Formation and distribution of volcanic hydrocarbon reservoirs in sedimentary basins of China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 35, Issue 3, June 2008 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2008, 35(3): 257–271.

RESEARCH PAPER

Formation and distribution of volcanic hydrocarbon reservoirs in sedimentary basins of China ZOU Cai-neng*, ZHAO Wen-zhi, JIA Cheng-zao, ZHU Ru-kai, ZHANG Guang-ya, ZHAO Xia, YUAN Xuan-jun Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China

Abstract: Volcanic hydrocarbon exploration in China has experienced three phases: accidental discovery, local prospecting, and all-round exploration. There are mainly Carboniferous-Permian, Jurassic-Cretaceous, Paleogene-Neogene volcanic rocks and lava, pyroclastics, and karst reservoirs in the oil- and gas-bearing basins in China. Volcanic rocks cannot generate organic hydrocarbons, and the combination of volcanic rocks, source rocks, and seals are the key controlling factor of the primary lava plays. The near-source play is most favorable for hydrocarbon accumulation. Distribution of oil and gas is controlled predominantly by the hydrocarbon generating center. The play requires communication with faults or unconformities. Near-source plays are in the faulted basins in eastern China. Structural-lithologic hydrocarbon reservoirs are formed in the higher place of faults and lithologic hydrocarbon reservoirs are formed on the slope. Two types of plays are developed in central and western China. The near-source play is most favorable for the formation of large stratigraphic hydrocarbon reservoirs. Key words: volcanic reservoir; eruption environment; volcano-deposition tectonic sequences; controlling factors

1

Research review

There is an exploration history of over 120 years abroad for volcanic hydrocarbon reservoirs, as the new domain of hydrocarbon exploration. Volcanic reservoirs have caught the attention and interest of the petroleum industry and scholars. At present, volcanic reservoirs have been globally found in the Jatibarang basalt reservoir in Indonesia, the Cenozoic volcanic reservoirs in Japan, the Muradkhanli trachybasalt and Andesite reservoirs in Azerbaijan and so on.[1] They have the characteristics of great pay thickness, high production rate, and large reserves, Therefore, they have become important targets. Volcanic rocks are widely distributed in the Chinese sedimentary basins and their surrounding regions (Fig. 1) [2]. They have a large distribution scale of volcanic massif that has developed during the Yanshan movement in the east part of China, and the area of the volcanic rock that has developed during the Yanshan movement in the southeast coastland is more than 50 × 104 km2, and the area of the Daxing’anling volcanic rock belt is more than 100 × 104 km2, thus it forms a better base for the exploration of the volcanic rock reservoir [3]. It has been more than 50 years since the

exploration of volcanic rock reservoirs in China, and one batch of volcanic rock reservoirs have been found successively in 11 basins at the northwest margin of the Junggar basin, Bohai bay basin, and so on. Especially since the year 2000, important breakthroughs in the exploration of volcanic rock reservoirs have been obtained in succession, in Bohai bay basin, Songliao basin, Erlian basin, Junggar basin, Tarim basin, Sichuan basin and so on [4, 5]; Also, new domains for hydrocarbon searches have been found in the distribution region of Mesozoic volcanic rocks in the east part of Zhejiang-Fujian-Guangdong and the developing region of Mesozoic-Cenozoic volcanic rocks in the Changjiang depression, sea reef bulge, Qiantang depression, Oujiang depression and so on in the shelf basin of the East China sea [6, 7]. At present the volcanic rocks have being explored in all aspects as an important exploration domain and it just forms a scale for the two volcanic rock hydrocarbon provinces of East China and North Xinjiang. Moreover, preliminary matching technologies have been formed for volcanic rock distribution, reservoir prediction, volcanic rock reservoir evaluation, and so on. The volcanic rock is one type of important reservoir with a complex specialty, and to form reservoirs it must match

Received date: 27 December 2007; Revised date: 28 February 2008. * Corresponding author. E-mail: [email protected] Foundation item: “Eleventh Five-year” Major Scientific and Technological Project, PetroChina (07-01A-01). Copyright © 2008, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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Fig. 1 Distribution of volcanic rocks in hydrocarbon bearing basins of China (rearranged according to the geologic maps by Shang [2] Xiangru )

effectively with the source rock. The reservoir formation conditions and main controlling factors are more complex, and there is scope for further study. For the development characteristics of volcanic rocks and their distribution pattern, especially for the knowledge of aspects such as, pore structure characteristics in volcanic conformation, and their formation, mechanism, and distribution pattern, the type and oiliness of volcanic rock reservoirs, the type of hydrocarbon reservoirs, and reservoir forming conditions, they are still in the beginning stage. On the basis of the investigations of the exploration abroad, of volcanic hydrocarbons, and the available studies, some aspects, such as, the exploration characteristics, accumulation features, main control factors on distribution and forming, and so on are summarized systematically for the volcanic hydrocarbon reservoirs in the sedimentary basin of China. The aim is to promote the exploration of volcanic rock hydrocarbons in China and to study it further.

2 Exploration progress and characteristics of volcanic rock hydrocarbon reservoirs abroad and in China 2.1 Review of exploration of volcanic rock hydrocarbon reservoirs abroad 2.1.1

Exploration phases

Since the first discovery of a volcanic hydrocarbon reservoir in 1887, in the San Joaquin basin, in California, U.S.A, more than 300 reservoirs or hydrocarbon, relative to volcanic rocks, have been globally found at present, in which there are 169 volcanic rock hydrocarbon reservoirs with proven reserves[1]. The study and recognition of exploration of volcanic rock hydrocarbons abroad can be roughly summarized into three phases: Early phase (before the 1950s): Most of the volcanic rock hydrocarbon reservoirs were found occasionally during exploration of other reservoirs in shallow formations, and it

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was considered that they would not be of any economic value, therefore, no evaluation study was carried out and no attention was paid to it. The second phase (from the beginning of 1950s to the end of 1960s): It was recognized that the hydrocarbon accumulation in volcanic rock was not just an occasional phenomenon, and a certain amount of attention was given to them. Exploration was carried out with an aim, in the local region. In 1953, the Lapasi oilfield was found in Venezuela. The highest production for a single well was up to 1 828 m3/d. This was the first volcanic oilfield in the world that was explored with an aim and gained success. This finding marked the recognition of volcanic reservoirs, which has increased to a new level. The third stage (since the 1970s): The exploration of volcanic rock reservoirs have been carried out worldwide. Many volcanic hydrocarbon reservoirs (fields) have been found in U.S.A, Mexico, Cuba, Venezuela, Argentina, former Soviet Union, Japan, Indonesia, Vietnam, and so on. Among them the more famous are the Binehe-Bikenya volcanic hydrocarbon reservoir in Arizona, America, Samugeli-Pata’erzu tuff reservoir in Georgia, Muradkhanli andesite and basalt reservoirs in Azerbaijan, Jadibarang basalt reservoirs in Indonesia, Jijing-Dongboyi rhyolite reservoir in Japan, Baihu granite reservoir in the Neritic area, in the southern part of Vietnam, and so on. 2.1.2

Exploration and study levels

The levels of exploration and study of volcanic reservoirs abroad is generally lower, though many reservoirs have been found, but most of them are found occasionally or by local exploration, and have not been considered as the main domain for extensive exploration or further study. The present proven hydrocarbon reserves of volcanic reservoirs, globally, only share about 1% of the total proven reserves. The study of volcanic rocks has a much longer history in geology, but the study of volcanic reservoirs is still at the starting stage. 2.1.3 Accumulation time and areal distribution of hydrocarbon reservoirs The aspect of “time”, with regard to the formation of volcanic hydrocarbon reservoirs abroad, is new. From the time statistics for the volcanic accumulations found, it can seen that most volcanic hydrocarbon accumulations are found in Neogene, Paleogene, Cretaceous, and fewer are found in the formations before Jurassic. The exploration depth is generally several hundred meters to about 2000 m, seldom more than 3000 m. The Circum Pacific region is the main distribution area of volcanic hydrocarbon reservoirs. From America, Mexico, and Cuba in North America to Venezuela, Brazil and Argentina in South America, then to China, Japan, and Indonesia in Asia, it generally goes through in the shape of campagiform. Furthermore, in the central Asian regions, currently, volcanic hydrocarbon reservoirs are found in Georgia, Azerbaijan,

Ukraine, Russia, Rumania, Hungary and so on. Also volcanic hydrocarbon reservoirs have also been discovered in the periphery of Africa, such as, Egypt, Libya, and Morocco in North Africa and Angola in Central Africa. The structural setting of the generation of volcanic reservoirs is mainly on the continental marginal basin, as also on the intercontinental rift basin. For example, the volcanic hydrocarbon reservoirs discovered in North America, South America, and Africa are mainly distributed in the continental marginal basins. The volcanic hydrocarbon reservoirs are mainly of Intermediate-basic basalt and andesite, and among them basalt reservoirs share 32% of all the volcanic reservoirs and andesite share 17%; reservoir space is of major primary or secondary porosity, and fractures with various origins that generally develop, have a deciding effect on the reservoir improvement. 2.1.4

Scale of volcanic hydrocarbon reservoirs

The scale of volcanic hydrocarbon reservoirs abroad is generally small, however, there are also giant oilfields and gas fields, with high production (Table 1). From the production statistics of the representative volcanic oilfields and gas fields abroad, it can be seen that the oilfield with the highest oil production is the Cristales oilfield in the North Cuba basin of Cuba. The gas field with the highest gas production is the Yoshii-Kashiwazaki gas field in the Niigata basin of Japan (Table 2). 2.2 2.2.1

Exploration process in China Exploration stages

The volcanic hydrocarbon reservoirs in the Chinese sedimentary basin were first discovered in 1957, in the northwest margin of the Junggar basin, and the exploration of volcanic hydrocarbon reservoirs in this region have been witnessed for over 50 years. At present, volcanic hydrocarbon reservoirs have been discovered in 11 hydrocarbon bearing basins, such as, Bohai bay, Songliao, Junggar, Erlian, Santanghu, and so on. (Fig. 2). The exploration of volcanic hydrocarbon in China has experienced three stages: (1) Stage of occasional discovery (1957–1990): Mainly centralized in the northwest margin of the Junggar basin and in Liaohe, Jiyang, and other depressions of Bohai bay basin. (2) Stage of local exploration (1990–2002): With the enhancement of geology recognition and improvement in exploration technology, target explorations have started to take place in the Bohai bay, Junggar basin, and so on. (3) Stage of general exploration (since 2002): The exploration deployment of volcanic hydrocarbon reservoirs was carried out in full scale in Bohai bay, Songliao, Junggar basin, and so on, and great achievements and breakthroughs were obtained. By the end of 2006, the proven oil reserves of volcanic rock submitted by PetroChina had gone up to 47 821.3 × 104 t, geological reserves for dissolved gas had gone up to 229.4 × 108 m3, and the proven natural gas reserves in the place had risen to 1 249.2 × 108 m3. The total hydrocarbon

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Table 1 Reserve statistics for the volcanic reservoirs in global[1] Country

Field

Basin

Fluid property

Australia Indonesia Namibia Brazil Congo America Algeria Algeria Russia Georgia Italy

Scott Reef Jatibarang Kudu Urucu area Lake Kivu Richland Ben Khalala Haoud Berkaoui Yaraktin Samgori Ragusa

Browse NW Java Orange Solimoes

Oil, gas Oil, gas Gas Oil, gas Gas Gas Oil Oil Oil Oil Oil

Monroe Uplift Triassic/Oued Mya Triassic/Oued Mya Markovo-Angara Arch Ibleo

Reserve Gas/108 m3 Oil/104 t 3 877 1 795 764 16 400 849 330 1 685 498 399 >3 400 >3 400 2 877 >2 260 2 192

Lithology Plateau basalt Basalt, tuff Basalt Diabase deck Tuff Basalt Basalt Basalt, diabase Tuff Gabbro deck

Table 2 Global production statistics for volcanic reservoirs [1] Country

Field

Basin

Fluid property

Cuba Brazil Vietnam Argentina Georgia America Venezuela Argentina New Zealand Japan Brazil Australia

Cristales Igarape Cuia 15-2-RD 1X YPF Palmar Largo Samgori West Rozel Totumo Vega Grande Kora Yoshii-Kashiwazaki Barra Bonita Scotia

North Cuba Amazonas Cuu Long Noroeste

Oil Oil Oil Oil, gas Oil Oil Oil Oil, gas Oil Gas Gas Gas

Fig. 2

North Basin Maracaibo Neuquen Taranaki Niigata Parana Bowen-Surat

Oil/(t·d–1) 3 425 68–3 425 1 370 550 411 296 288 224 160

Reserve Gas/(104m3·d–1)

3.40

1.10 49.50 19.98 17.80

Reservoir lithology Basaltic tuff Diabase Altered granite Vesicle basalt Tuff Basalt, agglomerate Volcanic rock Fractured andesite Andesitic tuff Rhyolite Plateau basalt, diabase Crack andesite

History of reserve increasing and exploration for onshore volcanic rock in China

equivalent proven for volcanic rocks in the whole country was about 73 000 × 104 t. 2.2.2 Major exploration characteristics for volcanic hydrocarbon reservoirs in China Compared with the exploration status of volcanic hydrocarbon reservoirs abroad, the exploration of volcanic hydrocarbon reservoirs in China has chiefly followed three characteristics: (1) Full-scale exploration has been carried out for volcanic hydrocarbon reservoirs, as important new domains in China. From 1980s to 1990s, volcanic hydrocarbon reservoirs have

been discovered successively in Junggar, Bohai bay, Subei, and other basins of China, such as, the Karamay basalt hydrocarbon reservoir in the northwest margin of the Junggar basin, Abei andesite hydrocarbon reservoir in the Erlian basin of Inner Mongolia, Fenghuadian Mesozoic andesite hydrocarbon reservoirs, and Zaobei Sha3 basalt hydrocarbon reservoirs in the Huanghua depression of Bohai bay basin, the Shang 741 diabase hydrocarbon reservoirs in the Jiyang depression, and so on. From the beginning of the twenty-first century, the exploration of volcanic reservoirs has been enhanced in China, and the exploration domain extends continuously.

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Other batches of scale hydrocarbon reservoirs were also discovered in the Liaohe east depression of the Bohai bay basin, in the deep formations of the Songliao basin, Junggar basin, and the Carboniferous-Permian of the Santanghu basin, in particular. A great breakthrough was obtained at well Xushen 1 in the north of the Songliao basin. Taking this breakthrough as a marker, a large-scale exploration of volcanic hydrocarbon reservoirs was brought along comprehensively, and at present, volcanic rock has become an important exploration domain in China. (2) All the volcanic rocks of different periods and types of basins could generate volcanic hydrocarbon reservoirs. In the case of volcanic hydrocarbon reservoirs discovered in China, in the east part, they mainly developed in Mesozoic and Cenozoic and the rock type was mainly intermediate acidity volcanic rock. In the west, they mainly developed in the Palaeozoic and were mostly made of intermediate basic volcanic rocks, but all volcanic rocks could form hydrocarbon reservoirs. Volcanic hydrocarbon reservoirs mainly developed in the environment of the continental rift basin, such as the Bohai bay basin, Songliao basin, and so on, although they also universally developed in the foreland basin and island transitional facies basins, such as the Northwest margin of the Junggar basin and the region of Ludong-Santanghu. With the hydrocarbon reservoir type and scale in view, in the eastern part, it was mainly of a lithology type, and could superimpose and distribute in a blanket form, thus forming a large-scale oilfield and gas field with a large area, such as, the Xushen gas field in the deep Xujiaweizi formation of the Songliao basin. In the western part it was mainly the stratigraphic type, which could form a large-scale, mono-block oilfield and gas field, such as, the large Kelameili gas field in the Junggar basin, the large oilfield in the Northwest margin, and so on. The distribution of the volcanic hydrocarbon reservoirs had a close relation with the sedimentary basin. (3) Since the “Tenth Five-Year Planning”, the matching technologies for exploration and development, such as, the seismic reservoir prediction for volcanic rocks, large scale fracturing, and so on, have improved continuously in PetroChina, and the technology series aiming at volcanic hydrocarbon reservoirs have formed in the preliminary, which means a four-step method in the prediction of volcanic hydrocarbon reservoirs: (1) Regional prediction of volcanic rock, focus on high precision gravity-magnetic-electric and 3D seismic; (2) Target identification of volcanic rock; (3) Prediction of volcanic reservoirs; (4) Prediction of volcanic rock.

3 Characteristics of volcanic reservoirs in Chinese sedimentary basins 3.1 Concept system of volcanic reservoirs in hydrocarbon bearing basins Since the discovery of volcanic hydrocarbon reservoirs in the hydrocarbon bearing basins from the 1950s, in China, much research has been carried out on many aspects, such as,

rock characteristics of volcanic reservoirs, lithofacies combination, development environment, storage space and quality, controlling factors, and so on. However, most researchers will name them according to the research purpose and research target, therefore, there are the problems of disorder in classification and naming systems, and obscure concepts in the research of hydrocarbon reservoirs, causing the phenomenon of same name with different meaning, same meaning with different names, and so on. These will cause difficulty in interregional volcanic rock comparison and the communication of fundamental geologic study [8, 9]. The uniformity of volcanic rock classification and Nomenclature system concept, relative to hydrocarbon reservoirs, is the problem that must be resolved urgently at present. It is necessary to set up one set of systematic conceptual systems for volcanic reservoirs on the basis of uniform classification and nomenclature for rocks and lithofacies in the geological research. The operability and practicability of this system during the study of the lithology and lithofacies of volcanic reservoirs in hydrocarbon bearing basins must be emphasized, as well as stress placed on the relation between the lithofacies and reservoir properties. Also consideration must be given to the practicability of hydrocarbon exploration and communication in the domain of fundamental studies. The classification of volcanic lithofacies should follow the common classification principles and consider their practicability. The authors provided the classification scheme of five facies and 15 subfacies for volcanic lithofacies on the basis of a comprehensive study on volcanic lithofacies of hydrocarbon basins in China (Table 3). In this article, the volcanic eruption will be described by the third-order classification system of volcanic rhythm, volcanic cycle, and volcanic sedimentary structural sequence. One volcanic cycle can include multiple near sequential eruptions, which can develop many volcanic rhythms. There are certain discontinuities between two adjacent cycles, which usually present as structural unconformity, eruption unconformity or regional normal sedimentary interbed, with a great deal of thickness. Volcanic sedimentary structural sequence refers to the development of the three stages during a certain structural activity, that is, before volcanic activity, during volcanic activity, and after volcanic activity. The infill volcanic rock and sedimentary have a thickness of several hundreds to several thousands of meters. Multiple eruptions and depressions can be seen and there are many volcanic cycles. 3.2 Development pattern and distribution of volcanic reservoirs in the Chinese hydrocarbon bearing basins 3.2.1 Eruption mode of volcanic rock in the continental hydrocarbon bearing basins The eruption of continental hydrocarbon bearing basins in China is mainly of the central vent eruption, chiefly in layered volcano. There are two kinds of eruption environments, onshore and underwater. The combination of underwater eruption sedimentary is easy to contact with the hydrocarbon

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Table 3 Classification of volcanic lithofacies for Chinese hydrocarbon bearing basins (compiled according to literature [10–13]) Facies

Cryptoexplosion breccia

Formation depth Surface-magma layer or volcanic source region About 3 km below surface About 3 km below surface

Fallout

Land surface

Thermal base-surge

Land surface

Pyroclastic lava flow

Land surface

Top, middle, bottom

Land surface

Massive volcaniclastic rock Undulated structure tuff, volcanic brecciate tuff Welded volcaniclastic rock Various lava

Centre, transition, margin Sedimentary volcaniclastic rock Volcaniclastic sedimentary containg Extraclast Rehandling volcaniclastic sedimentary

Land surface

Lava and brecciate lava

Land surface

Effusive rock, sedimentary volcaniclastic rock, volcaniclastic sedimentary rock and sedimentary rock

Subfacies Vent-volcanic neck

Volcanic conduit

Eruption

Effusion Leaching

Volcanic eruption sedimentary facies

Subvolcanic rocks

Rock type

Forming mode

Occurrence status

Lava, volcaniclastic lava and volcaniclastic rock

Volcanic edifice is denuded, filler in volcanic channel outcrop

Long-cycle, cycle or polygon rock neck on plane

Lava, brecciate lava, breccia Cryptoexplosion breccia

Synchronous or late intrusion Subsurface explosion caused by magma riches in volatile constituent invading fractured rock

Deck, dike, typhon, apophysis, and cryptoexplosive breccia body Tubular, laminate, veined, arborescent, fracture filling

source rock tightly, and is most favorable for hydrocarbon to form reservoirs. The major rock types are basic and acidic rocks, and the minor are neutral and alkaline rocks. The volcanic eruptions in the east during the Yanshan period are all central eruptions. One hundred and fifty-three palaeovolcanos have been restored in the Southeast Coast, in which 102 palaeovolcanos have caldera. A single edifice of volcanic rock from the Yingcheng formation in the Songliao basin is mainly generated by the central vent eruption, and it is also controlled by regional big fracturing on the whole, and presents in the shape of a moniliform, for development in a plane[14]; but there are also views considering that both fissure eruption and central vent eruption develop in the volcanic rock of Yingcheng formation, with their horizontal thicknesses having great variation. Volcanic rock facies is mainly of effusion facies and volcanic sedimentary facies, and a volcanic cone often develops. The volcanic rock of the Huoshiling formation, mainly of fissure eruption, develops widely in a horizontal manner, its thickness variation is relatively more homogeneous, many edifices develop, and volcanic facies is mainly of effusion facies[12, 13, 15, 16] . The Mesozoic volcanic activity in the Erlian basin belongs to the continental eruption; Late Jurassic is a fissure eruption; the Aershan formation in the Bayanhua group of Early Cretaceous is a fissure-central vent eruption; the early part of the Late Cretaceous is a fissure eruption. Cenozoic is a fissure

Product of volcanic eruption

Volcanic eruption Products of volcanic effusion and flooding Land product of extrusive volcanic neck lava Sedimentary product during volcanic eruption inter-period and minor period

Blended multiphases system of volcanic debris, surrounding rock debris and steam Incandescent clastic deposit by volcanic eruption Rock flow, rock quilt; pahoehoe, columnar, slaggy, etc. Needle, plug, kupola, and dome Continental facies, basin facies, laminate or lenticular sedimentation etc.

eruption; the Pleistocene of Quaternary is a central intermittent eruption[17]. The volcanic rock in Liaohe depression develops along the faulting, which is the product of underwater multiple effusions, synchronous with the sedimentary, belonging to the underwater intermittence multiple effusion along the faulting[18]. During the Tertiary volcanic activity, the strongest is the sedimentary period in the Fangshenpo formation. According to the eruption intensity and time-space distribution of the volcanic rock, the effusions are divided into 12 eruptions in four periods. The volcanic rock eruptions in the Nanpu depression are divided into five stages, which are central vent eruption, fissure eruption, and effusion along the faulting. The volcanic rock in the Dongying depression is mainly of lava flow, lava quilt, and mainly effusion, very rarely there are explosive and extrusive facies. The eruption environment is mainly of the continental facies, which also has underwater eruption, and is mainly with central vent eruption. The minor one is controlled by faulting, present as a linear for arrangement, and the faulting joint is the strongest eruption center for volcanic activity[19-21]. The Minqiao volcanic rock of the Gaoyou depression in northern Jiangsu is formed by the continental central vent effusion, and in the late period it is of subaerial eruption, flowing into water, accumulating underwater, and forming volcanic rock. Nearer from the volcanic vent is the thicker volcanic rock[22,23]. The volcanic rocks in the Jianghan basin are mainly of underwater

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eruption, from Cretaceous to Eogene, whereas, in the structural highs of Jinjiachang in the Jiangling depression, the volcanic rock of the subaerial eruption is developed volcanic rock[24]. The Permian volcanic activity in the Tahe region of the Tarim basin has the features of an intermittent eruption. Its characteristic is the alternating of slow effusion with long time eruption and quick eruption with short time eruption, occasionally accompanied by much stronger explosive eruption, thus forming the volcanic eruption cycle from basalt to dacite or mono dacite[25]. The Carboniferous volcanic eruption in the northwest margin of the Junggar basin is a type of fracture central vent, as the brecciate lava distributes widely in the ventral Shixi region. Brown and mahogany volcanic rocks occupy the higher proportions, which are formed by eruptions meeting with atmospheric precipitation or formed by shallow underwater eruption. The base of the eastern Wucaiwan depression is mostly made up of Neopaleozoic Carboniferous volcanic rock (lava and volcaniclastic rock appear alternately). In total the color is much deeper, mostly gray green, there is scarcely any brecciate lava, welded breccia, interbedding with thin mudstone and sandstone, the sedimentary rock formation include marine fossils, belonging to the sedimentary environment of the continental sea. In general, the volcanic activity is relatively weak during Early Carboniferous, and relatively stronger during Late Carboniferous, which present as the continental intermittence volcanic eruptions, belonging to the continental sea volcanic sedimentary environment, having the characteristic of deep underwater eruption, and the volcanic rock erupts in deep water. From west to east the eruption environment of the volcanic rock has the trend changing from above water to underwater[26]. 3.2.2 Lithology of volcanic rock of various periods in hydrocarbon bearing basins of China In eastern China, the Mesozoic volcanic rock in the hydrocarbon bearing basin is mainly acidic. The Cenozoic volcanic rock is mainly intermediate basic; whereas, in the west, volcanic rock is mainly intermediate basic. Many types of the volcanic reservoirs develop in the hydrocarbon bearing basins of China, in which the lava consists mainly of basalt, andesite, dacite, rhyolite, trachite, and so on. The volcaniclastic rock includes agglomerate rock, volcanic breccia, tuff, welded volcaniclastic rock, and so on. The Xing’anling group in the Hailar basin can be divided into three intervals from bottom to top: the lower interval of intermediate acidity volcanic rock consists mainly of one set of intermediate acidity lava, volcaniclastic rock, gray yellow rhyolite porphyry, trachite, and gray green tuff. The middle is the interval of intermediate acidity volcanic rock interbedding with a coal bed. The lithology is made up of gray purple andesite, andesite basalt interbedding coal bed; the upper is made up of intermediate basic volcanic rock, and the lithology is of thick black-gray black basalt, interbedding with thin

black mudstone. Twelve kinds of volcanic rocks have been found in the Songliao basin, including rhyolite, andesite, dacite, basalt, basaltic andesite, andesitoid, rhyotaxitic breccia tuff, andesitoid volcanic breccia, dacite volcanic breccia, andesibasalt volcanic breccia, andesitic crystal tuff, and sediment volcanic breccia. In these intermediate acidity volcanic rock occupies 86% of the total samples, basic volcanic rocks occupy 14%, and mainly belong to the series of alkalic and calc-alkalic[18]. The volcanic rocks in the Bohai bay basin are mainly of basalt, Andesite, and trachite[19-21, 27-31]. For example, the Mesozoic volcanic rock in the Liaohe basin consists mainly of andesite, wheras, the Eogene volcanic rock is mainly made up of basalt and trachite. The Jurassic rocks in the Jizhong depression consist mainly of dark purple andesite and gray andesite, interbedding with tuff, the upper is basalt, andesitic breccia, and volcaniclastic sandstone. The bottom of Cretaceous is variegated volcanic breccia, the upper consists of gray tuffaceous glutinite, sandstone and andesitic breccia. In the Dongying depression it develops widely with basic volcanic rock, subvolcanic rocks, and volcaniclastic rock, the dominant rock type is olivine basalt, basalt, basaltic porphyrite, tuff, volcanic breccia, and so on, in the Fenghuadian region of the Huanghua depression. The volcanic rock is mainly of pantellerite, dacite rhyolite, rhyolite, and rhyolite dacite. In the Nanpu depression it consists of basic volcaniclastic rock, neutral volcaniclastic rock, and basalt. In the Gaoyou depression it is gray-black, gray-green, and gray purple basalt[22,23]. The dominant type for the Cretaceous-Eogene volcanic rock in the Jianghan basin is quartz tholeiite, olivine tholeiite, and basalt porphyrite (secondary basalt), followed by diabase and volcaniclastic rock[24]. In the Erlian basin, it chiefly develops into autoclastic breccia andesite, stomatal-amygdaloid lava, massive lava, tuff, breccia, and agglomerate rock[17,32]. In the Chagan depression of the Yingen basin, the volcanic rock is mainly of intermediate basic basalt, andesitoi and andesite, minor of tuff, welded breccia, and diabase[33]. The Permian volcanic rock in the Sichuan basin is mainly of plagiobasalt, tuff, tuffaceous breccia. and so on[34]. The Permian volcanic lava in the Tarim basin includes basalt and dacite, and dacite is in dominance, occupying 80.3% of the total volcanic rock thickness. The minor one is breccia dacite, there are also ones with less breccia basalt, brecciated tuffaceous dacite, brecciate tuffaceous basalt, tuffaceous breccia, and volcaniclastic breccia, crystal-vitric tuff, crystal-lithic tuff, and crystal tuff, sediment tuff, sediment volcanic breccia, tuffaceous argillaceous siltstone, tuffaceous ger llton, tuffaceous, and pebbled sandstone[25]. The rocks in the region of Ludong-Wucaiwan in the Junggar basin are dominant in basalt, andesite, dacite, rhyolite, volcanic breccia, tuff, and so on, whereas, in the region of the northwest margin the Carboniferous lithology is mainly of andesite, basalt, andesitic basalt, volcanic breccia, tuff breccia,

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Table 4 Types and characteristics of storage space for volcanic reservoirs Type of storage space Gas pore

Intergranular pore Primary porosity Intercrystal and intracrystal pore Condensate shrinkage pore

Secondary porosity

Corresponding rock type Andesite, basalt, breccia, brecciate lava

Genesis

Characteristics

Gas expanse and spill out during diagenetic process

Volcanic breccia, agglomerate rock, volcanic sedimentary rock Basalt, andesite, Autoclastic brecciate lava Basalt

Interclast porosity left after compaction

Most distribute in the top and bottom of rock flow layer, with various shapes and sizes Often seen in volcaniclastic rock

Devitrification pore Feldspar emposieu

Pyromeride

Lava ash emposieu Carbonate emposieu Cavern

Tuff, welded tuff, volcanic breccia Rock of various types

Explosive fracture Shrinkage facture

Rock of various types

Framework of rock forming mineral Volume shrinkage during condensation for melt lava Formed after devitrification of hyaline Feldspar denudation often developed along cleavage fissure Lava ash denudation Calcite, siderite dissolve

Basalt, andesite, brecciate lava, breccia

Weathering, leaching, and denudation

Autoclastic brecciate lava, buried volcanic rock Basalt, andesite, autoclastic brecciate lava

Autoclastic or subvo1cboc explosion

Fracture Structure facture

Rock of various types

Weathering fracture

Rock of various types

Upper lava is destroyed by the upswelling of the bottom lava during magma cooling Tectonic stress Various weathering

welded breccia, tuff, agglomerate rock, and so on.[26, 35]. The Permian volcanic rocks in the Santanghu basin are of basalt, andesite, dacite, rhyolite, tuff, volcanic breccia, and so on .[36, 37] . 3.3 Genesis and characteristics of volcanic reservoirs in Chinese sedimentary basins 3.3.1

Type and origin of storage space

According to the origins, the storage spaces in volcanic reservoirs can be classified into three categories, referring to primary pore (gas pore, intergranular pore, intercrystal pore), secondary porosity (dissolved pore, cavern), and fracture (Condensate shrinkage pore, explosive fracture, structure hogging fracture, shear fracture and weathering fracture) (Table 4). Primary porosity includes the gas pores formed when volcanic material was blown out of the land surface, the residual porosity was incompletely filled by the amygdaloid material, intercrystal microspore, pores between volcanic breccia, and so on (Fig. 3a).

Most distribute in the middle of rock flow layer, with small pores No certain direction, irregular shapes Microporosity, but with good connectivity Irregular porosity shapes Small porosity in large amount, good connectivity Large porosity Developed along fracture, autoclastic detrital rock belt, and structure high location Retrievable

Hydrocarbon-bearing property Better hydrocarbon bearing when connected with fracture or cavern Good hydrocarbon bearing

Contain no oil for most Filled with hydrocarbon when connected with gas pores Better gas storage space One of the main storage spaces Can form good reservoirs Good hydrocarbon bearing property Good hydrocarbon bearing property Better hydrocarbon bearing property

Columnar cleavage, open, planar fracturing, less slippage

Better hydrocarbon bearing property in general

Developed nearby fault, mostly straight, most with high angle fracture Connect with emposieu, denudation fracture, cavern and structural fracture

Relative to action time of structure Certain storage significance

Secondary porosities are chondrule rhyolite devitrification pores; emposieu in feldspar (including the feldspar by divitrification, phanerocryst, crystal, and microlite), and lava ash, emposieu in clay mineral; emposieu in carbonate amygdaloid filled by gas pores, carbonate vein filled by fracture, and rocks with carbonation (Fig.3b to Fig.3e). The forming of secondary porosity is mainly the result of weathering, erosion, and denudation on volcanic rock. Fractures can be the chief flow channel of volcanic rocks and part of the accumulation space (Fig.3f), its forming reason lies in: volcanic action and diagenesis forms, explosion fracture, and shrinkage joint; the action tectonic stress allows the volcanic massif to transform and slippage, thus forming a structural fracture. The action of weathering, erosion, and denudation, as well as the action of ablation and damage by structure stress on volcanic rock, supplement and complement each other, and superimpose each other, even if the volcanic rock is covered by an overlying formation. A large amount of water or organic acid solution can also filtrate into volcanic rock along the fault

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Fig. 3 Types of storage space for volcanic reservoirs

of the fracture, and denudation will occur in the deep part, thus generating emposieu and corroded fissure. In general the gas pore and emposieu in the volcanic reservoirs have a lot of oil, whereas the structure fissure and weathering fissure have the effect of connecting gas pores. Emposieu and other storage spaces have the main action of conduit during hydrocarbon migration, and they can also be storage spaces by themselves, but the scale is small; in general, various storage spaces cannot exist separately and will present in a certain combination. The mingling of pores, fractures, and cavities can form a favorable hydrocarbon storage space, and different storage intervals have different storage space combinations.

3.3.2

Typing of volcanic reservoirs

In China, volcanic rocks develop in most hydrocarbon bearing basins, and they have a wide distribution area with thick formations. The neoformation includes three types: volcanic action, diagenesis, and tectonic action. According to the genetic feature the reservoirs can be classified into four types, that is lava reservoir, volcaniclastic reservoir, denudation reservoir, and fracture reservoir (Table 5). There are obvious differences in the aspects of occurrence location, distribution form, porosity type, and porosity permeability features for each type of reservoir. For example, the volcanic

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Table 5 Neogenesis and types for volcanic reservoirs Controlling action

Storage space

Volcanism

Primary pore

Diagenesis

Secondary pore

Tectonism

Fracture

Reservoir type

Distribution and occurrence

Volcanic lava Subvolcanic rocks Volcaniclastic rocks Weathering karst Buried karst Alteration Fracture

Effusion facies, laminate Shallow intrusive facies, tubular explosion facies, stacked or cyclical Inside reservoir, thickness can be 300 m Acidic fluid denudation, depth not limited Deck, typhoon, alteration zone Structure highs, fault belt

Reservoir type Lava volcaniclastic rock

Denudation Fracture

Fig. 4 Sketch of various lithology microfractures in the northwest margin of Junggar basin

rocks of different lithofacies in the northwest margin of the Junggar basin, in the west part of China, can form better denudation reservoirs through weathering, leaching, porosity, and microfracture development (Fig. 4). 3.3.3

Properties for volcanic reservoirs

Palaeozoic and middle Cenozoic volcanic rocks are widely developed in the hydrocarbon bearing basin of China, which have the characteristics of extensive area and long geological perdurability, but do not have the exclusive property of these rock types, no matter whether they are basic rock, neutral rock, acidic rock, volcanic rock or intrusion rock, and no matter whether they are lava or volcaniclastic rock. There are better reservoirs of them from Cenozoic to Archaean[9], such as, the Yingcheng formation in the Songliao basin, Suhongtu formation in the Yingen basin, Xing’anling group in the Abei oilfield of the Erlian basin, Middle Cenozoic in the Bohai bay basin, Middle Cenozoic in the Jianghan basin, Middle Cenozoic in the North Jiangsu basin, Carboniferous in the Karamay oilfield of the Xinjiang province, Carboniferous in Ludong Wucaiwan, and the Permian volcanic reservoirs in Tarim, Santanghu, and Sichuan basins, and so on (Table 6). The porosity of volcanic reservoirs is less affected by depth. This is because the skeleton of the volcanic rock is more solid than other rocks, thus it has stronger compaction resistance ability, and is less affected by mechanical compaction during burial, making it easier for the porosity of volcanic rock to be preserved when compared with other rocks, also allows the total continental exploration depth in China to go down to about 1000–2000 m. At the same depth, the porosity of clastic rock is less than the porosity of volcanic rock, for example, when the depth of Carboniferous volcanic rock in the Shixi oilfield of Junggar basin is more than 3800 m, the porosity of the volcanic rock is 8.46%–19.78%, with an average of 14.4%, whereas, the average porosity for clastic rock is about

7.13%[26]. 3.4 Main controlling factors for the formation of volcanic reservoirs in Chinese sedimentary basins The evolution process of generation, development, blockage, and regeneration, at different stages, for the storage space of volcanic rock is very complex. Primary porosity and fracture are mainly controlled by initial eruption conditions, which are volcanic rock subfacies. Under the same tectonic stress action, the development and preservation degree of the structure fracture are also controlled by initial eruption conditions (subfacies). Though the volcanic rock is formed by condensation, welding, compaction, and solidifying after volcanic eruption, it has primary gas pores, but they do not connect with each other, therefore, it has no permeability. Only after the transformation by various geological actions, at different stages, can they have storage ability. On the whole, volcanic action, tectonic movement, weathering, leaching, and fluid action are the main geological actions affecting and controlling the development degree of reservoirs. 3.4.1

Control of volcanic action on reservoirs

The volcanic action not only controls the dynamic scale of reservoirs and the relation between each other, it also controls the type of storage space and mineral component features of reservoir rock. The storage ability of volcanic reservoirs in the same region is mainly controlled by the rock type and lithofacies. The volcanic reservoirs with different rock types develop different types of storage systems. For example, in the basal volcanic rock of the Wucaiwan depression in the Junggar basin, the volcaniclastic rock has the highest porosity (value is 1.26%–30.08%, average 9.84%), followed by andesite (porosity is 8.14%), tuff (porosity is 7.92%), with basalt having the lowest porosity (5.89%); tuff has the highest average permeability (2.09 × 10-3 Pm2), followed by andesite

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Table 6 Characteristics of volcanic reservoirs in Chinese hydrocarbon bearing basins Erathem

System

Group, formation, member

Cenozoic

Neogene

Yancheng group Bottom of Guantao formation

Paleogene

Sanduo group Sha1 member Sha3 member

Sha4 member Xingouzui formation Kongdian formation Mesozoic

Cretaceous

Yingcheng formation Qingshankou formation

Jurassic

Palaeozoic Carboniferous -Permian Permian

Suhongtu formation Xin’anling group

Basin, Lithology depression Gaoyou Gray black, gray green, gray purple depression basalt Dongying Peridotitic depression Huimin Peridotitic depression Gaoyou Basalt depression Dongying Basalt, andesitic basalt, volcanic breccia depression Huimin Peridotitic basalt depression Liaohe east Basalt, andesitic basalt depression Zhanhua Basalt, ndesitic basalt, volcanic breccia depression Jiangling Gray black, gray green, gray purple depression basalt Weibei Basalt, tuff depression Songliao basin Basalt, andesite, dacite, rhyolite, tuff, volcanic breccia Qijia-gulong Intermediate acidity volcanic breccia, depression tuff Yingen Basin Basalt, andesite, volcanic breccia, tuff Erlian Basin Basalt, andesite Halar Basin Pyroclastics, rhyolite porphyry,trachyte tuff, andesite, Andesitic basalt, basalt Junggar Basin Andesite, basalt, tuff, volcanic breccia Tarim Basin Dacite, basalt, volcanic breccia, tuff Santanghu Basin Andesite, basalt Sichuan Basin Basalt

and volcanic breccia. Basalt has the lowest permeability (0.89 × 10-3 Pm2)[26]. Volcanic rock facies is an important factor affecting reservoirs. Different lithofacies and subfacies have different porosity types, and the reservoir properties of different subfacies with the same lithofacies may have great difference, this is because there exists a great deal of difference in the rock texture and structure among various subfacies. Rock texture and structure control the combination and distribution of primary and secondary porosity, as well as fracture. The properties of volcanic reservoirs and the type, characteristics, and variations of storage space are mainly controlled by volcanic rock subfacies. The storage space of volcanic conduit facies is mainly of insular gas pores and pores among the volcaniclastic, for example, the lava of volcanic conduit facies at intervals of 2 541.71–2 548.27 m below well Shang 74-6 in the Jiyang depression has the measurement porosity of 13.1%–16.4%, permeability of 106.95 × 10-3 Pm2, and in the lava, a columnar cleavage can often be found, thus forming better storage space. Volcanic eruption facies takes the occurrence of volcaniclastic rock as its characteristic. The impulsive force during explosion will break the roof and surrounding rock, thus forming abundant fracture and crackles, as well as forming volcanic breccia. The volcanic breccia intergranular pores and gas pores are well developed; in

Porosity/%

Permeability /10–3 Pm2

20

37

25

80

25

80

22

19

25.5

7.4

10.1

13.2

20.3–24.9

1–16

25.2

18.7

18–22.6

3.7–8.4

20.8

90

1.9–10.8

0.01–0.87

22.1

136

17.9 3.57–12.7 13.68

111 1–214 6.6

4.15–16.8

0.03–153

0.8–19.4 2.71–13.3 5.9–20

0.01–10.5 0.01–17

addition, because the volcanic eruption facies are generally located in the fossil erosion highland, they easily suffer the weathering and leaching, therefore emposieu (cavity) and corroded fissures develop, thus forming favorable accumulation regions. for example, the volcanic breccia porosity in the interval of 1975–1979.6 m at well Shang74-12 in the Jiyang depression is 20.7%–33.1%, the permeability is 11.2 × 10-3–140 × 10-3 Pm2, the volcanic breccia porosity in the interval of 1 830–1 838 m at well Shang 74-6 is 16.7%–37.4%, permeability is 0.988 × 10-3 –3 170 × 10-3 Pm2. This facies belt is also a very favorable accumulation facies belt. Volcanic effusion facies forms during the entire period of volcanic eruption and lava primary gas pores develop. The secondary porosity is mainly represented as the porosity produced by volume, reducing after the forming of minerals (such as feldspar, quartz, and so on), by feldspar denudation and hyaline devitrification. According to statistics, the upper subfacies of the effusion facies is the lithofacies belt with the best properties in the Songliao basin, the Xingcheng and Shengping reservoirs. The subfacies storage space in the middle belt of the extrusive facies is mainly made up of fractures, dissolved pores, and micropores of intercrystal pores, and the storage property is good. Moreover, it is a favorable storage facies belt [19, 20, 29].

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3.4.2

Effects of volcanic eruption on storage space

The eruption environment has a great effect on the formation of volcanic rock storage space, for this the Carboniferous volcanic reservoirs in Shixi oilfield and Wucaiwan depression in the belly part of Junggar basin can be taken as examples. The volcanic rock in the Wucaiwan depression alternates with sedimentary rock, and sedimentary rock layers contain marine facies fossils, belonging to the epeiric sea sedimentary environment. The volcanic rocks erupted in deep water, because of the great deepwater hydrostatic pressure. The volatile constituents dissolved in the magma could not escape easily and it was difficult to form gas pores, therefore, the primary gas pore was not developed. In addition to the symplesiomorphic action of the water body, obvious alterations (chloritization) and packing actions occurred in the volcanic rock, allowing the previous less primary porosity to decrease; whereas, the brecciate lava widely distributed in the Shixi oilfield erupted in the environment of shallow water or on continent, especially when atmospheric precipitation occurred during eruption. On the one hand, the volatile constituents dissolved in the magma could escape in large amounts, thus forming primary gas pores, and on the other hand, when the incandescent magma suddenly met the water body, a quenching action occurred, leading to the formation of a large number of primary microfissures, to connect the primary pores in the well, thus building a better primary storage space. Therefore, different eruption environments led to a great difference in reservoir properties, for example, the porosity (8.14%) and permeability (1.14 × 10-3 ȝm2) for the volcanic rock in Wucaiwan were both not as good as the one for the Shixi oilfield (porosity and permeability were 14.77% and 2.08 × 10-3 ȝm2, respectively)[26]. 3.4.3

Control of diagenesis on reservoirs

Similar to sedimentary rocks, the diagenesis of volcanic rocks is complex and varied, and the type of diagenesis mainly includes compaction, packing, dissolution, metasomasis, and so on. They have different effects on the forming of reservoirs. Packing can decrease the porosity and permeability of reservoirs, and is unfavorable for the development of volcanic reservoirs; compaction is unfavorable for the formation, preservation, and development of reservoirs, it especially has an obvious effect on volcaniclastic rock. A lot of common diagenetic alteration includes chloritization, calcite metasomasis, zeolitization and so on. This not only has a negative effect on the formation of volcanic reservoirs, but also has a positive effect. Generally the gas pore in the volcanic rock cannot become a storage space directly, instead first it is packed with chlorite, zeolite, calcite, and so on, and in the following denudation by groundwater, it becomes a reservoir by the connecting of fractures. Volcanic action, tectonic movement, and hydrocarbon displacement can all cause large scale fluid action. The direct effect of fluid on volcanic rock allows the materials to be

brought in and out and allows the volcanic rock to stay in the open system. The fluid can be divided into hydrothermal fluid and acidic fluid, relative to organic materials. The fluid allowed the porosity structure of the volcanic rock to change, thus significantly improving the property of volcanic reservoirs, and making the type of storage space of the volcanic rock more complex and varied. The direct result of the hydrothermal action leads to the alteration and denudation of former minerals, as well as forms new minerals, and leads to the occurrence of secondary cement and packing. Alteration and denudation increases the porosity of volcanic rock, and cement and packing decreases the porosity, especially permeability. Nearly all volcanic rocks undergo weathering and leaching of different degrees. In view of most volcanic rocks, the developmental degree of porosity has a close relation with leaching. Leaching can not only crack rocks, but also allows the chemical component to have obvious alterations, such as, dissolution, oxidization, hydration, carbonization of minerals, and so on. For example, the purple andesitic welded tuff in the upper part of the Yingcheng formation at Shengshen 2 well, because of weathering and leaching will allow the previously tight tuffaceous lava of the explosive facies, to become extremely loose, which will represent the shape of a soybean residue in cores. Their porosity is more than 15%, and they have better permeability. Therefore, leaching is not only an important factor affecting the storage property of volcanic rock, but also a geological phenomenon generally existing in the volcanic rock. The distance from the top of the volcanic rock to the unconformity becomes an important controlling factor for the development of storage space in weathering, leaching, and denudation reservoirs. Denudation is mainly represented as the bringing out process of materials. The total effect is the increasing of porosity. All of the secondary porosity has a relation with denudation, which is another important factor to control the storage ability of volcanic rock, under the action of acidic water (organic acid and inorganic acid). The action of denudation allows the unstable components in the volcanic rocks to dissolve, thus forming secondary porosity. 3.4.4

Control of tectonic movement on reservoirs

The tectonic movement and tectonic position have a dominant effect on the formation of faulting and the development degree of a fracture. The formation of a fracture has three aspects of effects on the development of reservoirs: (1) forming a fracture in the development belt of a gas pore amygdaloid enhances the connectivity of gas pores, increases permeability, and more importantly, surface fresh water or ground water dissolves and reworks the volcanic rock along the fracture, forming a large amount of emposieu, or even a cavern on the basement of a gas pore, residual gas pore, or matrix intercrystal pore; (2) forming a fracture at tight intervals, can form a pure fractured reservoir, and under certain conditions it can develop dissolved pores, even caverns; (3) the existence of a fracture can improve the features of

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ground water distribution and flow, thus promoting the occurrence and development of dissolution, such as, the secondary dissolved pores that develop along the structural fracture, seen in the core or under a microscope. This is just the result of this action. For example, the Carboniferous—Permian volcanic rock in the Santanghu basin develops at least two stages of tectonic fractures. The ĉ stage fractures are formed in them earlier, on a large scale, and have a great effect on the reservoirs (is the infiltration flow channel for diagenetic water developed by dissolved pore and cavern), but the fracture itself is almost packed; the Ċ stage fracture is of a smaller scale, and the effect on reservoir reforming is not as good as the I stage, but because most of the factures in this stage are open fractures, they have less packing materials, therefore, have more significance on the improvement of the reservoir quality.

4 Geologic characteristics and distribution pattern of hydrocarbons in volcanic rock of Chinese sedimentary basins 4.1 Source and origin of hydrocarbons in volcanic reservoirs The hydrocarbons in the volcanic reservoir not only have organic resources, but also inorganic origins, although mainly of organic resource. The biology resource and nonbiology resource can be distinguished by the į13C value of hydrocarbons (usually CH4) for organic and inorganic hydrocarbons, because the hydrocarbon of biology resource is deficient in 13C (į13C is lower than –30‰), and the hydrocarbon of the nonbiology resource is enriched in 13C (į13C is about –27‰)[38, 39]. The development of volcanic rock generally corresponds with a certain stage in the sedimentary basin evolution. The volcanic vent, volcanic collapse, and volcanic lake formed during volcanic evolution can usually provide better sedimentary basin development foundation, for the development of latter lakes; at the same time, during the evolutionary process of the sedimentary basin, more volcanic movements are generated with the extending of tensional, fault-depression in the sedimentary basin, and the development of deep, big faults. The development of organic matters are affected by the volcanic movement in three aspects: (1) During volcanic eruption, the wide distribution of lava ash can cause the die out of biology in great quantities, thus leading to the preservation of organic matters; (2) Before and after volcanic activity, with the effusion of a large amount of thermal liquid, it often accompanies the transition metals Ni, Co, Cu, Mn, Zn, Ti, V and materials of N, P and so on. The materials in the thermal liquid and gas liquid have a positive effect on the growth and multiplication of organics, the maturity of organic matters, transforming of organic matters, and so on. (3) Volcanic activity, pyrogenesis, hydrothermal activity and so on, can all promote the maturing of organic matters, allowing them to generate hydrocarbon creating matters, thus providing oil and gas to the volcanic rock.

According to the existing studies, in the development region of the volcanic rocks, under the environment of the current onshore land surface, the lakes contain hydrocarbon enrichment sediments. Kirkham considered that the natural gas in the Rattlesnake gas field located in Washington state of USA may have come from the lacustrine sediments inner basalt[40], because the natural gas includes a considerable amount of Nitrogen. Some volcanic activity and pyrogenesis can also provide anorganogene natural gas for volcanic rock, and the scale of this type of gas reservoir may also be considered to a large extent. There are mainly three kinds of viewpoints for anorganogene hydrocarbons: (1) Directly from the mantle[1]. Hydrocarbons can be synthesized by Fischer-Tropsch through CO or CO2 with H2; Maybe during the forming of the earth, the aggregate cosmogenic materials (nebula particles and hydrocarbons etc.) were preserved in the mantle; (2) From the late period of magmatic activity to the period after the magmatic stage, when the temperature is lower than 600 ć, the original fluids containing CO2 generate hydrocarbons in close systems[41, 42],; (3) After the magmatic stage, the reaction of mineral fluid (such as serpentinization) can generate hydrocarbons [1, 43-45]. Nearly all the oil and gas from the volcanic hydrocarbon reservoirs in the continental sedimentary basins of China are from the organics in the sedimentary rocks, but hydrocarbons of inorganic origin can also be discovered; about the effect of volcanic activity on the forming and evolution of organic matters, further researches are scarce at present. The studies on CO2, CH4, He, and stable carbon isotope for the deep natural gas in the Songliao basin show that the gas of organic origin is dominant, but inorganic origin gas is also present. In a particular region, the content of inorganic CO2 is more than 60%, in which all the He is of mantle inorganic origin. In the depth of the Songliao basin, four sets of hydrocarbon source rocks have developed, which are of Huoshiling formation, Sahezi formation, Yingcheng formation, and Denglouku formation. The content of organic carbon in the hydrocarbon source rock is 1.58%–2.4%, and the organics belong to the type of ċ, Ro is 1.15%–1.6%, which is a favorable gas source rock. The volcanic rock of Yingcheng formation is the main reservoir; except the hydrocarbon gas reserves that have been discovered, of 1.0 × 1011 m3, more than 1.0 × 1011m3 of CO2 has also been discovered. The CO2 in the Songliao basin has two main origins: (1) Inorganic origin, formed by magma degasification and decomposition of carbon enriched rock in crust, where the į13C of the CO2 is more than –8‰; (2) Organic origin, formed by decomposition of organic matters, where the į13C of the CO2 is less than –10‰. The discrimination chart shows that the sample CO2 from the Changling faulted depression is of inorganic origin (Figs.5 and 6). The Discrimination chart of the He isotope also shows that the CO2 gas reservoir in the Changling faulted depression is of mantle-magma origin. Therefore, one should also look for an inorganic origin gas in these regions, except that the emphasis should be on the discovery of organic origin natural gas.

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4.2 Hydrocarbon enrichment and reservoir forming conditions for volcanic rocks 4.2.1 Space configuration of hydrocarbon source rock and volcanic rock traps is the key for reservoir formation The forming and distribution mechanism of volcanic rocks is very different from that of sedimentary rocks. Because the volcanic rock itself cannot form organic hydrocarbons, the leading condition for the formation of volcanic rock hydrocarbon reservoirs is to associate it with a hydrocarbon source sock, that is, volcanic rock is located at the top or bottom of hydrocarbon source rock systems, or there are hydrocarbon generation depressions in the vicinity, thus volcanic rock accumulations can have a better chance to form matching relations with the hydrocarbon source rock in the

Fig. 6

Fig. 5

Illustration of genetic model of natural gas in Changling

fault depression (rearranged according to the data of Mi [46] Jing-kui )

Reservoir forming patterns of the CO2 reservoir in Changling fault depression

sedimentary layers. Providing enough oil sources is the necessary condition for the formation of volcanic rock reservoirs. For the main continental oil-bearing basins, with the development of volcanic rocks, in China, such as Songliao, Bohai bay, and Junggar basins, the volcanic rock layers alternate with sedimentary layers, forming favorable volcanic-sedimentary sequence combinations for the reservoir generation, which are favorable for the formation of proximal source hydrocarbon reservoirs, therefore, the hydrocarbon reserves in the volcanic rock are in great abundance. For volcanic hydrocarbon reservoirs formed through paragenesis or adjacency with sedimentary rocks (hydrocarbon source rock), the hydrocarbon is from the mature hydrocarbon source rocks in the sedimentary rocks. In the early development stage of the rift basin, the strong tafrogenesis leads to volcanic eruption, on the other hand, it also leads to the rapid sinking of the basin, deeply increasing the water body, with rapid accumulation of sediments, thus forming the hydrocarbon source rock. The

forming of the volcanic rock is nearly in syntonization with the development of the hydrocarbon source rock, thus allowing the volcanic rock to locate in or nearby the hydrocarbon source rock layers, which is very favorable for the forming of volcanic rock[15-18, 31]. The formation of volcanic hydrocarbon reservoirs must also have the conditions of generating, storing, covering, trapping, migrating and preserving, accumulating, and they also need favorable allocation of time-space, except its specialty in reservoir generation pattern and distribution. The types of the volcanic hydrocarbon reservoirs found presently are varied[15, 16] , and are mainly structural litho-stratigraphic hydrocarbon reservoirs. For example, the volcanic hydrocarbon reservoirs in the Xujiaweizi fault depression of the Songliao basin are superimposed gas reservoirs, there is no uniform gas-water contact, the connectivity between gas reservoirs is poor, and the height of the gas plug exceeds construction amplitude. All these show that it is a lithology gas reservoir. On the northwest margin of the Junggar basin, weathering vugular

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Fig. 7 Sketch of volcanic hydrocarbon reservoir plays in the main hydrocarbon bearing basins of China

fracture reservoirs of Permian volcanic rock have developed, various types of rocks can all form better reservoirs, and hydrocarbon enrichment is controlled by unconformity (weathering crust), belonging to the stratigraphic hydrocarbon reservoirs[3]. Volcanic rock adjacent to hydrocarbon source rock can form proximal reservoirs, and the hydrocarbon is most enriched. Analysis from the reservoir generation mechanism shows that volcanic rocks mainly have two reservoir generation modes: (1)near source type (in source, below source), which is the main generation mode for volcanic rock, such as, the Eogene Mesozoic in the Bohai bay basin, Cretaceous in the Erlian basin and Hailar basin, Cretaceous in the Yingen basin, deep layer of Songliao basin, inside the Junggar basin, and Carboniferous-Permian of Santanghu basin; In the near-source combination, hydrocarbon source rock is located at the top, bottom, and lateral part of the volcanic reservoirs. Volcanic reservoirs distribute into the hydrocarbon generating depression or nearby, the oil and gas generated from the hydrocarbon source rock have the biggest opportunity of contact with reservoirs. Generally speaking, near source play will allow volcanic rocks to have much more favorable conditions, which are most favorable for hydrocarbon enrichment. (2) distal play (source top), such as the Sichuan basin and Tarim basin. The depression in the east of China is mainly because of proximal play. In the high location, the structure lithology hydrocarbon reservoirs, mainly of explosive facies, develop, whereas, in the slopes,

lithology hydrocarbon reservoirs, mainly of effusion facies, develop. In the Central and western part, these two kinds of plays develop and it is most favorable for proximal largescale stratigraphic hydrocarbon reservoirs (Fig.7). 4.2.2

Better reservoirs

Most types of volcanic rock reservoirs belong to this type of fracture porosity. Storing pores are mainly of porosity (cavity) and fracture with different origins. The forming of porosity (cavity) is not only controlled by volcanic lithofacies, but also depends on the secondary action to a great degree, such as, denudation and fracturing action. Fracture is an important factor in forming effective accumulations, fractures with different scales connect different types of porosities (cavity), thus generating a porosity-fracture network, which is a necessary factor to form volcanic hydrocarbon reservoirs. Fractures can also promote migration and accumulation of hydrocarbons. The action of weathering and leaching can transform accumulations effectively because of the generated dissolution porosity. Their developing thickness can be up to hundreds of meters, even thousands of meters. In summary, favorable volcanic rock accumulation is the main factor for the enrichment and high production of volcanic hydrocarbon reservoirs. 4.2.3

Better caprock conditions

The mudstone overlying volcanic rocks can be the

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high-quality caprock. If this set of mudstones is also the hydrocarbon source rock simultaneously, it can also provide hydrocarbon for the volcanic rock, which is very favorable for hydrocarbon accumulation. In addition, the volcanic rock itself also has compact intervals, which can construct the favorable accumulation covering combination together with porosity developed intervals. 4.2.4

Hydrocarbon enrichment

The volcanic hydrocarbons are mainly enriched in the structure highs nearby the big faulting and in the regions superimposed by proximal volcanic eruption and effusion facies. Most proximal vent facies distribute along the big faulting, and the vicinity of the faulting belt is the location where fractures develop in concentration. The footwall of a normal fault in a depression basin is the structure high, and the accumulation of a volcanic eruption can further cause a high landform at the location of the proximal vent. The structure high is exposed for a long period, and easily suffers the weathering, leaching, and denudation, thus forming good accumulations. Structure highs are also the directional region of hydrocarbon migration, therefore, the areas where faulting belts, structure highs, and volcanic rocks of proximal vent facies are superimposed, they are the most hydrocarbon enriched regions. In a faulted basin, in general, volcanic eruption facies are much more developed in the inherited highs of the proximal vent, and structure fractures can develop easily in proximal faulting. Thus the accumulation properties are better generally and are the favorable location for hydrocarbon accumulation. However, in the location of the slope, volcanic rocks of effusion facies develop, and are nearer to the oil source, mainly in the lithology hydrocarbon reservoirs. On a relatively structural high location in kraton or intracontinental depression basins of the central-west region, there will be stronger weathering and denudation in general, forming denudation reservoirs with large areas, which can form large scale mono-block oilfields and gas fields, such as, the Junggar and Santanghu basins, in which there is stronger weathering and denudation on the paleo-nose uplift belt, and large-scale fracturing develops, which is favorable for the creation of fractures and better reservoirs, and there are favorable conditions to catch hydrocarbons in the long run, which is the main direction to look for, when looking for volcanic hydrocarbon reservoirs. In the northwest margin of the Junggar basin and Santanghu basin, large stratigraphic hydrocarbon reservoirs have been discovered, which have relations with weathering and leaching. 4.3 Distribution of volcanic hydrocarbon reservoirs in Chinese sedimentary basin Lithology hydrocarbon reservoirs are mainly developed in the east part of China, whereas, stratigraphic hydrocarbon reservoirs are mainly developed in the west part of China. The volcanic rocks develop in the eastern faulted basin along the fracture with the shape of banding. Furthermore

most volcanic reservoirs are preserved in situ, thus are less reformed later. Reservoirs of explosion facies develop near the fracturing and the effusion facies in the slope part develop in large areas. Generally in the earlier stage of each cycle, the volcanic activity movement is stronger, volcanic rock develops widely, basalt of effusion facies is developed, as well as diabase of the hidden volcanic rock facies, volcanic breccia, and tuff of explosion facies are also developed. They have a large area and big thickness, and are mainly of fissure eruption; whereas, late magmatic activity becomes weak, showing that the central vent eruption is dominant. The development of volcanic rock is relatively limited and it is thin, hidden volcanic rock facies is relatively developed, to the lesser extent is the effusion facies, and volcaniclastic rock is scarce. In general the volcanic rock on the fault rift edge is older in time and it is newer in the center of the fault rift, and has relations with the evolution characteristics of faulting. In the eastern faulted basin, the faulting on the faulted depression border controls the development of the basin; as well as the development of the volcanic rock and the distribution of the facies belt are also controlled by it (Fig.8). Near the big faulting, the explosive facies and the proximal volcanic vent effusion facies are more developed, although in the location of the synclinal slope and depression, the effusion facies is distributed in large areas. Generally speaking, the reservoirs near the faulting can be easily transformed by faulting and form fractures, thus they can be improved; except for the relation with effusion and diagenesis, the development of the effusion facies is also controlled by the secondary activity, such as denudation and so on. Because the proximal faulting location is also generally the relatively higher location on the structure, it is mainly a structure-lithology hydrocarbon reservoir; and on the slope, hydrocarbon reservoir of lithofacies is mainly formed. Most volcanic rocks in the Superposition basins of the central-west part witness multiple tectonic movements. Along the unconformity it develops the reservoirs of weathering and leaching type with large areas, thus forming large scale mono-block stratigraphic hydrocarbon reservoirs. The development of volcanic rocks in the basins of the central-west region has a close relation with the forming of Paleo-Asian Ocean, Paleo-Tethys Ocean and their closure, as well as orogenic activity. For example, the Carboniferous volcanic rock of different lithology in the northwest margin of the Junggar basin can form better reservoirs after weathering and leaching, and reservoirs are most developed in the arrangement of about 600 m under the erosion surface. The development depth of reservoirs can be more than 1000 m (Fig. 9), thus forming large scale stratigraphic oilfield and gas field of weathering and leaching type. In the Ludong region of the Junggar basin, the volcanic rock is mainly of neutral lava, also having basic and acidic lava; volcaniclastic rock is more developed, and its distribution is controlled by faulting. Volcanic rock developed along the faulting in the shape of a moniliform.

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5 Exploration trend for volcanic rock reservoirs in Chinese sedimentary basins In the filler of sedimentary basins, volcanic rocks occupy considerable proportion, and they can contribute up to 25% to the total fillers in these types of basins. Therefore, sedimentary basin volcanic rock easily accepts the hydrocarbons from sedimentary rocks, and it has an extensive prospect of hydrocarbon exploration in volcanic rocks. Fig. 8 Development patterns of volcanic rock in Songliao basin

Fig. 9 Reservoir Section at the northwest rim of the Junggar basin (Rearranged according to literature[3])

The volcanic rock distributes widely in the Chinese sedimentary basin, and new discovery is continuously met by exploration in recent times, and exploration domains also extend continuously. Volcanic rock reservoirs have become the important exploration targets in China, and the source for the hydrocarbon reserves growth. Three sets of volcanic rocks have developed in Chinese sedimentary basins, Carboniferous-Permian, Jurassic-Cretaceous, and Tertiary. Volcanic rock is mainly formed in the environment of the intracontinent rift and arcuate islands; volcanic rocks are mainly found in the central and composite eruption along the faulting, eruption phase and the effusion phase is more developed. Volcanic massif is medium and small in general and throughgoing in large areas, in groups and in banding; there are two kinds of eruption environments onshore and underwater. The assembly of underwater eruption– sedimentation is the most favorable. The volcanic rocks in the sedimentary basins of east China are mainly of the intermediate type, whereas, in west China they are mainly of the middle basic type. Volcanic rock can form four types of reservoirs, namely, Laval reservoir, volcaniclastic rock reservoir, dissolution reservoir, and fracture reservoir, under volcanic action, diagenesis, and tectogenesis. Initial eruption phase

volcaniclastic rock and effusion phase lava are the most favorable accumulation phase belts. Rocks with different lithologies can all form well denudated reservoirs through weathering and eluviations. The forming of volcanic rock reservoirs is mainly controlled by the lithology, lithofacies, and secondary action during volcanic eruption, and is less affected by compaction. Therefore, reservoir properties are less changed with the increasing of depth. Volcanic rock itself cannot generate organic hydrocarbons, and the key for reservoir forming is its matching with effective source rocks. Proximal play is most favorable for reservoir forming, because the hydrocarbon distribution is controlled by the center of hydrocarbon generation, whereas, distal play needs a fault or unconformity surface to communicate. Fault subsidence volcanic rock reservoirs in east China are mainly of proximal play, accumulations of eruption phases develop along the fracturing high, and a structure lithology reservoir is formed; effusion phases are distributed on slope part with large areas. This will be favorable for accumulation after fracture transforming, and mainly lithology reservoirs are formed. In the central and west of China, two kinds of plays are developed, proximal play is most favorable for large stratigraphic hydrocarbon reservoirs, the accumulation of weathering and eluvial type distribute along the unconformity,

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and can also form large stratigraphic hydrocarbon reservoirs. The generation of natural gas in the Chinese sedimentary basin is caused because of many reasons. The gas in them, with high CO2, is mainly from the inorganic mantle, which is mainly distributed near the deep-large fracturing belt during the late movement. There are six new trends for the exploration of volcanic rock in China at present: (1) in view of the regions, expand from Bohai bay basin in the east to deep formation of the Songliao basin, and expand rapidly from point to area in regions of western Junggar basin, Santanghu basin, and so on; (2) in view of the exploration position, extend from eastern Mesozoic-Cenozoic to western Neopaleozoic; (3) in view of the exploration depth, extend from middle-shallow formation to the middle-deep, even deep formation; (4) in view of the exploration location, extend from structure high to slope and depression; (5) in the view of lithology and lithofacies, extend from monolith to multi-lith, from proximal vent to distal vent; (6) in the view of hydrocarbon reservoir types, extend from structure, lithology hydrocarbon reservoir to lithology, stratigraphic hydrocarbon reservoir. Geological studies show that the volcanic rocks in China have extensive distribution areas. The total area is up to 215.7 × 104 km2, the predicted favorable exploration area is 36×104 km2, which shows the substantial potential for the exploration domain for volcanic rock hydrocarbon reservoirs. According to the present exploration achievement, in the primary, the predicted total petroleum reserves in the volcanic rocks will be above 60 × 108 t oil equivalent. Therefore, there are abundant reserves remaining in hydrocarbon bearing volcanic rocks in China, thus exploration has much potential, and is the important new domain for future hydrocarbon exploration.

Acknowledgments The authors would like to acknowledge together, the support and help from Zhao Zheng-zhang, Du Jin-hu, Fang Chao-liang of PetroChina, the support and help from Feng Zhi-qiang, Zhao Zhi-kui, Kuang li-chun, Liang Shi-jun, and others of the relative oilfield, and the support and help from the experts of RIPED.

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