Effects of Flow Processes, Sediment Composition and Resultant Texture on the Reservoir Quality of Deep-Marine Sandstones

Here we present new results from a detailed petrographic analysis of deep-marine sandstones deposited by different subaqueous gravity-flow types from the Fram area (northern North Sea) and the Vøring Basin (Norwegian Sea), and their resulting reservoir properties. All studied sandstones are subarkoses dominantly composed of quartz, K-feldspar, mica, heavy minerals (trace amounts) and rock fragments, and contain low to moderate volumes of quartz cement (less than 1% to 12%). However, significant textural variations are observed for the different sandstone-types. Classical high- and low-density turbidites are relatively better sorted (moderately to well-sorted), compared to hybrid event beds (poorly to moderately-well sorted). Dominant grain size decreases from medium to coarse for the high-density turbidites to upper-fine to lower-medium for the low-density turbidites and to very-fine to lower-medium grained in hybrid event beds. The last important difference between the studied deep-marine sandstones is detrital clay content. Although variable between wells, overall larger volumes of detrital clay are present in ‘non-classical’ turbidites than in classical turbidites, and high-density turbidites contain smaller clay proportions than low-density turbidites. Measured porosities of the different bed types span the same interval, but overall the classical turbidites have a higher proportion of samples with porosities higher than 20%, compared to the non-classical beds. Permeabilities, in contrast, are significantly different for classical and non-classical turbidites, with the high- and low-density turbidites having permeabilities of about two orders of magnitude higher than hybrid event beds and debrites. A comparison of the observed variations in textural parameters with reservoir porosity and permeability indicate that hybrid event beds and debrites deposited by composite/cogenetic flows and debris flows, respectively, have much poorer reservoir potential than high- and low-density turbidites deposited by fully turbulent flows, due to the overall smaller grain sizes, poorer sediment sorting and higher proportions of detrital clay in ‘non-classical’ compared to classical turbidites. Similarly, reservoir quality of the high-density turbidites is significantly better than for the low-density turbidites.

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