Oil and gas reservoirs in clinoforms of the Kolgan strata in the Volga–Ural oil and gas province: Forecast and survey technologies

In view of depletion of conventional hydrocarbon resources and due to exhaustion of reservoirs in the main oil- and gas-producing regions, the new oil and gas field prospects are connected with poorly explored areas influenced by extreme climate and tough economy, and with deposits of complex geological structure. The article presents the studies on the late Frasnian–early Famennian Kolgan strata within the boundaries of the Tsarichanka area in the Kolgan–Borisovo depression, which detail the geological structure of the test area. The paleotectonic reconstructions have identified the drift paths of terrigenous turbidites as the southeast northwestward trends, which conforms with the general geology data and with the tectonic history of the area. Application of seismic data proved to be highly efficient in prediction of facies zones and in detection of lithological traps. The facies analysis specificated the geological structure and its nonuniformity for the Kolgan strata. The interpretation of the data on the reflection horizons confirmed the clinoform structure of the strata. The paleoreconstructions determined the source of the sediment drift in the Kolgan–Borisovo paleo-depression—Sol-Iletsk anticline, and confirmed the conceptual model of formation of the Kolgan reservoir. The facies analysis of the late Kolgan sediments identified the facies zones of alluvial cones and noncontinuous flows as promising reservoirs. The hydrocarbon-promising zones represent isolate lithological traps of alluvial cones, identified from the comparison of the seismo-facies analysis, core analysis, geophysical well logging and predictive estimates of effective thickness and effective porosity.