Recognition Criteria for Distinguishing Between Hemipelagic and Pelagic Mudrocks in the Characterization of Deepwater Reservoir Heterogeneity
Determination of turbidite event magnitude and frequency
remains subjective and difficult to define. This is because turbidite
sedimentation events often include both sand and mud, with the mud component
often excluded from bed thickness studies because of the inability to establish
a genetic link to the turbidity current. Natland (1976) defined pelagic mudstone
as fine-grained marine sediment derived primarily from biogenic particles,
whereas hemipelagic mudstone includes both biogenic and terrigenous particles.
Unfortunately, these compositional definitions do not account for differences
in depositional process.
Scanning electron microscopic (SEM), field emission microscope (FEM), and X-ray diffraction analyses of 70 samples from outcrop and core illustrate this distinction. Furthermore, these laboratory measurements are calibrated to 192 outcrop samples to provide a robust method for field identification of clay fabric and mineralogy to define turbidite sedimentation units.
Pelagites show organized layering of clay platelets, few flocculates, and a lower proportion of high-density minerals. Hemipelagites have disorganized and chaotic clay fabrics characterized by visible flocculates and contain a higher proportion of denser particles. There also can be a corresponding change in clay mineralogy, e.g., smectite in pelagites versus kaolinite in hemipelagites. These results indicate a settling velocity > shear velocity in pelagites, whereas hemipelagites record the opposite condition. Turbidity currents support and suspend denser grains, generate disorganized and chaotic clay fabrics and provide more time for flocculation. Discrimination between pelagites and hemipelagites has important implications to determination of turbidite event frequency and magnitude, which affects vertical connectivity and continuity of sand, deposited from multipartite turbidity currents.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California