Figure
1.
Comparison of petroleum accumulation system and petroleum system.
| Citation: | Zhenxue Jiang, Xiongqi Pang, Zhijun Jin, Dongxia Chen. Division of Gas Accumulation System and Laws Controlling Distribution of Natural Gas in Typical Petroliferous Basins of China. Journal of Earth Science, 2002, 13(4): 306-312.
|
Considering the existing problems of the petroleum system, this paper brings forward the concept of natural gas accumulation system and presents the dividing principles. Then detailed statistics on the accumulation factors of the 32 typical natural gas accumulation systems in China and studies on the laws controlling distribution of gas are collected. The research shows that the petroleum accumulation system is the basic unit controlling petroleum generation, migration and accumulation. Generating intensity, generating amount, accumulating efficiency and migration distance plays an important role in the distribution of natural gas. Through analysis on results of resources evaluation, discovered reserves and residual reserves, potential areas in middle-scaled petroliferous basins in China are forecasted in this paper. Ordos, Sichuan, Tarim and Qaidam basins are found out to be the main basins developing and enriching gas accumulation systems.
The petroleum system (Magoon and Dow, 1994; Magoon, 1992, 1990; Demaison and Huizinga, 1991) emphasizes on the distribution and evolution of source rocks. A petroleum system has only a set of source rocks if it is mainly focused on the formation, distribution and evolution of the source rock. Most basins in China are superimposed basins, in which it is difficult to divide petroleum system. Moreover, investigations of the laws of petroleum distribution in the system are insufficient currently, which is a disadvantage to instructing exploration (He and Zhao, 2000; Zhang, 1999).
We define petroleum accumulation system as the organic combination of one or several suits of source rocks with the evolving petroleum accumulation plays being formed during the formation of the basin. Although both of them adopt means of system analysis, the biggest difference between them is the view of analyzing problems. Adopting theories of system (Zhou, 1988; Zang, 1985), and according to the formation and evolution of trap, "petroleum accumulation system" systematically analyzes petroleum accumulation laws based on the formation, distribution and evolution of source rocks.
A petroleum system may contain one or more petroleum accumulation plays; a petroleum accumulation play may be located between two petroleum depressions and belong to two petroleum systems. This brings about difficulties to the application of the petroleum system. If taking petroleum accumulation plays as the nuclear factor in the systematic analysis of petroleum accumulation, the difficulties can be solved. Meanwhile, it is much more practical, for system analysis is conducted closely with trap, which is the nucleus of exploration. If there is one suit of source rock in the petroleum accumulation system, petroleum accumulation system is the subunit of the petroleum system. If a petroleum accumulation system is located between two source rocks, this petroleum accumulation system comprises some parts of the two petroleum systems (Fig. 1). Thus, a close relation between petroleum accumulation system and petroleum system is expected. The former is the development of the latter while the latter is a basis when analyzing petroleum accumulation system.
Because of the complication of geological process, it is difficult to establish the division criteria of the petroleum accumulation system. Whether the criteria are over detailed or over simple or not, it may bring inconveniences to the operator, Jin (1997) summed up common principles as the followings (CNPC, 1997. "Nine Five year" plan project "Quanititative Models of petroleuj Accumulation in Major Fields (960007) " medium-term report) : (1) Both source rocks and main petroleum accumulation zones should be considered on division; (2) A reservoir with uniform interface of oil, gas and water can only be attributed to one petroleum accumulation system; (3) For superimposed basin with several source rocks and accumulating periods, accumulation cycle (the combination of the source, reservoir and cap) should be firstly divided according to structural layers and abnormal pressure strata, then petroleum accumulation system; (4) For each accumulation cycle, commonly take accumulation unit as the nucleus and the migration channels as main clue, and the location of source rocks as boundary to divide the petroleum accumulation system.
The concrete division method of petroleum accumulation system is: firstly, with the help of the paleo-tectonic maps of the basin comprising oil and gas fields, the migration direction and accumulation zones can be identified based on the distribution of potential source rocks according to the principle of division groove; secondly, petroleum accumulation system in each oilfield can be divided into combination of development and evolution of tectonic unit and/or traps (Jiang et al., 1999).
We divide the petroleum accumulation systems of the main petroliferous basins in China based on principles foregoing, area, average intension of gas generation, total gas generation amounts, the efficiency of migration and accumulation and migration distance for every petroleum accumulation system are calculated, the detailed data are given in Table 1.
|
DownLoad:
CSV
Gas accumulation system is the basic unit constraining gas generation, migration and accumulation, and plays an important controlling role on the gas distribution.
Intensity of gas generation is plotted against the maximum gas field reserves of gas accumulation systems in mid-large gas fields (Fig. 2). It is observed that there is an increasing trend of the maximum gas field reserves increasing with the accretion of gas generation intensity. It is also found that there is a minimum limit of the gas generation intensity to form a large gas reservoir, and it is impossible to form a gas reservoir if the gas generation intensity is smaller than the minimum level. The reason is that natural gas will be depleted by dissolution, diffusion and adsorption in the process of migration and accumulation. Gas reservoirs can beformed only if the amount of gas generated is larger than the amount depleted, which reflects that the accumulation of gas is more difficult than that of oil from this point.
Figure 3 is the correlation map between gas generation intensity and gas field reserves in mid-large gas field accumulation system. Obviously, there is an increasing trend of gas field reserves with the accretion of gas generation intensity, and there is a critical minimum limit of gas generation intensity, which shows that in the area with the same gas generation intensity, the risk of oil exploration is lower than the gas.
Figure 4 is the correlation map between the total gas generation amount and the accumulation amount of the mid-large gas field accumulation system. It shows that there is an increasing trend of gas field reserves with the accretion of the gas generation intensity. Itcan be seen that, to form a large-scale gas reservoir, the total gas generation amount must reach or exceed a critical amount. For example, to form a gas reservoir with 100×108 m3 gas, the total gas generation amount must reach or exceed 20 000×108 m3, and to form a gas reservoir with 1 000×108 m3 gas, the total gas generation amount must reach or exceed 50 000×108 m3.
Figure 5 shows relationship between migration and accumulation efficiency and gas field reserves in gas accumulation systems of mid-large gas fields. It demonstrates that gas reserves are larger in the gas accumulation system with higher accumulation efficiency. Migration efficiency reflects geological accumulation condition of a gas accumulation system. Favorable geological conditions tend to form larger gas reservoir.
Migration distance is also an important factor controlling the laws of gas distribution in gas accumulation system. Three conclusions could be acquired according to Figs. 6 and 7.
① More than 95 % of mid-large gas fields are discovered within 100 km from the source area. There is a decreasing trend of the number of gas fields of mid-large scale with increasing of the migration distance (Fig. 6). Moreover, more than 95 percent gas reserves of mid-large gas fields are distributed in areas within 50 km from the source area.
② More than 95 % of the numbers and reserves of mid-large gas fields concentrate in an area within 50 km from source area, and decrease with the increasing of migration distance. This shows that source rocks control the distribution of natural gas (Fig. 7). Therefore, to trace efficient source rocks is the premise for exploring large gas fields.
③ Most mid-large gas fields are discovered within 15-90 km from the source area. It demonstrates that gas migrates more easily than oil. The number and reserves of large gas fields increase with the migration distance firstly, then decrease. It shows that the mi-gration distance within 20-60 km from source area is favorable. This is probably not the real case as the reserves for deep basin gas accumulations, which are very likely to occur in the central part of the source rock area, not taken into consideration.
After a comprehensive study on gas accumulation system in which mid-large gas fields have been discovered and those in which mid-large gas fields are hopeful to be discovered, the evaluation and ranking are carried out based on their remaining resource potential. The research and exploration advances made by former specialists and scholars are referred and when determining gas accumulation system in which mid-large gas fields are hopeful to be discovered.
Firstly, predicting the potential distribution of mid-large gas fields by using the results of resources evaluation discovered reserves and remaining resources potential. Secondly, using the distributive law of mid-large gas fields discovered to predict the favorable potential distribution areas, which include large-scale paleo-upheaval area, the structure belt within large subsidence center, large-scale structure belt of anticline, large-scale fault zone, large-scale slope belt, large-scale lithologic pinchout zone, large-scale unconformity zone, large-scale ancient buried hill, the reservoir bed and the salt-roof rock with large thickness. Moreover, the favorable potential areas are predicated by fully applying petroleum geological theories of China with advanced petroleum geological theories abroad, such as theory on terrestrial originated oil, theory of source controlling, theory of superimposed accumulation play, theory of coal-related hydrocarbon, and theory of deep basin gas. Also, exploration for mid-large oil and gas fields under complex geological conditions can be conducted by fully applying advanced explorative technology, such as seismic technologies for mountainous areas, elaborate seismic technology for low-height trap with deep buried depth, further study and technologies of reservoir modification on carbonate reservoir of buried hill, and beach and offshore seismic exploration.
Natural gas exploration will focus on four basins: Sichuan, Ordos, Tarim and Qaidam in China. According to the investigation of diverse geological conditions, twenty-three accumulation systems favorable for mid-large gas fields are selected for evaluation and ranking. The detailed results are shown in Table 2. It indicates that the accumulation system of middle gas field in Ordos basin is the most favorable for the formation of mid-large gas fields, whose predicted resource is 10 920×108 m3 and the residual resource is 8 024×108 m3. The next is gas accumulation system of western area of Chuandong with residual resource of 68 052×108 m3, gas accumulation system of eastern area of Chuandong with residual resource of 6 143×108 m3, and gas accumulation system of Hetian River with residual resource of 4 492×108 m3.
|
DownLoad:
CSV
(1) Applying the principle and methodology of the petroleum accumulation system, the authors divide 32 gas accumulation systems in the main petroliferous basins of China.
(2) The research indicates that the petroleum accumulation system is the basic unit to control hydrocarbon generation, migration and accumulation. The intensity and amount of gas generation, and the efficiency and distance of gas migration etc. play important role in the distribution of gas.
(3) Based on analysis of diverse geological conditions, twenty-three accumulation systems favorable for mid-large gas fields are selected for evaluation and ranking. The gas accumulation system in the Ordos basin is the most favorable for formation of mid-large gas field. The gas accumulation system of western area of Chuandong and eastern area of Chuanxi and Hetian River rank the second, the third and the fourth respectively.
| Demaison G, Huizinga B J, 1991. Genetic Classification of Petroleum System. AAPG Bulletin, 75(10): 1626-1643. |
| He D, Zhao W, 2000. Characteristics of Composite Petroleum System of Superimposed Basins in China. Earth Science Frontiers, 7(3): 23-36. http://en.cnki.com.cn/Article_en/CJFDTOTAL-DXQY200003003.htm |
| Jiang Z, Liu H, Huang Z, 1999. Division and Evaluation of Petroleum System in Tuha Basin. Journal of Daqing Petroleum Institute, 23 (4): 1-5. http://en.cnki.com.cn/Article_en/CJFDTOTAL-DQSY199904000.htm |
| Magoon L B, 1990. Identified Petroleum Systems with in the United States-1990. The Petroleum System-Status of Research and Methods. USGS Bulletin 1912, 1989b: 2-9. |
| Magoon L B, 1992. The Petroleum System—Status of Research and Methods. USGS Bulletin 2007, 1992a: 98. |
| Manoon L B, Dow W G, 1994. The Petroleum System. AAPG Memoir 60, 3-24. |
| Zang Z, 1985. The Method of System. Shenyang: People of Liaoning Province Publishing Company |
| Zhang H, 1999. New Concept of Petroleum System and Its History-Genetic Classification. Journal of Chengdu University of Technology, 26(1): 14-16 |
| Zhou S, 1988. Guide of Science System. Beijing: Earthquake Publishing Company |
Figures(7) / Tables(2)
Copyright © 2013-2020 Journal of Earth Science 鄂ICP备15021562号-2
Tel: +86-27-67885075 Fax: +86-27-67885075 E-mail: xbb@cug.edu.cn
Address: Editorial Office of Journal, China University of Geosciences, Yujiashan, Wuhan, Hubei 430074, P. R. China
Supported by:
Beijing Renhe Information Technology Co. Ltd
E-mail:
info@rhhz.net
Zhenxue Jiang, Xiongqi Pang, Zhijun Jin, Dongxia Chen. Division of Gas Accumulation System and Laws Controlling Distribution of Natural Gas in Typical Petroliferous Basins of China. Journal of Earth Science, 2002, 13(4): 306-312.
|
|
DownLoad:
CSV
|
|
DownLoad:
CSV
DownLoad:
DownLoad:
