Analysis of Fractures and Tectonic Structures in Core

Byron R. Kulander, Stuart L. Dean

Core analysis applied to characterization of fractured reservoirs should include interpretation of drilling-induced fractures, natural fractures, and tectonic structures. Cored natural fractures may possess geometric and genetic relationships with induced fractures, primary sedimentary features and tectonic structures that show cumulative effects of paleostresses and anisotropies active from initial basin development, through subsequent orogenic-epeirogenic phases, to the present. Drilling-induced fracture frequencies and orientations are related to rock anisotropies, rock mechanical properties, in-situ stresses, drilling stresses and preferred sonic and natural fracture directions.

Induced fractures form in the core barrel, at the scribe knives or bit, and before the bit. Those leading the bit are subsequently cored and can be present in the borehole wall. Different types of drilling-induced fractures possess unique developmental histories and orientations. In addition, surface structure geometry on drilling-induced fracture faces, as well as morphology of these fractures, shows distinct relationships to core geometry. In contrast, surface structures and morphology of natural fractures show no geometrical relationships to core parameters. Recorded information should be shown on a fracture-tectonic log designed for easy visualization and ready comparison with other data. Final synthesis of information from fracture-tectonic logs, downhole logs, regional fracture- ectonic investigations, analysis of remote sensor images, reservoir production facts, and results of laboratory tests on core samples provide a comprehensive data bank. Such data can be useful for planning stimulation-secondary recovery programs, modeling mass transport of fluids, locating additional wells, and developing a plan for regional exploration.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.