Folding and Pre-Folding Natural Fractures in the Cupido Carbonate Platform and Speculative Fracture Characteristics in Equivalent Deep-Water Carbonate Facies

Abstract

Abstract

There is increased recognition that many natural fracture systems develop in beds that have not suffered significant strain. For example, drilling in extensive areas of unconventional resources has demonstrated the ubiquitous presence of fractures in the absence of significant folding or faulting. Experience shows that these fractures impact well cost and operational safety, well placement strategies, and expected well performance. However, exact prediction of natural fractures in the subsurface is challenging because effective methods of natural fracture prediction and interactions with hydraulic fractures still need to be developed and tested.

The Cupido Formation, cropping out in the Sierra Madre Oriental near Monterrey, Mexico, contains fractured limestones and dolomites of Cretaceous Albian-Aptian age and is an analogue for pre-folding fractures. We show that the most obvious natural fracture sets in the Cupido likely formed before the spectacular folding that is observed in outcrops. Furthermore, the abundance of natural fractures appears to be primarily related to lithology and not to structural characteristics or bed thickness. Fracturing likely started early in the Cupido burial, prior to Laramide folding. Diagenesis, in which dolomite replaced some limestone, was the most important process that affected the pre-folding fractures in Cupido rocks.

We used structural geology methods to demonstrate that these fracture sets are best explained by a pre-folding deformation stage, and that fracture original orientation is related to regional-scale basin geometry during Cupido deposition. The results suggest that the shape of the carbonate platform is an important control on natural fracturing in the inner to outer platform in the Cupido. Fracture characteristics are different in shallow and deep water facies of the Cupido Platform. Deep water facies equivalent to the Cupido do not exhibit the same pre-folding natural fracture patterns or the fracture abundance of shallow water facies. In contrast, natural fracture patterns in deeper water facies appear geometrically related to the folds and are the latest of natural fractures. Similarly, the geometry of individual fractures varies for different sedimentary facies, different fracture timing, and the different geomechanical characteristics of the rock during fracturing. Fracture clustering also appears to vary with apparently longer and more clustered natural fractures developed in deep water facies.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016