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The Role of Depositional and Early Compaction History in Primary Migration: Example from the Upper Devonian Catskill Delta Complex, New York State

Lash, Gary G.1 and Terry Engelder2
1 Department of Geosciences, SUNY-Fredonia, Fredonia, NY 
2 Department of Geosciences, The Pennsylvania State University, University Park, PA

There are two distinct mechanisms for creating the pathways of primary migration in the early mature to mature Upper Devonian Dunkirk black shale of the Catskill Delta Complex, western New York. One pathway is horizontal and the other is vertical. The active mechanism depends on (1) the distribution of incompressive detrital silt grains (i.e., confined to discrete laminae versus more or less evenly distributed throughout the clay matrix) and (2) the degree to which clay floccules collapsed during early burial-related compaction of the sediment. Hydrocarbons were generated preferentially in organic-rich clay laminae that are interlayered with very thin silt-rich laminae in the lower part of the Dunkirk shale. Clay layers are characterized by a tight, planar microfabric that precludes migration except through occasional microchannels formed by interconnected macropores. The conversion of flattened kerogen/bitumen inclusions to oil caused overpressurization of the low-permeability clay laminae and induced horizontal mode I microfractures. This process sustained primary migration parallel to bedding and may have funneled hydrocarbons into interlayered porous silt laminae. Poorly laminated, moderately bioturbated black shale becomes more abundant higher in the Dunkirk shale. These deposits exhibit a relatively porous, more randomly organized clay microfabric, a reflection of sediment homogenization by burrowing organisms and the shielding effect of dispersed silt grains during compaction. The higher degree of porosity preserved in these deposits enabled them to drain during catagenesis without fracturing. Here, primary migration was vertical and occurred through a tortuous network of interconnected pores.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004