CHAPLIN, JAMES. R., Oklahoma Geological Survey, Energy Center, 100 East Boyd Street, Norman, OK
The Cottage Grove Sandstone of fluvial-deltaic origin in this geographic setting, consists of a heterogeneous, 121-ft thick sandstone-dominated succession. Subordinate lithologies consist of interlaminated and interbedded very fine-grained sandstone and black, fissile shale. Three distinctive lithofacies serve as reservoir and seals characterizing the Cottage Grove Sandstone and the overlying and underlying lithostratigraphic units: (1 ) a lower sandy, micaceous, calcareous siltstone and black fissile shale (offshore prodeltaic facies); (2) a middle very fine-grained sandstone succession deposited as part of a prograding delta front facies (distributary channel mouth bar); and, (3) a capping sequence of marine black, laminated "hot" shale deposited as a marine transgressive offshore facies (major flooding surface).
Compositionally, the sandstones vary from lithic subarkoses to subarkoses and feldspathic lithoarenites. The sandstones are dominantly very fine grained and moderately well sorted. Samples contain significantly more feldspar (5-17 percent) and rock fragments (1-19 percent) than do overlying and underlying formations.
Samples are quite variable in porosity ranging from 2 to 20 percent with an average of 16 percent. Porosity is dominantly a 3 combination of primary intergranular and secondary grain-dissolution types. Permeabilities range from 0.007 to 97 md with an average of 15.3 md. Clay content, primarily illite and smectite, varies from 1 to 39 percent with an average of 18 percent, and is inversely correlated with grain size.
AAPG Search and Discovery Article #90936©1998 AAPG Southwest Section Meeting, Wichita Falls, Texas