No description available.

No description available.

No description available.

No description available.

No description available.

High precision oxygen isotope analyses of quartz cements, in combination with quantitative petrography and Quantitative X-Ray Diffraction (QXRD) as well as timetemperature history modelling, were employed to constrain the timing and source of porosity reducing quartz cement in the Jurassic Ness Formation (North Sea) and the Palaeocene Wilcox Group (Texas). Temperature history modelling indicates maximum burial temperatures in the chosen Ness samples to be 109 to 165oC and 27 to 210oC in the Wilcox samples. In both sample areas the volume of quartz cement is controlled by temperature and quartz surface area. The volume of quartz cement recorded ranges from 1.3 to 22.5 %bv in the Ness Formation and from 4.4 to 18.8 %bv in the Wilcox sandstone. With the notable exception of the hottest Wilcox samples (>143oC), cement volume increases with temperature in both basins. In situ Secondary Ion Mass Spectrometry (SIMS) was performed at a spatial resolution of 12 μm and 2 μm on quartz overgrowths. δ18O(cement) measurements ranged from +27.7‰ to +19.3‰ in the Ness and +28.5‰ to +18.3‰ in the Wilcox. Precision for the 12 μm and 2 μm analysis was better than ±0.27‰ (2SD) and ±0.67‰ (2SD) respectively. δ18O(cement) measurements indicate that the initial 8% of cement in both the Ness and Wilcox sandstones precipitated below ~50oC. The remaining 90% of cement observed in both basins precipitated above 60-80oC in diagenetically evolving waters where δ18O becomes heavier during burial. In all cases cementation appears to continue through to maximum burial temperatures, from 60 to 143oC and can be accurately predicted using current cementation models. However, δ18O(cement) measurements in Wilcox sandstones sampled above 143oC indicate that cementation may be inhibited at high temperatures as a response to the late precipitation of diagenetic minerals such as illite and ankerite which act to reduce available quartz surface area. Within the quartz cementation window (50-143oC), silica producing reactions including the illitisation of kaolinite and smectite as well as the dissolution and albitisation of feldspars have been recorded in shale units adjoining both the Ness and the Wilcox sandstones. However, mass balance calculations indicate that insufficient silica is produced within these shale units to account for the total volume of cement quantified in the sandstone. These findings indicate that the majority of quartz precipitates above 60oC and is sourced internally within the sandstone body.

The importance of shales and mudstones to applied geosciences and in particular to fluid migration in sedimentary basins has never been more recognized than today. Prominent examples are conventional or unconventional petroleum systems, where shales and mudstones act as source, reservoir or cap rock, but also CO2 and nuclear waste storage or hydrogeology. Despite their importance, shales and mudstones are yet not as far well understood as sandstones or carbonate rocks. In particular, the influence of heterogeneity on fluid migration has been poorly addressed in the past, although many authors have identified and studied heterogeneities in shales and mudstones. Nevertheless, their flow properties are fairly well understood when treated as homogeneous on sample scale (centimetre-scale). Typical flow relevant heterogeneities are grain size and thus petrophysical property (e.g. porosity, permeability, capillary entry pressures) variations due to spatial lithological variation induced by primary and secondary sedimentary structures. In this study we investigate flow relevant heterogeneities of shales and mudstones on submetre scale derived from core and borehole images from an off-shore gas field in the Western Nile Delta, Egypt. Thereby, we combine latest models and published measurements of sample-scale petrophysical properties with interpretation, quantitative analyses, advanced modelling and numerical fluid flow simulation to assess the influence of shale and mudstone heterogeneity on fluid flow and hence, fluid migration, retention and mudstone seal capacity. Additionally, the set of mudstone heterogeneities used in this study has been derived from a combined visual and geostatistical interpretation of more than 500 m of mud-rich core and borehole images. As final results, we deliver stress-dependent effective porosity-permeability relationships for a broad range of shale and mudstone heterogeneities, representative model sizes and resolution as well as measures of uncertainty for each heterogeneity type. Moreover, probability density functions describing where and how these heterogeneities appear in larger scale geological units, such as seismic facies or local depositional environments, are provided. As a key result, heterogeneity and lithological variation have great influences on effective permeability and effective permeability anisotropy (Kh/Kv). Furthermore, our results indicate that mudstone heterogeneity is very common in all investigated larger scale geological units (hemipelagites, levees, channels). Modelling of fluid flow through mud-rich sedimentary basins without inclusion of these sub-metre scale heterogeneities of mudstones can therefore lead to misleading results. Thus, effective porosity-permeability (anisotropy) relationships are provided for different lithological variations and mudstone heterogeneities as a final result.

Siliceous spicules from the extinct sponge Rhaxella are found within the Mid to Late Jurassic Fulmar Formation. Rhaxella palaeoecology and palaeobiology are investigated in this study, along with the controls on Rhaxella sponge distribution. The way in which the presence of Rhaxella spicules affects both diagenesis and reservoir quality is also analysed. Spicules are observed in depositional settings ranging from the shoreface to the offshore transitional zone. Sponge colonies originally grew in shoreface settings. The spicules found in more distal environments represent re-worked, allochthonous accumulations. At the shoreface the sea was warm and contained low amounts of suspended argillaceous material. Energy levels were low, but substantial enough to provide a constant supply of nutrients to the filter feeding sponges. Rhaxella sponges first colonised the Outer Moray Firth area during the Callovian, then spread south through the North Sea into the northern margins of the Tethys Ocean. This expansion was followed by a retreat back into the North Sea Central Graben. Volcanic activity and water depth are proposed as important controls on the distribution of sponge colonies. Elevated silica levels were used by Rhaxella sponge colonies in the construction of their skeletons. Three of the wells studied display a trend between relative sea level and spicule volume. It is found that spicules are more commonly observed in shallower-water regimes, and are less common in sediments deposited in deep-water settings. The quantity of authigenic silica is highly variable throughout the study wells, but statistical analysis indicates a strong positive correlation between the abundance of intergranular silica cement and siliceous spicules. Seven types of silica cement have been identified, with two diagenetic pathways for silica cementation being recognised. It is proposed that the initial volume of spicules has a controlling influence in the type of silica cement that developed. Reservoir quality is significantly improved by the occurrence of spiculitic moldic porosity in all of the studied wells. The occurrence of spicules may however, also have detrimental effects on reservoir quality. Results presented in this thesis suggest that reservoir quality has been significantly reduced throughout intervals that exhibit abundant preserved spicules (>20% total volume), coupled with a low volume of 'spiculitic moldic porosity'. In such units all moldic porosity has been occluded and pore throats are significantly blocked, lowering permeability.

No description available.

Overpressure estimation methods that use sonic velocity as a proxy for porosity only account for excess pressure due to disequilibrium compaction; the influence of unloading processes in generating larger excess pressure observed in most basins is ignored. Wireline log data and pore pressure measurements from wells across the Central Graben and the East Shetland Basin, North Sea, have been used to find out whether velocity is sensitive to the contribution of unloading processes to observed overpressures. The approach was to focus on fine-grained sediments, chalk and mudstones, and establish a relationship between sonic velocity and other petrophysical parameters, necessarily including porosity and vertical effective stress, when the latter variables are treated as independent. Investigation of the Chalk in the Central Graben has shown that velocity has no detectable dependence on vertical effective stress when porosity and effective stress are treated as independent variables. The significance is that velocity in Chalk cannot be used to detect the presence of any overpressure caused by unloading. It is suggested that the absence of an observable velocity reduction in unloaded Chalk is due to cementation. Analyses in the Lower Cretaceous and Jurassic mudstones show that gamma ray count and depth can usefully be taken as additional parameters in overpressure estimation. In both the Cromer Knoll and in the Heather formation, there is a small but significant dependence of velocity on vertical effective stress when porosity and effective stress are taken as independent variables together with gamma ray count and depth. The sensitivity factor is 21.8 m/s/MPa in the Cromer Knoll and 17.4 m/s/MPa in the Heather. The contribution of the vertical effective stress with associated independent variables (gamma-ray and depth) produced RMS errors between measured and forward-calculated values of sonic velocity of 101 m/s for the Cromer Knoll and 107 m/s for the Heather Formation. The discrepancies may be attributed to the contributions of other rock parameters that were not taken into account.