Variations in Organic Content of the Kolje Formation in the Western Barents Sea: A Stochastic Modelling Approach

Abstract

During the Early Cretaceous, the western Barents Sea was a tectonically active area including faulting and differential basin subsidence. In this setting, the Kolje Formation was deposited over the Barents Sea shelf as shales and claystones. Borehole data indicate that the formation thickness varies significantly from only a few to about 750 metres. The organic content of the formation, demonstrated by total organic carbon (TOC), shows large spatial variability, ranging from 0.6 to 7.8 wt% in the Hammerfest Basin and the Fingerdjupet Sub-basin. In addition to spatial differences, the TOC values vary from the base to the top of the formation.

The aim of this study is to explain why the organic content and depositional environments of the Kolje Formation varied significantly over the shelf and during its deposition. In the approach taken here, that involves reconstruction of the Early Cretaceous sedimentation rates, initial rock properties, palaeo-water depths, and marine primary productivity. This was conducted by using a new Monte Carlo-based software StoRM (Stochastic Rock Modelling). The program models 1D deposition history by using semi-random sedimentological parameters (including rock ages, sedimentation-erosion rates, initial rock properties) for millions of modelling runs. Then it filters ‘likely’ runs and their parameter values (improved distributions) by comparing the modelled rock column thickness to available measurements. The improved input parameter distributions are then used for modelling of palaeo-water depths and marine primary productivity. The large number of exploration wells and available well data make the Barents Sea area ideal for conducting such a type of study.

The study results show deep-marine and spatially-variable water depths at start of the deposition of the Kolje Formation. It was found that variable deposition rates of the Kolje Formation were controlled by the uneven bathymetry and sediment supply from the adjacent subaerial Loppa High. The TOC content in the Kolje Formation was inferred to be mostly related to nutrient supply from the Loppa High. The regional distribution of primary productivity suggests that organic nutrient supply did not change during deposition of the Kolje Formation in the Hammerfest Basin. In the Fingerdjupet Sub-basin the nutrient supply was largely reduced from base to the top most probably due to a landward coastline shift in the northern Loppa High.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018