Play Analysis of the Middle Jurassic of the Danish North Sea, an Integrated Basin Modeling Study

D. Schwarzer, T. Fronval, L.F. Clausen, and R. Ensley
Maersk Oil and Gas, Copenhagen, Denmark

A comprehensive evaluation of the Middle Jurassic prospectivity in the Danish North Sea formed the technical basis for an assessment of the Middle Jurassic play and the controlling factors for hydrocarbon and reservoir occurrence.

The first phase of the play analysis included the generation of a new stratigraphic framework based on integration of seismic and well data. This introduced a regional unconformity at the base of the Callovian dividing the reservoir section into a primary Callovian section and a secondary Bathonian section.

A post-mortem analysis of 22 wells revealed that 7 wells were either tested or documented to have movable hydrocarbons, 3 had major gas or oil shows. The analysis involving an evaluation of play parameters demonstrated that source and migration are the most critical factors for the play.

The Middle Jurassic play comprises reservoirs at depths of 7,000 ft to 15,000 ft, interpreted to be formed by alluvial braided and fluvial sand prone channel systems on a coastal floodplain. The distribution and quality of reservoir sandstones were evaluated using a combination of seismic attribute data, electric logs, core descriptions and porosity-permeability data. The final facies model suggests that the sandstones formed in tributary river systems flanking the incipient Central Graben and in a north-south trending axial system. Gross thickness, Net/Gross, net porosity of pay and permeability are within the ranges of 25-125 ft, 0.11-0.76, 10-30% and 1-1000 mD, respectively. Seismic interpretation demonstrated that the play involves a variety of trap types, all of which were in place during Late Jurassic times and pre-dated hydrocarbon migration. The majority of the traps are rotated fault blocks with closures created by a combination of fault seal (reservoirs juxtaposed against Middle Graben or Lola formations) and dip closure down the fault-block flank.

Rock-Eval, geochemistry, and isotopic data integrated with the seismic interpretation formed the basis of the source rock evaluation, which, together with distribution of hydrocarbon-shows suggest an active Middle Jurassic Bryne:Bryne petroleum system which had been regarded as subordinate in a regional context until now.

With the aforementioned detailed analyses, a regional 3D basin modelling study was performed to assess the timing of source rock maturation, hydrocarbon generation, and subsequent migration and charge of the identified prospects. Two major phases of hydrocarbon generation can be recognized: Late Jurassic to Lower Cretaceous and Late Miocene to present-time. These phases account for 75% and 25%, respectively, of the total amount of hydrocarbons generated. The second phase was recognised to be crucial for a final charge of the structures, and the volumes expelled during this period are enough to charge most of the identified prospects. As calibration to the migration model existing discoveries were used.

Structures located close to or within the expulsion front, which is the maximum extent of actively generating and expelling source rocks, had a strong focus of charge. However, structures that were located off the expulsion front at this time, had to rely either on a carrier system, such as the distribution of the Callovian channels, or on fill-and-spill from neighbouring structures.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands