Alf Ryseth

High-Resolution Stratigraphy and Seismic Attribute Mapping of a Fluvial Reservoir: Middle Jurassic Ness Formation, Oseberg Field

Continental successions of alternating fluvial reservoir sandstones and fine-grained interfluvial deposits make hydrocarbon production planning and reservoir management difficult. The vertical and lateral distribution of fluvial sandstones within the surrounding nonreservoir rocks, in particular, is a critical factor that needs to be investigated before production or injection wells can be successfully drilled. In the Oseberg field of the Norwegian North Sea, about 21.7 million Sm3 (standard cubic meters) of recoverable oil is stored within fluvial channel sandstones of the Middle Jurassic Ness Formation. The Ness Formation reservoir is characterized by a relatively low proportion of good-quality reservoir rocks (approximately 30% of the total rock volume). Sedimentological studies indicate that reservoir sandstone bodies are developed mainly as elongate, laterally restricted sandstone bodies concentrated within certain stratigraphic levels and geographic areas. Identifying and accurately mapping these sandstone-rich intervals are important in defining possible drilling targets. The distribution of reservoir rock within the upper Ness Formation has been the focus of detailed geological and three-dimensional (3-D) seismic mapping. Inverted seismic data, integrated with geological well data, give an empirical correlation between acoustic impedance and sandstone proportion. In turn, these relationships provide the basis for defining areas with a higher probability of encountering reservoir sandstone. Subsequent production drilling into the Ness Formation has confirmed the existence of reservoir sandstones within areas defined by the integration of high-resolution stratigraphic mapping and the geophysical analysis. The integration of sedimentological and 3-D seismic techniques presently offers the best potential for defining drilling targets within the Ness Formation in the Oseberg field.

Reservoir quality and burial diagenesis in the Statfjord Formation, North Sea

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.

Uncertainty in well test and core permeability analysis: a case study in fluvial channel reservoirs, northern North Sea, Norway

Reservoir permeability is one of the important parameters derived from well test analysis. Small-scale permeability measurements in wells are commonly made using core plugs or, more recently, probe permeameter measurements. Upscaling of these measurements for comparisons with the permeability derived from drill stem tests (DSTs) can be completed by statistical averaging methods. DST permeability is commonly compared with one of the core plug averages: arithmetic, geometric, or harmonic. Questions that commonly arise are which average does the DST-derived permeability represent and over what region is this average valid? Another important question is how should the data sets be reconciled where there are discrepancies? In practice, the permeability derived from well tests is commonly assumed to be equivalent to the arithmetic (in a layered reservoir) or geometric (in a randomly distributed permeability field) average of the plug measures. These averages are known to be members of a more general power-average solution. This pragmatic approach (which may include an assumption on the near-well geology) is commonly flawed, owing to several reasons that are expanded in this article. The assessment of in situ reservoir permeability requires an understanding of both core (plug and probe) and well test measurements in terms of their volume scale of investigation, measurement mechanism, interpretation, and integration. This article presents a comparison of core and well test measurements in a North Sea case study. We undertook evaluation of three DSTs and associated core plug and probe data sets from Jurassic fluvial channel sandstones in a single field. The well test permeabilities were generally found to differ from the core estimates, (Begin page 1930) and no consistent explanation could be found for the group of wells. However, the probe permeameter data were able to further constrain the core estimates. This study highlights the uncertainty in effective in situ reservoir permeability, resulting from the interpretation of small (core) and reservoir (well test) scale permeability data. The techniques used are traditional upscaling combined with the Lorenz plot to identify the dominant flowing interval. Fluvial sandstones are very heterogeneous, and this exercise is instructive in understanding the heterogeneity for the guidance of reservoir models in such a system.

Alluvial architecture and differential subsidence in the Statfjord Formation, North Sea; prediction of reservoir potential

Statfjord Formation sandstones (Rhaetian-Sinemurian) form important reservoirs in the North Viking Graben, and are still a target for exploration drilling. Sedimentological analyses from the Tampen Spur and Horda Platform show that the reservoir potential is controlled by the distribution, density and stacking pattern of multistorey/multilateral fluvial channel sandstones within successions of interbedded fluvial and interfluvial deposits. Formation thicknesses vary from about 50 m to more than 500 m within the study area. Empirical relationships linking the sandstone content to succession thickness, show that the proportion of channel deposits varies systematically with thickness, and indicate that the most sandstone-prone reservoirs associate with low to moderately high formation thickness. This suggests that differential subsidence influenced the architecture of the Statfjord Formation. Sandstone body stacking patterns derived from correlation panels show a sequential ordering of multilateral/multistorey sandstone sheets and intervals with a higher portion of mudrock and more isolated sandstone bodies. Sequence stratigraphic boundaries are recognized at the bases of multilateral sandstone sheets, and stacking patterns form a basis for detailed correlations within structural compartments with constant formation thickness. However, the number of recognizable sequences increases with increasing succession thickness, and the correct correlation of sequences across major fault systems pose major problems to stratigraphic studies.

Sequence Stratigraphy of the Lower Jurassic Dunlin Group, Northern North Sea

Sequence stratigraphic analyses of the Lower Jurassic Dunlin Group of the North Viking Graben provides a consistent basinwide framework for a better understanding of the sedimentary facies distributions. Nine sequences are defined on the basis of temporal changes in depositional environments, extensive biostratigraphic information and log correlation. The bounding surfaces of these sequences represent maximum transgressive surfaces (mts) and mark the points between a series of regionally identifiable regressive-transgressive episodes in the Early Jurassic (latest Sinemurian to Toarcian) interval of the North Viking Graben. The biostratigraphic information has enabled the sequences to be calibrated to standard schemes established elsewhere within the North Sea Basin and the ammonite-dated Lower Jurassic type section on the Yorkshire coast. The sequences have thicknesses of between 10 m and 100 m and a duration of approximately 1-4 Ma. They consist of facies representing deposition in a variety of shelf, shoreline and estuarine systems. The significance of estuarine facies within this interval is highlighted, most notably within the J14 and J15 sequences. The sediments generally have a source direction from the Horda Platform and Lomre Terrace areas on the eastern basin margin. The sandstone distribution within these sequences is primarily controlled by variations in sediment supply, accommodation potential and tectonic subsidence and represents a history of repeated progradation and retrogradation.

Sedimentology and palaeogeography of the statfjord formation (Rhaetian-Sinemurian), North Sea

Facies analysis of the Statfjord Formation (Rhaetian-Sinemurian) in the Tampen Spur, Horda Platform and Utsira High areas of the North Sea Viking Graben, shows that alluvial sediments on the Utsira High are finer grained and contain a higher proportion of non-pedogenic, subaqueously deposited sediments than time-equivalent deposits located further north. This tendency is accompanied by regional variations in the lithofacies composition of the Statfjord Formation, indicating that the Early Jurassic drainage system was dominated by a southerly dipping continental palaeoslope that eventually terminated in shallow marine environments to the south. Furthermore, data from basins in Denmark and offshore Britain show that the Late Triassic-Early Jurassic period in northwest Europe was characterized by marine incursions from the south, with gradual onlap of marine strata to the north. This regional pattern supports the interpretation of a southerly dipping palaeoslope in the Viking Graben during the Early Jurassic. The proposed palaeogeographic model links the dominantly continental Statfjord Formation to age-equivalent coastal and marine deposits in the southern North Sea, and implies that the contemporary shoreline was located up to 350 km southwards of proximal fluvial environments in the north Viking Graben.

Differential subsidence in the Ness Formation (Bajocian), Oseberg area, northern North Sea: Facies variation, accommodation space development and sequence stratigraphy in a deltaic distributary system

The Ness Formation (Bajocian) consists of the alluvial distributary plain deposits of the Middle Jurassic Brent delta. The formation comprises fluvial channel sandstones intercalated with fine-grained floodplain deposits. The sandstone bodies are significant hydrocarbon reservoirs in the Oseberg field and surrounding smaller structures in the Norwegian North Sea, to which the present study pertains. The alluvial succession shows significant thickening across normal faults, reflecting syndepositional differential subsidence. The thickness proportion of fluvial sandstones varies with the succession thickness. Where the succession is relatively thin, it is characterized by a large variation in the content of fluvial sandstones. Where thicker, the succession shows less variation in the proportion of fluvial sandstones. The proportion of fluvial sandstones tends to stabilize or even decrease in the thickest profiles of the formation. These findings are in contrast to theoretical alluvial stratigraphy models, w...

Differentiation of incised valley systems from mobile streams: some examples from the oseberg field, Norwegian North Sea

The Statfjord Formation (Rhaetian-Sinemurian) and the Ness Formation (Bajocian) in the Oseberg Field (Viking Graben) feature interstratification of fluvial sandstone bodies, and packages of mudrock-dominated floodplain deposits. Together, these two formations contain a set of different continental depositional environments, including braided stream (Statfjord Formation) and humid-climate delta plain deposits (Ness Formation). A comparative study offers a potential of testing sequence stratigraphic concepts in a broad range of continental environments, including the problem of distinguishing incised valley fill from mobile stream deposits. Thick sandstone bodies (tens of metres) of fluvial origin may represent incised valleys cut during base-level falls and filled during subsequent periods of base-level rise. Consequently, their basal bounding surfaces attract attention as likely sequence boundaries within alluvial units. This simple allocyclic model contrasts with current autocyclic models of alluvial stratigraphy, which are based on rivers being erosive and mobile and able to sweep across entire alluvial plains. Using the autocyclic models, thick fluvial sandstone bodies are likely to form due to vertical aggradation of fixed channels during periods of rapid subsidence, or by vertical stacking of mobile channel belts during periods of slower subsidence. The latter models also relate channel deposit abundance and sandstone body geometry to such factors as avulsion period, floodplain width and deposition rate, in addition to subsidence. This study investigates fluvial sandstone bodies of variable thickness on a local scale, as a contribution to establishing sedimentological criteria for differentiating between deposits that can be related to valley incision and those that are the product of deposition within mobile streams that erosively sweep across depositional plains. Sandstone body thickness and width, character of the encasing deposits, and the relationship between erosional surfaces and scours at sandstone bases and palaeosol units in the surrounding sediments form the basis for interpretations regarding the origin of these sandstone bodies. In both formations, the existence of incised fluvial valleys can be disputed, and the observed alluvial architectures indicates that deposition was governed by generally declining long-term rates of accommodation space development.