Porosity and Packing of Holocene River, Dune, and Beach Sands (1)

Abstract

The porosity and packing of 174 samples of surficial and shallowly buried (to 17 m), unconsolidated Holocene sands (medium to coarse-grained) were determined by point counting the upper surface of thin sections of epoxy-impregnated samples in reflected light. Average depositional porosity for 124 surficial sands from beaches, river point bars and braid bars, and eolian dunes, respectively, is 47, 48, and 49%. The porosity values range between 40 and 58%. Beach sands exhibit an average packing value (CI = contact index) of 0.79, fluvial sands an average CI of 0.91, and eolian sands an average CI of 1.02. Beach sands typically contain 5.5% oversized pores, river sands 3.8% oversized pores, and eolian dune sands 4.0% oversized pores. Most oversized pores are depositional packing defects, although trapped air bubbles are common in some beach sands. Extrapolation of our data for buried samples indicates depositional oversized pores are destroyed at a depth of less than 100 m. Average porosity is 46% for eolian ripple strata, 50% for grain-fall strata, and 51% for grain-flow strata. Differences in packing are significant for grain-flow versus grain-fall strata and for ripple versus grain-flow strata for samples of similar grain size. Porosity is more uniform within individual beds from a river and a beach than among different beds at the same localities. Porosity of ancient sandstones from a particular bed or facies may be as uniform as sampled Holocene sands, unless cements having patchy distribution, such as carbonate or anhydrite, are present. Compaction associated with burial to 17 m is minor. Packing is tighter, but our technique of measuring porosity is not sensitive enough to show any decrease in porosity. Changes in packing and porosity upon burial to common hydrocarbon reservoir depths will depend on individual histories of cementation, compaction, and secondary porosity development. However, if only physical compaction of quartzose river, dune, and beach sands takes place, porosity would approach a limiting value of 26% and CI would approach 5.5.