Borehole Breakout Data Research Articles

Active stress along the ne external margin of the Apennines: the Ferrara arc, northern Italy

We have analysed borehole breakout data from 12 deep wells in order to constrain the direction of the minimum and maximum horizontal stress in a part of the Po Plain, northern Italy, characterised by a ∼N–S prevailing compressional stress regime, and in order to shed light on the regional state of stress and on the correlation between the active stress field and the orientation of tectonic structures. The results have been compared with seismological data relating to 1988–1995 crustal seismicity (2.5< M d<4.8) and to the 1983 Parma ( M s=5.0) and the 1996 Reggio Emilia ( M s=5.1) events. Plio-Pleistocene mesostructural data are also described in order to better define the present-day stress field and to understand the active tectonic processes in particular stress provinces. The borehole breakout analysis, in accordance with the seismicity and mesostructural data, shows the presence of a predominant compression area, characterised by approximately N–S maximum horizontal stress, along the outer thrust of the Ferrara arc. Particularly, the breakout analysis indicates a minimum horizontal stress, N81W±22° relative to a total of eleven analysed wells, with 3746 m cumulative total length of breakout zones. Among these, nine wells are located in the same tectonic structure, consisting of an arc of asymmetric folds overthrust towards the NE. The breakout results for these wells are quite similar in terms of minimum horizontal stress direction (∼E–W oriented). The other two wells are located in the outside sector of the arc and one of them shows a different minimum horizontal stress direction, probably distinctive of another tectonic unit. On the basis of these new reliable stress indicators, the active compressive front in this area is located along the termination of the external northern Apenninic arc.

Regional stress alignments in the Kutai Basin, East Kalimantan, Indonesia: a contribution from a borehole breakout study

Borehole breakout data from 134 wells located within the Kutai Basin region, East Kalimantan were analyzed to determine the present day regional horizontal stress alignments. The data were extracted from various types of dipmeter logs. The study reveals that the majority of the data give a coherent picture of breakout orientation. The mean azimuths for the entire-unweighted, ellipticity-weighted and magnitude-weighted data sets are preferentially aligned in the regional-mean direction of 48.9° N or 128.9° N. Most of the data have a low dispersion value ( So) and the ranking of reliability in Zoback’s classification is ‘A’. There is no significant azimuth variation with depth. These relatively consistent alignments of breakout azimuths indicate that the maximum regional stress direction in the study area is NW–SE. This regional-mean of breakout azimuths deviates from the axis of the anticlinorium trends and from the strike of the thrust–fault patterns in the region. It is believed that these structural patterns are influenced by reactivation of weak zones related to sediment loading (structural inversion).

In situ stress data from the Jura Mountains — new results and interpretation

Recently completed in situ stress measurements using the borehole slotter at 33 new test sites within the Swiss and French Jura Mountains are presented. When combined with previously published stress data, these new measurements allow a detailed description of the contemporary state of stress in this fold‐and‐thrust belt and the adjacent foreland. The data now allow the recognition of five stress provinces, within which two general and different orientations of maximum horizontal stress SH can be distinguished: the Central and the Southwest Provinces, with a NNW–SSE to NNE–SSW orientation, and the East, Northwest and Southeast Provinces, orientated E–W to NW–SE. Each province has its own characteristic orientation. Measurements in three study areas are presented for comparison with already published acoustic anisotropy measurements, borehole breakout data, overcoring tests and triangulation measurements. The results show a generally good agreement with the new borehole slotter measurements. The tectonic implications of these stress data are discussed in terms of a large‐scale stress reorientation of the NW–SE orientated contemporary Central European stress field in the foreland of the western Alpine arc.

群発地震発生域直上における地殻応力測定

Crustal stress measurements by the hydraulic fracturing method were carried out using a 1000m- and a 100m-deep boreholes drilled at Inagawa town, Hyogo prefecture, where earthquake swarm activities with very shallow hypocenters have intermittently occurred since July 1994. On borehole televiewer record, borehole breakouts were clearly observed at a number of depth intervals of the 1000m-deep borehole. The borehole breakout data as well as the hydraulic fracturing data was used to estimate orientation of the maximum horizontal compressive stress.Magnitudes of the horizontal stress down to 700m in depth are comparable to the standard stress gradients with depth in western Japan derived by TANAKA (1986). On the other hand, the maximum and minimum horizontal compressive stresses at about 950m in depth are about 70MPa and 40MPa, respectively, which are about twice as large as the standard stress gradients with depth in western Japan. The maximum horizontal compressive stress below 600m in depth is generally oriented E-W to NW-SE.

Orientation of crustal stresses in the North Sea and Barents Sea inferred from borehole breakouts

The orientations of the greatest horizontal principal stresses (S H) derived from borehole breakouts are presented for 30 boreholes located in the Central Graben, the Viking Graben, the Møre-Vøring region, and the Barents Sea area. Additional borehole breakout data from 20 other wells for which information about breakout length was available have been used to calculate the mean azimuth of S H for the four areas of investigation. The mean azimuth of S H is calculated from the length-weighted distribution of the breakout orientations. In the Viking Graben and the Central Graben a S H orientation of about N 78° and N 97° was found, respectively. For the Møre-Vøring region a mean S H orientation of N 127° has been evaluated. Borehole breakout data from the Barents Sea area indicate a S H orientation of about N 177°. The S H orientations presented in this study are consistent with the results from previous studies which suggests that the direction of the maximum compressive horizontal stress S H differs off the coast of Norway markedly from the well documented NW-SE trend of S H in Western Europe. The observed S H orientation changes significantly across the investigated regions which may have to be attributed to their complicated geomechanical setting.

Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia

The Portuguese mainland territory is located close to the Azores‐Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmax trend. These differences in stress trend suggest a regional progressive rotation of the SHmax direction from NNW‐SSE to WNW‐ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW‐SE SHmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.

Crustal Stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms

ABSTRACTThe regional stress field in the northern North Sea (offshore western Norway) has been studied through the acquisition and analysis of directions of maximum horizontal compression (s̀H) as extracted from borehole breakouts and from earthquake focal mechanism solutions.The results indicate that the regional stress field is dominated by NW‐SE compression, with good consistency between shallow borehole breakouts (2–5 km depth) and deeper earthquakes (10–25 km depth). The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the results are in good agreement with the western Europe trend found in earlier investigations.The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW‐SE and NE‐SW. From Tampen Spur in the west to the Sogn graben in the east an anomalous stress field is indicated, with NE‐SW oriented maximum horizontal compressions. This anomaly is clearly seen both in the borehole breakout data and in the earthquake data. Possible sources for this anomaly are discussed, and include postglacial uplift and/or lateral variations in the physical properties of the crust.

Observations of shear-wave splitting from marine VSPs in the southern North Sea

SUMMARY Four offset marine Vertical Seismic Profiles (VSPs) are analysed for shear-wave splitting in a southern North Sea gas reservoir using mode-converted shear waves. Automatic estimates of fast shear-wave polarizations and time delays indicate the presence of stress-induced cracks striking at between N35W and N6QW, corresponding to independent measurements of stress from borehole breakout data and earthquake studies. Time delays suggest that shear-wave splitting occurs only in the overlying sealing rock with no measurable anisotropy in the reservoir sands. All measurements are contaminated by scattered energy, probably resulting from non-planar interfaces and/or lateral variations in lithology. More reliable results may have been obtained if the VSP offsets were not almost parallel and perpendicular to the maximum horizontal stress direction.

Australia-Banda Arc Collision and In Situ Stress in the Vulcan Sub-Basin (Timor Sea) as Revealed by Borehole Breakout Data

Boreholes drilled in the search for oil in the Vulcan Sub-basin (Timor Sea, North West Shelf, Australia) commonly exhibit an elliptical cross-section believed to be the result of wellbore failure known as borehole breakout. Breakouts form by compressional shear failure in response to stress concentration around the borehole due to the prevailing in situ stress. The bore wall becomes elongated in the direction of least horizontal compressive stress. The orientation and shape of the breakouts are measured by the four-arm dipmeter tool. The azimuths of the long axes of breakouts in the Vulcan Sub-basin show a reasonably consistent 130?170°N trend implying that maximum horizontal compressive stress (SHmax) is oriented 040?080°N. This NE-ENE SHmax orientation in the Vulcan Sub-basin is not controlled by compression transmitted from the nearby Australia/Banda Arc collision zone. However, it is consistent with theoretical models of stress distribution in the Indo-Australian plate based on the plate-driving forces at all of its boundaries. The present SHmax orientation is consistent with either strike-slip or normal movement on the pre-existing NE-trending faults in the basin.

Structural Controls on Ochoan Salt Dissolution and Delaware Mountain Group Oil Field Permeability Trends in the Delaware Basin, West Texas and Southeast New Mexico

A Landsat TM image of the Delaware basin, west Texas and southeast New Mexico, reveals geomorphic lineaments and tonal anomalies with preferred northwest-southeast and northeast-southwest orientations. Lineament orientations are the same as the trend of joints and fractures observed in Delaware Mountain exposures and from subsurface borehole break-out and televiewer data. These data suggest that lineament trends are controlled by subsurface joints and fractures. Petrographic data indicate that Delaware Mountain Group porosity/permeability development is controlled in large part by the occurrence of calcite/dolomite cement and chlorite/corrensite clays. The unimodality of grain size, sorting, and framework grain mineralogy, along with the virtual absence of detrital clays, favors a diagenetic control on cementation patterns. These observations coupled with formation water chemistry, and cement carbon-oxygen isotope and fluid inclusion data suggest that the occurrence of Delaware Mountain Group cements is related to diagenetic alteration by waters that have dissolved Ochoan halite and potash salts. Hydrodynamic fluid flow along joint and fracture systems coupled with rock-water interactions are proposed that account for the coincidence of salt dissolution fronts and oil field permeability barriers as well as formation water chemical trends and cement isotopic signatures. Preliminary data suggest fracture systems provide conduits for hydrodynamic fluidmore » flow capable of extensive Ochoan salt dissolution and the transport of reactive solutions to remote horizons both laterally and vertically in the basin.« less