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- Ask, Maria
(författare)
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In-situ and laboratory stress investigations using borehole data from the North Atlantic Ocean
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This doctoral thesis presents results from investigations ofin-situ and laboratory stress from borehole logging andlaboratory deformation experiments on drill-cores. The datacome from hydrocarbon exploration and production wells in theDanish Sector of the North Sea, and from boreholes drilled bythe Ocean Drilling Program (ODP). The drill-core samples werecollected from two ODP boreholes in the equatorial and easternAtlantic Ocean. The objectives for this project are: (1) to collect datafrom the North Atlantic Ocean where few or no stressmeasurements exist, and to incorporate the data into the WorldStress Map (WSM); (2) to determine the source of stress thatgenerates the mean orientation of the maximum horizontal stressin individual boreholes in ocean areas; and (3) to study themechanical responses of sediments and rocks in order tounderstand the nature of stress and stress paths that producedeformation of the ocean floor. The orientations of in-situ horizontal stresses weredetermined from analyses of borehole breakouts anddrilling-induced fractures (DIFs) as determined from four-armcaliper and Formation MicroScanner (FMS) data. Logging datawere analyzed using graphical and automated programs, usingbreakout and DIF criteria, directional statistics, and theborehole data were ranked according the WSM ranking system. Stress data from 44 oil wells and ODP boreholes in the NorthAtlantic Ocean represent important additions to the WSM fromareas where few stress data were available. Despite of theirrelatively shallow total depths, it is suggested that thestress orientation in many of the ODP boreholes are influencedby plate boundary forces. The observed variation in downholedistribution of breakouts in the oil wells and the lack ofregional trends indicate that local and regional stress sourcescontrol the stress field in these wells. Detailed laboratory stress measurements were conducted in atriaxial cell mounted in a servo-hydraulic load frame, usingreconsolidation and triaxial tests on small sub-samples fromtwo drill-core samples of different lithologies. Results from tests on a claystone from the basal section ofthe ocean-continent transition in West Iberia margin suggestthat the in-situ effective stresses are low because of the highpore pressure. The fractured nature of the underlying basementand breccia suggest that the pore fluids originate from deepersections. Results from tests on a calcarenite from about 1 kmdepth show that the elastic versus elastic-plastic behaviordiffers significantly, but that the rock behavior can bedescribed by critical state soil mechanics.
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| 2. |
- Ask, Maria
(författare)
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In situ stress at the Cote d'Ivoire-Ghana marginal ridge from FMS logging in Hole 959D
- 1998
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Ingår i: Proceedings of the Ocean Drilling Program. Scientific results. - College Station, TX : Texas A&M University. ; , s. 209-223
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The orientation of the in situ stress between 550 and 925 meters below seafloor in Hole 959D has been derived from the analysis of caliper and electrical images of Formation MicroScanner data. Because of a high hole deviation (5°-13°), drill pipe wear has enlarged the hole in a northerly direction. As a result, the four-arm caliper analyses cannot distinguish between mechanically and stress induced borehole elongations. The FMS data show that one of the principal stresses is parallel to the borehole axis and that mainly tensile wall fractures have formed in the borehole wall. The tensile wall fractures demonstrate a 168° ± 8°N weighted mean orientation of the maximum horizontal stress. The upper 750 mbsf of the borehole comprises the majority of the tensile wall fractures, whereas they only occur sporadically below this depth. The change in the downhole distribution of fractures coincides with the transition from weaker to more compacted, stronger sedimentary rock. Furthermore, the integration of seismic and physical properties data suggests that this depth corresponds to the boundary between passive margin evolution and continent/ocean transform faulting. The orientation of the maximum horizontal stress in Hole 959D is almost perpendicular to the east-west maximum horizontal stress orientation of the first-order stress province in western and north central Africa. Through comparisons with (1) the orientations of the maximum horizontal stress in Hole 959D, (2) the maximum horizontal stress of three focal mechanisms from earthquakes in Ghana, and (3) the strike of the continental margin, it is suggested that lateral strength contrasts across the Côte d’Ivoire-Ghana Transform Margin are the most probable source for the observed stress orientations. It is further suggested that sediment loading and lateral variation in crustal thickness and density may only have a minor influence on the stress field in this area.
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| 3. |
- Ask, Maria, et al.
(författare)
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In-situ stress determination from breakouts in the Tornquist Fan, Denmark
- 1996
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Ingår i: Terra Nova. - : Wiley. - 0954-4879 .- 1365-3121. ; 8:6, s. 575-584
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Tidskriftsartikel (refereegranskat)abstract
- The Tornquist Fan, a fan-shaped region in Denmark and Western Baltic, is situated in the transition zone between the Western and Northern European Stress Provinces. Breakout data from 20 wells (0.3-3.6 km) were analysed. The fan can be divided into three stress provinces: (i) The area south of the Romo Fracture Zone is part of the Western European Stress Province and has NNW-SSE orientation of the maximum horizontal stress, (ii) The sediment cover in the Norwegian-Danish Basin is dominated by ENE-WSW orientated maximum horizontal stress, (iii) The maximum horizontal stress is sub-parallel to the strike of the Sorgenfrei-Tornquist Zone. Deviations from the regional stress field were observed in wells close to faults and salt diapirs. In wells south of the Sorgenfrei-Tornquist Zone, breakout occurrence decreases with increasing age of the stratigraphic units. The downhole breakout distribution seems to correlate with lithology and thickness of the sediment layer.
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| 4. |
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| 5. |
- Ask, Maria
(författare)
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In situ stress from breakouts in the Danish sector of the North Sea
- 1997
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Ingår i: Marine and Petroleum Geology. - 0264-8172 .- 1873-4073. ; 14:3, s. 231-243
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Tidskriftsartikel (refereegranskat)abstract
- Borehole breakouts from 26 wells situated in the Danish Central Graben and one well situated in the northwestern section of the Ringkobing-Fyn High have been studied. The data provide information on the orientation of the contemporary principal horizontal stresses down to depths of 4.7 km. Statistics were used to obtain a mean azimuth of breakouts for each well, and each individual breakout was weighted by its length. Data quality of breakouts is assigned according to the World Stress Map quality ranking scheme. The breakout data reveal that SH orientations in the Danish Central Graben are extremely scattered; no regional trend in mean SH orientation can be identified as a result of the large standard deviation (±77°). Similar scatterings of SH orientations have been reported from adjacent regions of the North Sea. The results indicate that the state of stress in the sedimentary cover of the Danish Central Graben is influenced by a local stress generating mechanism rather than platewide stress transmissions mechanisms. Three units with different states of stress have been identified and found to correspond somewhat with the lithostratigraphic divisions of the Post Chalk Group, Chalk Group and Pre Chalk Group.
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| 6. |
- Ask, Maria, et al.
(författare)
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Physical properties data at Hole 959D : comparison of core and log measurements and a proposed revision of lithologic units
- 1998
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A revised division of lithologic units is proposed for Hole 959D in the interval from 400 to 1000 meters below seafloor from Ocean Drilling Program Leg 159 based on combining the geophysical and sedimentological information. This new division is essential for further geological studies of the Cote d’Ivoire-Ghana Transform Margin area. A comparison is made between shipboard laboratory measurements made on core samples and in situ downhole logging data. Logging data, in conjunction with physical properties data, provide a more detailed record of the variation in the sediment column, which enables a more qualitative assessment of lithotype distribution. Emphasis is placed on distinguishing between the lithologic factors influencing the geophysical signals and artifacts resulting from imperfect data collection. Good overall agreement between core and log data for rotary core barrel cored sediments at Hole 959D is observed, and effects of core expansion caused by the removal of overburden pressure do not appear significant.
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| 7. |
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