Pre-Late Cretaceous structural development of the Danish Central Trough

The structural development of the Danish Central Trough area from the Carboniferous to the Early Cretaceous was studied to test how far back in geological time a graben development can be justified or proved by data.

Upper Jurassic dinocyst stratigraphy in the Danish Central Trough

The English dinocyst zonations established by Woollam and Riding (1983) and Riding and Thomas (1988), and the Danish-English zonations of Davey (1979, 1982), are unified for the Danish North Sea area and the subzones of the Endoscrinium luridum Zone are redefined. Two standard sections for the Upper Jurassic are described and correlations to five other wells in the Danish North Sea area are demonstrated.

Diagenetic aspects of Upper Jurassic sandstones in the Danish North Sea sector

Studies of Upper Jurassic sandstone formations, the Heno Formation and the Gert unit, in the Danish North Sea sector have revealed two diverging diagenetic trends, which appear related to sedimentary environments: 1 ) Distal marine sands (lower shoreface to offshore) end up as reservoirs with fair porosities but low permeabilities, 2) Marginal marine sands (beach, barrier-island complex) create reservoirs with fair to high porosities and fair to high permeabilities.

Preface

In June 1989, Dansk Olie- og Gasproduktion A/S (DOPAS) sponsored a well-attended symposium on the Jurassic in the Southern Central Trough held at the DOPAS premises in Hørsholm, Denmark. The background for the symposium was a desire among geologists working in the Dutch, German and Danish sectors of the North Sea to establish a forum to discuss problems of lithostratigraphic correlation across the sector boundaries and eventually arrive at a common nomenclature. A number of lectures were held at the symposium and an informal working group was established to deal with the details of lithostratigraphic correlation. The present publication, sponsored partly by DOPAS and partly by the Geological Survey of Denmark (DGU), contains extended abstracts of the lectures.

Upper Jurassic-Cretaceous stratigraphy of the Vlieland Basin, and adjacent areas, the Netherlands

This contribution presents an extended abstract intended for the proceedings of the symposium: The Jurassic in the southern Central Graben, Hørsholm, (Denmark) June 15-16, 1989. The full text entitled "Tectonostratigraphic development of the Vlieland Basin. The Netherlands" will appear in the Proceedings 1st Conference European Association of Petroleum Geoscientists (Berlin: May 29th - June 2nd, 1989). The Vlieland Basin is located in the northern Netherlands; it covers the NW'ern part of the province of Friesland to the offshore area of the Frisian Island of Vlieland. It is a NW-SE striking structure which is bounded to the W, S and SE by the Texel-IJsselmeer High, and flanked by the Vlieland High, an extension of the Schill Grund High, in the north, and the North Netherlands High in the east. In the Vlieland Basin the major Zuidwal gasfield is located, which recently started production from the Lower Cretaceous Vlieland Sandstone.

Petroleum geology of the Dutch Central North Sea Graben

This contribution presents an extended abstract intended for the proceedings of the symposium: The Jurassic in the Southern Central Graben (Hørsholm, Denmark, June 15-16, 1989). It generally covers the stratigraphic and petroleum geological aspects of the paper entitled: "Late Jurassic" petroleum geology of the Dutch Central North Sea Graben presented by Th. E. Wong, Th. H.M. Van Doorn and B.M. Schroot at the 78th annual meeting of the Geologische Vereinigung (Jiilich, febr. 24-26, 1988). The full text has been published in Geologische Rundschau, 78, 1989, pp. 319-336. The structural and seismic aspects are delt with in detail by B.M. Schroot in the present volume.

Structural development of the Dutch Central Graben

After Heybroek (1975) described the structure of the Dutch Central North Sea Graben and NAM and RGD (1980) published a stratigraphic nomenclature of the Netherlands containing a stratigraphic model for the Jurassic sediments, further work continued, both within the industry and the Geological Survey. Because Herngreen and Wong (1989) set up a revised stratigraphy for the Late Jurassic of the Central Graben and neighbouring areas, it was felt necessary to have a closer look at the structural geology of the Central Graben and to review its hydrocarbon potential. The results of this study have been reported by Wong et al. (1989). This work just about coincided with an increased interest of the industry in the south-eastern extension of the Graben because of the Dutch seventh round of exploration licenses.

Discussion of Jurassic lithestratigraphy in the Danish, Dutsch, and Norwegian Central Graben Areas

Based on recently published data dealing with the Jurassic lithostratigraphy from the Danish, Dutch, and Norwegian sectors of the Central Graben areas, it seems possible to establish a more consistent and dynamic stratigraphic scheme for these regions. A preliminary proposal of a south-north lithostratigraphic section, from the Dutch to the southern Norwegian sector is discussed. It is suggested to initiate an international cooperation in order to study the common geological features in detail.

Biostratigraphy of the Jurassic strata in the Dutch Central North Sea Graben

This contribution presents an extended abstract intended for the proceedings of the symposium: The Jurassic in the southern Central Graben, Hørsholm (Denmark) June 15-16, 1989. The full text entitled "Dinoflagellate, sporomorph, and micropaleontological zonation of Callovian to Ryazanian strata in the Central North Sea Graben, The Netherlands" has been published in the proceedings of the 2nd International Symposium on Jurassic Stratigraphy, Lisbon 1989, p. 745-762.

Geochemistry of the Upper Permian Ca-2 Deposits of the Løgumkloster-1 Well, South Denmark

Rock samples of the Upper Permian Ca-2 unit of the Løgumkloster-l well have been subjected to chemical analysis. The Ca-2 interval was undoubtedly originally an aragonite which have undergone several episodes of diagenesis including a dolomitization and a heavy anhydrite mineralization. Two original facies types are recognized, an oolitic shoal facies, and a lagoonal carbonate facies. The samples have been characterized by neutron activation, atomic absorption, Xray diffraction, palaeomagnetism, optical and electron microscopical examination. Samples of the drilling mud has been analyzed as well, and it is found that contamination of the samples included in the present study from the drilling mud is unlikely to have happened. Imprints of both the original facies and the diagenetic events are found in the present work. The Sr and Ba abundances reflect the original facies-bound distribution, which is also reflected in the normalized REE distribution patterns. A low-Sr anhydrite component is clearly distinguishable in several parameter cross plots. Iron, Sc, REE and several other elements are found to be situated in the EDTA-insoluble residue, which has been identified primarily as a muscovite or clay mineral phase. Pyrite is present, although of small relative volume it is thought to be responsible for the occurrence of As, Sb, and possibly Se. A small fraction of the Cr, As, and Sb may originate from the heavy part of the crude oil, the introduction of which was the last process of alteration the rock experienced. A geomagnetically reversed period is encountered in the middle of the Ca-2 interval, and is tentatively identified as one of the Tatarian reversed events.