Volcanic Reservoirs in Petroleum Exploration (eBook)

Figure 1.1 Global oil and gas distribution in volcanic rock, among which there are 169 reservoirs, 65 cases with oil and gas shows, and 102 cases with oil seepage.

1.1.1.1 Overseas Exploration for Volcanic Reservoirs and Division of Research Period

Most reservoirs in volcanic rocks were the accidental by-products of traditional shallow oil reservoirs, and were considered to be of no commercial value, and therefore were ignored without appraisal or study.

Researchers eventually became aware that hydrocarbon accumulation in volcanic rocks is not an abnormal phenomenon and conducted oil and gas exploration intentionally in volcanic rocks in some local areas. La Paz oil field, discovered in Venezuela in 1953 with the highest oil production in a single well (up to 1828 m3 d− 1), was the first successful case in the world of exploration aimed at volcanic rock. From this point on, the study of volcanic oil reservoirs entered a new phase.

After the discoveries of oil fields in volcanic rock, volcanic reservoir exploration was conducted extensively around the world, giving birth to many discoveries of oil and gas reservoirs (fields) in the United States, Mexico, Venezuela, Argentina, former Soviet Union, Japan, Indonesia, and Vietnam. Well-known cases include the tuff oil reservoir in Samgori (Georgia), oil reservoir in eruptive rock in Muradkhanli (Azerbaijan), andesitic oil reservoir in Jatibarang (Indonesia), rhyolitic reservoir in Yoshii to east Kashiwazaki (Japan), and granitic reservoir in White Tiger oil field in shallow seas (south Vietnam).

1.1.1.2 Features of Overseas Research

Overseas volcanic reservoirs have undergone a long history of exploration with many discoveries of oil and gas reservoirs, but most were discovered by chance or in localized areas. Little attention has been paid to this kind of reservoir; therefore, volcanic reservoirs have not been extensively explored as a whole and their contribution to the global total of reserves only accounts for 1% or so. Research remains at a low level. Relevant papers and studies specializing in volcanic reservoirs are very few at home and abroad, resulting in a lack of understanding.

Neogene, Paleogene, and Cretaceous volcanic rocks are relatively rich in oil and gas, while little occurs in rocks Jurassic and older. Volcanic rocks that have formed reservoirs are likely to occur in strata from hundreds of meters to 2000 m deep, and seldom deeper than 3000 m.

Discovered volcanic reservoirs mainly occur in a circular distribution around the Pacific Rim, from the United States and Mexico in North America, to Cuba, Venezuela, Brazil, and Argentina in South America, and then to China, Japan, and Indonesia in Asia. They also have been discovered from Central Asia to Eastern Europe, including Georgia, Azerbaijan, Ukraine, Russia, Romania, and Hungary. Some countries in North Africa and Central Africa, e.g., Egypt, Libya, Morocco, and Angola, have reported discoveries of volcanic reservoirs as well.

In view of the structural setting of hydrocarbon-bearing basins, volcanic reservoirs mainly distribute at passive continental margins, e.g., those discovered in North America, South America, and Africa, and can also develop in inland rift basins.

With respect to rock type, volcanic reservoir rocks mainly include intermediate to basic rock, such as basalt and andesite. Primary and secondary pores serve as reservoir space. These develop from fractures and dissolved pores of various geneses and are crucial to the improvement of reservoir properties.

In general, oil and gas reservoirs have been reported to be limited in volcanic rock, but large, high-yield oil and gas fields do exist. Table 1.1 lists 11 overseas volcanic oil and gas fields with recoverable reserves all higher than 2000 × 104 tons oil equivalent, among which Jatibarang oil and gas field in the northwest Java Basin, Indonesia, is the largest oil field with recoverable oil reserves of 1.64 × 104 tons. Scott Reef oil and gas field in Browse Basin, Australia, is the largest gas field with recoverable gas reserves of 3877 × 108 m3. Table 1.2 lists the production of 12 overseas volcanic oil and gas fields, among which daily oil production of Cristales oil field in the North Cuba Basin is the highest, up to 3425 tons, and daily gas production of the Yoshii-Kashiwazaki gas field in Niigata Basin, Japan, is the highest, up to 49.5 × 104 m3.

1.1.2.1 Scott Reef Oil and Gas Field, Australia

Browse Basin lies in the northwestern sea area of Australia with seawater depth between 80 and 300 m and a basin area of 14 × 104 km2. Several oil and gas fields have been discovered in the basin, among which Scott Reef volcanic oil and gas field has the largest reserves (Table 1.2).

Browse Basin has a Paleozoic to Cenozoic deposition thickness greater than 15,000 m, with the development of several stages of a Mesozoic depositional basin. Most of the basin lies in continental shelf. There were six periods of structural evolution: (1) inward extension of the craton into a half-graben basin in the later Carboniferous to the early Permian, (2) heat subsidence in the later Permian to the Triassic, (3) tectonic reversal in the later Triassic to the early Jurassic, (4) extension in the early and middle Jurassic, (5) heat subsidence in the later Jurassic to the Cenozoic, and (6) reversal in the middle and later Miocene.

Source rocks mainly developed in the lower Permian, Jurassic, and lower Cretaceous, which were predelta onshore-plain, fine-grained sediment of coal-bearing mudstone. Reservoir rock includes river delta sandstone and Campanian-Maastrichtian marine sandstone of the middle and lower Jurassic, among which there are large amounts of volcanic interbeds in the Jurassic with good reservoir properties. Burial depth of the reservoir rock can reach 4000-5000 m. The depth of the basin margin is 3000-3500 m. In Scott Reef field, the burial depth of the reservoir rock lies between 3934 and 4695 m. Reservoir trap types primarily include fault, anticline, buried hill, and stratigraphic overlap.

1.1.2.2 Jatibarang Oil and Gas Field, Indonesia

Jatibarang oil field, northwest of Java, Indonesia, was discovered through well JTB-44 drilled in 1969. with zone of interest at 2011 m. The field entered the development stage in 1973. Daily oil production of a single well is from 250 to 3000 bbl and burial depth is from 2000 to 2300 m.

The Upper Cretaceous Jatibarang Formation pay zone is 1200 m thick and composed of andesite and dacite with interbeds of clay, glutinite in the lower part, and andesitic volcaniclastic rocks and altered volcanics in the upper part. Volcanic rock develops in an onshore fluvial environment. The reservoir is fractured volcanic rock, including lava (andesite/basalt), tuff, volcanic breccias, and agglomerate. Fractures, intergranular pores, and intercrystalline pores form the reservoir space. The reservoir rock is characterized by high heterogeneity and well-developed fractures with porosity varying between 16% and 25% and permeability of 10 × 10− 3 μm2. The burial depth of the reservoirs lies between 2700 and 4000 m. There are structural reservoirs and lithologic-stratigraphic reservoirs in this field.

1.1.2.3 Kudu Gas Field, Namibia

Kudu gas field in Namibia, discovered in 1974, lies in the western sea area of Namibia at water depths of approximately 170 m. Basalt of effusive facies distributes extensively at the African and South American continental margins with lateral extension from 60 to 100 km. This extension inclines seaward and forms the...