There is a complex paragenetic sequence including brecciation, silica dissolution, fracturing and silica precipitation which are interpreted as resulting from subaerial exposure. Late diagenetic features attributed to hydrothermal fluid flow include dolomite filled fractures and sphalerite. Geochemical data are inconclusive in terms of providing evidence for alteration by externally derived fluids. Timing of the CRM is consistent with dates found for the nearby Tri-State MVT mineralization, which along with the evidence for hydrothermal alteration, suggest that the CRM acquisition may be attributed to alteration by external fluids.