Heartburn in Predicting Natural Fracture Orientations: The Effects of the Different Fracture Susceptibilities in Heterogeneous Lithologies
John C. Lorenz
Formations where natural fractures change orientation by 20-25 degrees from bed to bed and/or laterally within a bed are not uncommon. Some heterogeneous formations may even have entirely different fracture sets within different layers, making exact predictions of fracture orientations in the subsurface even more difficult. An example of this is the basal Wasatch Formation in the Piceance basin of western Colorado. Two stress events affected these strata at different times, superimposing two oblique regional fracture sets on many of the clastic, fluvial lenses. However, some beds fractured during the early stress event but were not susceptible during later stresses, whereas other beds did not fracture until the later event and contain fractures with entirely different ori ntations. Some beds contain both fracture sets and display obvious older-younger age relationships. The differences in fracture susceptibility that are caused by variable lithologies (and thus mechanical properties), can have significant influence on the fracture character of a reservoir. In the case of the Wasatch Formation, conglomerates were resistant to fracturing during the early stress event and thus contain only the later fractures, whereas sandstones commonly contain both sets. Fracture character may even change locally within a bed as the lithology changes from a pebble conglomerate at the base of a fluvial channel to a sandstone in higher zones.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California