TNO is commissioned by the Dutch Ministry of Economic Affairs to provide free access to public domain information on the subsurface, in order to stimulate the exploration of oil and gas reserves in the Dutch sector of the North Sea. With this in mind, the institute is developing a comprehensive 3D subsurface model. Special attention is being paid as a whole to the reliability of the structural model.
The model encompasses a detailed 3D structural framework, including essential stratigraphic units and related faults, and improves on the initial regional subsurface model 'NCP-1'. Rock and fluid parameters and 3D burial history analysis are being used to perform a petroleum systems analysis. The offshore area is subdivided into seven regions along the outlines of major Mesozoic structural elements. The regions are being modelled separately and will then be combined into one composite model for the entire offshore area. Interpretation of 3D and 2D seismic surveys provides input for mapping the major stratigraphic units and 3D fault systems. Stratigraphic interpretations of available wells aid in the identification of horizons in the seismic data. Faults are only interpreted when they can be traced over a substantial distance and have resulted in a significant offset of the horizons they affect. The compilation of all faults and horizons into a single 3D model is an important and elaborate part of the modelling process.
As the interpretation and modelling proceeds, a more refined insight into the structuration of the offshore is emerging. These new insights are captured in revised structural element maps. Besides the improved outline of major structural elements, the modelling is yielding a higher resolution of intra-element structures. Two important sources of uncertainty in constructing the 3D structural model are the seismic interpretation and the velocity modelling. The reliability of the seismic interpretation depends on the quality of the seismic data, the availability of wells for correctly identifying seismic horizons for interpretation and, of course, the skill of the interpreter. Since the seismic interpretations are being interpolated to surfaces, regions with a low density and irregular distribution of seismic data (e.g., regions only covered by 2D surveys) will be less reliable in areas removed from the data, especially when these regions are characterised by a structurally complex subsurface. Seismic velocities introduce uncertainty because they are based on sparse well data only and have to be interpolated to regional surfaces.
Dissemination and progress
Standard deliverables from the structural model are published per mapping region in NLOG (www.nlog.nl) and comprise maps of depth and thickness for each major stratigraphic unit and several reservoir layers, cross-sections, structural element maps and corresponding tectonostratigraphic charts, and fault maps. File formats comply to industry standards (e.g., Zmap, Esri Arc-Gis, PDF and Excel). For some areas, reports of special studies are published. Areas NCP-2A and NCP-2E are now available on NLOG. NCP-2B and NCP-2C will be published later this year, and NCP-2D, NCP-2F and NCP-2G are planned for 2009 and 2010. The complete article on '3D Subsurface mapping of the Dutch offshore – Results and progress' can be downloaded (PDF).