The Triassic System of West Siberia formed in two stages. In the first (Early Triassic) the climate was arid to semi-arid and sedimentary deposition was accompanied by volcanic activity. In the second (Late Triassic) the climate was humid.
Three sedimentary series are recognized in the Triassic of West Siberia: Tampey, Turin, and Chelyabinsk. The volcanic- sedimentary Turin Series occurs in graben-like depressions in the region of the middle and lower course of the Ob River and also to its southwest. It was deposited largely under arid and semi-arid conditions as redbeds. Its age is in the upper half of the Induan and in part Late Triassic. The Chelyabinsk clastic-coaly sediments (Rhaetic-Lias) were deposited in grabens along the east flank of the Urals and the west and east flanks of the Central Kazakstan Shield. The Tampey Series consists of silt-clay rocks in downwarps largely in the north of West Siberia.
The present study determined the catagenic level of organic matter in Triassic clastics of 14 exploration areas in various parts of West Siberia based in vitrinite reflectance. The level of catagenesis (maturation) is lowest in the southeast of the study area with Ro=0.55 (MK-1). To the west of the Ob River in the Sherkalin area of the Uvat-Khanty-Mansiysk microplate the maturation level is similar at Ro=0.64. Then to the very south in the Omsk area the maturation level is the same for both the top of the Triassic and base of the Jurassic at Ro=0.76.
Nikol well 1 of Omsk Region penetrated a 1.2 km thickness of Triassic. In the upper part of the section maturation of the organic matter is at Ro=1.2. This parameter increases downward in the section, although slowly. At depth of about 4 km Ro=1..4, and at bottom hole at 4.5 km - Ro=1.56. Contact alteration of the organic matter (Ro=3.85) is found for a distance of a few tens of meters in direct proximity to an igneous body at depth of 4.2 km.
Relatively high levels of maturation of the Triassic organic matter are found in the north of West Siberia, where these rocks occur at greater depths. For example, in well Nadym-7 in the Nadym mega-depression the maturation level at the top of the Triassic at -4.5 km is at Ro=2.5 (AK-1). A similar situation obtains in the Srednepur mega-downwarp where the level is at Ro=2.45 in well Samburg-700 at -5.4 km.
This study found that there is no abrupt increase in level of catagenesis at the transition from Jurassic to Triassic. There is, however, a significant difference in this parameter between the Triassic and Paleozoic. Consequently, the Triassic experienced maximum catagenesis at the same time as did the Jurassic.
The Triassic rocks are still in the oil window along the margins of the West Siberian platform. A humic composition of the organic matter offers little favorability for liquid hydrocarbons; however, conditions are very favorable for gas over much of the region.
Taken from Fomin, 2001; digested in Petroleum Geology,
vol. 36, no. 4, one map, in preparation.
Copyright 2002, James Clarke. You are encouraged to print out
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