The document summarizes the geology of the Oseberg oilfield in Norway. Four wells were analyzed using well logs to identify geological features like reservoir intervals. A stratigraphic cross-section showed formations thinning from north to south, likely due to uplift. A structural cross-section showed formations intersected at lower depths from north to south, indicating normal faults. Fluid analysis of logs identified oil in most formations except some gas in the Ness Formation. The biggest reservoir is the Drake Formation, which contains oil across the field. Rifting caused multiple faulting that influenced the geology.

1. Introduction
Oseberg oilfieldwas discovered in 1979 and productionstarted in 1988. Oil and gasin thisfieldis produced
from sandstone of middle Jurassic age in the Brent group. The Brent group consist of sandstone from
Oseberg, Etive, Ness and Tarbert and serve as the main reservoir. The basic structural framework of the
northern North Sea is mainly as a result of repeated periodsof crustal stretching and thinning and relative
post-rift subsidence during Permian to Early Triassic and Late Jurassic (Ziegler, 1990; Færseth 1996; Faleide
et al.,2010). As aconsequence theareaischaracterized bylargerotated fault blocksandsedimentarybasins
in asymmetric half grabens, formed during the lithospheric episodes of extension and crustal thinning
(Badley et al., 1984; Færseth 1996; Faleide et al., 2010).
Objective
The objectivesof thisreport is to identifyand describe thevarious geologicalfeaturessuch as the geologic
tops, flooding surfaces, reservoir intervals found within thewells and also to predict possiblefluids present
in the reservoir. This is accomplished by interpreting the well log data made available in Petrel.
Data Method
Four wells out of the eight available were used for our study. The wells stretched laterally from north to
south and were widely spaced in order to get a broader perspective on of the field’s geology as shown in
figure 1 below. Geological features within the study area was identified using geophysical logs. The main
welllogsused included gammaray,resistivity,and density logsmerged with reversed neutron porositylogs.
Forthedataset,NPDwelltopswere used as reference for thereservoir formations.Thevariations ingamma
rayand resistivityserved as indicatorsforour reservoir, maximum flooding surfaces andmaximum reservoir
intervals. Thedensity logs in combinationwith thereversed neutron porosity logswere used to predict the
type of fluid present within the reservoir. Maximum Flooding Surface one (MFS1) was flattened and used
for thestratigraphicwellcorrelation.For the structural well correlation,thedepth was converted to subsea
true vertical depth (SSTVD) and flattened on depth to get the elevation of the wells at the same level.

2. Fig 1 Map of the field showing selected wells
Fig 1. Stratigraphic Cross Section of the Oseberg Oil Field

3. Fig. 2 Structural Cross Section of the Oseberg Oil Field
Observation
Themaximum flooding surface one (MFS1) from figure 1 was used for thestratigraphiccross section. MFS1
was used for this correlation because it is a widespread deposit s that were easily visible in the log data
across the various wells. It also serves as the genetictime boundary which separates younger strata’s that
forms above it from older ones that forms below in order to bring it to a common datum.
For theStratigraphicCorrelationwhich isshownon Figure 1. One ofthefirst observationswas spotting how
from the stratigraphic cross section, the formation sandstones, maximum flooding surfaces and reservoir
decreases in thickness from wells 30/6-7 located in the northmost part of the field to well 306/1 to 30/6-9
and then down to 30/9-19 which lay southward in that order respectively.
For theStructural well correlationwhich isshown on Figure 2. It can beobserved that the Ness, Etive,Drake
and Cook formation are all intersected at lower depth in well 30/6-1 as compared to well 30/6-7. They are
thenintersected at alowerdepth inwell30/6-9 and inan even lowerdepth inwell30/9-19. It canbeobserved
that theOseberg and Tarbert formationare not present in allthewells. Tarbert formation is present in well
30/6-7 and 30/9-19 and Oseberg formationis onlypresent in well30/6-7. Cookformationis present in allthe
wells but well 30/9-19.
For our fluid presence study, when observing the combined neutron porosity and density logs of the
different formations, we observed various distinctions in the displacement gaps between both logs. We
took notice that in some formations we had close gaps in displacements in between the logs throughout
theformation.Also, in some cases withinthesame formations we hadabiggerdisplacement betweenboth
logs after the formation tops and little of no gaps as it increased in depth and thickness downward.
Interpretation
From the stratigraphic cross section, observations made indicates that the thickness of the various
formations getsthinner inwell 30/6-1 andthenthethickness starts increasing againas it trends towardsthe
south. This was be as a result of tectonic activities possibly uplift which caused the sediments to stretch
resulting in the thinning of its layers in the area were well 30/6-1 is located. It is also possible that
unconformity also set in after the uplift which would explain why the Tarbert formation is only present in
thewells located at thenorthern andsouthern part ofthefieldbut thenabsent from thewells in thecentral
part.
From the structural cross section, it can be observed that all the formations were intercepted at a higher
elevation in well 30/6-1 relative to wells 30/6-7 and 30/6-9. The formations were all intercepted at a much
lower depth in well30/9-19 thanin well30/6-9. Thismight be as a result offault that set in after deposition.
The fault that exist between the two northern most wells might be a reverse fault and the ones that exist
between well 30/6-7 and 30/6-9 and well 30/6-9 and 30/9-19 are normal faults.
From the combined neutron porosity and density it can be interpreted that the fluid that exist in the
Tarbert, Oseberg, Drake, Etive formations are oil. Ness being the exception holds mix of gas and oil with
the formation as it stretches to the various wells.
Conclusion
From the report, it can be concluded that
 Drake Formation is our biggest reservoir, it contains oil and can be found though out the various
wells with the field.
 Rifting which occurred in theViking basin resulted in multiplefaulting activityoccurring within the
field.