Geology of the Volga-Ural petroleum province and detailed description of the Ramashkino and Arlan oil fields

The Volga-Ural petroleum province is in general coincident with the Volga-Ural regional high, a broad upwarp of the east-central part of the Russian (East European) platform. The central part of the province is occupied by the Tatar arch, which contains the major share of the oil fields of the province. The Perm-Bashkir arch forms the northeastern part of the regional high, and the Zhigulevsko-Orenburg arch makes up the southern part. These arches are separated from one another by elongate downwarps. The platform cover overlies an Archean crystalline basement and consists of seven main sedimentation cycles as follows: 1) Riphean (lower Bavly) continental sandstone, shale, and conglomerate beds from 500 to 5,000 m thick deposited in aulacogens. 2) Vendian (upper Bavly) continental and marine shale and sandstone up to 3,000 m thick. 3) Middle Devonian-Tournaisian transgressive deposits, which are sandstone, siltstone, and shale in the lower part and carbonates with abundant reefs in the upper; thickness is 300-1,000 m. In the upper carbonate part is the Kamsko-Kinel trough system, which consists of narrow interconnected deep-water troughs. 4) The Visean-Namurian-Bashkirian cycle, which began with deposition of Visean elastics that draped over reefs of the previous cycle and filled in an erosional relief that had formed in some places on the sediments of the previous cycle. The Visean elastics are overlain by marine carbonates. Thickness of the cycle is 50-800 m. 5) Early Moscovian-Early Permian terrigenous clastic deposits and marine carbonate beds 1,000-3,000 m thick. 6) The late Early Permian-Late Permian cycle, which reflects maximum growth of the Ural Mountains and associated Ural foredeep. Evaporites were first deposited, then marine limestones and dolomites, which intertongue eastward with clastic sediments from the Ural Mountains. 7) Continental redbeds of Triassic age and mixed continental and marine clastic beds of Jurassic and Cretaceous age, which were deposited on the southern, southwestern, and northern margins of the Russian platform; they are generally absent in the Volga-Ural province, however. The Volga-Ural oil and gas basin is a single artesian system that contains seven aquifers separated by seals. The areas of greatest hydraulic head are in the eastern parts of the basin near areas where the aquifers crop out on the western slopes of the Ural Mountains. The Peri-Caspian basin is the principal drainage area of the artesian system. Approximately 600 oil and gas fields and 2,000 pools have been found in the Volga-Ural province. Nine productive sequences are recognized as follows: 1) Upper Proterozoic (Bavly beds), which are promising but not yet commercial, 2) Clastic Devonian, which contains the major reserves and includes the main pays of the super-giant Romashkino field. 3) Carbonate Upper Devonian and lowermost Carboniferous, which is one of the main reef-bearing intervals. 4) Visean (Lower Carboniferous) elastics, which are the main pays in the super-giant Arlan field. 5) Carbonate Lower and Middle Carboniferous. 6) Clastic Middle Carboniferous Moscovian. 7) Carbonate Middle and Upper Carboniferous. 8) Carbonate-evaporite Lower Permian, which contains the major gas reserves and the lower part of the Melekess tar deposits. 9) Clastic-carbonate Upper Permian, which contains the major part of the Melekess tar deposits. The Volga-Ural province is divided into several productive regions on a basis of differences in structure, distribution of reservoir and source-rock facies, and general composition of the petroleum accumulations. These regions are the Tatar arch, Birsk saddle, Upper Kama depression, Perm-Bashkir arch, Ufa-Orenburg monocline, Melekess-Sernovodsko-Abdulino basin, ZhigulevskoOrenburg arch, Ural foredeep, and north borders of the Peri-Caspian depression, Exploration activity has declined in recent years; however, interest remains high in several parts of the province, particularly in the Ural fore-deep, Perm-Bashkir arch, Upper Kama area, and north border of the Peri-Caspian depression. The super-giant Romashkino oil field is in the middle of an oily belt that extends along the eastern margin of the Russian platform. For twenty years it was the largest producer in the U.S.S.R. The field covers an area of 3,600 sq km, and more than 10,000 wells have been drilled. Although in decline, production in 1981 was still 400 million barrels. The main pay zone is a complex succession of beds and lenses of sandstone, siltstone, and shale in the Middle Devonian Frasnian stage. The trap is a broad, hummocky structural plateau on which the crest of the main pay zone is at -1,435 m and closure is about 60 m. In addition to the Devonian pays, flows of oil have been recovered from five additional pays higher in the Paleozoic section. Initial formation pressure in the Devonian pools at Romashkino was 175 atm, and saturation pressure was 81-95 atm. The gas-oil ratio ranges from 41 to 54 m^/rn3 , and density is about 0.805. Sulfur content is 1.5-2.1 percent, paraffin is 2.6-5.4 percent, and tar content is 30-48 percent. The field was divided into 23 independent production sectors and worked by water flood from the very beginning. Facies changes within the pays are so abrupt, however, that three, and in places four, rounds of infill drilling have been necessary. The giant Arlan field is the second largest producer in the Volga-Ural province. The field is 90 km long and 15-20 km wide. Production in 1981 was 62 million barrels. The main pay zones at Arlan are Lower Carboniferous Visean elastics that, along with underlying Tournaisian limestones, drape over Upper Devonian reefs. The draping of the Visean pays is enhanced further by their having been deposited on a pre-Visean erosion surface where channels had been cut in the area between the reefs. Initial formation pressure in the main pays at Arlan was 142 atm, and saturation pressure was 99 atm. Gas-oil ratio was 16-21 m^ per ton. The oil is very viscous; values range from 15 to 20 cps. Density is 0.874-0.881. Sulfur content is 2.8-3.4 percent. Formation pressure has been maintained by water flood.