Porosity Pathways for Devonian Reservoir Sandstones of Pennsylvania

SMOSNA, RICHARD, and BRUNER, KATHY R., West Virginia University, Morgantown, WV

The present lithology of Devonian sandstones can be compared to the rocks' composition at earlier stages of diagenesis, allowing the porosity to be quantified at different periods in their burial history. Point-count data are collected for framework grains, cements, and primary pores; secondary pores are identified and the volume of leached grains determined; and the amount of compaction is quantified. With this data we trace the diagenetic pathway of Lock Haven sandstones through time: from their composition at deposition, to that after compaction, to that after dissolution of unstable grains.
Fluvial litharenite sandstones, rich in ductile rock fragments of shale and phyllite, suffered extreme pore-space reduction because of compaction (from a depositional porosity of 40% to a post-compaction porosity of 6%). Cementation destroyed an additional 5% porosity, but minor leaching of rock fragments and feldspars added 3% secondary porosity. Sublitharenite sandstones of distributary-mouth bars and offshore shelf, with fewer ductile grains, underwent less compaction (post-compaction porosity of 9%) but greater cementation (7% porosity loss). A significant volume of rock fragments and feldspars were leached adding 7% secondary porosity. Beach sandstones are almost quartzarenites. With few ductile grains compaction was minimal (post-compaction porosity of 15%) although cementation was extensive (11% porosity loss). With few soluble grains secondary porosity added only 2%.
Although the Warren, Speechley, Bradford and Elk reservoirs belong to the same formation, were deposited as part of the same deltaic system, and are within a few hundred meters stratigraphically of one another, sandstones of the various depositional facies have followed different porosity pathways.

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