The influence of the Gronau and Boekelo faultzones on the Jurassic and Lower Cretaceous geology of SE Twente.

Abstract

The characteristics of the overburden are important in the determination of the inherently safe height of the salt caverns in southeast Twente, as well as important for method of abandonment of these wells. Thickness and strength of the different geological units are the two most important characteristics. Using seismic, gamma ray data and lithology the properties of Jurassic and Lower Cretaceous sediments have been determined as well as the influence of the Boekelo and Gronau fault zones. The shallowing of the stratigraphy towards the east, and local lows directly affect the thickness of the overburden. The current strength measurement of 15 MPa is very unlikely representative for the Jurassic and Lower Cretaceous sediments. Characteristics are a result of the depositional environment and tectonics. Both the Boekelo and the Gronau fault zone have been a major influence on the regional stratigraphy. During the deposition of the Niedersachsen sediments Europe and America started breaking apart to create the Atlantic Ocean, giving the Niedersachsen deposits their syn-rift nature. This is also likely to be the reason for its heterogeneic character, as the extension created local depocentres. The Altena group was deposited pre-rift in the large Permo-Triassic basin. The limited appearance (both in area and thickness) of these deposits are a result of the erosion that followed the Cretaceous (Sub-Hercynian) inversion.