Oil and gas reservoirs of weathering crusts and Paleozoic basement in the southeast of Western Siberia (forecasting of hard-to-recover reserves)

Abstract

The study and development of hard-to-recover reserves in the pre-Jurassic petroleum of Western Siberia is a new focus for development of the resource base and oil production in Russia, identified the necessity of assessing a special strategy, a conceptual scheme for forecasting and exploration. The aim of the study is to identify and assess the primary research areas of hydrocarbon reservoirs in pre-Jurassic basement by the example of underexplored areas of the Ust-Tym mega-depression and structures of the Nurol’ka mega-depression which are located on the well-developed oilfield infrastructure areas of Tomsk region. Object of study is the potential pre-Jurassic reservoirs of Permo-Triassic weathering crust and Paleozoic basement. The accumulations of weathering crust are associated with reservoirs formed on clay and siliceous, carbonate and acid volcanic rocks, and, under certain conditions, by ultrabasites. The reservoirs of bedrock are associated with secondary reservoirs formed by limestones, granites, rhyolites. The main oil source formation is the Lower-Jurassic Togur sediments. The study is carried out based on the concept of mainly vertical hydrocarbon migration. The research area is located in boundaries of the oil source formation. Thermal history reconstruction of oil source formation, cumulatively taking into account the temperatures of the localized source kitchen, is carried out by solving the direct and inverse problems of geothermy. The studies are based on a special technology that includes an integrated analysis of mapping results of the Togur oil source kitchen by the geotemperature criterion and predicting mapping of the weathering crust and the Paleozoic basement reservoirs according to the data of deep drilling, oil geological zoning and perspective areas ranking. In the territory of the Ust-Tym megadepression as the primary research area for the weathering crust reservoir the zone of the northeastern side of the megadepression has been selected. As the priority area for exploration of Paleozoic basement reservoirs the zone uniting the north-eastern slope of North-Parabel megamonocline and the adjacent southern part of Pyzhinsk mesodepression has been selected. As the perspective research area for the weathering crust reservoir the land of the south beads of Kulan-Igay and Tamrad basins and its adjacent areas for the territory of Nurol’ka megadepression have been identified. As the priority areas for research and development of Paleozoic basement reservoirs the south beads of Kulan-Igay basin and adjacent zone of Tamrad basin have been identified, as well as the lands of the south-eastern beads of Nurol’ka megadepression and the adjacent lands of Chuzik-Chizhap mesosaddle. The reliability of the forecast for perspective areas of the pre-Jurassic petroleum play is identified by comparison with the results of well testing and is about 70—80 %. It argues the previously formulated resource-efficient strategy and technology of exploration for oil deposits in the pre-Jurassic basement of Western Siberia.