Roel Verreussel

Integrated chronostratigraphy of the Pliocene-Pleistocene interval and its relation to the regional stratigraphical stages in the southern North Sea region

Time-stratigraphic interpretations of Late Pliocene to Early Pleistocene sediments from onshore locations and from marginal marine settings of the North Sea Basin often refer to the subdivision of the Dutch and British ‘Quaternary’ regional stratigraphic stages. Since age control for these stages and their stage boundaries are based on relative dating methods, in this study pollen, dinoflagellate cysts and foraminiferal assemblages were investigated to correlate the regional stratigraphic stages independently to the global chronostratigraphy and the paleomagnetic timescale. The data were obtained from eight boreholes located in the depocentre setting of the Late Pliocene North Sea Basin comprising a 1000 m thick sedimentary succession. The British Gedgravian and Waltonian stages, the Dutch Reuverian to Brunssumian as well as published foraminiferal zones (NSB 14, FB and the lower part of the FA2 zone) fall within the Zanclean and Piacenzian. The lower boundaries of the Pre-Ludhamian and Pretiglian stages and of the NSB 14 to 15 zones are close to the paleomagnetic Gauss-Matuyama boundary. The Pre-Ludhamian, Ludhamian, Thurnian and the Pretiglian, Tiglian A and Tiglian B stages presumably cover the marine isotope stages 103 to 95. It is proposed that the Ludhamian, Thurnian and the Tiglian A were short lasting, warm, periods during which sea level highstand facilitated sedimentary deposition at the marginal areas of the North Sea Basin. The lower boundary of the paleomagnetic Olduvai subchron is situated in the Tiglian C1-4b stage while the TC4c stage is found within the Olduvai subchron. Foraminiferal NSB 15 and NSB 16 zone as well as the upper part of the FA2 and FA1 zone fall within the Gelasian and cover the Matuyama chron as well as the lower part of the Olduvai subchron. Comparison with formerly dated North Sea sediments shows a good agreement between foraminiferal zonations on a broader scale but significant differences in absolute ages occur. Strontium isotope values indicate approximately 1 Ma younger ages as expected from our chronostratigraphic model. This discrepancy is explained by the dominance of freshwater from river discharge contributing high amounts of eroded material to the basin, leading to an increase of the 87 Sr/ 86 Sr ratio in the shelf-sea water.

Revision and update of the Callovian-Ryazanian Stratigraphic Nomenclature in the northern Dutch offshore, i.e. Central Graben Subgroup and Scruff Group

Exploration in a mature basin requires a detailed classification and standardisation of rock stratigraphy to adequately comprehend the depositional history and prospect architecture. The pre-Quaternary Stratigraphic Nomenclature of the Netherlands compiled by Van Adrichem Boogaert &Kouwe in 1993 provided a consistent framework for use by the Dutch geological community. Over the past twenty years, new biostratigraphic techniques and continued exploration in the Netherlands have provided additional stratigraphic information. Based on this information the Late Jurassic lithostratigraphy in particular, shows significant inaccuracies. The Callovian-Ryazanian strata from the northern offshore of the Netherlands’ territorial waters, termed the Central Graben Subgroup and Scruff Group, reveal a complex sedimentary history. The combination of non-marine to shallow marine lateral facies changes, repetitive log and facies characteristics in time, sea-level and climate change, salt tectonics and structural compartmentalisation hamper straightforward seismic interpretation and log correlation. Recognition of three genetic sequences by Abbink et al. in 2006 enabled an improved reconstruction of the geological history. Further improvements in refinement and reliability of the stratigraphy together with new information on the facies and ages of the successions and about the subsequent tectonostratigraphic development of the northern Dutch offshore area form the basis of the present revision. As a result, earlier lithostratigraphic models have been changed and new lithostratigraphic relationships and names are introduced in this paper.

Sweet Spot Identification and Smart Development - An Integrated Reservoir Characterization Study of a Posidonia Shale of a Posidonia Shale Outcrop Analogue

Shale gas reservoir stimulation procedures (e.g. hydraulic fracturing) require upfront prediction and planning that should be supported by a comprehensive reservoir characterization. Therefore, understanding shale depositional processes and associated vertical and lateral sedimentological variability is key in predicting the character and position of sweet spots and to estimate their geomechanical properties. The Toarcian Posidonia Shale Formation is considered to be the main shale gas prospect in the Netherlands. However the scale at which reservoir heterogeneities occur is unknown and not easily estimated from the limited subsurface datasets. Therefore, a multidisciplinary reservoir study was performed on an outcrop analogue in the Cleveland Basin (UK), including sedimentological, biofacies, petrophysical and geomechanical analysis. The results help developing predictive models of sweet spot detection and allow to generate pseudo logs that can be compared with surface data. In spite of the apparent mudstone uniformity at outcrop and hand samples it is clear that the Toarcian succession presents complex patterns of heterogeneity at mm- to outcrop scales. Climatic control on run off and the input of riverine material and coinciding changes in production of organic material appear the main controls on the distribution of sweet spots and zones of high fraccability.

Late Jurassic Rifting in the Southern Central Graben - A Complex Story Simplified

In order to be able to predict the distribution of reservoir sands and of grainsize and porosity trends in a hydrocarbon province, it is essential to understand the basin evolution in detail. In this study, an attempt is made to reconstruct the complex basin evolution of the southern Central Graben area by careful correlation of sedimentary successions from the different structural elements. The Late Jurassic rift phase is complex: a change in tectonic regime occurs, subsidence varies dramatically through time and space and former depocenters may become sources for erosion. In this paper a break down is proposed for the basin evolution in three discrete steps, each of which is closely related to changes in the tectonic regime. The basin fill of the southern Central Graben area is displayed in Wheeler type of diagrams, showing the facies relationships between the various parts of the basin.

Integrated Reservoir Characterization of a Posidonia Shale Outcrop Analogue: From Serendipity to Understanding

Shale gas reservoir stimulation procedures (e.g. hydraulic fracturing) require upfront prediction and planning that should be supported by a comprehensive reservoir characterization. Therefore, understanding shale depositional processes and associated vertical and lateral sedimentological variability is key in predicting the character and position of sweet spots and to estimate their geomechanical properties. The Toarcian Posidonia Shale Formation is considered to be the main shale gas prospect in the Netherlands. However the scale at which reservoir heterogeneities occur is unknown and not easily estimated from the limited subsurface datasets. Therefore, a multidisciplinary reservoir study was performed on an outcrop analogue in the Cleveland Basin (UK), including sedimentological, biofacies, petrophysical and geomechanical analysis. The results help developing predictive models of sweet spot detection and allow to generate pseudo logs that can be compared with surface data. In spite of the apparent mudstone uniformity at outcrop and hand samples it is clear that the Toarcian succession presents complex patterns of heterogeneity at mm to outcrop scales. Climatic control on run off and the input of riverine material and coinciding changes in production of organic material appear the main controls on the distribution of sweet spots and zones of high fraccability.