Horizontal stresses from well-bore breakouts and lithologies associated with their formation, Oklahoma and Texas Panhandle

Abstract

Orientations of crustal stresses are inferred from stress-induced well-bore breakouts in three areas in the south-central United States: the eastern part of the Anadarko basin in central Oklahoma, the Marietta basin in south-central Oklahoma, and the Bravo dome area of the central Texas Panhandle. Inferred directions of maximum horizontal principal stress (SH max ) are ENE for the eastern Anadarko basin, and NE for the Marietta basin and the Bravo dome area. For the Bravo dome area, the magnitudes of the three principal stresses (S 1 , S 2 , S 3 ) are known from existing hydraulic-fracturing (hydrofrac) measurements, and a normal-faulting stress regime (S V > SH max > SH min ) is implied. For the eastern Anadarko basin and the Marietta basin, the magnitudes of the principal stresses are not known. Because Quaternary left-lateral oblique slip on the Meers fault in south-central Oklahoma suggests strike-slip (SHmax > Sv > SHmin) and reverse faulting (SH max > SH min > S V ), the study region is inferred to be a possible transition zone between areas of extensional and compressional stresses. Breakout data from the eastern Anadarko basin yield a single consistent SH max orientation. Data from the Marietta basin and the Bravo dome area have bimodal-orthogonal distributions consisting of breakouts and orthogonal sets of well-bore enlargement orientations. Orthogonal trends in the data are probably related to drilling-induced hydraulic fracturing of the well bore, or to preexisting natural fractures or joint sets intersecting the well bore. On the dipmeter log, breakouts and fracture enlargements have elliptical cross sections of similar size and shape. Orthogonally oriented well-bore enlargements are differentiated by comparing their long-axis orientations with directions of known or inferred horizontal stress. Dispersion, or data scatter, among enlargement orientations (bimodal data sets) increases the standard deviations for many well data sets from the Marietta basin and the Bravo dome area. In these two areas, some dispersion may reflect variation in stress conditions across fault-bounded blocks and the orientations of fractures or joints within these blocks. Although breakouts and fracture enlargements formed in all parts of the thick sequences of sedimentary rocks logged, they occurred primarily in limestone, shale, and dolomitic rocks, reflecting the abundance of these rock types in the study areas.