HYDROCARBON PETROLEUM SYSTEM

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 ROCK Two main sources Oil-Prone Draupne Formation Gas Prone Heather Formation DRAUPNE FORMATION Main Shale rock that forms the hydrocarbon source in this field Physical Characteristics Include Brownish Black Medium of Dark Olive Grey Non- Calcareous Mudstones Thickness of Formation typically 50- 250 m but may exceed 1200m in localized area

HEATHER FORMATION RESERVOIR ROCK Physical Characteristic Dark grey Silty Mudstones with thin Carbonate layers Thickness formation ranges up to 1000 m It is typically gas prone Total organic carbon ( TOC ) values are typically between 2-2.5 % RESERVOIR ROCK TRIASSIC AND LOWER JURASSIC Occurs in tilted fault blocks with varying degree of Jurassic Cretaceous erosion and onlap. The main reservoir intervals comprise of thick , fluvial channel and sheetflood deposit. Porosity range from 20-24% Permeability 300-2000 mD

MIDDLE JURASSIC Present in the Northen Sea are arkoses and subarkoses with quartz , clay minerals and Fledspars consisting about 95 % of the total miner Sandstones are both quartz and calcite cemented at depths exceeding 2500 m. Reservoir form a thick clastic wedge comprising laterally extensive interconnected fluvial Deltaic and coastal depositional systems with porosities 20-30% and permeability 50 – 500 mD. UPPER JURASSIC Its is rarer in the Gullfaks field.

TRAPS AND SEALS Most trapping mechanism is provided by rotated faults sealed by fine grained post rift sediments. These sediments draped on to the structures to form seals Lateral trapping and sealing is formed where reservoir rocks are juxtaposed with non reservoir rocks at fault contacts Most seals are closed to hydraulic fracture. Migration Primary migration in the North Sea is through pressure driven flow of a discrete hydrocarbon phase through pores and micro fractures Secondary migration is mostly by buoyancy resulting from difference in density between the hydrocarbon and water

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 across Micro fracture systems Classification into five modes as follows Short distance migration Migration in rotated fault blocks Migration through faults and micro fractures Up-flank migration from the graben Unconformity and multiple conduit migration

Migration in Gullfaks

Depositional Environment and Facies Analysis

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 subsidence The 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 300 - 400 m.

Cretaceous Cretaceous 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 sedimentation Tarbert There 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.

Ness Ness 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.

Etive Etive 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.