Patterns of Distribution of Giant Oil and Gas Fields in the Earth’s Crust

S. N. Gulyaev

(Presented by Professor A. V. Aksarin)

The largest and giant oil and gas fields determine the principal reserves and production of hydrocarbons on Earth, and their development yields the maximum economic effect.

Many researchers have dealt with the analysis of the distribution of oil and gas fields in individual regions [1, 2, 6, 8, 9, 12], and the general patterns of hydrocarbon distribution have been examined by M. K. Kalinko [7]. However, discoveries of recent years make it possible to substantially refine our ideas about the patterns of distribution of unique fields in space and time (Table 1).

In Cambrian sediments only one giant oil field has been discovered: Hassi Messaoud, located in Algeria on the El Biod arch. It is confined to a large (43 × 30 km) dome-shaped uplift, clearly expressed in Lower Paleozoic deposits and weakly expressed in Triassic deposits [5]. The main productive horizons, composed of quartz sandstones at depths of 3300–3500 m, have been truncated by erosion and sealed by the clayey-halogenic sequence of the Triassic. Such a structural-stratigraphic trap indicates a post-Triassic time of field formation and, possibly, an influx of hydrocarbons from Mesozoic sediments, which at lower hypsometric levels also adjoin rocks of the Lower Paleozoic. It is also noteworthy that the Lower Paleozoic deposits in the Algerian Sahara are represented chiefly by sandstones and siltstones and constitute a single hydrodynamic complex.

No unique fields have been discovered in Ordovician and Silurian deposits, and even the largest accumulations are rare in them. This, in our opinion, is explained by the absence in them of sufficiently

Table 1

Distribution of giant oil and gas fields in the Earth’s crust

of thick, regionally persistent permeable complexes under favorable tectonic conditions or by the poor degree of study of the regions.

Individual giant oil fields have been discovered in Devonian sandstones on the Russian Platform, where their position is controlled by the crest and swell of the southeastern part of the Tatar arch, by the area of distribution of the Eifelian–Lower Frasnian terrigenous complex and by the Kynov–Sargaev regional seal overlying it. Moreover, the largest pools here occur in sediments of the lower part of the transgressive cycle beneath this seal, which is composed of clays and clayey carbonates.

To the northeast of them, toward the swell situated in the most uplifted part of the Birsk saddle, a single oil field in Carboniferous deposits is confined; it is likewise associated with the regional terrigenous sequence.

The clear connection of the fields considered with the most, or relatively, uplifted positive structures and with persistent terrigenous complexes indicates that their formation occurred to a considerable extent through lateral upward migration from adjacent depressions and troughs. Confirmation of this for the Devonian pools is also the evident discrepancy in the amount of oil,

that migrated from the rocks of the terrigenous Devonian and is situated on the southern culmination of the Tatar arch, where there is much more of it [10]. Since each petroleum giant is an integral function of all migration flows from the beginning of field formation, migration criteria, which are a consequence of the tectonic and lithological conditions of the region, must be used in prospecting for them.

No giant fields have been discovered in Middle Paleozoic carbonates, which is apparently explained by the absence in them of persistent porous permeable sequences that would ensure the accumulation of hydrocarbons from large oil-and-gas catchment areas and serve as reservoirs for unique accumulations. As L. U. Illing [14] notes, the cementation of carbonates, along with compression, compaction, and other factors, is also influenced by their age.

In Permian formations the first gas giants appear, both in terrigenous and in carbonate rocks. The Permo-Carboniferous carbonates, occurring beneath thick (100–1200 m) halogen sequences in the lower part of the regressive cycle, contain the largest gas accumulations on the North American and Russian platforms (the Panhandle-Hugoton and Orenburg fields) and in the outer zone of the Cis-Ural marginal trough. The presence of permeable strata in the Permo-Carboniferous at the Vuktyl and Orenburg fields is explained to some extent by the position of the former in the marginal trough, and of the latter adjacent to it, where increased tectonic mobility caused fracturing of the carbonates. However, the enormous (200 × 56 km) lithologic Hugoton accumulation in Permian dolomites and limestones, confined to the gentle (1–1.5°) monocline of the western flank of the Dodge City basin of the North American Platform [1], shows that not only tectonic factors were decisive in the creation of unique reservoirs in Upper Paleozoic carbonates. Evidently, at that time favorable conditions arose in an arid climate for the formation of persistent sequences of porous and permeable carbonates, which, in our opinion, are determined by a sharp decrease in the carbon dioxide content of the atmosphere. The unique accumulations of hydrocarbons in Triassic terrigenous sediments are associated with the large uplifts of the African Platform and of the Alaskan marginal trough.

The gas pools of the Hassi R’Mel and Rhourde Nouss fields in Algeria are located in the lower part of a transgressive Triassic complex that unconformably overlies Lower and Middle Paleozoic sediments. The productive sandstones here are separated from one another by clay beds, which serve as zonal seals. The regional seal for the gas pools is a 500-meter Triassic halogen sequence composed of anhydrites and salt.

The Prudhoe Bay oil field, located onshore and partly offshore in the Arctic Ocean, in the Alaskan marginal trough, is a large (60 × 30 km) gentle buried uplift, where Upper Paleozoic, Triassic, and Lower Jurassic sediments are overlain by Lower Cretaceous argillites. The structural-stratigraphic pools, occurring here at depths of 2430–2650 m, are located in Carboniferous limestones and Jurassic sandstones, but 85% of the reserves of the entire field are contained in the Triassic terrigenous rocks [13].

In Jurassic deposits the first oil giants appear in carbonates, and their number increases sharply. Five fields are situated on the Arabian Platform and one on the Turan Platform. In the eastern marginal part of the Arabian Platform, where the thickness of the sedimentary cover is 4–10 km, the Ghawar, Abqaiq, Khurais, Manifa, and Umm Shaif fields are confined to contrasting local uplifts

Fig. 1. Relationship of the types of pools, seals, and their lithological composition in the largest oil and gas fields in sedimentation cycles:
1 — sands, sandstones; 2 — oil-bearing or gas-bearing sands; 3 — siltstones; 4 — clays, argillites; 5 — clayey limestones; 6 — limestones; 7 — dolomites; 8 — oil-bearing or gas-bearing carbonates; 9 — anhydrites; 10 — salt.

or even swells, and contain, beneath the regional halogen seal of the Hith Formation, unique accumulations of oil in Upper Jurassic fractured dolomites and dolomitized limestones [2, 11] and in fine-clastic limestones (calcarenites) with primary porosity [11].

In terrigenous Jurassic deposits, the largest accumulation of oil is controlled by a local uplift situated on the northern flank of the South Mangyshlak–Assakeudan depression.

The greatest number (16 out of 45) of oil and gas giants, and the largest of them (Greater Burgan, Safaniya-Khafji, Urengoy, Medvezhye), occur in Cretaceous terrigenous and carbonate sediments of young and ancient platforms (Table 1). Here they are more often encountered on contrasting uplifts of the submerged part of a platform adjoining a marginal trough (the eastern slope of the Arabian Platform), on slopes and megaswells surrounded by deep depressions and troughs (fields of Western Siberia), and rarely on uplifts situated on the flanks of depressions, such as Sirte in Libya and the Amu Darya in the USSR. Only one oil field, East Texas in the USA, has a structural-stratigraphic trap and is confined to a broad structural nose on the western slope of the Sabine arch, where the Upper Cretaceous productive sandstones are truncated by erosion and sealed by clay shales and limestones. The largest Cenomanian gas pools of the West Siberian and Turan platforms occur in the lower part of the Albian–Upper Cretaceous transgressive complex beneath regional clay seals with thicknesses ranging from 104 m at Gazli to 400–700 m at the Urengoy, Medvezhye, Yamburg, and Komsomolsk fields. On the Arabian Platform, unique accumulations of oil in the Albian–Cenomanian sands and sandstones at the Greater Burgan, Raudhatain, Wafra, and Safaniya-Khafji fields in Kuwait and Saudi Arabia are associated with this same complex, which rests with a regional unconformity on Aptian limestones of the Shuʿaiba. The productive interval, represented in its lower part by coarse- and medium-grained, and in its upper part by fine-grained glauconitic sands and sandstones, is reliably overlain here by clays and argillaceous limestones of the Ahmadi and Magwa formations of Cenomanian–Turonian age. In Libya, a massive oil pool likewise occurs in Albian–Cenomanian basal sandstones beneath variegated clay shales with interbeds of anhydrites, overlain in turn by marine argillites and limestones of the Upper Cretaceous [4]. The facts cited from different regions indicate a clear connection of these fields with the maximum Upper Cretaceous transgression, that is, with their position in the lower part of the transgressive complex, the upper part of which, composed of marine clays and sometimes carbonates, often serves as a regional seal for the richest oil and gas pools.

However, the fields of Western Siberia, Murban, Lulu-Esfandiyar, and Rumaila on the Arabian Platform contain beneath zonal clay seals the largest accumulations of oil in Neocomian sandstones and carbonates located in the lower part of the regressive cycle. Thus, on platforms, optimal conditions for the formation of oil and gas giants exist in the lower parts of transgressive and regressive cycles, above and below sediments corresponding to the maximum transgression.

In addition to platforms, the largest accumulations of oil in Cretaceous limestones have been established in fields of the inner zone of the Mesopotamian marginal trough and the Maracaibo intermontane basin. However, in the trough, at the Iranian fields Agha Jari, Bibi Hakimeh, and Gachsaran, a hydrodynamic connection exists [3] between the Upper Cretaceous limestones and the Asmari of Oligocene–Lower Miocene age, indicating the presence in the Cenozoic–Upper Cretaceous sequence of single massive pools with heights from 1500 to 2200 meters. Such pools formed here as a result of high tectonic mobility, which created regional fracturing of the upper carbonate sequences and large (45–60 × 3–6 km) contrasting uplifts. The presence in the fields of southwestern Iran of large gas caps and a unique oil—

of the Pazenun gas field, with gas reserves of 1,274 billion m³, attest to the high qualitative characteristics of the halogen regional seal of the lower Fars, beneath which they occur.

In the Maracaibo Alpine intermontane basin, at the Bolivar-Coastal field, which is a combination of stratigraphic, structural, and lithological pools, the principal oil reserves are concentrated in Cenozoic terrigenous sediments. The Paleogene sands and sandstones are the richest in oil.

In other areas of the Alpine folded regions, in the intermontane basins of Los Angeles in the USA and Central Sumatra in Indonesia, the oil giants Wilmington [15] and Minas are confined to contrasting uplifts and are associated with Neogene terrigenous strata.

All the intermontane basins noted are filled with thick (several kilometers) sequences of sediments and open into the waters of seas and oceans.

Thus, the analysis carried out makes it possible to establish the following patterns in the distribution of oil and gas giants:

1. All of them occur in areas with a thick (more than 3 km) sedimentary cover or in regions adjacent to them on platforms (35 out of 46), in marginal troughs and coastal intermontane basins of Alpine folded regions, and, with the exception of Bolivar-Coastal, are controlled by structural traps.

2. In ancient, Lower–Middle Paleozoic and Lower Carboniferous sediments, oil giants on platforms occur only in regional terrigenous complexes (Cambrian–Ordovician of the Algerian Sahara; Eifelian–Lower Frasnian and Visean coal-bearing strata of the Volga–Ural region), in which conditions exist for the collection of hydrocarbons from large areas, their long-distance migration, and accumulation in unique pools on positive structures.

3. In carbonate rocks, the distribution of giant pools is limited by stratigraphic and paleogeographic boundaries. All carbonate strata containing such oil and gas pools formed only in an arid climate, mainly during marine regression, when conditions existed for the formation of persistent oolitic and clastic primary-porous limestones and cavernous sedimentary-diagenetic dolomites. The appearance of the first unique pools from the Upper Paleozoic onward in carbonates is apparently explained by a sharp decrease in carbon dioxide in the atmosphere at that time, a change in the ratio of dolomite to calcite in fresh sediment, and, as a consequence, the emergence of persistent zones of cavernous porous metasomatic dolomites in limestones.

In marginal troughs, the presence of fields in carbonates is determined to a considerable extent by their regional fracturing.

1. On platforms, fields occur in the lower parts of transgressive and regressive cycles, above and below the sediments corresponding to the maximum of transgressions. The greatest number of oil and gas giants is associated with the Albian–Cenomanian terrigenous basal sequence of the Upper Cretaceous maximum transgression. In marginal troughs they are located in the lower part of the regressive cycle.

2. Gas giants on platforms occur beneath thick (100 m and more) clayey and halogen regional seals, whereas in marginal troughs they occur only beneath halogen seals, which is a necessary condition for their formation.

References

1. A. A. Bakirov. Oil-and-gas-bearing regions of North and South America. Gostoptekhizdat, 1959.

2. A. A. Bakirov, A. M. Pronina. Oil-and-gas-bearing regions of the Near East and Southeast Asia. Gosgeolizdat, 1962.

3. V. G. Vasiliev, A. V. Aleksandrov, A. D. Brents. The raw-material base of the trans-Iranian main gas pipeline.—In: Geology of exploration and development of gas and gas-condensate fields. Moscow, 1969.

4. M. I. Varentsov, G. E. Ryabukhin, E. R. Alieva. New data on the oil-and-gas potential of Libya.—Geology of Oil and Gas, No. 6, 1969.

5. M. I. Varentsov, M. V. Doroshko, E. L. Melamud. On the geological structure and oil potential of the Cambrian deposits of North Africa and their significance for assessing the prospects for oil-and-gas potential of Eastern Siberia.—In: Tectonics of oil-and-gas-bearing regions of Siberia. “Nauka,” Moscow, 1967.

6. F. G. Gurari, A. E. Kontorovich, et al. Some regularities in the distribution and conditions of formation of oil and gas accumulations in Mesozoic deposits of the West Siberian Lowland and the Siberian Platform.—Transactions of ZapSibNIGNI, issue 3, 1967.

7. M. K. Kalinko. Basic regularities in the distribution of oil and gas in the Earth’s crust. Moscow, “Nedra,” 1964.

8. V. A. Klubov, S. K. Nechitailo, et al. Geological regularities in the distribution of oil and gas fields of the Volga–Ural region.—Transactions of VNIGNI, issue LXVII, “Nedra,” 1968.

9. N. T. Lindrt. Reserves of large and medium oil and gas fields abroad and the basic regularities of their distribution.—In: Results of scientific-research work on the search for and exploration of oil and gas. Transactions of VNIGRI, issue 272, 1969.

10. S. G. Neruchev. Oil-producing formations and migration of oil and gas. “Nedra,” 1969.

11. G. E. Ryabukhin, E. R. Alieva, V. A. Rudik. Oil-and-gas-bearing offshore areas of the Persian Gulf.—Geology of Oil and Gas, 1969, No. 7.

12. A. A. Trofimuk, V. S. Vyshemirskii, A. N. Dmitriev, V. V. Ryabov, O. P. Vyshemirskaya, I. A. Ril, T. I. Shtatnova. On a comparative study of giant oil fields using logical-discrete analysis.—Geology of Oil and Gas, 1969, No. 6.

13. Basye Dale. Secrece shround lifts on giant Prudhoe Bau. fild. Oil and Gas J., 67, No. 47, 1969.

14. Jlling L. U., Wood G. U and Fuller G. C. Reservoir rocks and stratigraphis traps in non-reef carbonates. 7-th World Petrol. Congr., Mexico, 1967.

15. Szasz S. E., Adent W. A., Frantozi J. H. Special development problems of Long Beach unit, Wilmington. Oil and Gas J. 67, No. 30, 1969.