Abstract

In siliciclastic hydrocarbon reservoir rocks, economic gas and oil production may depend on the attributes of natural fractures, and, with the advent of horizontal drilling, fractures are increasingly exploration and development targets; yet reliable information on such key fracture attributes as orientation (strike) is sparse because few fractures intersect vertical well bores. This paper describes how fracture strike can be documented on a bed-by-bed basis even in well bores where few or no visible fractures are directly sampled. Quartz-lined opening-mode microfractures (lengths of microns to millimeters) in quartz-cemented sandstones commonly are not visible using standard petrographic methods, but systematic mapping of these microfractures is possible using photomultiplier-based electron beam-induced luminescence (scanned cathodoluminescence) imaging. As shown by observations, primarily from three natural gas plays and one oil play in the United States, microfracture strike is a good guide to the strike of large fractures (macrofractures) that formed concurrently. Because microfractures are widespread and small specimens can be used to get accurate fracture-strike data, this approach can be applied to samples obtained from wireline-conveyed rotary (drilled) sidewall coring devices, as well as to samples from full-diameter core.

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