Former Soviet Union Countries

There are said to be numerous fields in the FSU producing from fractured basement reservoirs (Kenny, 1996), but very little detail has been published in the West. Kenny (1996) states that more wells have been drilled into crystalline basements within the FSU than all other nations combined with the consequence of greater production. For example, the Caspian district has a total of eighty fields producing from crystalline basements. Unlike the majority of drilling operations which cease as soon as basement rocks are encountered (Aguilera, 1995b), Krayushkin et al (1994) state that all of the hydrocarbon fields within the FSU producing from crystalline basements were developed intentionally.

Published articles from a working conference on oil in granite held in Kazan, Tatarstan, Russia in late 1997, (see latest reference section), refer to basement oil shows in the Chibuiuskoye, Verkhnechutinskoye and Iskosgorinskoyeoil fields, together with the Zelenetsky, Chernorechensky, Lekkemsky and Timansky oil productive areas. Production statistics from individual wells or fields were not made available.

One such example is discussed by Krayushkin et al (1994) involving an exploration project on the flanks of the Dnieper-Donets Basin. An initial geological study of the sedimentary, metamorphic and igneous rocks in the 'Northern Monoclinal Flank' of the Dnieper-Donents Basin concluded that there was no potential for hydrocarbon production. The conclusion was made because of the absence of any source rock and the presence of active, strongly circulating artesian waters.

However, the exploration and drilling programme which followed the initial study resulted in the discovery and development of 12 fields with oil reserves equal to 219 million metric tons of oil equivalent, the major part of which, according to Krayushkin et al (1994), is produced from the PreCambrian crystalline basement. However, this is difficult to demonstrate, partly because of multiple completions in basement and overlying cover (Kitchka, pers. comm., 1999).

The fields were discovered in an area covering 30-35 km by 400 km where the oil and gas bearing rocks are Carboniferous sandstones and PreCambrian granites, amphibolites and schists of the crystalline basement complex. The exploration programme also resulted in the discovery of a gas field with reserves of 100 billion cubic metres.

From a total of 61 wells drilled in a corridor 35 km wide by 400 km long, 37 produced commercial quantities of hydrocarbons (an exploration success rate of 55%). Initial flows from the productive wells varied between 40 and 350 metric tons/day of oil and 100,000-1,600,000 m3/day of gas. Production interval depths within the PreCambrian basement varied between 3,135 m and 4,041 m. Recently we have learnt of a new discovery in PreCambrian basement called Goshinovskoye field (Kitchka, pers. comm., 2000). Near Khark another corridor 30 km wide by 100 km long is associated with 3.5 Tcf reserves (Kitchka, pers. comm., 1998)

Tatarstan

A well at Novoyelkhovskaya penetrated crystalline basement at 1845 m and significant hydrocarbon shows have been reported below 4500 m (Kitchka, pers. comm., 1998)

Georgia - Samgori Field

The Samgori field is located near Tbilisi, in the Republic of Georgia. Production of more than 165 M bbl of oil has come from a middle Eocene laumontite tuff which is enclosed within a thick layer of andesite-basalt tuffs and tuffites which act as a seal (Grynberg et al, 1993). Productivity is dominantly controlled by fracture porosity and permeability. The fractured tuffs are relatively isolated from the action of geostatic and geotectonic loads which would otherwise have resulted in closure of fractures and microfractures. In addition, the fluid pressure tends to maintain open fractures.

Russia - Shaim Field

Located on the western side of the West Siberia basin on the Shaim uplift, the Shaim oil field is situated on the eastern slope of the Ural Mountains. Oil was first found in a basement fracture in 1959 (from Well 2). Well 7, on the west flank of the uplift, produced 25 bbl/day in April 1960 from basement fractures. Well 11, also on the west flank, produced 28 bbl/day from fractured basement rock. The basement rock consists of Paleozoic metamorphic and igneous rocks mainly composed of sericitic/siliceous schists intercalated with fine sandstone, fine conglomerate, marble, granite and gneiss (P'An, 1982). Well 6 on the east flank flowed at 2,380 bbl/day in June 1960 from the Vogulkin sandstone (Upper Jurassic). Oil from the very productive wells in the Shaim oil field does not come from basement rock. The basement rock (producing from wells 7 and 11) yields only several tons of oil per day (about 25 bbl/day).

Kazakstan - Oimasha (Oymasha) Field

The Oimasha oil field is located in the South Mangyshlak Trough, in a sub-basin sometimes referred to as the Peschanomyss-Rakushechnoe Uplift. The field was undeveloped as at 1995. However, Well 12 tested approximately 1,382 bbl/day from fractured/weathered granite at a depth of 3,720 m to 3,752 m. The granite intrudes into Early Palaeozoic metamorphics (Reisser, 1996).