FRACTURE NETWORKS IN CARBONATES AS A FUNCTION OF FACIES, DIAGENESIS AND LARGE-SCALE STRUCTURE

Carbonate reservoirs such as the Miss. Lime in the western US are typically drilled in order to exploit fracture networks. However, the characterization of subsurface fractures in varying carbonate lithologies and across structures is still a challenging exercise. This study takes outcrop data from carbonates in the Sawtooth Range, Montana, together with core from exploration wells drilled the foreland, and assesses the fracture styles as a function of lithology, diagenesis and large-scale structure. Methods include field observations, core descriptions and thin-section analysis.

The carbonate rocks in the cores range from tidal flat facies to shelf facies and shelf crest facies. Material from the tidal flat facies has been dolomitized, is heavily stylolitized and develops an extensive network of small-scale fractures. Many of these fractures are oil stained. Material from the shelf facies shows evidence of dissolution of bioclasts, leaving a network of unconnected vuggy porosity. Some of the material has been dolomitized. Small-scale fractures are found in this facies, but not as extensively as in the tidal flat facies. Small-scale fractures and vugs are frequently oil stained. Material from the shelf crest facies shows extensive dissolution of bioclastic material, leaving partially connected vuggy porosity. Some zones of this facies have been partially silicified. This facies is shows no small-scale fracturing, but is extensively oil stained. All facies present in the cores are cut by larger-scale, through-going fractures, similar to the fracture network that can be identified in the field. The through-going fractures are rarely oil stained. In the field, the orientation of through-going fractures is affected primarily by structural position, whether on the backlimb, forelimb or crest of an anticline. Material from the tidal flat facies and shelf facies is exposed on the anticline, and shows a similar development of small-scale fractures as observed in the core.

These results suggest that oil-prone fracture networks and porosity are intimately connected to facies in carbonate systems, and that a complete fracture network characterization should account not only for large-scale through-going fractures, but for variations in fracture intensity related to the carbonate stratigraphy.