Petroleum Systems Modelling as a Screening Tool in the Evaluation Cycle, A Case Study From the Helgeland Basin, Mid-Norway

S. DÖRING¹, K.C. FISCHER¹, U. MÖLLER¹, and B. UJETZ^2
1Wintershall Kassel
²Wintershall Norge AS

As part of a regional study for Mid-Norway, several open blocks bordering the sparsely drilled Helgeland Basin on its western and northwestern side were evaluated geologically. The area was about 140 x 175 km in size and included 9 blocks and 3 half blocks (one block is 15 x 28 km) open for application. Since the 6 wells in the Helgeland Basin drilled valid and sealed structures without finding commercial hydrocarbon accumulations, the highest risk was allocated to the hydrocarbon charge from the beginning of the evaluation. Hence a regional 3D petroleum systems model was built from a very early point which then guided the evaluation process.

For the general understanding of the basins thermal history, pseudo-well locations were defined and modelled. These gave quick insights on timing and temperature evolution and guided further study objectives. Results confirmed the low maturity of the proven source rocks (Lower and Upper Jurassic) and therefore an alternative source rock had to be taken into account. Assumptions in this respect were based on the geochemistry of oil shows within the Helgeland Basin which point towards a Pre-Jurassic source and on geochemical data from Permian and Triassic source rocks which were drilled in a shallow borehole to the NE.

Regional seismic interpretation and depth conversion gave clues on the structural evolution of the area but were still lacking detailed information on the petroleum system. Mapping of the Triassic salt, the basement, and an alternative source interval below the salt became crucial input for the model in order to define potential pre-salt opportunities. Uncertainties concerning the depth of a probable Pre-Jurassic source rock remained high since seismic interpretation and correlation towards the shallow borehole in the NW was very difficult based on the existing data.

First results related to 3D secondary migration helped to focus on certain blocks and therefore reduced the amount of work. Especially for the detailed structural mapping and volumetrics, a considerable amount of time was saved.

Final model results helped to assess the risk on the identified leads in a quantitative way, but the scope of risk assessment was still very much regional. The greatest unknowns for lead or prospect risking of the pre-salt play are, besides the depth of a potential source rock, the carrier and the source rock properties. Concerning the carriers, it came clear that diagenetic modelling should also be taken into account. A source rock depositional forward model and better seismic data would be of great value in order to reduce the uncertainty on source rock depth, presence and quality.

 

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

 

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