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672 RAGG ET AL.' TECTONIC STRESS IN SICILY
Figure 2. (a) Contour plot of the well Chiesa (well 18). (b) occur within the thick succession of undeformed carbonates
Contour difference plot of the well Chiesa. The measured
caliper diameters are plotted symmetrically under their (Figure4). The SHmaxorientationsare very homogeneouasnd
correspondingazimuth in the contour plot. In the contour arequalitiesA or B exceptCarrubowith a D quality(Table 1).
difference plot the difference between the maximum and This can partly be traced back to a tool damage as it was
minimum caliper is plotted under the azimuth of the maximum indicatedfor a part of the log. The breakoutanalysisshowsa
caliper. Here diameterdata of a 300-800 m depthinterval of the NW to NNW SHmax orientation, which dominates on the
well Chiesa are stacked. southernandeasternpart of the HybleanPlateau.
The wells SettefarineandManfriaweredrilledthroughthe
(Figure 2). During data processing the orientations of the
caliper arms are stackedfor each individual breakoutinterval. allochthonouscover into the downbendingHyblean Plateau.
The dataqualityis A (Settefarinea) ndB (Manfria) (Figure5
The breakout orientation is determined from cross sections of andTable 1). Within the Hyblean Plateauthesewells indicate
a significantchangeto a NE-SW orientationw, hichis nearly
the borehole wall with an accuracy of -10 ø. For the perpendicular to the data described above. However, in the
computationof the mean breakout orientation, for each well allochthonouscover, any conclusionon the occurrenceand
we used the Mardia [1972] statistics for azimuthal data and orientationof the stressfield is impossiblebecauseonly a
weighted the individual breakout angles with the depth shortdepthintervalof allochthonouws as logged.
interval coveredby the breakoutzones.
The wells Vittoria Ovest (well 7), Biddine (well 8),
To be identified as a breakout,the following criteria [Plumb ContradaUlmo (well 9), Margi (well 10), andUrsitto (well 11)
and Hickman, 1985] have to be satisfied: (1) Tool rotation in the southwestersnegmenot f the Gela-CataniaForedeepare
of D quality becausethe beakout orientationsin the individual
should cease in the breakout interval. The tool should rotate wells are scattered(Figure 5 and Table 1). The wells Biddine
and Vittoria Ovest lie in proximityof NNE trendingfaults
above and below the breakout interval. (2) In the breakout
interval one diameter should be significantly greater than the 0 Hole Azimuth 270 360
diameter of the drilling bit (bit size). (3) The other diameter
shouldbe almostequal to bit size in the breakoutinterval. (4) 2500 90 180 ; •ß
The directionof the hole azimuth (projectionof the borehole 280O- ß
to the horizontal) should differ from the direction of the 3100- ;
regarded borehole elongations in wells which are deviated 3400- 8 I7
from vertical. This criteria helps to avoid a possiblebiasing
due to mechanicalabraisonof the boreholewall inducedby 7• ]5
drilling operations(Figure 3). All data are quality rankedinto
classesA to D (with A asbestquality)accordingto the quality '4 I4
ranking criteria of the World StressMap project [Zoback,
19921. I3
3700- /
4000- /
4. Resultsand Interpretation 4300- "' Ir:o: 1
For both southeasternand western Sicily the data have ,2
been investigatedwith respectto the different tectonicunits
which they cut through (Figures 4-8). In none of the wells
could a significanttrend of stressrotation with depth be
detected.
4.1. Southeastern Sicily ........ Devlation
The breakouts of six wells (from east to west: Cassibile Figure 3. Plot of the individual breakout orientations with
(well 1), Capo Negro (well 2), Bimmisca (well 3), Cozzo depthfor the well Settefarine(well number12). The breakout
Scalia (well 4), Carrubo (well 5), San Barrolo (well 6)) have intervals are numbered.The mean breakout orientation for this
been analyzed in the Hyblean-Malta platform. The breakouts
occurover a largedepthrangebetween200 and4500 m. They well is Nl10øEwith a standardeviationof 13ø. The plot
showsfor comparisonthe direction of the hole azimuth and
the deviation.
