Geochemical Characteristics of the Source Rocks in Mesozoic Yanchang Formation, Central Ordos Basin

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1 Journal of Earth Science, Vol. 4, No., p , October 13 ISSN X Printed in China DOI:.7/s Geochemical Characteristics of the Source Rocks in Mesozoic Yanchang Formation, Central Ordos Basin Senhu Lin ( 林森虎 ), Xuanjun Yuan ( 袁选俊 ), Shizhen Tao ( 陶士振 ) Zhi Yang* ( 杨智 ), Songtao Wu ( 吴松涛 ) Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 83, China ABSTRACT: The shale of Yanchang Formation in Upper Triassic is the most important source rock for the Mesozoic petroleum reserviors in Ordos Basin. and Chang 4+ members are the major source rock formation. Source rock samples, obtained from 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.36% to 19.%, 8.9% 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 and Chang 4+ members, which suggests that the shale has higher TOC, especially the highest lower members. The abundance of organic matter of Chang 4+ and members is both richest in southeast basin. The kerogen type of Chang 4+ and upper members is type II 1, the counterpart of middle and lower member is type I. During the burial history, the total hydrocarbon-generating quantity of member is much more than that of Chang 4+ members. KEY WORDS: hydrocarbon-generating potential, geochemical characteristics of source rock, Mesozoic Yanchang Formation, Ordos Basin. INTRODUCTION Ordos Basin is rich in both oil and natural gas. The petroleum exploration in the basin has last for more than a century since 197. At present, the exploration range has expanded to the whole basin and the exploration depth has extended to Paleozoic. There This study was supported by the National Technological Key Project of China (No. 11ZX1). *Corresponding author: yangzhi9@petrochina.com.cn China University of Geosciences and Springer-Verlag Berlin Heiderberg 13 Manuscript received December 1, 1. Manuscript accepted May 1, 13. are 3 petroleum systems in Ordos Basin: Lower Paleozoic carbonate oil, Upper Paleozoic gas and Mesozoic oil systems. Mesozoic Yanchang Formation is the earliest found and most productive oil layer. Recently, several large continuous oil plays have been found in Yanchang Formation, such as Jiyuan, Huaqing, Zhenbei and Heshui plays. Each of these plays has oil reserves more than 3 8 t. Especially, Huaqing play has submitted demonstrated reserves of t. Large area, multiple oil layers, low porosity and low permeability characterize this kind of continuous oil play. Many studies have demonstrated that the most of oil and gas in these continuous plays are derived from main sources: Chang 4+ and shale, and

2 West obduction Geochemical Characteristics of the Source Rocks in Mesozoic Yanchang Formation, Central Ordos Basin 8 (a) 4 km Jingbian N Geng Luo3 Luo4 Yuan19 Huanxian Mu14 Zhen89 Zhen71 Luo8 Li147 Li38 Yuan43 Li Qingcheng Bai478 Bai17 Wuqi Yuan4 Bai46 Baibao Shan Zhuang7 Zhuang176 Bai46 Zhidan Hydrocarbon Kitchen Wu8 Zhang Ansai Yan an (b) Yimeng uplift Tianhuan depression Yishan slope Jingbian Yulin Wuqi Ansai Zhidan Yan an Qingcheng Weibei uplift Jinxi fluxion Xi an Dark mudstone thinkness Unit: m City Well Figure 1. The isopach map of source rocks in Ordos Basin. shale contributed more (Lu et al., 6; Zhang et al., ). We have analyzed organic matter from drilled cores of both members to determine which rock unit is the most productive source rock. Improved understanding of the abundance, types and thermal evolution of organic matter, and the hydrocarbon potential that controls the oil occurrence in Mesozoic rocks, has important implications for future petroleum exploration and resource assessment in the center of Ordos Basin. GEOLOGICAL SETTING Geographically across the Shaanxi, Gansu, Ningxia, Mongolian and Shanxi provinces (areas), Ordos Basin is located between east longitude 6 ' 1 3' and north latitude 34 ' 41 3', has an area about 4 km. It is a large craton basin built on the Archaeozoic Early Proterozoic basement. Ordos Basin transformed from a rift basin in Late Proterozoic into a platform during Early Paleozoic, and then became a coastal plain in Late Paleozoic. After that, in Mesozoic the basin shifted to a lacustrine

3 C C 86 basin, finally became a faulted depression in Cenozoic (Yang, ). According to the tectonic structures, Ordos Basin is divided into 6 units: Yimeng uplift, West obduction, Tianhuan depression, Yishan slope, Jinxi fluxion and Weibei uplift (Fig. 1b). Yanchang Formation of Upper Triassic in Ordos Basin is a set of lake deltaic deposition. There are some geological differences between the North Yanchang and South Yanchang bounded by latitude 38 N. Rock particles are finer and strata are thicker in South Yanchang. The thickness is 7 m in the north, and m in the south. In some regions of southwest basin (Weibei uplift), Yanchang Formation is completely absent. Based on lithology, Yanchang Formation is divided into sections from the bottom to the top: T 3 y 1, T 3 y, T 3 y 3, T 3 y 4 and T 3 y. In terms of lithology, resistivity, oil saturation etc., Changqing Oilfield Company divides Yanchang Formation into members from the bottom up: Chang Chang 1 (Fig. ). In Late Triassic, the Ordos Lake level fluctuated several times. Two main transgressions coincided with the deposition time of and Chang 4+ members. Moreover, during the deposition of member, Ordos Lake had the deepest water and widest surface (Li, 4), developed huge source rock (Fu et al., ) (Fig. 1a). SAMPLES AND ANALYSIS According to the intervals of interest on hydrocarbon generation study, a total of 3 core samples were selected from Yanchang Formation in wells, which are all located in the central Ordos Basin (Fig. 1a), including 4 Chang 4+ samples, Chang 6 samples and 4 samples. The depth is between 96 and 79 m. Most of the samples are shale, with low porosity and extremely low permeability. All samples were subjected to Rock-Eval pyrolysis and chloroform extract content analysis. We chose 14 samples from 1 wells to do vitrinite reflectance measurement (Ro) and 1 samples from 18 wells to do carbon isotope testing. All the testing was done in Petroleum Geology Research and Laboratory Center (PGRL) of RIPED, PetroChina. The results are listed in Table 1. Senhu Lin, Xuanjun Yuan, Shizhen Tao, Zhi Yang and Songtao Wu Formation Upper Triassic Jurassic Yanchang Formation Section Zhifang Formation C Shale T3 T3 T3 T3 T3 y y y y y Sandstone Thickness (m) Carbonaceous mudstone Member Chang 1 Chang Chang 3 Chang 4+ Chang 6 Chang 8 Chang 9 Chang Lithology Silty mudstone Siltstone Coal Figure. The strata and lithology of Yangchang Formation in Ordos Basin. RESULTS AND DISCUSSION Abundance of Organic Matter Total organic carbon (TOC) values range between.36% 19.%, 6.6% on average, with the minimum value observed in a silty mudstone at depth about 16 m, which are indicative a good to excellent potential source rocks (Huang et al., 3) (Table 1). The potential of generating hydrocarbon (S 1 +S ) ranges from. to mg/g, 16. mg/g on average, which indicates part of source rocks did little contribution to hydrocarbon generation. The TOC values of Chang 4+ samples range from.87% to 3.36%. Their chloroform bitumen A values range from.6 1% to.79 4%. The S 1 +S ranges from 1.67 to 1.1 mg/g. The hydrogen index (HI) ranges from 11 to 76 mg/g. The results illustrate that all samples have high content of total organic carbon (TOC), which indicates Yanchang Formation is abound in organic matter (OM). According to the Chinese terrestrial hydrocarbon source rocks evaluation criteria (Huang et al., 3), Chang 4+ source rocks are barely worth consideration as good source rock. Figure 3 shows these average values compari-

4 Table 1 Analytic data of source rock samples in Yanchang Formation, central Ordos Basin Depth (m) Member TOC (%) Tmax R c Ro S 1 S S 1 +S PI HI PC D S 1 /TOC Chloroform δ 13 C ( ) (%) (%) (mg/g) (mg/g) (mg/g) (mg/g) (%) (%) (mg/g) bitumen (PDB) A (%) Bai Chang Luo Chang Wu Chang Zhang 96. Chang Bai Chang Zhuang Chang Bai Upper Bai Upper Luo Upper Yuan 19. Upper Yuan Upper Zhuang Upper Li 71. Middle Yuan Middle Zhen Middle Zhen Middle Bai Lower

5 Continued Depth (m) Member TOC (%) Tmax R c Ro S 1 S S 1 +S PI HI PC D S 1 /TOC Chloroform δ 13 C ( ) (%) (%) (mg/g) (mg/g) (mg/g) (mg/g) (%) (%) (mg/g) bitumen (PDB) A (%) Geng 7. Lower Geng 6.1 Lower Geng 64.7 Lower Geng 7. Lower Li Lower Li Lower Li Lower Li 7.3 Lower Luo Lower Luo Lower Mu Lower Shan Lower S 1. free volatile hydrocarbons (HCs) thermally flushed from a rock sample at 3 (free oil content); S. products that crack during standard Rock-Eval pyrolysis temperatures (remaining potential); Tmax. temperature at peak evolution of S HCs; HI (hydrocarbon index). S /total organic carbon (TOC); PI (production index). S 1 /(S 1 +S ) (or transformation ratio); S1/TOC. normalized oil content; R c. equivalent vitrinite reflectance, equal.18 T max 7.16, after Jarvie et al. (1); Ro. vitrinite reflectance measurement; PC. effective carbon, equal.83 (S1+S ); D. degradation rate, equal PC/TOC %.

6 Geochemical Characteristics of the Source Rocks in Mesozoic Yanchang Formation, Central Ordos Basin 89 son between Chang 4+ and source rock samples, which indicates the abundance of organic matter (AOM) of Chang 4+ source rocks is obviously lower than. Precisely, member can be divided into 3 units. Not every unit has high AOM. Within upper member, the source rocks are interbedded thin mudstone with mudstone, have poor hydrocarbon generation potential. TOC values range from.36% to.6%. S 1 +S range from.41 to.99 mg/g and chloroform bitumen A ranges from.4 3% to. 8%. The core samples from middle member are shale. Those TOC values range from.6% to 13.4%. S 1 +S range from 6.7 to 39.9 mg/g and chloroform bitumen A ranges from.133 4% to.87 1%. They have good hydrocarbon generation potential. Lower member consists of oil shale, whose TOC values range from.63% to 19.%. S 1 +S range from 9.8 to 91.6 mg/g and chloroform bitumen A ranges from.7 8% to 1. %. The source rocks of lower member have the best hydrocarbon generation potential. Figure 3 shows that the Upper source rocks are even poorer than Chang 4+ source rocks. Horizontally, the highest AOM region of Chang 4+ members is located in southeast basin, which is similar with member. The highest AOM region of the best source rocks, oil shale of lower Chang 7 member, lies in Jiyuan area. The distribution of AOM has close correlation with the thickness of shale. Kerogen Type Kerogen types can reflect the hydrocarbon-rich level and indicate the hydrocarbon generation potential. With the modern testing methods of organic petrology, kerogen microscopy, infrared analysis, elementary analysis, carbon isotope analysis, group components analysis, hydrocarbon generation kinetics etc., researchers have done some meticulous discussion on the parameters of organic matter in Yanchang Formation (Gang et al., ; Yao et al., 9; Han et al., 8; Ji et al., 7, 6; Ma et al., ; Yang and Zhang, ). They generally agreed that the formation of the oil prone kerogen is controlled by TOC (%) Chang 4+ mg/g S1 Chang 4+ S S1+ S A (%) Chang 4+ TOC (%) Upper Middle Lower mg/g Upper Middle Lower S1 S S1+ S A (%) Upper Middle Lower Figure 3. Averaged geochemical data comparison among core samples used in this study.

7 8 depositional environment, biological agent and climate: (1) oil prone kerogen is formed during the basin subsidence, preserved in aphytal and bathyal lake zone; () oil prone kerogen has close correlation with aquatic organism in slack-water lake; (3) mild and wet climate is of benefit to the formation of oil prone kerogen. During the medium term of Late Triassic, it was warm and humid at Ordos Basin. Aquatic organism bloomed, which resulted in the formation of very thick, wide and high hydrocarbon-generation shale. The main kerogen type of shale is sapropel. However, the kerogen type of Chang 4+ mudstone is humic, due to the climate change (Ji et al., 7). In light of the cross plot of pyrolytic peak temperature (T max ) and hydrogen index (HI), kerogen can be classified into 4 types: I, II 1, II and III. The dominating type of kereogen in study area is type II (Fig. 4). In detail, kerogen type of Chang 4+ and upper Chang 7 source rocks is type II 1, kerogen type of middle and lower source rocks is type I (Fig. 4). It is well known that the change of carbon isotopic composition (δ 13 C) in the source rocks thermal evolution is not obvious (Wang et al., 1997; Lewan, 1983). In addition, it can be influenced by the geological period, sedimentary environment, matrix component and some other factors. Therefore, δ 13 C just is a supplemental indicator for kerogen classifiction. Generally speaking, δ 13 C of type I kerogen HI (mg/g) I II1 Ro=.3% 3 II Ro=1.3% III ( o C ) T max Ro=.% Ro=1.% Chang 4+ Upper Middle Lower Figure 4. Kerogen type of core samples used in this study. Referred to Wu et al. (1986). Senhu Lin, Xuanjun Yuan, Shizhen Tao, Zhi Yang and Songtao Wu derived from phycophyta is lighter, commonly less than minus 31 ; δ 13 C of type III kerogen derived from Terrestrial plants is heavier, more than minus 6 ; hybrid type falls in between the two types mentioned above (Xiao and Jin, 199; Huang et al., 1984). δ 13 C of samples has a wide range from -3.6 to -8.8, whose dominant frequency ranges from -31 to -3 and secondary frequency ranges from -33 to -3. It means half of shale samples are type I, rest are hybrid type. δ 13 C of Chang 4+ samples is narrowly distributed from -3.9 to -3.3, whose dominant frequency ranges from -3 to -31. So the Chang 4+ mudstone samples are hybrid kerogen. Furthermore, δ 13 C of each unit in member is different. It is the lightest in lower, which is a little lighter than that of middle. Upper and Chang 4+ source rocks have the basically same δ 13 C, which is heavier (Fig. ). Heavy isotopic composition is poor for oil generation. On the whole, in central Ordos Basin, the kerogen type of source rocks from Yanchang Formation is type II, which favors the oil generation. The kerogen type of shale is better for oil generation than that of Chang 4+ mudstone. Especially, the kerogen type of lower and middle member has the best oil generation potential. Thermal Evolution of Organic Matter We prefer the oil prone kerogen in Triassic, because the source rocks of Yanchang Formation have entered oil window (Table 1). The maximum reflectance of vitrinite (Ro) is one of the most direct indexes of source rocks maturity. In central Ordos Basin, Ro values range from.87% to 1.3%, which means all samples have developed at a mature stage. Moreover, the source rocks are a little maturer than Chang 4+ source rocks (Table 1). It s worth noting that the residual crude oil and bitumen kept in the micropores may result in a lower maturity in shale than reality (Zhang et al., 6). Besides Ro, the pyrolytic peak temperature (T max ) has positive correlation with maturity, which rises with the depth increases. However, T max of Chang 4+ and samples don t show significant difference (Table 1).

8 Geochemical Characteristics of the Source Rocks in Mesozoic Yanchang Formation, Central Ordos Basin 811 After the source rocks enter the oil threshold, as the burial depth increases, the content of organic hydrogen decreases rapidly with the kerogen thermal degradation, which can be shown on the sharp drop of hydrogen index (HI) (Xu, 1993). The average HI of source rocks is mg/g, less than the average HI of Chang 4+ source rocks ( mg/g on average), which indicates that the source rocks of Chang 7 member have higher maturity than those of Chang 4+ members. In addition, the hydrocarbon index (S 1 /TOC) bigger, the maturity higher (Xu, 1993). Table 1 shows that the hydrocarbon index increases from upper to lower member, which suggests that the maturity of source rocks in lower member is the highest as well. These characteristics may be related to kerogen type as well with geothermal gradient. In the whole basin, Chang 4+ source rocks have the highest maturity in the southwest. The maturity of source rocks is highest in Baibao-Huachi area (Figs. 1a, 6), decreasing westward. The peak maturity location change from period to Chang 4+ Chang 4+ N=4 3 Upper N=6 6 Middle & lower N=9 Frequency 1 Frequency 1 Frequency δ 13 C (%) δ 13 C (%) δ 13 C (%) Figure. δ 13 C frequency of core samples used in this study. Table Total hydrocarbon generation of source rock samples used in this study Well Formation HGP (kg/t) Thickness (km) THGQ ( t/km ) Well Formation HGP (kg/t) Thickness (km) THGQ ( t/km ) Bai 46 Chang Zhen 89 Middle Luo 8 Chang Bai 46 Lower Wu 8 Chang Geng Lower Zhang Chang Geng Lower Bai 46 Chang Geng Lower Zhuang 7 Chang Geng Lower Bai 17 Upper Li 147 Lower Bai 478 Upper Li 147 Lower Luo 8 Upper..1.8 Li 147 Lower Yuan 19 Upper Li 38 Lower Yuan 4 Upper.9.1. Li Lower Zhuang 176 Upper Luo 3 Lower Li Middle Luo 4 Lower Yuan 43 Middle Mu 14 Lower Zhen 71 Middle Shan Lower HGP. Hydrocarbon genetic potential, kg hydrocarbon/t rock; THGQ. total hydrocarbon-generating quantity, t/km.

9 81 Senhu Lin, Xuanjun Yuan, Shizhen Tao, Zhi Yang and Songtao Wu N 4 km Yanchi Jingbian Jiyuan Luo3 Luo4 Luo8 Yuan43 Hujianshan Geng Huanxian Li147 Bai46 Li Bai478 Mu14 Huachi Li38 Bai17 Shan Zhen89 Yuan19 Wuqi Yuan4 Bai46 4 Qingcheng Zhen71 Zhuang176 3 Wu8 1 Zhidan Ta erwan Zhang 3 Ansai Fuxian Zhenyuan Heshui Zhengning 4 Hydrocarbongenerating quantity 4 1 Unit: t/km 3 4 City Well Figure 6. Total hydrocarbon generation isopleth of Yanchang Formation, central Ordos Basin. Unit: t/km. period may be relevant to the migration of depocenters. Total Hydrocarbon-Generating Quantity The total hydrocarbon-generating quantity during geological history decides the source rocks contribution to reservoirs accumulation. In order to restore the hydrocarbon-generating quantity per unit mass source rocks, we used the following formula (Wu et al., 1986) TOC ini =TOC res /(1 D) Q=TOC ini (D/.83) H d TOC ini is initial TOC (%); TOC res is residue TOC (%); D is degradation rate (%), see Table 1 for details; Q is hydrocarbon-generating quantity per unit mass source rocks (kg hydrocarbon /t source rocks);.83 is the conversion factor from carbon (%) to hydrocarbon (kg/t); H is thickness of source rocks (km); d is density (.3 9 t/km 3 ). The THGQ (total hydrocarbon-generating quantity, t/km ) of shale samples is 44.7 t/km on average, which is much higher than the average THGQ 7.7 t/km of Chang 4+ samples (Table ). In the whole basin, the Chang 4+ source rocks have the highest THGQ at southeast part. The

10 Geochemical Characteristics of the Source Rocks in Mesozoic Yanchang Formation, Central Ordos Basin 813 maximum THGQ of source rocks are located in southern area (Fig. 6). In consideration of the source rocks evaluation and the tight reservoir in the Yanchang Formation, the interbedded sandstone and siltstone in member could be the optimum layer for tight light oil exploration in short term. Starting from the south central basin (e.g., Heshui area) is available. CONCLUSION Geochemical characteristics indicate that the source rocks of Yanchang Formation in the center of Ordos Basin are organic rich, with average TOC value of 8.9%. It indicates that this formation is an excellent potential source rock for hydrocarbon generation. In detail, source rocks have higher organic matter abundance than that of Chang 4+ source rocks. Moreover, the organic matter abundance of lower is the highest, middle secondary, upper lowest. According to Rock-Eval data, the shale is dominated by Type I and Chang 4+ mudstone is dominated by Type II 1, containing oil- and gas-prone lacustrine organic matter. Vitrinite reflectance indicates that all source rocks have entered oil window and source rock is mature and better for oil generation. The calculation of total hydrocarbon generating quantity turns out that source rocks is 6 times the hydrocarbon-generating capacity of Chang 4+. The maximum THGQ of Chang 4+ members lies at southeast basin. The maximum THGQ of member is located in south central basin. REFERENCES CITED 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, 7(1): (in Chinese with English Abstract) 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), 34(6): (in Chinese with English Abstract) Han, Z. Y., Xu, W., Fang, Q. H., 8. Comparison and Analysis of Hydrocarbon Source Rocks in Yanchang and Yan an Formations, Longdong Area. Fault-Block Oil & Gas Field, 1(1): 1 4 (in Chinese with English Abstract) Huang, D. F., Zhang, D. J., Wang, P. R., 3. Genetic Mechanism and Accumulation Condition of Immature Oil in China. Petroleum Industry Press, Beijing (in Chinese) 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, (3): (in Chinese with English Abstract) Huang, D. F., Thermal Evolution and Hydrocarbon Generation Mechanism of Terrestrial Organic Matter. Petroleum Industry Press, Beijing (in Chinese) Jarvie, D. M., Claxton, B. L., Henk, F., et al., 1. Oil and Shale Gas from the Barnett Shale, Fort Worth Basin, Texas (abs.). AAPG Annual Meeting Abstracts, Denver. A Ji, L. M., Wu, T., Li, L. T., 6. Paleoclimatic Characteristics during Sedimentary Period of Main Source Rocks of Yanchang Formation (Triassic) in Eastern Gansu. Acta Sedimentologica Sinica, 4(3): (in Chinese with English Abstract) Ji, L. M., Wu, T., Li, L. T., 7. Geochemical Characteristics of Kerogen in Yanchang Formation Source Rocks, Xifeng Area, Ordos Basin. Petroleum Exploration and Development, 34(4): (in Chinese with English Abstract) Lewan, M. D., Effects of Thermal Maturation on Stable Organic Carbon Isotopes as Determined by Hydrous Pyrolysis of Woodford Shale. Geochimica et Cosmochimica Acta, 47: Li, D. S., 4. Return to Petroleum Geology of Ordos Basin. Petroleum Exploration and Development, 31(6): 1 7 (in Chinese with English Abstract) Lu, J. C., Li, Y. H., Wei, X. Y., 6. 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), 36(6): (in Chinese with English Abstract) 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, 3(3): 1 4 (in Chinese with English Abstract) Wang, W. C., Xu, Y. C., Schidlowski, M., The Geochemical Characteristics of Carbon and Hydrogen Isotopes of Kerogens of Various Maturity and Depositional Environ-

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