The main pay zone of Salym oil field of West Siberia is the Upper Jurassic Bazhenov Formation, which is one of the prime oil plays of this oil-gas province. The Bazhenov consists largely of dark-brown or black bituminous shale. Thickness is 35-40 m. The rock is altered Radiolarian ooze composed of clay minerals, various modifications of silica, fine-grained carbonate, and organic matter. Porosity of samples is 1-8 percent; however, it is up to 14 percent after extraction by organic solutions.
Post-sedimentary alteration was in the form of solution and redeposition of new silica modifications of the organic silica of the rock-forming Radiolaria; formation of authigenic kaolinite and some glauconite; intensive pyritization; calcitization of Radiolaria and diagenetic dolomitization, and leaching. The main changes in structure of the pore space, leading to conversion of isolated pores into open porosity in Salym field, took place during catagenesis - maturation.
The main factor in generation of bitumen in the Bazhenov was formation temperature. Commercial oil pools are found in zones of anomalous high temperatures, and these are characterized by open porosities up to 8 percent. Outside these zones the porosity is limited to 1.5 percent. Development of the reservoirs took place largely by combining existing closed pores, which were commonly sealed by heavy bitumen.
Important to the development of interconnection between pores were micro-cavities. During consolidation the Bazhenov sediments experienced strong dessication and compaction, which is recorded by presence of compacted recrystallized tests of Radiolaria, some in the form of lamellae. Connecting of the pores was aided by micro-channels and micro-fractures that formed during redistribution of hydrocarbons during maturation - intra-pore pressure increased due to increase in volume of the hydrocarbons.
Permeability is favored by long, predominantly horizontal fractures. These fractures are a necessary condition for flow of oil to the wells, and, like the micro-farctures, they provide capacity for the reservoirs.
The composition of the reservoirs, the processes of creating the effective porosity, and the quantitative and qualitative character of the channels for transmission of fluids indicate that the entire thickness of the Bazhenov Formation of Salym field is a single oil-saturated target, productivity of which is uneven due to variations in hydrodynamic properties. The highest values of permeability are associated with two north-south-trending zones, which coincide with highs on the structure. The most productive wells are in this zone. (Taken from Dorofeyeva (1980); digested in Petroleum Geology, vol. 18, no. 5, one map showing distribution of porosity and permeability).
The Bazhenov is generally assumed to be the source bed for the super-giant oil fields in the Lower Cretaceous of West Siberia. I disagree. I think that the Bazhenov is too "tight" to give up its oil, and that the oil present in the Cretaceous pools was derived from source beds of the Bazhenov type that were to the west in a starved. basin. These source beds interfinger with the Cretaceous reservoirs. -J. C.
Copyright 2001 James Clarke. You are encouraged to print out this
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