Petroleum Geoscience - Volume 4, Issue 2, 1998

Dipmeter data, core data, stratigraphic isochore data and seismic data provide the geologist with several different tools to obtain information on the orientation of bedding. When these data are integrated into a single database, the seismic interpreter can significantly improve the structural control of seismic interpretation by using all of the available information. In addition, any inconsistencies in orientation measurements from one or more data types are easily recognized, and the cause of the mismatch can be further examined. Experiences from the Gullfaks Field, northern North Sea, show that inconsistencies related to core data include non-optimal slabbing of cores, poorly developed bedding and misinterpreting sedimentary features as structural dip. Estimates of the orientation of bedding from stratigraphic isochore/isopach thickness estimates are poorly understood, or if repeated or missing sections related to faulting are not recognized. Bedding orientations from dipmeter data may be erroneous if the quality of the data are poor and the data points show significant scatter. Local variations in bedding orientation may be at a scale that will not be reflected in the seismic interpretation. In addition, in areas of poor seismic quality, seismic noise may interfere with real reflections and cause erroneous interpretation.

An analysis of the formation and distribution of oil pools within the Varandey-Adz'va Zone in the northeast of the Timan-Pechora Basin has revealed differing patterns of oil migration and accumulation in various parts of the area. Oil fields in the north and centre of the Sorokina Swell and in the Gamburtseva Swell are reservoired over a wide stratigraphic ranger: from the Triassic through the Lower Devonian. This suggests the possibility of uninterrupted upward transit of fluids from lower stratigraphic levels (Ordovician-Lower Devonian). The oils found in the Upper Devonian at the Saremboy-Lekkeyaga Swell and in the south of the Sorokina Swell are syngenetic, mixed with oils brought by inflow from underlying strata. Oils in the other complexes are epigenetic. Triassic through Lower Permian strata have not yet entered the main oil window and cannot be the sources of commercial hydrocarbons.

Geostatistical inversion is developed for constraining a 3D geostatistical realization of acoustic impedance using a 3D seismic block. The geostatistical realization is constructed using a sequential variogram-based approach in such a way that the convolutional response of the realization fits with the actual 3D seismic data. The method is illustrated with a 3D synthetic example. The initial acoustic impedance model is generated using an unconditional simulation of one million grid cells. Then, using five wells 'drilled' in this model and the synthetic seismic obtained by convolution, the geostatistical inversion method is applied to reconstruct the initial acoustic impedance model. The 3D synthetic seismic block is matched in a matter of half-an-hour CPU on a standard workstation. Analysis of the results shows that the input geostatistical model (variograms, means and standard deviations) controls the higher and lower frequencies that are not present in the seismic amplitudes. The use of a '3D earth modelling' tool allows efficient management of the '3D earth model' constructed by geostatistical inversion, and visualization of the various inputs and outputs.

In the current debate on the ability of oil emplacement to halt quartz cementation in sandstone oil reservoirs, this paper supports a "yes, it can" answer. Although we find some of the empirical data petroleum fluid inclusion abundances, quartz-depth trends) to be inconclusive, we have made progress in this debate by examining theoretical controls for three end-member quartz cementation scenarios. (1) Silica externally sourced via advection. Oil emplacement must halt quartz cementation because of the associated massive reduction in relative permeability to water. (2) Silica supplied internally, water-wet reservoir. Quartz cementation will be inhibited because the rate of diffusion will be reduced by orders of magnitude, making diffusion the rate controlling step. The degree of inhibition will depend on rock fabric and oil saturation; at very high oil saturations quartz cementation could effectively cease. (3) Silica supplied internally, oil-wet reservoir. Quartz cementation will be halted as water cannot access to grain surfaces.

Barremian-Aptian carbonate sediments in southern Provence belong to a drowning sequence within an intra-shelf basin and display organic-carbon-rich horizons corresponding to the demise of a rudists platform system and the onset of dysaerobic conditions. These horizons depart from the classical anoxic model accepted for most marine organic-carbon-rich deposits. They have a rich and diverse fauna documenting nutrient-rich waters with low oxygen content--an environment in which organic matter is preserved from both biological and chemical degradation. Sedimentological, geochemical and palaeoecological investigations suggest that the organic-carbon-rich carbonates reflect dysaerobic conditions favourable for organic matter preservation, the amount of dissolved oxygen being lower than the geochemical threshold for organic matter decay. These organic-carbon-rich sediments are the result of high sea surface productivity and sea bottom conditions favouring preservation. The kerogen is mainly amorphous sapropelic organic matter, essentially algal, with a high hydrogen index and is of marine origin, deposited during high sea-level.

Natural gas sourced from Carboniferous coal-bearing strata is produced from Carboniferous, Permian and Triassic reservoirs in the Southern Permian Basin in northwest Europe. The composition of this gas has been reviewed. In all reservoirs a distinct area rich in non-hydrocarbons, principally nitrogen, coincides with the basin depocentre in northern Germany. Furthermore, the average nitrogen concentration increases upwards in the stratigraphy. Carbon dioxide is a minor component which is geographically restricted and occurs mainly in Carboniferous and Zechstein reservoirs. It is generated indigenously in these reservoirs and does not migrate into other reservoirs. Hydrogen sulphide occurs only significantly in the Zechstein and is areally restricted to the basin margin facies of this reservoir. It is formed by thermochemical sulphate reduction. The maturity of the source rock, timing of the trap formation relative to the gas source maturation and reservoir lithology appear to be the dominant controls on the non-hydrocarbon gas composition in the Southern Permian Basin. For very high nitrogen gases (50-100 vol. %), an admixture of inorganic nitrogen to the thermogenic gas from organic sources cannot be excluded.

The UK Department of Trade & Industry (DTI) recently sent out a questionnaire (see Appendix) to North Sea operating oil companies to survey the different methods used in the estimation of ultimate hydrocarbon recovery and production forecasting. The feedback revealed significant differences in approach between operators in estimation methods and also a wide range of perceptions of the value and accuracy of ultimate recovery and production forecasts. This paper reports the findings of the survey and also takes a historical look at how accurate ultimate recovery estimates have actually been for UK Continental Shelf oil fields developed over the last decade. Historical reserves estimates have been rather inaccurate, with many fields exhibiting changes in estimated ultimate recovery over a ten-year period of over 50%. These inaccuracies have had a severe effect on facilities requirements for many large North Sea fields. The fields studied have required an average of 60-80% more wells than anticipated at the time of Development Plan submission and up to 400% average increase in platform water handling capacities has been necessary. Average field lifetimes have been over a decade longer than originally expected. Accommodating unexpected changes of this magnitude within existing offshore facilities has proven a very expensive exercise and we therefore suggest there is a need for a more unified approach by the industry to ultimate reserves estimation.

Synthetic samples of different grain size fractions of quartz were prepared and the thermal conductivities measured with a divided bar apparatus. The measurements were carried out on dried and water-saturated samples both perpendicular and parallel to the main stress direction during compaction of the samples. A clear correlation was observed between the porosity and the measured thermal conductivities. For water-saturated samples the thermal conductivity decreases more with increasing porosity than for ideal isotropic material as predicted by the geometric mean model. A clear positive correlation was also found between the thermal conductivity and the grain sizes of the samples. When the expected effect of porosity on the thermal conductivity was excluded, a linear correlation between the logarithm of the grain size and the measured thermal conductivities of the water-saturated samples was found. An anisotropy effect (a = k/k 1 ) in thermal conductivity was observed for the samples, being most prominent for the coarsest ones. The measured anisotropy is assumed to be mainly due to the thermal anisotropic nature of the quartz crystal.

Tertiary deep-water sandstones are major reservoirs in the North Sea, and display an upward gradation from sheet-like bodies to increasingly channelized units. 3D seismic horizon attribute maps at near Top Forties and Top Chalk levels within the Central North Sea portray peculiar textures, which were found to be genuine, and not the result of artefacts caused by the use of autotracking in seismic interpretation. The semi-circular fabric of the near Top Forties event is the result of pre-existing NW-SE structural trends, depositional processes and differential compaction. The pockmark pattern displayed by the Top Chalk horizon is associated with small-scale fractures related to reactivated shear zones and to episodic overpressure release. Use of attribute maps and time slices highlights elusive features, thus refining fairway delineation and understanding of fracture network properties, leading to improved reservoir description and management.