Abstract: Porosity Type is Related to Sandstone Composition is Related to Depositional Setting

Richard Smosna, Kathy R. Bruner

Reservoir and nonreservoir sandstones of the Devonian Lock Haven Formation in central Pennsylvania are classified as sublitharenites and litharenites. Rock fragments of shale and phyllite originally constituted from 4 to 43% of the total rock volume (before diagenetic leaching), and the mean quartz/feldspar/rock fragment was 76/4/20. The amount of rock fragments was largely controlled by depositional processes: fluvial sands had a high lithic content (average of 22%); distributary-mouth bar and offshore-shelf sands, an intermediate content (17%); and beach sands, a low content (7%).

Primary porosity is indirectly related to compaction of the ductile lithic grains, decreasing from a maximum of ^phgr = 13% down to zero as lithics increase. Secondary porosity, created by leaching of the chemically unstable rock fragments, is greatest (^phgr = 13%) for sandstones with an intermediate lithic content. With a lower volume of rock fragments, there were not many unstable grains to remove by leaching. With a higher volume of rock fragments, compaction was extreme, leaching fluids could not enter the rock to dissolve the unstable grains, and secondary porosity dropped to zero.

Because of these interrelationships between depositional processes, mineral content, and porosity, sandstones of different sedimentary environments now exhibit characteristic porosity types and amounts. Fluvial and distributary-bar sandstones of reservoir quality (6% or more total porosity) have predominantly lithmoldic porosity. Beach-barrier island sandstones have mostly primary porosity, and shelf sandstones have subequal amounts of primary and secondary porosities. The best reservoirs are those with an intermediate content of rock fragments, say 5%<RF<21%, which display a favorable combination of both pore types.

AAPG Search and Discovery Article #90984©1994 AAPG Annual Convention, East Lansing, Michigan, September 18-20, 1994