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STAGES OF SAND RESERVOIR FORMATION IN NIZHNE-KARMALSKOYE SUPER-VISCOUS OIL DEPOSIT

Э А Королев

Abstract

Nizhne-Karmalskoye super-viscous oil deposit is located within the western slope of the South Tatar arch of the Volga-Urals anteclise. In the structural plan, the deposit is a sedimentary accumulative uplift-bar of 13 km in length, 3-5 km in width and 35-50 m in height. The uplift-bar was formed at the end of the Ufa age. According to one of the hypotheses, the sand body has a delta-baric origin. The body is composed of well-sorted fine-grained graywacke sandstones, which are now reservoir rocks of the super-viscous oil deposit. The study of sand reservoirs has shown a complex geological history of their formation. At the stage of sedimentation, in conditions of active hydrodynamics of shallow water, sandstones with potentially high capacitive-filtration properties formed within the bar. Over time, the pore space was filled with authigenic calcite, in the structure of which impurity ions Mn2+ and paramagnetic centers SO3- were accumulated. The association of structural defects in the early diagenetic carbonate cement of sandstones indicates that the pore solutions of the bar structure were represented by desalinated sea water. The areas of the bar preserved in unchanged form show that before the arrival of the oil-water fluids, the calcite cement completely filled the pore space of the sandstones. The introduced water-carbonic solutions, preceding the water-oil fluids, dissolved calcite cement, creating a capacitive space for the arrival of hydrocarbons. Carbonic solutions, when interacting with effusive fragments, activated the hydrolysis processes. The product of hydrolytic reactions was analcite. Aggregates of analcime formed secondary cement in graywacke sandstones. The incoming hydrocarbons that filled the pores of the reservoir, inhibit all subsequent processes. The next stage of transformation of reservoir rocks is associated with the oxidation of hydrocarbons. Biochemical reactions of oxidation of hydrocarbons facilitated the saturation of pore solutions with carbon dioxide and hydrogen sulphide. As a result of this, rhombohedral crystals of dolomite and calcite began to form in the porous space of sand reservoirs. The relics of the early diagenetic calcite cement were metasomatically replaced by pyrite. Thus, there are three stages of the sand reservoirs formation in Nizhne-Karmalskoye super-viscous oil deposit. In the first, sedimentation-diagenetic stage, the primary structural framework of sandstones, cemented with calcite, was formed. In the second, fluid dynamic, the processes of carbonate cement dissolution and hydrolysis of fragments of effusive minerals became more active. In the third stage, biocost-metasomatic stage, dolomite, calcite and pyrite were formed in the sand collectors.

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