WHITAKER, AMY E., Pennsylvania State University, Department of Geosciences, University Park, PA

ABSTRACT: The Effect of High Sedimentation Rates on Density and Lithological Distribution of Natural Fractures in Carboniferous, Clastic Rocks of the Western Ouachita Mountains

Natural hydraulic fractures occur where elevated pore pressures overcome the compressive forces of the earth and induce a net tensile stress. The goal of this project is to map the mechanical response of sandstone of a continental deltaic sequence to compaction-induced fluid pressures under sedimentation rates exceeding 1 km/Ma. Specifically, this investigation aims to develop a model that predicts the distribution and density of natural hydraulic fractures in sandstones as a function of specific sedimentation rates and to further constrain the conditions under which pore fluid pressures become large enough to fracture the rock.

We hypothesize that natural hydraulic fractures develop in sediments where the sedimentation rate exceeds the ability of the sediment to drain trapped pore fluids. As the rate of sediment accumulation increases, this threshold should be attained in increasingly permeable sediments. In eastern Oklahoma, the Carboniferous sequence of Stanley Group, Jackfork Group and Atoka Formation was deposited at sedimentation rates up to and, exceeding 1 km/Ma. These units are ideal for elucidating the relationship between fracture development and sedimentation rate, as their relative sedimentation rates are well documented.

The first phase of this project, undertaken in June 2000, was a field investigation in Oklahoma, visiting Stanley, Jackfork, and Atoka outcrops and characterizing joint development. The field data will be used to produce a joint probability analysis for the various lithologies for specific sedimentation rates and then compared to joint data sets from the Appalachian Plateau, where Alleghanian tectonic compaction rates were comparable.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid