Subsurface Fracture Analysis Using FMI Logs: Implications for Regional State of Stress Prediction in the Black Warrior Basin, Alabama, USA

Abstract

Fracture characterization is a matter of great importance in sedimentary basins as the orientation of new fractures is systematically related to the orientation of the regional (paleo)stress field, and/or the trend of faults. Fracture sets represent deformation at the sub-seismic scale, and thus cannot be easily mapped in the subsurface using standard geophysical methods. Adjacent to the northwest-trending Ouachita and the northeast-trending Alleghanian fold and thrust belts, northwest-trending thin-skinned normal faults are the dominant structures in the triangular shaped Paleozoic Black Warrior foreland basin. Researchers have attributed the thin-skinned normal faults to flexural extension related to the Ouachita orogeny. However, little is known about the regional state of stress and subsurface fracture architecture in the basin. This study uses Formation Micro Imager (FMI) resistivity logs collected in a 1498 m (4915 ft) well in Walker County, Alabama, to interpret the direction of subsurface natural fractures and related stress orientations, and to clarify the existence of Ouachita-related structural features in the basin. A total of 58 pairs of borehole breakout and 33 pairs of drilling-induced fractures were observed in the well, indicating a present-day maximum horizontal stress direction of 55°N (±10°). The origin of the wellbore-derived stress orientation determined in this study is consistent with those measured in the Appalachian Basin, indicating that both basins are under the same stress conditions. The image log data revealed 202 open fractures. This is dominated by three sets of open fractures with strikes of (O1) 30–45°N, (O2) 60–80°N, and (O3) 110–140°N. The FMI interpretation also revealed 142 healed fractures striking in a wide variety of azimuths, but dominantly (H1) 35–50°N, (H2) 60–80°N, (H3) 100–130°N, and (H4) 0–5°N. Fracture sets O2 and H2 are consistent with being formed and open in the present-day stress field. Fracture sets O1 and H1 strike parallel with the regional systematic joints, and fracture sets O3 and H3 are parallel with the regional fold-normal systematic joints, indicating northwest- converging Alleghanian-related stress field. The existence of N-S oriented fracture set H4 might is interpreted to be related to a pre-Alleghanian origin, most likely the north-converging Ouachita thrust belt. This interpretation is highly supportive of a Ouachita-related stress field in the Black Warrior Basin.

AAPG Datapages/Search and Discovery Article #90194 © 2014 International Conference & Exhibition, Istanbul, Turkey, September 14-17, 2014