♦Abstract

♦Figures

♦Geology

♦Accumulation

♦Exploration prospects

♦References

♦Abstract

♦Figures

♦Geology

♦Accumulation

♦Exploration prospects

♦References

♦Abstract

♦Figures

♦Geology

♦Accumulation

♦Exploration prospects

♦References

♦Abstract

♦Figures

♦Geology

♦Accumulation

♦Exploration prospects

♦References

♦Abstract

♦Figures

♦Geology

♦Accumulation

♦Exploration prospects

♦References

♦Abstract

♦Figures

♦Geology

♦Accumulation

♦Exploration prospects

♦References

Geological Setting of Songliao Basin

The tectonic evolution of the Songliao Basin can be divided into four phase: 1) a pre-rift phase, 2) a syn-rift phase, 3) a post-rift phase, and 4) compression phase (Cheng et al., 2006) (Figure 1c). According to the characteristics of rises and depressions, the SLB can be divided into six first-order tectonic units: central depression zone, north plunge zone, west slope zone, northeast uplift zone, southeast uplift zone, and southwest uplift zone (Gao et al., 1994) (Figure 1b).

The basement of the SLB consists of Precambrian to Paleozoic metamorphic and igneous rocks and Paleozoic to Mesozoic granites (Wang et al., 2006; Pei et al., 2007). Unconformably overlying the basement are up to 7000 m of Mesozoic and Cenozoic terrestrial strata, unevenly distributed across the basin (Gao et al.,1994). A simplified lithostratigraphy of SLB is presented in Figure 1c.

The Upper Jurassic and Cretaceous strata in SLB are commonly subdivided into two sequences separated by a regional unconformity at the top of the Yingcheng Formation (K1y). The lower sequence includes the Upper Jurassic Huoshiling Formation (J3h) and the Lower Cretaceous Shahezi (K1s) and Yingcheng (K1y) formations (Figure 1c). These formations are composed of volcanic/volcaniclastic rocks and alluvial/lacustrine sedimentary rocks. The upper sequence consists of seven formations, in ascending order including Denglouku (K1d), Quantou (K2q), Qingshankou (K2qn), Yaojia (K2y), Nenjiang (K2n), Sifangtai (K2s) and Mingshui (K2m) formations (Figure 1c). The Denglouku Formation (K1d) is composed mainly of alluvial and fluvial deposits, while the Quantou Formation (K2q) consists of coarse clastic rocks of fluvial origin in the lower part and lacustrine mudstones in the upper part. The lacustrine sandstone, mudstone, and shale of the Qingshankou (K2qn), Yaojia (K2y) and Nenjiang (K2n) formations contain the majority of the oil source rocks, reservoir rocks and seals of the Daqing Oil Field (Li, 1995) (Figure 1c). Unconformably overlying the K2n, the Sifangtai (K2s) and Mingshui (K2m) formations are mainly composed of siliciclastic rocks deposited from alluvial and deltaic environments during basin inversion.

The Qingshankou Formation (K2qn) and Nengjiang Formation (K2n) were deposited during the post-rift thermal subsidence stage. On the basis of lithofacies and biostratigraphy difference, the K2qn and K2n can be respectively divided into three and five members. K2qn1, K2n1 and K2n2 are mainly composed of deep lacustrine black mudstone and shale. The oil shale at the lower part of the K2qn1 and K2n2 are the most important marker for regional stratigraphic correlation. K2qn2, K2qn3, K2n3, K2n4 and K2n5 consist of interbedded black mudstone and gray- to celadon-colored siltstone. In general, the Qingshankou and Nenjiang formations show a shallowing-upward trend from deep lacustrine in the lower part to shallow lacustrine, and coastal and deltaic deposits in the upper part (Gao et al., 1994; Li, 1995).

Accumulation of Mud-Shale Oil and Gas in SLB

Source rock distribution and its geochenmical character
The majority of mud-shale oil and gas reservoirs which are found in SLB are distributed in two sets of source rocks of the Qingshankou and Nenjiang formations. Gross thickness of Nenjiang Formation mud-shale ranges from 70 to 240 meters, and Qingshankou Formation from 80 to 420 meters with a burial depth of about 300-1500 m. The mud-shale mainly consists of clay minerals, and sometimes includes some siltstone. Dark mudstone, light-colored siltstone, and ostracoda mostly form interbeds. Mud-shale total organic carbon (% TOC) content ranges from 1% to 5%, and as high as 13%. Qingshankou Formation mud-shale is thermally mature for hydrocarbon generation, vitrinite reflection (% Ro) ranges from 0.7% to 1.5%. Nenjiang Formation shale has relatively low maturity, Ro only as high as 0.9%. Figure 2 shows detailed geological and geochemical parameters of the black mud-shales in Songliao Basin.

Fracture systems
Fractures are favorable mud-shale reservoirs. Research shows multiple types of fractures, such as structural fractures, super-pressure micro-fractures, and diagenetic micro-fractures, are developed in SLB (Figure 3). Structural fractures form the main reservoir of mudstone (Zhang et al., 2002). Since structural fractures developed in the transition zone of fold structures and near faults, high-yield oil and gas wells are also mostly distributed in these areas. Super-pressure micro-fractures in mudstone are an important factor for formation of oil and gas reservoirs. Nenjiang and Qingshankou formations in SLB exist as overpressure phenomenon, which is reflected in the acoustic time logging deviation from the normal compaction trend line (Figure 4). Mud-shale oil and gas are mostly distributed in the over-pressure micro-fracture zone of the mudstone. Also in SLB, diagenetic shrinkage fractures are being found, including dehydration shrinkage fractures and mineral phase transitions shrinkage fractures.

Exploration Prospects of Shale Oil and Gas in SLB

Most of the oil and gas generated by source rocks still remain in mudstone, while a small portion migrates into reservoirs. Songliao Basin source rocks hydrocarbon expulsion coefficients were generally less than 20%. For example, in the Xin4 well the hydrocarbon emission factor was 19% (Sheng Zhiwei,1993).

The two sets of black shales have high pressures during drilling. Studies have shown that hydrocarbon generating pressure-boosting is one of the main factors generating overpressure. Gas measurements showed obvious abnormity in Nenjiang and Qingshankou formations mud-shale (Figure 4). On the basis of comprehensive evaluation of geology, geochemistry, well logging and seismic data, it is a favorable zone for mud-shale gas and oil exploration in Xinli, Da-an and Qian-an, etc.

Drilling results reveal that the two sets of black shales have oil and gas shows. Even in some wells, commercial amounts of oil and gas have been produced from the mud-shale. Mud-shale exploration has broad prospects, and is expected to become a new exploration play in Songliao Basin.

Selected References

Gao, R.Q., Y. Zhang, and T.C. Cui, 1994, Cretaceous Petroleum Bearing Strata in the Songliao Basin: Petroleum Industry Press, Beijing, p. 1-333.

Hou, D, M. Li, and O. Huang, 2000, Marine transgressional events in the gigantic freshwater lake Songliao: paleontological and geochemical evidence: Organic Chemistry, v. 31/7-8, p. 763-768.

Huang, Y., P. Wang, G. Men, and H. Tang, 2007, Division of volcanic cycles and stages of the Yingcheng Formation of the Songliao Basin – Take the cross sections at the margin and the boring holes as examples: Journal of Jilin University Earth Science Edition, v. 37/6, p. 1183-1191,

Li, M. and Z. Pang, 2004, Contentious petroleum geochemical issues in China’s sedimentary basins: Petroleum Science, v. 1/3, p. 4-22.

Li, D.S., 1995, Hydrocarbon habitat in the Songliao basin, in J.J. Lambiase, (ed.) Hydrocarbon habitat in rift basins: Geological Society of London Special Publication, v. 80, p. 317-329.

Pei, F., W. Xu, D. Yang, Q. Zhao, X. Liu, and Z. Hu, 2007, Zircon U-Pb geochronology of basement metamorphic rocks in the Songliao Basin: Chinese Science Bulletin, v. 52/7, p. 942-948.

Wang, P.J., W.Z. Liu, X.L. Shan, W.H. Bian, Y.G. Ren, X.D. Du, C.B. Wan, and R.H. Cheng, 2001, Depositional Events: Introduction, Example, Application. Jilin Science and Technology Press, Changchun, p. 1–182 (in Chinese with English abstract).

Wang, Y., F.Q. Zhang, D.W. Zhang, L.C. Miao, T.S. Li, W.Q. Jie, Q.R. Meng, and D.Y. Liu, 2006, Zircon SHRIMP U-Pb dating of meta-diorite from the basement of the Songliao Basin and its geological significance:  Chinese Science Bulletin, v. 51, p. 1877-1883.

Zhang, X., Y. Zou, and Q. Huo, 2008, Application of fluid inclusions to period research on oil and gas accumulation in Songliao Basin: China Petroleum Exploration, v. 13/4, p. 50-55.

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