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Abstract Understanding the shallow gas system is critical for assessing its potential as an energy source, for evaluating the possible hazard of shallow gas for drilling and wind farm locations and for evaluating the effect of gas emissions at the seabed on marine ecosystems and climate. This paper presents the key elements and processes of the microbial shallow gas system in the Plio-Pleistocene Dutch Southern North Sea Delta based on recent findings from different projects. Geochemical and carbon isotopic composition of shallow gas occurrences in the delta are indicative of a microbial origin of the gas. Shallow gas mainly occurs in stratigraphic traps and stacked anticlinal structures above salt structures, as indicated by identified direct hydrocarbon indicators on seismic, such as bright spots. Organic matter in the delta deposits is of predominantly land plant origin with TOC values varying between <1% and 5%, and mostly between 1 and 2%. Simulations of temperature and burial history in combination with dedicated 1D simulations of microbial gas generation reveal that gas generation in the delta started in Early Pleistocene Calabrian times and is still ongoing. Simulated volumes of gas generation are more than enough to fill published estimated volumes of shallow gas prospects in the delta. The geometry of the E-W prograding delta sequences and the close interbedding of interglacial silty/sandy and glacial clayey/silty sediments focus gas migration updip through the foresets towards the topsets of the delta sequences, and ultimately into the anticlinal stacked traps. Grain-size based calculations of the permeability and capillary seal capacity of clayey/silty seals of the stratigraphic and anticlinal traps provided first estimates of permeability values ranging from 2.8E-20 m2 to 1.1E-18 m2 and capillary seal capacity values between 10m and 24m. Comparison of gas column heights derived from grain-size based calculations, cross plots of neutron and density logs and pressure measurements, with trap heights derived from seismic bright spots suggests that many traps in stacked bright spots are not filled to structural spill point. This suggests that filling of the stacked reservoirs is not related to fill-spill migration, but rather to leakage through the top seal. In absence of fault and fracture zones crossing the seal, this leakage is related to the capillary seal capacity and permeability of the top seal. The leakage extends, locally, to the seabed. The microbial gas system in the delta today is a highly dynamic system driven by ongoing burial of the delta sediments and microbial gas generation. Highlights • Integrated evaluation of all key elements and processes of the microbial system. • Modelling results of microbial gas generation. • Evaluation of permeability and capillary seal capacity of intra-delta clay/silty layers. [ABSTRACT FROM AUTHOR]

The principal vertical stress at a certain depth is equivalent to the weight of the overburden (rocks and pore fluids) at that depth. Vertical stress trends will be similar across areas with a simple layer-cake lithostratigraphic sequence and similar burial history. These conditions are not fulfilled for the lithostratigraphy and burial history of the onshore and offshore Netherlands. Burial histories vary over short distances and resulted in highs, platforms and (inverted) basins with different lithostratigraphic sequences and porosity distributions. Density log measurements and basin modeling results were used to assess the spatial variation of vertical stress in the Dutch subsurface. This paper presents the approach and assessed spatial variation in vertical stress. The 3D basin modeling proved to be a good tool for correcting and extending density log derived vertical stress-depth relations. Results show that the vertical stress varies spatially and that the vertical stress gradient is less than the standard gradient in large part of the area. The largest local deviations from the standard gradient occur in areas with a thick package of low density Cenozoic sediments and large low density salt structures, i.e. in the northern offshore. Relatively old and more dense stratigraphic units occur at relatively shallow depth on the highs and in strongly inverted parts of the Mesozoic basins. In these areas the vertical stress reaches and follows the standard vertical stress gradient at shallower depths than in other areas. [ABSTRACT FROM AUTHOR]

Data analysis in combination with 2D basin modelling is used to study the hydrogeologic and hydrodynamic response of the basin fill of the Broad Fourteens Basin to its geodynamic evolution and the significance of this response for the evolution of the petroleum systems in the basin. Data analysis resulted in the identification of the major mechanical processes involved in overpressure and fluid flow evolution in the basin (sedimentary loading and unloading, tectonic compression and extension, topography of the groundwater table) and provided a timeframework for the important permeability alterations in the basin and the distinct phases of fluid flow. Observed present-day diagenetic features and fracture characteristics in the basin indicate permeability alterations and paleo fluid flow during distinct periods of tectonic activity in the basin, e.g. during Late Jurassic rifting and Late Cretaceous inversion. The basin boundary fault zones were repeatedly reactivated during the periods of tectonic activity. The 2D basin modelling included sedimentary loading and unloading, topography of the water table and petroleum generation as pressure and fluid flow influencing mechanisms. In addition, the influence of time-dependent permeability of fault zones on the overpressure history and the evolution of petroleum systems was modelled and evaluated. The favoured modelling scenario, including a time-dependent permeability of faults, shows that syn-rift and post-rift sedimentation in combination with gas generation induced overpressured conditions in restricted parts of the basin, that is mainly in the deeper low permeable Carboniferous shales and Zechstein evaporites. The calculated pre-inversion overpressured conditions disappear during subsequent inversion of the basin. Predicted present-day pressures in the reservoir horizons are near-hydrostatic and reasonably in accordance with observed pressures. The favoured scenario also explains the known location of oil fields, and the location and saturation characteristics of the gas occurrences. The model results further show that the difference in timing of oil charging of the P9 and Q1 Lower Cretaceous reservoirs explains the observed differences in geochemical composition of the accumulated oils. The biodegraded and water-washed nature of the Late Cretaceous charged Q1 oil reservoir is consistent with the concentrated topography-induced groundwater flow through Lower Cretaceous reservoir units during the Late Cretaceous inversion period. Late charging of the Lower Cretaceous oil reservoir in the P9 area explains the non-biodegraded character of the accumulated oils. In general, this paper shows the importance of using data analysis to obtain an a priori characterization of the basin geofluid evolution and shows that paleohydrodynamic conditions and time-dependent permeability of the hydrogeological framework have influenced significantly the petroleum systems in the basin. [Copyright &y& Elsevier]