2.3.1.1 Reserves and Resources

(1) Oil and gas reserves in 1997

According to statistics made by relevant departments, up to the end of 1997, the country's accumulative explored geological reserves of oil whole were 19,066 million tons, and the recoverable reserves were ratified to be 5,499 million tons. With an average recovery rate of 29%, the accumulative oil production for the past years was 3,172 million tons, and the remaining recoverable reserves became 2,327 million tons, ranking 11th in the world.

Up to the end of 1997, the accumulative explored geological reserves of gas of the whole country were 1,697.796 billion cubic meters, and the recoverable reserves were ratified to be 1052.804 billion cubic meters, the accumulative production for the past years was 247.563 billion cubic meters, and the remaining recoverable reserves became 805.241 billion cubic meters, ranking 24th in the world.

(2) Oil and gas resources

The oil and gas resources include the discovered reserves and the undiscovered reserves, i.e. the total resources as are commonly termed.

In the 1980s, China's energy and mineral resources departments made a comprehensive assessment and quantitative prediction of the oil and gas resources in the country by using a number of methods. The estimated data have been published separately: the oil resources of the whole country are 61.47 billion tons (former Ministry of Geology and Mineral Resources (MGMR), 1987) and 78.75 billion tons (former Ministry of Petroleum Industry (MPI), 1987); the gas resources are 26.8 trillion cubic meters (former MGMR, 1987) and 33.3 trillion cubic meters (former MPI, 1987). In the early 1990s, CNPC (China National Petroleum Corporation) and CNOOC (China National Offshore Oil Corporation) carried out another nation-wide assessment of the petroleum resources.

This paper presents a general description based on the new round of assessment and some literature on the resources and their characteristics.

According to statistics, there are 319 basins of various sizes in China. The new round of resources assessment covered 145 of them, including 25 basins with discovered oilfields, 42 with oil and gas shows and 78 with no findings. The results give out that the total oil resources of the whole country are 93.032 billion tons, and the total gas resources are 37.92 trillion cubic meters.

At the end of 1997, the explored geological reserves of oil comprised 20.5% of the total resources, and the ultimate recoverable reserves were 14.0 billion tons. By deducting the 5,499 million tons of explored reserves, the recoverable reserves to be proved would be some 8.0 billion tons. The explored geological reserves of natural gas accounted for 4.5% of the total resources, and the ultimate recoverable reserves were 13.2 trillion cubic meters. Deducting the 1.05 trillion cubic meters of explored reserves, the recoverable reserves to be explored would be some 12.15 trillion cubic meters.

In addition, according to the data published by Fushun Institute of Petroleum and the Ministry of Geology and Mineral Resources in 1997, there are also 31.482 billion tons of explored reserves of oil shale in China, with the prognostic reserves reaching 452.0 billion tons. Based on the empirical conversion that 33~35 tons of oil shale produces 1 ton of shale oil, the above reserves may yield 917 million tons and 13.29 billion tons of shale oil respectively, which add up to about 14.2 billion tons. This figure is equivalent to the ultimate recoverable reserves of China's natural crude oil.

Another unconventional natural gas resource abundant in China is the coalbed gas (coalbed methane). As predicted by the North China Petroleum Bureau of CNSPC (China National Star Petroleum Corporation), the recoverable resources of coalbed gas (methane) at depths shallower than 2,000 m throughout China amount to 32.63 trillion cubic meters, almost equivalent to the total conventional natural gas resources of the country.

(3) Distribution of oil and gas resources in terms of scale

In China, 82% of the oil resources are concentrated in 13 large basins, each with resources exceeding 1 billion tons, and 86% of the natural gas resources are also distributed in those 13 basins (Table 2.3.1). The oil and gas resources can be divided into four categories according to their scales, as shown in Tables 2.3.2 and 2.3.3.

There are only 3 basins with oil resources exceeding 10 billion tons, which are the Bohai Bay (North China), the Songliao and the Tarim basins. Their resources amount to 42.233 billion tons, accounting for 45.4% of the total resources of the country. The offshore shelf basins have a total amount of 19.153 billion tons of resources, comprising 20% of the total.

Table 2.3.1 China's oil and gas resources

Table 2.3.2 Distribution of oil resources

Table 2.3.3 Distribution of gas resourcesScale of resources

There are 9 basins with natural gas resources over 1 trillion cubic meters, namely, the Tarim, Ordos, Sichuan, Pearl River Mouth, East China Sea Shelf, Bohai Bay, Yinggehai, Qiongdongnan and Junggar basins. Among them, four are offshore shelf basins, with the resources amounting to 11,691.114 billion cubic meters, 30% of the total gas resources of the country; while the Tarim, Ordos and Sichuan basins have 19,750 billion cubic meters of resources, accounting for 52.1% of the total.

Table 2.3.4 Localities of China's oilfield and gasfields

Note: LL=superlarge; L=large; M=medium; S=small.

There are 9 basins with natural gas resources over 1 trillion cubic meters, namely, the Tarim, Ordos, Sichuan, Pearl River Mouth, East China Sea Shelf, Bohai Bay, Yinggehai, Qiongdongnan and Junggar basins. Among them, four are offshore shelf basins, with the resources amounting to 11,691.114 billion cubic meters, 30% of the total gas resources of the country; while the Tarim, Ordos and Sichuan basins have 19,750 billion cubic meters of resources, accounting for 52.1% of the total.

(4) Geographic distribution of oil and gas resources

The 145 basins with petroleum resources assessed spread all over the country. Among them, 25 petroleum provinces with oil/gas fields and reserves explored are distributed in 25 basins (22 are shown in Fig. 2.3.1, the other three are the Lunpola Basin in Tibet, and the Wan'an and Zengmu basins in the southern South China Sea). Besides, there are 42 basins in which oil/gas pools or shows have been found. They are scattered over 25 of the provinces, municipalities or regions of China (Fig. 2.3.1). Information on their names, pay beds, scales etc. are given in Table 2.3.4, and their explored reserves are shown in Table 2.3.1.

According to statistics, the distribution of petroleum resources in the 6 geographic regions is as follows: in the eastern region (Northeast China, North China and the Yangtze-Haihe area) there are 36£¬340 million tons, comprising 39.1% of the total; while in the western region (Xinjiang, eastern Qinghai and Gansu), and the sea area there are 24£¬789 and 24£¬675 million tons respectively, amounting to 53.2% of the total. The above three regions occupy 92.3% of the total resources, whereas the central, southern and Qinghai-Tibet regions have only 7.7%.

Natural gas resources are mainly distributed in central China (Shaanxi, Gansu, Ningxia, Sichuan and Chongqing),amounting to 11.36 trillion cubic meters; secondly in the western region, 10.78 trillion cubic meters. Resources in the above two regions make up 58% of the total, those in the offshore shelves 30%, and those in eastern China (4.83 trillion cubic meters) 12% of the total.

To sum up, in terms of the reserves already explored, oil is mainly concentrated in eastern China while natural gas mainly occurs in central China.

2.3.1.2 Characteristics of Oil and Gas Resources

(1) Quality

Among the 93£¬032 million tons of total oil resources, conventional oils amount to 81£¬000 million tons, or 87.1% of the total. The others are unconventional resources: 5,657 million tons of oil in low-permeability reservoirs, 6.1%; 1£¬907 million tons of heavy oil, 2.0%; and 4£¬462 million tons of low-mature oil, 4.8%. The latter two amount to 6£¬369 million tons and are mainly distributed in the western Liaohe and Damintun depressions, the Dongying depression in North China, the Biyang depression in Henan, and the Pearl River Mouth and Junggar basins.

In the 37.92 trillion cubic meters of natural gas resources, there are: oil-type gas, 33.5 trillion cubic meters, 88.4%; coalbed gas, 4.2 trillion cubic meters, 10.8%; and biogas, 0.31 trillion cubic meters, 0.8%. The coalbed gas mainly occurs in central China (3.9 trillion cubic meters), and the biogas, mainly in the Qaidam Basin (0.29 trillion cubic meters).

(2) Conditions for development

Except for a small number of oilfields, such as Daqing, the conditions for the development of the resources are not ideal. The Daqing Oilfield is endowed with very good conditions with the resources fairly concentrated. In the 5,400 million tons of explored geological reserves, those of the three main oilfields, Lamadian, Saertu and Xingshugang alone make up 80%. They are large in area (totaling 920 km2), with thick oil reservoirs (20~72 m), good physical properties (porosity 23%~35% and permeability 145¡Á10-3~3600¡Á10-3¦Ìm2), high abundance of reserves (4.54 million tons/km2) and moderate depths of reservoirs (750~1500 m), which make it easy to exploit. Conditions are more complicated for other regions.

(3) Economic geographic conditions

China's petroleum resources in the plain area amount to 28,090 million tons, accounting for 30.2% of the total petroleum resources in the country. Among them, 9,700 million tons are explored and 18,390 million tons are unexplored. The latter are less than those in the sea areas (24,670 million tons) or the deserts (18,850 million tons). The others are distributed in swamps and drainage networks (9,660 million tons), mountainous areas (4,290 million tons) and plateaus (5,060 million tons). The gas resources distributed in the plain areas comprise only 11.8% (4,387.8 billion cubic meters) of the total; they occur mainly in the sea areas, 21.46% (8,159.1 billion cubic meters); the deserts, 24.22% (9,210.7 billion cubic meters); and the mountainous areas, 25.87% (9,854.8 billion cubic meters). The others are distributed in: plateaus, 12.17% (4,620.6 billion cubic meters); swamps and drainage networks, 1.92% (729.8 billion cubic meters); and beaches, 2.52% (956.9 billion cubic meters). To sum up, only one third of China's petroleum resources are distributed in good economic geographic conditions, whereas the others lie in complicated regions.

(4) Geologic conditions for development

If assessed in terms of the trapping conditions, about one third of oil resources belong to simple types (anticlinal, nose-structural oil reservoirs), and the others are of complex types (fault-block, lithologic, stratigraphic oil reservoirs etc.). In terms of regions, the proportion of complex types has raised from 44.8% in 1987 to the present 77.21% for the Northeast China region, from 47.7% to 86% for the North China region, and over 50% for the other regions. Similar is the situation for natural gas: 86.13% are of complex types in the eastern regions and about 50% in the other regions.

(5) Burial conditions of productive strata

In China, 49.57% of oil resources are buried at depths of 2,000~3,500 m, 23.26% at 3,500~4,500 m, and 15% shallower than 1,500 m. Four billion tons of oil resources (about 40%) in the Tarim Basin are buried at depths exceeding 4,500 m. Situation is similar for natural gas, most of which is buried at 2,000~3,500 m in the central and sea areas. Compared with other countries, China's petroleum resources are buried at greater depths and under more complicated geographic and geologic conditions, and hence more difficult to develop. That is why the average recovery factor is only 29% for the explored reserves of oil (42% in the Daqing Oilfield) and 62% for those of natural gas.

2.3.1.3 Reservoiring Characteristics

The long history and the polycyclicity and complexity of tectonic evolution in China have resulted in such features as the multistage formation of basins, polycyclic deposition, superimposition of basins controlled by various types of sedimentary environments and different tectonic regimes, and hence the hydrocarbon resources are characterized by their multi-source, multi-stage, multi-age and multi-type reservoirs. They can be summed up as follows.

(1) Wide span of reservoiring time intervals

Some 46.6 billion tons of oil resources are of Cenozoic origin, accounting for 50% of the total; and 19 trillion cubic meters of gas resources of the Paleozoic origin, also about 50%. These data agree with the distribution of explored reserves: 45.6% of oil reserves are found in the Cenozoic, while 44.8% of gas reserves are found in the Paleozoic (Table 2.3.5).

Table 2.3.5 Geologic ages of China's petroleum resources

(2) Oil being dominated by continental origin, gas by marine and continental origins

Most of China's basins are Meso-Cenozoic basins, in which are deposited mainly continental sediments. Continental lacustrine source-reservoir rock series are well developed, forming a great number of continental oil and gasfields. About 96% of the explored oil reserves are of continental origin, and nearly half of gas resources (18 trillion cubic meters) are of continental origin.

(3) Characteristics of formation of continental oil and Gas pools

Oil and gas in China are mainly "generated and reservoired in place", that is to say, in a continental basin with oil-generating conditions, there often occurs an oil/gas pool in which the source, reservoir and caprock structures are adjacent to each other, e.g., the main oilfields in eastern China, such as the Daqing Oilfield. Other features include reservoirs dominated by sandstones, oil-gas distribution controlled by oil-generating zones; and multi-system, polycyclic and multi-age formation of oil and gas pools and great variety of types of oil-gas pools.

(4) Characteristics of natural gas of marine origin

The source and reservoir beds are dominated by Paleozoic marine carbonate rocks, and the gas pools are characterized by multi-source and multi-stage generation, anisotropic reservoirs, and multi-stage and even late-stage reservoiring. The main factors controlling the enrichment and accumulation are the regional unconformity and paleohigh. Anticlinal gas reservoirs are the dominant type (20 out of the 26 large and medium-sized gasfields in China are of the anticlinal type).

(5) Large oil and gasfields being predominantStatistics of the 387 oilfields and 132 gasfields discovered in the early 1990s show that the reserves of large oilfields account for 62% of the total whereas those of large and medium-sized gasfields comprise 84% of the total (Table 2.3.6).

Table 2.3.6Statistics of oilfields and gasfields