Geochemical characteristics of the source rocks in Mesozoic Yanchang formation, central Ordos Basin

Abstract

The shale of Yanchang Formation in Upper Triassic is the most important source rock for the Mesozoic petroleum reserviors in Ordos Basin. Chang 7 and Chang 4+5 members are the major source rock formation. Source rock samples, obtained from 22 cored wells in central Ordos Basin, were geochemically analyzed to determine the organic matter abundance, kerogen type and thermal maturity. Total organic carbon values ranged from 0.36% to 19.10%, 8.09% on average, indicating a good source rock potential. In this area, the shale is mature, as indicated by vitrinite reflectance values. Rock-Eval data revealed that the samples are dominated by type II kerogen. Compare the Chang 7 and Chang 4+5 members, which suggests that the Chang 7 shale has higher TOC, especially the highest lower Chang 7 members. The abundance of organic matter of Chang 4+5 and Chang 7 members is both richest in southeast basin. The kerogen type of Chang 4+5 and upper Chang 7 members is type II1, the counterpart of middle and lower Chang 7 member is type I. During the burial history, the total hydrocarbon-generating quantity of Chang 7 member is much more than that of Chang 4+5 members.

Keywords

hydrocarbon-generating potential / geochemical characteristics of source rock / Mesozoic Yanchang Formation / Ordos Basin

References

[1] Fu J H, Guo Z Q, Deng X Q. Sedimentary Facies of the Yanchang Formation of Upper Triassic and Petroleum Geological Implication in Southwestern Ordos Basin. Journal of Palaeogeography, 2005, 7(1): 34-44.

[2] Gang W Z, Gao G, Han Y L. Analysis of Source Rock of Yanchang Formation in Hujianshan Area, Ordos Basin. Journal of China University of Petroleum (Edition of Natural Science), 2010, 34(6): 50-55.

[3] Han Z Y, Xu W, Fang Q H. Comparison and Analysis of Hydrocarbon Source Rocks in Yanchang and Yan’an Formations, Longdong Area. Fault-Block Oil & Gas Field, 2008, 15(1): 1-4.

[4] Huang D F, Zhang D J, Wang P R. Genetic Mechanism and Accumulation Condition of Immature Oil in China, 2003 Beijing: Petroleum Industry Press

[5] Huang D F, Li J C, Zhang D J. Kerogen Types and Study on Effctiveness, Limitation and Interrelation of Their Identification Parameters. Acta Sedimentologica Sinica, 1984, 2(3): 18-34.

[6] Huang D F. Thermal Evolution and Hydrocarbon Generation Mechanism of Terrestrial Organic Matter, 1984 Beijing: Petroleum Industry Press

[7] Jarvie D M, Claxton B L, Henk F, . Oil and Shale Gas from the Barnett Shale, Fort Worth Basin, Texas (abs.), 2001 Denver: AAPG Annual Meeting Abstracts, A100.

[8] Ji L M, Wu T, Li L T. Paleoclimatic Characteristics during Sedimentary Period of Main Source Rocks of Yanchang Formation (Triassic) in Eastern Gansu. Acta Sedimentologica Sinica, 2006, 24(3): 426-431.

[9] Ji L M, Wu T, Li L T. Geochemical Characteristics of Kerogen in Yanchang Formation Source Rocks, Xifeng Area, Ordos Basin. Petroleum Exploration and Development, 2007, 34(4): 424-428.

[10] Lewan M D. Effects of Thermal Maturation on Stable Organic Carbon Isotopes as Determined by Hydrous Pyrolysis of Woodford Shale. Geochimica et Cosmochimica Acta, 1983, 47: 1471-1479.

[11] Li D S. Return to Petroleum Geology of Ordos Basin. Petroleum Exploration and Development, 2004, 31(6): 1-7.

[12] Lu J C, Li Y H, Wei X Y. Research on the Depositional Environment and Resources Potential of the Oil Shale in the Chang7 Member, Triassic Yanchang Formation in the Ordos Basin. Journal of Jilin University (Earth Science Edition), 2006, 36(6): 928-932.

[13] Ma S P, Qi Y L, Zhang X B. Geochemical Characteristics and Hydrocarbon Generation Potential of the Source Rocks in Yanchang Formation, Xifeng Oilfield, Ordos Basin, NW China. Petroleum Exploration and Development, 2005, 32(3): 51-54.

[14] Wang W C, Xu Y C, Schidlowski M. The Geochemical Characteristics of Carbon and Hydrogen Isotopes of Kerogens of Various Maturity and Depositional Environ ments. Acta Sedimentologica Sinica, 1997, 15(Sl): 133-137.

[15] Wu L Y, Gu X Z, Sheng Z W. Source Rock Pyrolysis Quantitative Evaluation, 1986 Beijing: Science Press

[16] Xiao X M, Jin K L. A Petrographic Classification of Macerals in Terrestrial Hydrocarbon Source Rocks in China and Their Organic Petrological Characteristics. Acta Sedimentologica Sinica, 1990, 8(3): 22-34.

[17] Xu W M. Application of the Petroleum Geochemistry in the Oil and Gas Exploration, 1993 Beijing: Petroleum University Press

[18] Yang H, Zhang W Z. Leading Effect of the Seventh Member High-Quality Source Rock of Yanchang Formation in Ordos Basin during the Enrichment of Low-Penetrating Oil-Gas Accumulation: Geology and Geochemistry. Geochimica, 2005, 34(2): 147-154.

[19] Yang J J. Tectonic Evolution and Hydrocarbon Distribution in Ordos Basin, 2002 Beijing: Petroleum University Press

[20] Yao S P, Zhang K, Hu W X. Sedimentary Organic Facies of the Triassic Yanchang Formation in the Ordos Basin. Oil & Gas Geology, 2009, 30(1): 74-89.

[21] Zhang W Z, Yang H, Li J F. Leading Effect of High-Class Source Rock of Chang 7 in Ordos Basin on Enrichment of Low Permeability Oil-Gas Accumulation-Hydrocarbon Generation and Expulsion Mechanism. Petroleum Exploration & Development, 2005, 33(3): 289-293.

[22] Zhang Z L, Wu L Y, Shu N Z. Cause Analysis of Abnormal T(max) Values on Rock-Eval Pyrolysis. Petroleum Exploration and Development, 2006, 33(1): 72-75.