Within oil pools the biomarkers (chemical fossils) as a rule are close or even identical in composition. Some pools, however, exhibit a mixture of oils of different genetic type as, for example, having different pristane to phytane ratios. One such occurrence is the Lower-Middle Carboniferous pool in Zhanazhol field in the eastern part of the North Caspian depression. Two genetically different types of oils are present there - designated as type I and type II.
These oils differ from one another not only in composition of the biomarkers but also in other parameters. In contrast to type II, the type I oils have a higher density, elevated content of tar and asphalt, and lower concentrations of paraffin-naphthene components. The higher density and tar content are not due to oxidation but to other causes.
Type I oils occur in the western part of the pool, and type II in the eastern part. The boundary between them has a rather irregular form and does not conform with the structure contours.
Presence of two types of oils in the same pool with a distinct boundary between them is attributed to two main causes: First, introduction of oils from different zones of generation having different composition of the biomass; second, presence of a barrier in the pool that prevents mixing of the different genetic types.
Presence of the two types of oils in the Zhanazhol pool is related to lithofacies characteristics of the Early-Middle Carboniferous basin. The Zharkamys zone of oil-gas accumulation, within which Zhanazhol field occurs, is bounded on the west by a deep-water marine shelf complex and on the east by a shallow-water carbonate shelf complex. Type I oils were generated on the west by sapropelic organic matter, the sources of which were marine organisms with significant amounts of bacteria. In the biomass that generated the type II oils along with sapropelic material (planktonic and bottom forms) there was a greater amount of humic material. Accumulation of this organic matter took place in a more saline environment. The type I oils are catagenically more mature than type I oils.
Presence of different oils in the same pool may be due to a barrier within the reservoir. it cound be a zone of low permeability or a lithologic, stratigraphic or fault seal. A new type of seal has been proposed recently by Yeremenko and Chilingar (1996), citing the possibility of formation of a gas-water emulsion. Where dissolved gas searates into the free state it may form a gas-water emulsion, which under formation conditions may serve as a seal for gas pools. Emulsion seals may be much wider in distribution than is now assumed. (Taken from Botneva, Yeremenko, and Nechayeva, 1999; digested in Petroleum Geology, vol. 35, no. 2, 2001, in press; two maps).
Copyright 2001 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at our web page: http://geocities.com/internetgeology/ This News Letter is distributed without charge in the interest of our science of petroleum geology. To be added to the mailing please send your e-mail address to: jamesclarke@erols.com