Regional middle-upper Jurassic reservoirs in the Yenisei-Khatanga and eastern parts of the Gydan oil and gas bearing areas within the Siberian sector of Arctic: structural models and conditions of formation

Areas having different composition within the Aalenian-Bajocian, Bathonian, and Oxfordian regional reservoirs and ranging from mainly sandy to argillaceous-silty, which are regularly distributed across the region, are delineated. Environments where permeable sequences and impermeables associated with reservoirs were accumulated mainly in marine conditions are discussed. A set of thickness and composition maps is for the first time created for the permeable sequences throughout the region. It is shown that over the course of the Middle and Later Jurassic, transportation of silt-and-sand material from the Siberian Craton was the most intensive. Therefore, composition of the deposits from this provenance area is more sandy than in the neighbourhood of the Taimyr Orogen. Common features of reservoir quality variations depending on their depth of occurrence are discovered. Granular reservoirs occurring at the depth up to 3.5 km have the higher porosity and permeability. The reservoir porosity is reduced by 2-2.5 % with each next kilometre of depth. Starting from the depth of 4.5 km, the open porosity typically does not exceed 12-13 %; reservoirs occurring deeper than 5 km have open porosity close to their cutoff value. Reservoir permeability also decreases downward the section. Prediction of thickness of underexplored reservoirs is carried out using the available limited analytical and production logging data, the revealed regularity of variation in their porosity and permeability depending on depth of occurrence, and analysis of permeable rock associations’ composition. The authors delineated the areas of reservoir local occurrence and absence. The former can be classified into the areas having the highest, medium, decreased, and low reservoir thickness. Maps showing the quality of impermeables are for the first time created throughout the study area