Yamal-Gydan Source Rocks and Oils

Malvin Bjorøy¹, Ian L. Ferriday¹, Peter B. Hall¹, and Alla Nemchenko-Rovenskaya^2
1Fugro Geolab Nor AS, Trondheim, Norway.
²Russian Academy of Sciences Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOKHI), Moscow, Russia.

Fifty-seven oils and forty-eight core samples from the Yamal Gydan region of the West Siberian Basin were analysed. All cores were analysed for organic carbon content and by Rock-Eval pyrolysis. Visual kerogen composition, vitrinite reflectance analysis and pyrolysis GC were performed on samples with TOC more than 1%. All oils were analysed by whole oil GC. Solvent extraction, was performed on the source rock samples with the best potential. The extractable organic matter of the source rock samples and all the oils were deasphaltened, and then separated by MPLC. The saturated and aromatic hydrocarbon fractions of these samples were analysed by GC and GC-MS. Stable carbon isotope analysis was performed on all bulk fractions and whole oils were analysed by GC-IRMS. In addition, bulk physical and inorganic chemical data were obtained on most of the oils.

The analysed source rocks fall into two main groups, mainly gas-prone Cretaceous coals and carbonaceous shales and oil and gas prone Middle and Lower Jurassic shales of marine origin. The Cretaceous sediments are not mature enough to have generated significant amounts of oil or gas. The Jurassic source rocks show a wide range of maturities.

Reservoirs range from Jurassic to Cretaceous in age and occur at depths ranging from very shallow (< 1000 m) to about 3000 m. The majority of Yamal-Gydan oils are light with comparatively high wax contentsa and some are affecetd by biodegradation, which can be expected in any field shallower than 2200 m. Gas deasphalting and gas flushing of oil pools have also occurred in the Yamal-Gydan area, especially where gas-window mature Lower-Middle Jurassic sediments have generated large amounts of gas (i.e. in the main rift systems).

The geochemical data, particularly the tricyclic terpanes, steranes and bulk and individual compound isotopic compositions, permit separation of the oils into two main groups. Group 1 is found in the south and is most probably Bazhenov-sourced. Group 2 is mostly found in the north and in reservoirs older than the Upper Jurassic. Other oils have intermediate characteristics and probably have mixed sources.

AAPG Search and Discover Article #90096©2009 AAPG 3-P Arctic Conference and Exhibition, Moscow, Russia