| Abstrakt: |
Practically all oil and gas accumulations throughout the world are associated with some extensive productive series which consist of highly permeable rocks containing oil and gas pools, interbedded with shales and marls containing dispersed organic matter. All of the important recent discoveries between the Volga and the Urals were based on the observed association of oil and gas pools with certain lithologic-stratigraphic complexes of the Devonian, Carboniferous, and part of the Permian; although accumulation is related to many diverse Structural zones. Here, the productive series is the source rock as well, for the pelites contain dispersed organic matter including bitumen whose composition is related to the oils.Where accumulations occur in sediments deposited under oxidizing conditions, factors are sought which favor migration from possible source rocks along faults, unconformities, or surfaces of facies change. For example, in Azerbaidjan and Turkmenistan large oil and gas accumulations in Pliocene continental beds occur along structural zones that allowed migration from underlying, principally Paleogene and Miocene, deposits.As shales compact, their organic constituents gradually change; the coaly part from a ligniferous to a carboniferous stage and, ultimately, to graphite. The mobile bituminous substances formed by dissociation of organic matter and disproportioning of hydrogen, change from asphaltines and resins to oils and then to methane in the subcapillary pores of pelites, where they are in a dispersed, loosely bound state. Compaction causes slow removal of the loosely bound water and molecular migration with differentiation of the bituminous substances. After differentiation, the more mobile hydrocarbons are carried by the water into reservoir rocks. The bituminous substances move from small to large pores also by capillary forces. Migration in subcapillary pores apparently ceases when rocks become lithified and are no longer plastic.In the super-capillary pores and fissures of reservoir rock, free migration takes place as both molecular migration and movement in larger masses. Water circulates in reservoir beds from elevated areas of the outcrop toward lower discharge areas. When the hydrocarbon-bearing water moves into different environments of temperature and pressure, the hydrocarbons may come out of solution, rise to the top of the carrier-beds, and unite into oil and gas pools if traps are available. The location of oil and gas reflects the present equilibrium of fluids with the structural pattern and hydrogeological environment. --D. C. Van Siclen. |
