Petrology, Porosity Development, and Diagenesis of the Upper Devonian Elk Sands, Council Run Field, Centre and Clinton Counties, Pennsylvania
BILLMAN, DAN A., Eastern States Exploration Company, Snow Shoe, PA, and RICHARD SMOSNA and K. R. BRUNER, West Virginia University, Morgantown, WV
The Council Run field produces from sandstones in the Upper Devonian Lock Haven Formation and is located approximately 5 mi from the outcrop belt, west of the Allegheny front.
The reservoir sandstones generally are fine to medium grained, subrounded to subangular, well to poorly sorted, and well cemented. Cementation is in two forms: (1) formation of a "pseudomatrix" by compaction of ductile rock fragments, and (2) point-to-point cementation of quartz grains.
Quartz is dominant in the reservoir sandstones; however, much of this is polycrystalline or metamorphic quartz. Rock fragments are abundant. Metamorphic schist fragments make up the bulk of these, though some sedimentary rock fragments are present. Minor amounts of feldspars, marine fossils, and carbonate minerals are also present. Clays include chlorite, illite, and minor kaolinite. Due to the diverse mineralogy, rock types range from quartzarenites to litharenites to subarkose.
Porosities in the reservoir sandstones at Council Run field range from negligible to greater than 15%. Porosity is a combination of intergranular primary porosity and secondary porosity including (1) intragranular dissolution, (2) channel porosity due to dissolution of "psuedomatrix," and (3) fracture porosity.
The sandstones of the Lock Haven Formation have followed a complex diagenetic history due to structural position and diverse mineralogy. Illite coats on quartz grains have preserved some primary porosity whereas deformation of ductile micas and schist fragments destroyed it. This was followed by a combination of (1) the alteration of feldspars to dolomite, which continued to fill pores, and (2) the dissolution of the feldspars and rock fragments creating intragranular and channel porosity. Lastly, continued deformation created fracture porosity.
AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)