ABSTRACT

ABSTRACT

A recently-drilled ultra deep oil-well near the center of the Michigan Basin has provided an excellent opportunity to determine the state of stress and its variation with depth in formations ranging in age from Precambrian to Devonian. Except for the top test (1230m deep), results in the other three horizons (2806m, 3660m, 5110m) indicate a linear increase of sHmin yielding a constant ratio of 0.7 between sHmin and sV. The shallowest test in the Devonian Amherstburg Formation yielded a sHmin approximately equal to sV. Shallow measurements elsewhere in the Midwest have shown similar trends of relatively high horizontal stresses. The value of sHmax while not too consistent, tends to be equal or lower than sV at the three deepest levels tested. Hydrofracturing stress measurements have been carried out to about 0.4 km in two boreholes in Reykjavik, Iceland, on the flank of the Mid-Atlantic Ridge. The measurements indicate a dominant orientation of sHmax approximately perpendicular to the axial rift zone, in contrast to earthquake focal mechanism solutions from within the axial rift zone. In one hole (H32) a depth-dependent change in stress orientation is indicated, with the maximum compressive stress horizontal above a depth of about 0.25 km, and vertical below it; however the orientation of sHmax remains unchanged. The data thus suggest reconciliation of an apparent conflict between the dominantly compressive indications of shallow overcoring stress measurements and dominant extension as required by focal mechanisms solutions. The measured stresses are supported by the more reliable of overcoring measurements from southeast Iceland.

INTRODUCTION

Hydrofracturing as a method of n-u stress measurement was developed a decade ago based on theoretical work (Scheidegger, 1962; Kehle 1964; Haimson and Fairhurst, 1967), and laboratory experiments (Haimson, 1968; Von Schonfeldt, 1970). Additional theoretical and laboratory studies have been carried out during the present decade (Edl, 1973; Haimson and Avasthi 1.975; Roegiers, 1975). Initial field results based on oil-field hydrofracturing jobs (Scheidegger,-1962; Kehle, 1964; Haimson and Stahl, 1970), and small scale experiments (Von Schonfeldt, 1970) were very promising. The opportunity to test the method as a field stress measuring technique came at Rangely, Colorado. The nu stresses as determined by Haimson(1973) using the hydrofracturing technique in a 1900m deep borehole were in according with the nearby fault type and its slip direction as well as with surface stress measurements and other geological structures. More importantly, the determined stress magnitudes together with the slip criterion of the rock tested were used to accurately predict the level of formation pore pressure that would induce slip, thereby triggering local earthquakes (Raleigh et al, 1976). Since the Rangely tests hydrofracturing has become a familiar nu stress measuring method and it is still to date the only technique known for depths larger than 50m. Some twenty significant measurements have been carried out so far in the North American Plate (e.g. Haimson, 1974a, 1975, 1976a; Haimson et al 1976; Zoback et al, 1977). In this paper I am reporting on two sets of hydrofracturing measurements recently conducted in areas of particular geologic interest: the Michigan Basin and the Mid-Atlantic Ridge in Iceland.

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