| Popis: |
The intention of this chapter is to present a short description of the reservoir rocks, recognized types of the hydrocarbons traps, and a few chosen oil and gas fields in the basement and in the Miocene cover of the Carpathian Foredeep. Most of the oil produced from the Carpathian Foredeep basin has come from Oxfordian carbonates and Cenomanian sandstones, two of the most oil-productive reservoir rocks. The Devonian, Carboniferous, and Cretaceous carbonates are the secondary basement reservoirs. However, from the commercial point of view, the overlying Miocene clastic deposits cover has the prominent status. It consists of excellent source and reservoir rocks that have produced large amounts of gas. The primary reservoirs are sandstones of different depositional elements of submarine fans, sandstones of deltaic environments (large mouth bar, distributary channels, and others), the shallow-marine clastic deposits of estuaries, and sandy barriers. Sporadically, the gas accumulations are located in the secondary porous anhydrites. The most common basement oil and gas trap is the combination structural-stratigraphic type with varying systems of sealing. Traps are related mainly to the sub-Miocene and less to the sub-Cretaceous unconformity. The pinching-out stratigraphic traps are known from the Cenomanian sandstones. The stratigraphic traps in the carbonate buildups (reefs) have still-undiscovered potential. The traps for gas accumulations in the Miocene (Badenian and Sarmatian) deposits of the Carpathian Foredeep are related, first of all, to the paleomorphology of the pre-Miocene basement formed by the erosion supported by the faulting processes. This type of trap is classified as the compactional trap. The second, very productive structural traps were recognized beneath the Carpathian sole thrust, at the front of the Carpathians. The fault-related trapping mechanism is also known from several gas fields. The pinching-out trap types caused by the horizontal and the vertical facies changes are sparse and occur most commonly in the topmost part of the Miocene succession because of the more favorable facies and less compactional deformation of the strata. During last 50 yr of exploration, more than 120 gas and oil fields were discovered in the Carpathian Foredeep. The bulk of the produced hydrocarbons (97%, mainly gas) were contained in the Miocene deposits, with a further 3% in the Miocene basement. The Miocene contains only gas fields, whereas both oil and gas were found in the basement rocks. The Miocene gas fields are typically multihorizontal and saturated by gas with a very high methane content, normally from 95 to 99%. About 90 billion m3 (3.2 tcf) of this kind of gas has already been produced. The Przemysl gas fields group is the largest in Poland with gas initially in place (GIIP) of nearly 71 billion m3 (2.5 tcf), and the cumulative field production, as of December 31, 2002, amounted to 55 billion m3 (1.9 tcf). The Grobla and Plawowice are the biggest oil fields accumulated in the Oxfordian carbonates and Cenomanian sandstones of the Carpathian Foredeep basement. A few important gas fields, like Tarnow and Lubaczow, were also founded in the Oxfordian carbonates. According to the results of the latest deep wells, the basement of the Carpathian Foredeep is still highly prospective for hydrocarbons, especially the Devonian and Carboniferous carbonates in the central part of the foredeep and the Oxfordian buildups in the more western part. The deep wells like Hermanowa proved to be a very high source potential of the lower Paleozoic rocks, which allows for the prediction of new significant oil and gas discoveries in the nearest future. The improvement of methods, particularly the direct hydrocarbon indicators method, opened a new stage of exploration for gas accumulations in the Miocene deposits. In only the last 8 yr, nearly 20 new gas fields were discovered on the basis of such interpretation results. |
