Published March 2010
Primary gas accumulation zones in South Caspian depression
N.I. ImanovThe confined condensate accumulations with sufficient gas reserves with respect to the submerged Productive Series zones (Janub, Zira, Bahar, Khara-Zira, Kalmas, Shah-Deniz) are clearly observed in oil-and-gas fields maps of South Caspian depression. Such location is not occasional and connected with a number of regularities related to a square change of total gas and oil amount in deposits, gas content factor, chemical and isotopic gas composition, different geological factors affecting fluid composition, geological structure and development of South Caspian depression resulted in formation of known hydrocarbon accumulations. The paper deals with study of different geological-geochemical factors affecting gas content of oil deposits and gas condensate in the Productive Series of South Caspian depression. In schematic paleoprofiles constructed along the Kurdakhani-Shah-Deniz anticline zone it is observed that some little depth of Pliocene-Anthropogenic deposits 1000 to 2000 m were accumulated in the northwest area of the zone. In the southeast the depth of Pliocene-Anthropogen deposits increases from 3600 m in the structure of Gum Adasi to over 6000 m in Shah-Deniz. The depth of these deposits is characterized with higher values within synclines – up to 3000 m in the northwest and up to 10000 m in Shah-Deniz region. Such data together with studies on isotopic gas content have enabled to image the following scheme of carbohydrate deposit formation. As sediments are accumulated, the Productive Series rocks enter into the main oil formation phase in the submerged areas of South Caspian depression and Apsheron peninsula synclines. Here oil and gas generation occurs due to organic substance accumulated in the Productive Series rocks. When stratums are sufficiently inclined initial migration from clay to sandstones turns into lateral migration from the submerged areas to the elevated edge regions of the depression. At that, sufficient amount of gas transfers together with oil which due to bigger mobility passes ahead liquid hydrocarbons and is emitted to atmosphere in the Productive Series outcrops. Further submersion of central areas of the depression up to accumulation of anthropogenic deposits resulted in the main oil and gas generation phase in the Productive Series (PS to rich the main oil and gas generation phase and upper part of gas generation phase). The hydrocarbon gas generation volume is sufficiently increased here and at the border with the main oil generation phase there are light hydrocarbons – condensates that migrate to local structures with formation of gas condensate accumulations such as Shah-Deniz and Bahar. Primary gas accumulation zones of SCD considerably define energetic safety of Azerbaijan and is related to: preferably high-prospective gas zone in the submerged deposits of the Productive Series with low-risk of large and giant gas condensate deposits; gas zone perspective of mud-volcanic focuses in Miocene-chalk deposits reached by drilling in Western Apsheron and Gobustan; second prospective gas content zone of deeply submersed sediments in SCD; as well as methane deposits of the south depression of Caspian Sea and alternative sources of water-soluble gases, gas-hydrates and etc.
