DISTRIBUTION OF OIL AND GAS FIELDS AT GREAT DEPTHS

S. N. GULYAEV

(Presented by Professor A. V. Aksarin)

Intensive prospecting and exploration work in the European part of our country and abroad (USA, Western Europe) has identified most zones of oil-and-gas accumulation down to depths of 4–4.5 km [2, 9, 10], which has made it necessary to begin prospecting and exploration at great depths (4.5–10 km). These efforts have led to the discovery of numerous gas, gas-condensate, and oil pools and fields in the USA.

However, in the USSR only isolated commercial accumulations of oil and gas have so far been discovered. These are the Ust-Pogozhskoye, Antipovskoye, and Tashlinskoye oil fields in the near-flank part of the Precaspian syneclise; gas-condensate and gas-oil pools in Pliocene sandstones at the Karadag, Sangachaly, and Duvannyi-more fields in Azerbaijan; and oil pools in Upper Cretaceous carbonates of the Tersko-Caspian marginal trough [3]. But even these modest results of drilling indicate that commercial accumulations of oil and gas may be encountered at depths greater than 4.5 km, both in carbonates and in sandstones, although in the latter a deterioration of reservoir properties with depth is noted [1, 7, 8].

In France, in the Pre-Pyrenean oil-and-gas-bearing region, at a depth of 5400 meters in carbonates of Neocomian–Turonian age, the large Meillon gas field (80–100 billion m³) has been identified [10].

The largest number (more than one hundred) of gas, gas-condensate, and oil fields (including the largest among them—Gomez, Puckett, and Koyanoza) at great depths has been discovered in the USA. There, since 1966, about 400 wells have been drilled annually to depths greater than 4000 m; moreover, more than 50 of them over the last three years have had depths exceeding 6100 meters [11]. In March 1972, the world’s deepest well, No. 1 Baden, was drilled in the USA to a depth of 9159 m in the central part of the Anadarko Basin, Oklahoma. At bottomhole, the well penetrated Cambro-Ordovician dolomites of the Arbuckle Group, and the temperature and pressure in the bottomhole zone were, respectively, 204°C and 1400 atmospheres [11].

Drilling to great depths in the USA has led to the discovery of gas, gas-condensate, and oil pools in the Permian, Wichita, and Gulf Coast oil-and-gas-bearing provinces of the platform part of North America, and in the Green River and San Joaquin intermontane basins. The Green River Basin is a component part of the epiplatform region.

...of the Rocky Mountains; the San Joaquin basin lies within the Cenozoic folding of the Cordilleras [10]. Almost all fields are associated with structural traps.

In the Permian oil-and-gas-bearing province, in the Delaware and Val Verde basins, at depths of 4.5–8 km in the limestone-dolomite sequence of the Cambro-Ordovician Ellenburger formation, 32 gas and gas-condensate fields have been discovered, with explored reserves of 2.2 trillion m³, a significant part of which is concentrated in the largest fields: Gomez—420 billion m³, Puckett—234 billion m³, Coyanosa—144 billion m³, Lockridge—102 billion m³ [13]. The average porosity of the carbonates of the Ellenburger formation is 2–7%, and the high absolute open flows from wells, 3–22 million m³/day, testify to their good permeability. Gas-condensate pools here are traced down to a depth of 5200 m, and below that only gas pools occur. Some researchers [4, 14] believe that the gas and gas-condensate pools at great depths in the oil-and-gas-bearing province under consideration formed as a result of the destruction of oil, since at a depth of 1370 m oil with a density of 0.845 is encountered here in the Ellenburger formation; then, with depth, the density of the oil of this horizon in the pools decreases, reaching 0.800 at a depth of 3200 m, after which gas-condensate pools occur down to a depth of 5200 m, and below them only gas pools. This classic example of the phase state of hydrocarbons with depth in one horizon (the Ellenburger formation) convincingly indicates the influence of thermodynamic factors in the distribution of oil and gas pools in the Permian province.

In the Wichita oil-and-gas-bearing province, in the Anadarko and Ardmore basins, commercial accumulations of hydrocarbons have been found at great depths in the carbonates of the Hunton (Silurian-Devonian) and Arbuckle (Cambro-Ordovician) formations, and at the Gageby Creek field also in sandstones of the Simpson formation (Ordovician). The pools here, as in the Permian province, are predominantly massive, less often bedded, arched. The fields are mostly small, with reserves of 4–10 billion m³, but some of them are characterized by the very highest well yields. At the Gageby Creek field, from the carbonates of the Hunton formation (Silurian-Devonian), gas was obtained from a depth of 4573 m with an absolute open flow of 48.7 million m³/day, and at the Buffalo Wallow field, from a depth of 6072.5 meters—77.8 million m³/day. Noteworthy is the increase in porosity of the carbonates of the Hunton formation with depth from 8.5 to 18% at depths of more than 6000 meters [5]. The distribution of oil, gas-condensate, and gas pools here is controlled by the following depths. Down to a depth of 4900 m (temperature 115° C) oil pools still occur; lower, in the depth interval 4900–5800 m, gas-condensate pools predominate; and from a depth of 5800 m only gas pools have been found [5]. Thus, in the Wichita oil-and-gas-bearing province, as in the Permian, a vertical zonality is observed in the distribution of hydrocarbons, with a clearly expressed tendency for gas potential to increase downward in the section; only the gas zone in the former begins at a depth of 5200 m, and in the latter—from a depth of 5800 m.

The Gulf Coast oil-and-gas-bearing province is confined to the marginal basin of a young platform, where the thickness of Mesozoic-Cenozoic sediments reaches 15–20 m. Here, unlike the Permian and Wichita oil-and-gas-bearing provinces, along with gas-condensate and gas pools, oil pools are encountered almost throughout the entire section penetrated by wells (to a depth of 7000 m)—the Lake Washington, Lake Barre, and Richardson fields—and the reservoirs are sandstones.

Miocene. In all, several dozen fields and pools have been discovered here at great depths, of which more than 19 are oil-bearing [12]. Thus, at the Lake Washington field a productive oil bed was penetrated at a depth of 6542 m, with a reservoir temperature of 156°C. However, in the Gulf Coast province, within the Mesozoic–Cenozoic platform cover, an increase in gas occurrence with depth has also been established. Beginning at depths of 4500 m, the proportion of gas pools in the sedimentary cover increases.

The reservoir properties of the Neogene sandstones remain good. At the Lake Barre field, the porosity of sandy reservoirs at depths greater than 5000 m is 30%, and in the West Delta area (block 58) the sandstones have a porosity of 28%, with a permeability of 620 millidarcies [13].

In Florida, in recent years gas-oil and oil pools have been identified at depths of 4800–5000 meters. The pools here are confined to Jurassic carbonates (the Smackover Formation), and the rates of individual wells from them reach 189 t/day [15].

In addition to the oil-and-gas-bearing provinces considered, in each of which dozens of gas, oil, and gas-condensate fields occur at great (4.5–8 km) depths, individual gas and oil pools have been discovered in the intermontane depressions of Green River and San Joaquin in the USA and Maracaibo in Venezuela [6]; in the first of these, Paleozoic deposits are gas-bearing. In the Maracaibo depression, oil pools have been encountered in Cretaceous sediments, while in the San Joaquin depression (California), at the Coles Levee field, oil with a density of 0.751 was obtained from a depth of 5760 m from Eocene sandstones at a reservoir temperature of 141°C [4].

The review made of the distribution of gas, gas-condensate, and oil fields at great depths shows that in the depth interval 4000–6000 m, along with gas fields, oil pools may be identified, and the probability of discovering the latter increases as the age of the productive strata decreases.

At depths of 6000–7000 m, with a sharp predominance of gas pools, isolated gas-condensate and oil pools and fields occur only in young Cenozoic sediments.

The depth zone of 7000–8000 m is predominantly gas-bearing for all sedimentary basins.

The presence at great depths of 4.5–8 km of numerous gas and gas-condensate fields in the USA, with total reserves of several trillion m³, indicates the great potential gas-bearing capacity of the deep subsurface of the Caspian, Dnieper-Donets, and South Caspian depressions, the northern regions of the West Siberian and southern regions of the Turan plates, the northern part of the Timan-Pechora oil-and-gas-bearing region, and many regions of the Siberian ancient platform, where the thickness of the sedimentary cover is great.

REFERENCES

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1. Lodzhevskaya, M. I. Principal regularities in the distribution of deeply buried oil and gas pools.—Author’s abstract of Candidate’s dissertation. Moscow, 1971.

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9. Chaikovskaya, E. V. On the evaluation of the prospects for gas potential at great depths.—Proceedings of SevKavNIIgaz, issue 5. Ordzhonikidze, 1972.

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