Spatially Varying Characteristics in Mineralogy in Deepwater Lobe Deposits: Examples from Outcrop, Experimental, and Subsurface Studies

Submarine fans are pervasive hydrocarbon reservoirs. Reservoir quality, however, is variable within these depositional systems. Studies document that sediment gravity flows, which provide a majority of sand into deepwater basins, hydrodynamically sort and deposit grains based on size. No studies document how this process relates to mineralogy and texture. Minerals have varying shapes and densities, both of which affect settling velocity in turbulent flow. This, in turn, has direct impacts on reservoir quality. This research uses examples from outcrop, experimental, and subsurface studies to document spatially varying characteristics in mineralogy in deepwater lobes.

1) The Point Loma Formation, San Diego, California contains laterally extensive outcrops of lobe strata. Quantitative analysis of a bed in a lobe documents discrete, monotonic and non-linear axis-to-margin changes in facies, bed thickness, grain size, and mineralogy. The percentages of K-feldspar, plagioclase, biotite, and organic material relative to quartz increase toward the lateral margin. In contrast, the percentage of hornblende relative to quartz decreases toward the lateral margin. This spatial variability is interpreted to reflect hydrodynamic fractionation of grains based on size, mineral density and grain shape.

2) A physical experiment was conducted in Tulane University’s deepwater basin to document how turbidity currents spatially fractionate particles on the basis of density and shape. The experiment documents: (a) the percentage of high-density, spherical particles relative to low density, spherical particles decreases toward distal margins of the deposit; and (b) the percentage of angular, low-density particles relative to spherical, low-density particles increases toward the distal margins of the deposit.

3) An earlier subsurface study by Rush et al., (2011) documents spatially varying percentages of volcanic glass in the Aspen Field, northern Gulf of Mexico. Volcanic glass, which is highly angular, alters to clinoptilolite, and is highest in proportion in the margins of lobes. Clinoptilolite serves to clog pore throats at the expense of permeability.

The patterns documented above are interpreted to reflect that hydrodynamic fractionation of minerals on the basis of density and shape is a critical process active in deepwater systems. This process has significant consequences on primary and secondary reservoir quality.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California