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The Use of Stratigraphic Architecture and Rock Strength for Improved Carbonate Fractured Reservoir Modeling—Examples from Outcrop Analogs and Subsurface Reservoirs

Chris Zahm¹ and Charles Kerans²
¹Bureau of Economic Geology, University of Texas at Austin, Austin, TX, USA
²Department of Geological Sciences, University of Texas at Austin, Austin, TX, USA

Carbonate facies and the stratigraphic principles that determine their lateral and vertical distribution also define the most important heterogeneity within fractured reservoirs—how the reservoir is layered (i.e., stratal geometry and bed thickness) and how facies of differing rock strength are distributed within the volume of interest. Establishing the relationship between rock properties such as porosity, density, rock strength (e.g., unconfined compressive strength or elasticity), and facies or lithofacies can be critical for predicting how fracture intensity may change spatially within a reservoir. This study explores the relationship between the stratigraphic architecture, which controls the distribution of facies, porosity, and rock strength, and the development of fractures in outcrop analogs and in seismically-mapped faults in subsurface carbonate reservoirs.

We have established a relationship between sequence stratigraphy and mechanical stratigraphy that incorporates facies, rock properties, and bed thickness, herein referred to as vertical mechanical facies associations (VMFA). Outcrop and subsurface observations suggest nine stratal relationships that have significant influence on fracture style : (1) massive mound, (2) massive grainstone, (3) mixed grainstone packstone to grain-dominated packstone, (4) mixed grainstone to packstone, (5) mixed grain-dominated packstone to wackestone, (6) mixed wackestones and packstone, (7) wackestone and grain-dominated packstone to grainstone, (8) mixed shale and grain-dominated packstone to grainstone, and (9) very thin shale beds. These nine relationships become the building blocks of a mechanical stratigraphic model.

AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012