Nonproducing Carboniferous may have potential in Qaidam basin, NW China

Due to natural geography of high elevation, thick Cenozoic deposits, and complicated geological conditions, oil and gas exploration has been facing challenges for a long term.^1-3

At present, China National Petroleum Corp. and China National Petroleum and Chemical Corp. (Sinopec) are exploring the basin. No foreign companies are exploring there.

Through 2010, cumulative proved oil in place and recoverable reserves were 3.1 billion bbl and 750 million bbl, respectively, and cumulative proved gas in place and recoverable reserves were 10.2 tcf and 5.5 tcf, respectively. Qaidam production averaged 36,500 b/d of oil and 530 MMcfd of gas in 2010.

All oil fields are distributed in the Paleogene and Neogene of the western part and the Jurassic of the northern part of the basin (Fig. 2). Except one gas field in the north, other gas fields are centered in the eastern Quaternary.

Since the 1950s, thousands of wells have been drilled in the basin. The average Qaidam well went to 3,000 m, and Han-2, the deepest, went to 6,018 m.

Located in the southern part of the basin, the Geermu oil refinery has a capacity of 30,000 b/d.

The only gas pipeline starts from Sebei gas field, runs through Xining, the capital of Qinghai province, and ends in Nanzhou, the capital of Gansu Province.

As a frontier formation, the Carboniferous was formerly thought as a part of the metamorphic base of the basin. Since 2000, workers have been studying the basin's Carboniferous petroleum geology.^4-10 No Carboniferous reserves or production have been established in Qaidam.

Carboniferous exploration history

The exploration history of the Carboniferous in the Qaidam basin can be roughly divided into four phases.

During the first phrase (before 1994), the main exploration workloads included gravity and surface and airborne magnetic prospecting at 1:500000 and 1:200000 scales in the eastern part of this basin, and field geological surveys, electrical prospecting, aerial photogrammetry, seismic exploration, outcrop investigation, remote sensing, and geochemical exploration at 1:50000 scale and a combined 4,096 line-km of 2D seismic in local areas of the eastern part of Qaidam.

In the second phase (1994-98), the main workloads were centered in the southeast part of the basin and included magnetic airborne surveys at 1:100000 scale of 37,572 sq km, 3,884 line-km of 2D seismic, and one wildcat, Huocan-1, to a total depth of 2,150 m.

During the third phase (1999-2007), exploration workloads included seven magnetotelluric survey lines of 500 km, 248 line-km in three 2D seismic lines, and one wildcat, Gaqiu-1, with a total depth of 618 m, in the northeast.

The project of Carboniferous petroleum assessment in the Qaidam basin was in 2008-10. In 2010, CNPC drilled the Aibei-1 wildcat into the upper Carboniferous. The main workloads of this project included 1,700 km of field route surveys, actual measurement of strata profiles of 18 km, collection of 480 rock samples and 78 oil and gas samples, and 1,739 lab tests.

Oil and gas discoveries

In northeastern Qaidam, thick oil sand beds were discovered in the Carboniferous of Shihuigou, Chengqianggou, and Wangaxiu, and oil sands were also found in the Carboniferous carbonate cores of the Aibei-1 well.

Oil sands were found in the Carboniferous limestone of Alagertaishan in southeast Qaidam and Shiguaizigou in southwest Qaidam. At Nuliangshan in northeastern Qaidam are multiple oil sand beds in Carboniferous limestone and Quaternary accumulative rocks, which shows that oil is migrating to the surface.

Through biomarkers and other geochemical parameters features, the authors of this article performed geochemical studies on the oil sand samples and analyzed parent sources of these oil sands. Carboniferous hydrocarbon rocks were classified into six types, and oil sands were divided into four types.

Oil-source correlation showed no affiliation between Carboniferous oil sands and Cenozoic and Mesozoic source rocks, while geochemical features of these oil sands were similar to those of Carboniferous source rocks. Therefore, these oil sands were testified to originate from Carboniferous source rocks.

In light of biomarker features, some oil samples collected from oil fields in the north and west Qaidam were classified into two types of mixed-source oil. The first type of oil originates from both Jurassic and Carboniferous source rocks while the second obtains contributions from the Paleogene and Carboniferous source rocks. This confirms that oil and gas generation, migration, and accumulation take place in the geohistory.

Divisions and geological features

In light of hydrocarbon source rock evaluation, oil and gas shows, oil-source correlation, and regional structural division, the Carboniferous of the Qaidam basin was classified into four petroleum systems.

These are the northeast Qaidam, southeast Qaidam, northwest Qaidam, and southwest Qaidam petroleum systems.

In the northeast Qaidam petroleum system, a large proportion of mudstone, carbonaceous shale, coal, and limestone of upper Carboniferous and mudstone and limestone of lower Carboniferous are active source rocks, with medium-high value of total organic carbon of mudstone and carbonaceous shale, low value of total organic carbon of limestone, the kerogen type II₂ and III and the vitrinite reflectance of less than 1.3%.

In this petroleum system, Carboniferous reservoirs are mainly clastic (including siltstone, fine-medium sandstone, and conglomeratic sandstone) and carbonate rocks (mostly bioclastic limestone). Interbedded mudstone (carbonaceous shale, coal)-limestone-sandstone constitutes favorable source-reservoir-seal assemblages.

Multiple-staged overlapped folds form groups of structural traps. A large amount of faults and unconformities formed by tectonic movements are oil and gas conduits. Multiple-period tectonic movements are favorable for oil and gas migration and accumulation as well as tend to destroy original oil and gas pools. The end of Paleogene is the critical moment of the northeast Qaidam petroleum system (Fig. 3).

The Carboniferous of the northeast Qaidam petroleum system is entirely of shallow burial, and most of source rocks are currently in the oil window, so it is probable to find oil and gas in this system. With strong hydrocarbon generating of early phase while weak yielding of late phase, source rocks are presently at the stage of high and overmaturity, and the exploration object is assumed to be natural gas in the southeast Qaidam petroleum system.

In the large part of southwest and northwest Qaidam petroleum systems, the Carboniferous is buried more than 5,000 m during the period of the Cenozoic and source rocks are currently overmature, so it is probable to find natural gas in these two petroleum systems.

Resource assessment results

This research adopted geochemical and analog methods to evaluate oil and gas resources of every Carboniferous petroleum system in the Qaidam basin.

Carboniferous petroleum resources in place are 4.5-5.3 billion boe and recoverable petroleum resources are 1.1-1.26 billion boe in Qaidam.

Petroleum resources of the southeast Qaidam petroleum system are largest, while those of the northeast Qaidam are smallest. For the total basin, in-place petroleum resource richness is 36-49 million boe/1,000 sq km. In-place petroleum resource richness of the northeast Qaidam petroleum system is the largest while that of northwest Qaidam is the smallest (Table 1).

In light of the principle of maximum similarity, some assessment units in different basins, such as the south margin subsalt of the North Caspian basin and the Lodgepole of the Williston basin, having both Carboniferous source and reservoir rocks, and other similar geological conditions (including sedimentary environment, tectonic background, and petroleum generation, migration, and accumulation), were chosen to compare with every Carboniferous petroleum system of the Qaidam basin. Among the 21 geological units, four Carboniferous petroleum system of the Qaidam basin have medium oil and gas resource richness (Fig. 4).^11-12

Favorable exploration plays

Through petroleum geology and resource assessment, the Carboniferous of the Qaidam basin is thought to have favorable oil and gas exploration prospects.

In the northeast Qaidam petroleum system, the southern uplift is located in the oil and gas migration pathway for a long distance, and average burial depth of Carboniferous is 2,000 m, so it is the most favorable exploration area in the Qaidam basin.

In the southeast Qaidam petroleum system, oil and gas expelled from the central sag migrate and accumulate in the northern and eastern uplifts, so these two uplifts are favorable for petroleum exploration. However, due to deep Carboniferous reservoirs at more than 4,000 m), oil and gas exploration in the southwest and northwest Qaidam petroleum systems will face challenges.

Acknowledgments

The authors acknowledge contributions from Xiaofeng Wang, Chengming Yin, Jianjun Du, Taoyuan Fan, and Jiaying Yuan with the Institute of Geomechanics. We also thank Wangbin Gong, Wei Zhao, Zhiqiang Wang, Shaoqing Wang, Lili Xu, and Chuanqi Yu with the China University of Petroleum for their help.

References

1. Tang, L.J., Jin, Z.J., and Zhang, M.L, "Tectonic evolution of Qaidam basin in Sinian-Triassic," Scientia Geologica Sinica, Vol. 34, No. 3, 1999, pp. 289-300.

2. Yang, P., and Hu, Y., "Paleoecology and sedimentary environment of Carboniferous in Qaidam Basin," Xinjiang Petroleum Geology, Vol. 27, No. 3, 2006, pp. 280-84.

3. Niu, Y.B., Zhong, J.H., and Zhong, F.P., "Discovery of trace fossil Chondrites in the Carboniferous in south margin of Qaidam Basin and its geological significance," Journal of Palaeogeography, Vol. 10, No. 5, 2008, pp. 529-35.

4. Yu, H.J., Liu, L.H., and Zhao, L., "Study on source rocks of Palaeozoic in the Eastern Qaidam Basin," Journal of the University of Petroleum, China, Vol. 25, No. 4, 2001, pp. 24-30.

5. Wen, Z.G., Wang, Z.Y., and He, Y.B., "Evaluation on upper Carboniferous hydrocarbon source rock in the Northern margin of Qaidam Basin," Natural Gas Geoscience, Vol. 15, No. 2, 2004, pp. 125-27.

6. Wan, C.Z., Le, X.F., and Chen, Y.B., "Distribution Law and evaluation of hydrocarbon potential on Carboniferous in the east of Qaidam Basin," Natural Gas Geoscience, Vol. 17, No. 5, 2006, pp. 663-67.

7. Zhang, J.L., Zhong, J.H, and Li, Y.H., "Petroleum geologic conditions and exploration prospects of Carboniferous in the east of Qaidam Basin," Petroleum Geology & Experiment, Vol. 30, No. 2, 2008, pp. 144-49.

8. Shi, J., Zhen, Y.Q., and Wu, J.F., "Prospect of Carboniferous hydrocarbons in the Qaidam Basin," Geological Survey and Research, Vol. 31, No. 1, 2008, pp. 43-51.

9. Duan, H.L., Zhong, J.H., and Qiu, X.M., "Geochemical characteristics of carboniferous hydrocarbon source rocks in the Eastern Qaidam Basin," Journal of Basic Science and Engineering, Vol. 17, No. 2, 2009, pp. 219-29.

10. Niu, Y.B., Zhong, J.H., and Duan, H.L., "Relationship between Carboniferous sedimentary facies and source rock in Qaidam Basin," ACTA Sedimentologica Sinica, Vol. 28, No. 1, 2010, pp. 140-49.

11. China National Petroleum Assessment Team, "China national conventional petroleum resource assessment 2007," China Land Press, Beijing, China, 2009, 1,500 pp.

12. Charpentier, R.R., Klett, T.R., and Attanasi, E.D., "Database for assessment unit-scale analogs (exclusive of the United States)," U.S. Geological Survey Open-File Report 2007-1404, 2008, p. 61.

The authors
Chenglin Liu ([email protected]) was with the Research Institute of Petroleum Exploration and Development, Petrochina, from 1994 to 99, and has been with China University of Petroleum working on petroleum resources assessment since 2004. He received a BA degree in petroleum geosciences and a PhD degree in mineralogy, petrology, and mineral deposit geology from China University of Petroleum.

Gang Zhou ([email protected]) has been with China University of Petroleum studying for his master degree in petroleum geosciences since 2009. He received a BA degree in petroleum geosciences from China University of Petroleum.

Yinsheng Ma ([email protected]) has been with the Institute of Geomechanics, Chinese Academy of Geological Sciences working on geotectonics and crustal stability since 1998. He received a PhD degree in geotectonics from Chinese Academy of Geological Sciences.

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