Towards a
Quantitative Definition of Mechanical Units: New Techniques and Results from a
Field Test Site
Bertotti, Giovanni1, Nico J.
Hardebol1, Stefan M. Luthi2, Jose Taal2 (1)
Vrije Universiteit Amsterdam, Amsterdam, Netherlands (2) Delft University of
Technology, 2628 RX Delft, Netherlands
Physical properties of reservoirs are
strongly influenced by distributed fracture fields. Outcrop studies are often
used to determine them but have provided unsatisfactory results as the
definition of mechanical units, i.e. groups of layers displaying constant
fracture patterns (specifically spacing) is difficult and typically carried out
in a qualitative manner. We have developed an innovative methodology to acquire
and process fracture patterns in outcrops and to derive its mechanical
stratigraphy. The method makes full use of GIS capabilities and allows for
direct digital acquisition in the field leading to time-efficient acquisition.
Data are processed with a newly developed routine which permits an objective
description of changes of fracture characteristics along the stratigraphy of
the outcrop. The operator is then able to decide the most suitable mechanic
stratigraphy. Field data from a test site in the Karoo Basin (South Africa)
demonstrate that mechanical interlayers, i.e. layers devoid of fracturing, are
not the rule and that variations in fracture density and spacing often occur in
a continuous manner without the interposition of joint-free layers. The data
also show a negative, though weak correlation between spacing distance and bed
thickness. This is in contrast with data from the literature but compatible
with what is observed in scientific boreholes drilled in the same area. This
downhole validation of surface fracture fields strongly supports the use of
outcrop analogues for predicting subsurface fracture distributions.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California