Michele L. Cooke1,
Antonio J. Simo2,
(1) University of Massachusetts, Amherst, MA
(2) University of Wisconsin
(3) Montgomery - Watson
(4) Delft Technical University
Abstract: Carbonate cyclicity and fracturing - the mechanical stratigraphy connection
Carbonate strata often show repetitive facies patterns (cycles) reflecting temporal changes in depositional environments. These stratigraphic patterns can be expressed in terms of mechanical stratigraphy. Mechanical units, and the interfaces between units, control fracturing and thus fluid flow within low-matrix permeability rocks. Several case studies demonstrate that cycle boundaries commonly, but not always, act as mechanical interfaces, which terminate bed-perpendicular fractures. Additional factors such as intense lithification of the cycle caps, organic-rich partings and thick mud/shale cycle bases also influence whether cycle boundaries arrest fracture propagation. Thus, carbonate stacking patterns and characteristics of critical mechanical properties can be integrated to predict fracture networks in carbonate reservoirs.
The Austin Chalk, TX, contains cycles of fractured carbonate and relatively unfractured marl; in this case, the thickness of the marl layers controls fracture termination. Numerical experiments suggest that marl layers within the Austin Chalk as thin as 1 cm can act as mechanical interfaces. Fractures within the Silurian carbonate of WI abut against cycle and inter-cycle boundaries with little or no mud. In such carbonate stacking, the strength of stratigraphic horizons controls fracture termination. Organic partings and cycle boundaries are more effective at terminating fractures than mud horizons, facies contacts and inter-cycle boundaries. Empirically determined relationships can guide prediction of fracture network from the carbonate stratigraphy.
This interdisciplinary study demonstrates that carbonate depositional cycles aid in assessing fracture networks and predicting fluid flow. The key to predicting the fracture network lies in understanding the mechanical controls, which vary with different types of carbonate cycles.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana