--> Abstract: Microfabric Characteristics of Top Seal Deposits in the Upper Devonian Shale-Dominated Catskill Delta Sequence, Western New York State, by Gary G. Lash; #90039 (2005)

Datapages, Inc.Print this page

Microfabric Characteristics of Top Seal Deposits in the Upper Devonian Shale-Dominated Catskill Delta Sequence, Western New York State

Gary G. Lash
SUNY-Fredonia, Fredonia, NY

Upper Devonian black shales of the Catskill Delta Complex of western New York serve not only as source rocks for oil and gas accumulations in the Appalachian basin, but also as top seals. Inferences regarding fluid pressure generation and top seal development in this shale-dominated sequence are based largely on the preferential propagation of fluid-driven joints (natural hydraulic fractures) within and proximal to black shale units. The most efficient seals appear to be the most organic-rich (TOC>2.5%) basal deposits of each black shale unit, which comprise upper transgressive systems tract (condensed section) and/or early highstand systems tract strata. Inferred top seal black shales are characterized by very thin quartz silt laminae that alternate with organic-rich clay layers. Thin section and SEM examination of the latter reveals a strong bedding-parallel planar arrangement of clay grains and flattened (high aspect ratio) organic particles. Anoxic bottom conditions during accumulation of these deposits precluded bioturbation thereby preserving their finely laminated depositional fabric. The relative paucity of rigid silt grains in the clay laminae favored the gravitational compaction-induced collapse of clay floccules soon after deposition into a strongly oriented microfabric creating a permeability anisotropy and barrier to fluids migrating up from underlying heavily, more permeable, bioturbated gray shale. Less carbonaceous, moderately bioturbated deposits higher in the black shale units are characterized by a more random, open microfabric, and probably by a lesser seal efficiency. These results point to the important control of depositional environment and resultant microfabric traits on seal quality in shale-dominated sequences.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005