ABSTRACT:

A map of the in-situ major horizontal principal stress direction on- and offshore Norway reveal a stress rotation from N-S in the Barents Sea to E-W in the North Sea. The stress rotation is primarily explained by the ridge push force and the continental margin.

RESUME:

Une carte des pression horisontales des roches à Norvège se montre une rotation de la plus grande pression de N-S à la Mer Barents jusqu'a E-O à la Mer du Nord. La raison plus probale est une action de "ridge push".

ZUSAMMENFASSUNG:

Eine Spannungskarte von Norwegen zeigt eine Rotation der gröβten horizontalen Spannung, von N-S in dem Barents Meer bis E-W in der North See. Die Rotation ist hauptsachlich bei der "ridge push" Kraft und dem Kontinentalrand erklart.

1.
INTRODUCTION

An knowlegde of the in-situ rock stress field is important for almost all kinds of geomechanics work. Design of underground openings for mining and civil engineering purposes is often based on stress magnitude and orientation data. In petroleum exploration and production in-situ rock stresses are important for both petroleum retention, reservior behaviour and borehole stability. The in-situ stress field often seem to be linked to plate tectonic. Knowledge of the rock stresses thereby enhance our understanding of geodynamic mechanisms in the crust and is used for evaluation of seismic hazards and earthquake prediction. The first in-situ stress determination methods were developed in the 1950s (Hast, 1958). Prior to this, rock stresses had mostly been regarded as induced by gravity. Hast's measurements show anisotropic horizontal stresses by far exceeding what was expected from gravitational effects, and often the horizontal stresses exceeded the vertical stress. Today, high horizontal stresses are commonly accepted, as verified by many authors and measured by a variety of techniques.

2.
COMPILATION OF IN-SITU ROCK STRESS DATA

Randalli and Chandler (1975) compiled stress data from Scandinavia to determine the regional stress field. Based on Hast's measurements they concluded that the major horizontal stress direction was E-W in southern Fennoscandia and N-S in the northern Fennoscandia. Overeoring data (Myrvang, 1976) and earthquake focal mechanisms (Bungum and Fyen, 1979) from Norway seem to fit well into this pattern. Klein and Barr (1986) used stress data from all over Europe to identify a regional NW-SE trend in western Europe, but characterized the stress field in Fennoscandia as complicated. Based on overcoring and hydraulic fracturing measurements, mainly being done for engineering purposes, a Fennoscandian Rock Stress DataBase was compiled (Stephansson et al., 1987). Today, FRSDB contains more than 500 measurements from more than 120 locations in Finland, Norway and Sweden. These data were available for the the World Stress Map project (Zoback, 1992), which, when finished in 1992, contained more than 7300 stress data from 18 countries. The primary goal of WSM was to establish the global stress field within the plates excluding data thought to be influenced by local conditions. The WSM identified a well defined NW-SE trend in central parts of the European plate, whereas this trend is weaker and more dispersed in Fennoscandia and the North Sea (Mueller et al., 1992). Based on data from the FRSDB and WSM, together with new borehole breakout and focal mechanism data from the North Sea a new stress map for the North Sea and Scandinavia has been established. Four main provinces with regional trends have been identified (fig. 1). These trends have been tryed related to different possible stress generating mechanisms contributing to the observed stress field.

This content is only available via PDF.
You can access this article if you purchase or spend a download.