ABSTRACT:

Stress orientation in Denmark appear to be controlled by six stress sources: plate boundary forces, overpressure in sediment formations, detachments at formation boundaries, faults, salt diapirs and material properly variations. We predict that plate boundary forces control stress orientation in areas without detachments as well as below detachments. Faults, salt diapirs and material property variations appear to have the strongest influence on the stress orientation above detachments.

RESUME:

L'orientation des contraintes au Danemark parait être controlee par six sources de contraintes: forces aux marges des plaques, surpressions dans les formations sedimentaires, failles, dômes de sel et variations des proprietes des materiaux. 11est predit que les forces aux limites des plaques contrôlent l'orientation des contraintes dans des zones sans decollement et en dessous des decollements. Les failles, dômes de sel et variations des proprietes des materiaux paraîssent avoir une influence plus grande sur I'orientation des contraintes dans des zones au dessus des decollements.

ZUSAMMENFASSUNG:

Die Richtungen der Bergspannungen in Danemark sind wahrscheinlich von sex stueckSpannungsgruende beeinflusst: Krafte von die Plauenrander, Überdruck innen sedimentare Formationen, Abscherung bei Formationsrander, Verwerfunge, Salzdome und Eigenschafts variationen der Materialien. Wir Brechnungen, dass in Gebiete ohne Abscherungen oder unterhalb die Abscherungen, die Krafte von Plattenrander die Orienterung der Bergspannungen beeinflusst. Oberhalb die Abscherung scheint es uns dass Verwerfunge, Salzdome und Eigenschafts variationen der Materialien die hauptsachliche Gruende sind die ueber Spannungsrichtungen Einfluss haben.

1 INTRODUCTION

Determination of in situ stress is important for the extraction of hydrocarbons and the design of underground constructions. The in situ state of stress is the result from a combination of superimposed stress sources formed during the burial and subsequent history of sedimentary strata (Price, 1974). Stresses in the lithosphere arise from mechanisms operating on three scales: platewide, regional and local. Examples of platewide stress are plate boundary forces such as ridge-push and slabpull (e.g. Bott and Kusznir, 1984). Structural discontinuities (e.g. Hudson and Cooling, 1988; Evans et al., 1989; Yassir and Rogers, 1993; Yassir and Bell, 1994), pore pressure (e.g. Yassir and Rogers, 1993; Yassir and Bell, 1994) and material property variations (Warpinski et al., 1985; Evans et al., 1989), and remanent stresses (Voight, 1974; Evans et al., 1989) can generate stress. Regional stress occur at some point between the scale of platewide and local stresses. Methods for measuring in situ stress at shallow depths (<1 km) are, for example, hydraulic fracturing, overcoring and strain recovery. The borehole breakout method measures in situ stress at intermediate depths (0.5–6 km), and fault-plane solutions from earthquakes provide the state of stress at great depths (>5 km). Although results from hydraulic fracturing and borehole breakout measurements provide reliable results, in situ stress measurements may be both time-consuming and expensive. Because of this, it is of great importance to extract as much information as possible from every in situ stress measurement. Prediction of the state of stress near an investigated site is one method. Predictions are important at early planning stages for new hydrocarbon reservoirs and underground constructions. Moreover, the results would also be of importance for decisions whether further in situ stress measurements should be made or not. The objective of this paper is, first, to present potential stress sources in regions that have been investigated earlier. Secondly, we will make a prediction of the in situ state of stress in Denmark for areas that have not yet been investigated. Existing in situ stress data in Denmark include borehole breakout data and data from fault-plane solutions from earthquakes (e.g., Gregersen, 1992).

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