There have been differences in opinion as to the source of the hydrocarbons in sediments above the Lower Permian salt in the south and east of the North Caspian depression. Palynological study was made of the oils over a great stratigraphic range in both the sub-salt and supra-salt sediments of this region. The plant micro-fossils in these oils could have migrated only along with the oils. The study included oils from both clastic and carbonate rocks from more than twenty fields.
Oils from Carboniferous carbonates contain a small quantity of spores and acritarchs of Carboniferous age. No significant signs of vertical migration were noted in the areas studied; the spore assemblage is the same age as the host rock.
The micro-fossil assemblage in oils of the Artinskian rocks at Karatyube is mostly Permian, although 5-10 percent are Carboniferous. Oil from Kursay carries abundant Artinskian forms and 8 percent Carboniferous forms.
Oils from Kungurian sediments at Kenkiyak and Urikhtau carry abundant forms; however, they indicate an origin in the Artinskian. The pools here formed by migration from the Artinskian.
Two types of micro-fossil assemblage are found in oils from the Permian-Triassic sediments. The first is from Upper Permian sediments of Kenkiyak and Karatyube fields and consists almost entirely of Lower Permian pollen or with some spores from the host rock and individual Carboniferous and Devonian forms. This indicates that this oil was derived from sub-salt source beds. The second type is from Lower Triassic oils of Kenkiyak, Karaganda, and Kokzhide. The forms here are up to 50 percent Triassic, indicating a more complex origin for these pools.
The Jurassic oils carry 40-50 and in some places 90 percent Jurassic spores and pollen but also migrated assemblages of Permo-Triassic, Carboniferous, and even Devonian age. All the Jurassic oils contain 5-25 percent Paleozoic acritarchs and in some places up to 50 percent.
The Cretaceous oils carry migrated assemblages of Jurassic, Triassic, Permian, and Carboniferous age. The pools in the Cretaceous are therefore secondary, formed by migration from lower-occurring source beds. The presence in all the Cretaceous oils of light Paleozoic acritarchs carried by gas indicates considerable transfer of hydrocarbons in the gas from deep Paleozoic source beds.
The palynological data thus indicate presence of oil in pools above the Kungurian salt in the east of the North Caspian depression that has been derived by vertical migration from source beds below the salt. Rupture by salt tectonics provided avenues for this transfer. (Now a comment from me: Since hardly any Devonian forms are found, it seems likely that large Devonian pools may yet be discovered. - JC)
Taken from Medvedeva, Bulekbayev, and Dal'yan, 1993;
digested in Petroleum Geology, vol. 30, no. 3, one map.
Copyright 2002 James Clarke. You are encouraged to print out this
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