Petroleum Geoscience - Volume 2, Issue 4, 1996

Palynofacies characterization employing an innovative technique was successfully conducted on the sand-dominated lower Brent interval of the Ninian Field, which had previously been described as 'palynologically barren', and was used to aid in geosteering the field's first horizontal well. Pre-planning for the well included analysis on the cored interval of a neighbouring well where fifteen potential palynofacies/palynological events were recognized. All were distinct and consistent with the zonal breakdown and sedimentological description for the interval studied. The technique has been subsequently used at the well-site, in conjunction with Anadrill's GST (Geosteering Tool), to successfully maintain the well within its target window and close to its intended well path. More recently the technique has been used to successfully re-appraise and improve the stratigraphic understanding of wells within the field's structurally complex degradation sheet.

Compressional structures of Cenozoic age are ubiquitous features of the NE Atlantic margin between the western Barents Sea and offshore western Ireland. The structural suite includes simple domes or anticlines, reverse faults and broad-scale inversions. Our analysis focuses on a recently delineated group of structures in the Norwegian Sea, although these are placed in the context of similar features on the Barents margin, West of Shetlands, on the Faroes and their surrounding shelf, and in the Rockall Trough. Some (although not all) of the compressional anticlines were formed at the sites of pre-existing Cretaceous-Palaeocene depocentres. They show a multi-phase growth history. In the Norwegian Sea, particularly important phases occurred in the middle Eocene to early Oligocene and in the Miocene. We interpret the formation of these structures as a natural outcome of the transition to sea-floor spreading that occurred in the early Eocene. From this time, extremely thick sedimentary successions that had accumulated during some 300 million years of extensional tectonics were subjected to mild compression. The overall compressive stress field can be explained in terms of spreading in the adjacent ocean (ridge push) and by the distant effects of Alpine tectonics. In a plate-wide sense these effects can be regarded as two sides of the same coin. The origin of the Norwegian Sea structures is most easily visualized in terms of ridge push. NW-SE transfer zones, characteristic of the entire margin, are strongly implicated in these tectonics. A kinematic model is described that links significant structuring with a change in the spreading direction of Oligocene-Miocene age (35-20 Ma, A13-6). The compressional structures are mainly observed by their effect on sediments and volcanics of Cretaceous and Cenozoic age. They are frequently expressed in the present-day sea-floor relief, and in the case of the Faroe islands are probably responsible for present subaerial exposure. From the point of view of hydrocarbon exploration, the Cenozoic compressive anticlines have obvious potential as fourway dip closures or as components of structural-stratigraphic traps. The NW-SE fractures, orientated parallel or sub-parallel to the maximum horizontal stress direction, were probably periodically open for fluid flow from the time of NE Atlantic opening and onwards. They may therefore have facilitated migration from deeper source rocks or remigration from pre-existing hydrocarbon accumulations.

This paper presents models for reservoir quality prediction in the dominantly fluvial reservoir sandstones in the Lower Jurassic Statfjord Formation in the North Viking Graben. The models are based on theory and on empirical observations from wells from the Norwegian sector of the North Sea. At depths shallower than 3000 m, porosity and permeability in the Statfjord Formation sandstones show systematic trends versus burial depth and the net-togross ratio is mainly a function of the amount of sand deposited. Hence, confident reservoir quality prediction at shallow burial may be founded on regional or subregional models that address sandstone architecture and on regional or sub-regional porosity and permeability versus depth trends. Beneath 3000-3500 m, however, the reservoir quality is more variable due to variations in porosity and permeability reduction following compaction, quartz cementation and formation of fibrous illite. Between 3000 and 4000 m, formation of quartz cement and fibrous illite reduce the permeability and the net-to-gross ratio. Accordingly, permeability lower than 1 mD and net-to-gross ratios less than half the initial sandstone content are expected in many prospects below 4000 m. In some deeply buried reservoirs, porosity loss by quartz cementation and compaction is retarded by chlorite coatings or by high pore pressure. Good permeability may be preserved to depths greater than 4000 m if the porosity is preserved (i.e. by a high overpressure or chlorite coatings) and if illitization is hindered by limited potassium supply.

A new weak gel consisting of a non-ionic, high molecular weight polysaccharide and an organometallic activator was developed for harsh reservoir conditions. A first field application was performed on a candidate well in an oil field in the German North Sea tidelands in June 1992. The treatment had to be designed for the entire range of the perforated interval. The polymer solution was readily injectable and the activator was added to the last quarter of the injected volume. The treatment was considered successful as the initial increased water-cut could be maintained at a level of 65%, the overall production rate being comparable to that before treatment. However, the desired reduction in water production was not achieved, presumably because formation permeability was higher than estimated. A laboratory post-implementation evaluation was conducted to find out whether a possible further treatment with a stronger gel might be feasible. The results indicate that formulations yielding stronger gels would still be selective and easily injectable and would show a far greater water control potential.

In southern Italy the Apenninic-Maghrebian Orogen developed during the Neogene Africa-Europe collision as a regional-scale duplex structure. It shows an allochthonous hangingwall, widely overlying a mostly carbonate footwall, consisting of the External Thrust System. This formed since the Tortonian by the detachment of the sedimentary covers of the continental margins of the Africa and Adria plates. In the Southern Apennines the duplex geometry has favoured oil-trapping and fields have been detected in ramp-anticlines of the buried edifice. Similar geometries have been inferred in eastern and central Sicily. The re-interpretation of regional profiles in Sicily based on new field data defines deep-seated geometries showing several potential oil traps and the External Thrust System could represent a new target for petroleum exploration in the region.

Results from laboratory PVT measurements, special core analysis studies, and reservoir condition core-flood studies were used to develop and test the accuracy of a compositional simulation model in predicting near-multicontact miscible gas flood performance at the laboratory scale. Excellent agreement was obtained between the core-flood measured recoveries and GOR and the compositional simulation model predictions. No history matching was required to obtain this result. The choice of PVT and special core tests were critical to our ability to model the near-multicontact miscible displacement process without history matching. This result provides a high degree of confidence in using the compositional simulation model in cross-section and 3D element models to predict field-scale performance. Cross-sectional simulation sensitivities were performed in a geostatistical model of a dipping reservoir. Gas-flood recovery was on par with water-flood recovery for this cross-section.

The Rockall Trough is a large, lightly explored, deep water basin offshore West of Britain. Only the 132/15-1 well has penetrated Mesozoic section within the Rockall Trough encountering a syn-rift section, interpreted on seismic reflection data, of early Cretaceous age (Hauterivian-Cenomanian) lying directly on crystalline basement. Older rift events have been proposed in the Rockall Trough to explain the extensive crustal thinning (Beta = 3 to 5) and because of the presence of older rifts on adjacent basins. Although the evidence is not conclusive, analysis of seismic reflection data within the Rockall Trough and of rift system trends on an early Cretaceous plate reconstruction do not require or suggest older than Cretaceous rifts in the Rockall Trough but also do not preclude them. Structurally, the Cretaceous rifting is highly assymetric being dominated by faults downthrowing to the southeast. A model of a reactivated Caledonian foreland thrust belt is proposed similar to that reported on the adjacent Hebridean shelf.

During 10 years of production, crestal fluid pressures in the Magnus Field (UKCS) have dropped from 6650 psi to under 3500 psi leading to a two- to three-fold increase in the effective stress on the rock fabric. Petroacoustic measurements on cores indicate that this leads to a 12% increase in the acoustic impedance which, in theory, should be detectable using time-lapse seismic data. We analyse the difference between two 3D surveys shot over Magnus and relate this to dynamic changes in fluid pressure and saturation through time. The results demonstrate that, under certain conditions, fluid pressure changes may be detected in the reservoir and that fluid transmissibilities across faults can be deduced. Further, 4D seismic data may detect other dynamic processes, including thermal effects and cold water fracturing around injectors, and stress relaxation and fluid compositional changes around producers.

The Fife Field is located in the far southeastern part of the Central North Sea Basin close to the UK, Norwegian, Danish median line. The field is a shallow relief four-way dip closure formed by inversion during Late Cretaceous/ early Tertiary times. The reservoir consists of thick Upper Jurassic, heavily bioturbated sandstones which are considered to have been deposited in a similar setting to the Fulmar Formation. The depth to the top of the Upper Jurassic at the crest of the field is 8250 ft sub-sea with the oil-water contact at 8512 ft sub-sea. The seal to reservoir is provided by Volgian-Ryazanian shales of the Kimmeridge Clay Formation and Upper Cretaceous Chalk. Although Jurassic sandstones form the primary reservoir, additional hydrocarbons have been encountered in the Tor Formation of the Chalk Group which is fractured over the crest of the field. The Fife Field was discovered in 1991 and is currently under development. Production started in August 1995 via the 'Uisge Gorm' Floating Production Storage and Offloading facility (FPSO). STOIIP is estimated at 132 x 106BBL and ultimate recovery is predicted to be 34 x 106 BBL oil. The low mobility of the oil and the low vertical permeability of the reservoir contribute to the predicted low (26%) recovery efficiencies.