CONDITIONS FOR FORMATION OF SECONDARY DOLOMITES IN PALEOZOIC CARBONATE RESERVOIRS OF NORTH-OSTANINSK FIELD (WESTERN SIBERIA) ACCORDING TO MICROCRYOTHERMOMETRIC DATA

Abstract

The relevance of the research is determited by the development of new exploration strategies related to the non-anticlinal hydrocarbon deposits of the Paleozoic basement complex that is caused by the low efficiency of exploration activities carried out within traditional exploration structures. Given the significant decrease in oil-promising structures that provide an increase in hydrocarbon reserves from platform cover deposits, the identification of «non-structural» prospecting objects in the basement rocks based on a new understanding of the formation of Paleozoic carbonate reservoirs will contribute to the further development of the oil and gas industry in Western Siberia. The aim of the research is the analysis of new data from fluid inclusions of Middle-Devonian dolomites of the North-Ostaninsk field in order to study the mechanism of dolomitization of organogenic and bioclastic limestones, which are the main oil-promising object of the Paleozoic complex within the Nyurolskaya basin The research object is the carbonate bioherm from two wells at the roof part of the Paleozoic basement (horizon M1) of the NorthOstaninsk oil and gas condensate field. In the studied wells, the organogenic structure reaches the erosive surface of the basement and the roof part of the limestones, which are intensely dolomitized. Results. The formation of reservoirs of the North-Ostaninsk field is associated with secondary dolomitization of organogenic and bioclastic limestones related to the bioherm construction of the Paleozoic – horizon M1. Cryo- and thermometrical studies on two-phase primary fluid inclusion in dolomite crystals allowed determining the average homogenization temperature of 110–120 °C and an average salinity of 4,5 wt. % NaCl equivalent, which may indicate the formation of dolomites from moderate-temperature solutions of low salinity, formed as a result of a mixing of meteoric water and hydrothermal sources seeping from the surface along the fault system. The latter draining the weathering crust of volcanic rocks, saturate the water with magnesium and iron and contribute to the dolomitization of carbonate rocks.

Keywords

• microthermometry
• cryothermometry
• fluid inclusions
• pre-jurassic carbonate rocks
• dolomite
• western siberia