2 SOURCE ROCK DRAUPNE FORMATION This section is based on literature review as seismic data was not provided.Help us to know how hydrocarbons are produced and migrated.SOURCE ROCKTwo main sourcesOil-Prone Draupne FormationGas Prone Heather FormationDRAUPNE FORMATIONMain Shale rock that forms the hydrocarbon source in this fieldPhysical Characteristics IncludeBrownish BlackMedium of Dark Olive GreyNon- Calcareous MudstonesThickness of Formation typically m but may exceed 1200m inlocalized area
3 HEATHER FORMATION RESERVOIR ROCK Physical Characteristic Dark grey Silty Mudstones with thin Carbonate layersThickness formation ranges up to 1000 mIt is typically gas proneTotal organic carbon ( TOC ) values are typically between2-2.5 %RESERVOIR ROCKTRIASSIC AND LOWER JURASSICOccurs in tilted fault blocks with varying degree of JurassicCretaceous erosion and onlap.The main reservoir intervals comprise of thick , fluvial channel andsheetflood deposit.Porosity range from 20-24%Permeability mD
4 MIDDLE JURASSICPresent in the Northen Sea are arkoses and subarkoses with quartz , clay minerals and Fledspars consisting about 95 % of the total minerSandstones are both quartz and calcite cemented at depths exceeding 2500 m.Reservoir form a thick clastic wedge comprising laterally extensive interconnected fluvialDeltaic and coastal depositional systems with porosities 20-30% and permeability 50 – 500 mD.UPPER JURASSICIts is rarer in the Gullfaks field.
5 TRAPS AND SEALSMost trapping mechanism is provided by rotated faultssealed by fine grained post rift sediments.These sediments draped on to the structures to form sealsLateral trapping and sealing is formed where reservoirrocks are juxtaposed with non reservoir rocks at faultcontactsMost seals are closed to hydraulic fracture.MigrationPrimary migration in the North Sea is through pressure driven flow of a discrete hydrocarbon phase through pores and micro fracturesSecondary migration is mostly by buoyancy resulting from difference in density between the hydrocarbon and water
6 Sandstones of the Brent Group provide the main conduits of migration with the study area Three main mechanisms of primary migration as follows:(i) migration enhanced intercalation of source and reservoir(ii) migration due to juxtaposition of reservoir and source rocks across faults(iii) vertical migration acrossMicro fracture systems Classification into five modes as followsShort distance migrationMigration in rotated fault blocksMigration through faults and micro fracturesUp-flank migration from the grabenUnconformity and multiple conduit migration
9 Middle Jurassic deposits of the reservoirs in Gullfaks field are represented by the deltaic sediments with deposition strongly controlled by regressive/transgressive cycles and occurred during the late phase of post-rift subsidenceThe thickness distribution is consequently controlled by both the thermally driven subsidence and ongoing faulting of the Late Jurassic-Early Cretaceous episode of rifting.Basically the Gullfaks field is subdivided into 4 major stratigraphic units as Cretaceous, Tarbert,Ness and Etive formations.This petroleum system is a sequence of sandstones, siltstones, shales and coals with maximum thickness of m.
10 CretaceousCretaceous sediments were deposited unconformably on late Jurassic sediments of the North Sea.This major unconformity between the Jurassic and Cretaceous is called the Base Cretaceous Unconformity (BCU).the Lower Cretaceous deposits comprise shallow marine mudstones,calcareous shales and some sands.Planktonic carbonate algae then mainly dominated sedimentationTarbertThere is controversy exists regarding the boundary between Ness and Tarbert formation and depositional environment consequently.Consequently from that point of view the Tarbert formation is interpreted as pure controlled by the marine environment. Lithology varies from shales, siltstones and coal beds to medium-to coarse-gained sands in which calcite cementation is found.
11 NessNess and Etive formation is assigned by the first occurrence of a coal bed above the clean sands.Additionally several coarsening-upward sequences of sandstone with good reservoir quality propose the crevasse splay, crevasse channels and overbank flooding.The Upper Ness unit might be described as domination of siltstone/claystone and coal deposits with some lacustrine deposits.
12 EtiveEtive formation according to recent studies was interpreted as a barrier bar complex.On the basis of low-angle large scale cross-stratification, grain size, heavy mineral concentration and parallel lamination a high energy beach environment.Formation consisting primarily of medium-to coarse-grained sandstones varies in thickness of 15 to 40 m.