Oil and gas resource potential of the lower members of Eh3 in the lower Tertiary of the Biyang Depression, China

Petroleum Science, Jan 2012

The lower Tertiary Eh3 is divided into two sections: the upper members of Eh3 and the lower members of Eh3 in the Biyang Depression. The first section is generally regarded as a key target of oil and gas exploration, but the resource potential of the lower members of Eh3 has been neglected. We have obtained new knowledge about Eh3 from comprehensive geological research. The lower members of Eh3 are high-quality and main source rocks, which have good oil and gas resource potential. This is a new direction for oil and gas exploration. The geochemistry characteristics of source rocks of the lower members of Eh3 in the lower Tertiary of the Biyang Depression were analyzed in detail. A basin modeling method was applied to hydrocarbon generation of the lower and upper members of Eh3 source rocks, the oil and gas resource potential was comparatively analyzed, and then favorable tectonic zones were pointed out. In the lower members of Eh3, a set of semi-deep lake to deep lake high-quality source rocks occurs rich in algae organisms, mainly of type II1, with a high abundance of organic matter. Most of the source rocks are just in the peak stage of hydrocarbon generation, which is a favorable foundation for forming abundant oil and gas resources in the Biyang Depression. The comparative analysis of the hydrocarbon-generation quantities between lower and upper members of the Eh3 source rocks shows that the lower members of Eh3 have good oil and gas resource potential, and the hydrocarbon-generation quantity accounts for 51% of the total in Eh3. Specifically, the oil-generating quantity accounts for 50% of the total and the gas-generating quantity accounts for two thirds of the total. Therefore, source rocks in the lower members of Eh3 of the Biyang Depression have good oil and gas resource potential, which is a key factor for future deep oil and gas exploration.

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Oil and gas resource potential of the lower members of Eh3 in the lower Tertiary of the Biyang Depression, China

Corresponding author. email gwzcup.edu.cn Received July Oil and gas resource potential of the lower members of E in the lower Tertiary of the h3 Biyang Depression, China Gang Wenzhe 2 Wang Qingyu 1 Luo Jiaun 0 0 Research Institute of Petroleum Exploration and Development, Henan Oileld Company , SINOPEC, Henan 473132 , China 1 Exploration & Development Research Institute, Daqing Oileld Company , PetroChina, Heilongjiang 13712 , China 2 College of Geosciences, China University of Petroleum , Beijing 102249 , China The lower Tertiary Eh3 is divided into two sections the upper members of E h3 and the lower members of Eh3 in the Biyang Depression. The rst section is generally regarded as a key target of oil and gas exploration, but the resource potential of the lower members of Eh3 has been neglected. We have obtained new knowledge about Eh3 from comprehensive geological research. The lower members of Eh3 are high-quality and main source rocks, which have good oil and gas resource potential. This is a new direction for oil and gas exploration. The geochemistry characteristics of source rocks of the lower members of Eh3 in the lower Tertiary of the Biyang Depression were analyzed in detail. A basin modeling method was applied to hydrocarbon generation of the lower and upper members of Eh3 source rocks, the oil and gas resource potential was comparatively analyzed, and then favorable tectonic zones were pointed out. In the lower members of Eh3, a set of semi-deep lake to deep lake high-quality source rocks occurs rich in algae organisms, mainly of type II1, with a high abundance of organic matter. Most of the source rocks are just in the peak stage of hydrocarbon generation, which is a favorable foundation for forming abundant oil and gas resources in the Biyang Depression. The comparative analysis of the hydrocarbongeneration quantities between lower and upper members of the Eh3 source rocks shows that the lower members of Eh3 have good oil and gas resource potential, and the hydrocarbon-generation quantity accounts for 51% of the total in Eh3. Specically, the oil-generating quantity accounts for 50% of the total and the gas-generating quantity accounts for two thirds of the total. Therefore, source rocks in the lower members of Eh3 of the Biyang Depression have good oil and gas resource potential, which is a key factor for future deep oil and gas exploration. Geochemistry; source rocks; hydrocarbon-generation potential; Biyang Depression - The Biyang Depression is a relatively independent secondary structural unit in the Nanxiang Basin located between Tanghe and Biyang counties of Henan province, China. It is a down-faulted basin formed in the MesozoicCenozoic Eras. It covers only 1,000 km2 but its oil and gas reserves are abundant. The main oil and gas reserves are found in the Eogene Hetaoyuan Formation and the main exploration layer is Eh3 (Qiu et al, 2005b; Yu et al, 2007; Sun et al, 2008; Luo, 2003; Luo et al, 2008) . Achievements have been gained due to the high exploration degree and detailed research on the upper members of Eh3 (Wen et al, 200 ; Qin et al, 2005; Zeng et al, 2005) . However, relatively little study has been done on the lower members of Eh3 and more attention is needed. For deep hydrocarbon targets formed by the lower members of Eh3 source rocks in the Biyang Depression, the density of drilling is not adequate, and the exploration degree is relatively low. Many reservoirs are subtle reservoirs (Yu et al, 2007). Besides, information such as time and space distribution of the lower members of Eh3 source rocks, organic types, organic matter abundance, maturity and hydrocarbon generation history is not clear. Neglecting the existence of high quality hydrocarbon source rocks of the lower members of Eh3 leads to an underestimate of the deep-seated hydrocarbon resource potential. Therefore, based on the geochemical characteristics of the Eh3 source rocks, we compare the potential of oil and gas resource of the upper members of Eh3 with that of the lower members of Eh3, and clarify its distribution to provide guidance for future exploration. 1 Geological background A c c o r d i n g t o s t r a t i g r a p h i c s e q u e n c e , t e c t o n i c characteristics and development history, the Biyang Depression is divided into south abrupt slope area, deep depression area and north downslope area (Fig. 1) (Yu et al, 2007; Qin, 2004) . The Hetaoyuan Formation is the main hydrocarbon reservoir assemblage, with a thickness of 2,000 to 3,000 m. There are three geological units in the Hetaoyuan Formation, namely Eh1, Eh2, and Eh3. The Eh1 is only 400 to 500 m thick, mainly composed of greygrey green mudstone interbedded with oil shale and sandstone. The Eh2 is 700 to 800 m, mainly composed of grey dark grey mudstone and argillaceous dolomite and a small amount of grey brown light brown dolomite, oil shale, sandstone and evaporites with the grey grey green mudstone increasing from bottom to top. Grey blackdark grey mudstone is the main lithology of Eh3 which also contains argillaceous dolomite, dolomite and sandstone. There are evaporites, oil shale and calcareous shale in the upper layer of Eh3. The Eh3 Formation has the main oil and gas reserves in this basin, which includes two sub-formations and eight sand groups according to cyclic sedimentation and electrical differences. The upper members of Eh3 include four sub members Eh3-1, Eh3-2, Eh3-3, Eh3-4, the lower members of Eh3 include four sub members Eh3-5, Eh3- , Eh3-7, Eh3-8. Niuzhuang Tang 0 Fuwan FQ5 FQ15 Huangdian Fault block reservoir B145 GQ2GucheGnQg1 North B107 B137 B15 Fault block-litBhQo1l0ogic reservoir Bq50 B126 BG5G12 B121 BB212288 Yanglou B181 YQ1B106 B148B146 ZhudanogB1gw7u2anngsEwl1o7peWB62W2265 W21B226FBa22u2lt blocEkBw11t75r1apB244 Bq8 B105 B201 2 Correlation of crude oil and source rocks The Biyang Depression is a Meso-Cenozoic downfaulted lake basin with abundant oil and gas resources, and a thick hydrocarbon storage system of the lower Tertiary Eh formation. Nine oil & gas fields have been found so far, and reservoirs are concentrated in the Eh3 formation. Many scholars have published results on oil and source rock correlation of the Biyang Depression (Zhou and Wu, 2004; Yang et al, 2005) . Luo et al (2008) used biomarkers and carbon isotopes to analyze the geochemical characteristics of the upper and the lower members of Eh3 source rocks and the oil source differences between them, and suggested that the crude oil in the upper members of Eh3 was originated from those members, and the crude oil of the lower members of Eh3 was from the lower members of Eh3 source rocks. This result indicates that oil and gas origin in the Biyang Depression is very simple, and it is a typical self-generated and selfcontained reservoir where the lower members of Eh3 source rocks are effective. 3 Geochemical characteristics of the lower members of Eh3 source rocks 3.1 Types of the lower members of Eh3 source rocks In the early Eh3 deposition period, the Biyang Depression continued to subside, then the lake deepened and water expanded as the environment became warm and humid which resulted in abundant aquatic organisms. At the same time, the Biyang Depression was a semi-deep to deep water strongly reducing environment, which was favorable for the preservation of organic materials, so a set of typical continental lacustrine pelitic source rocks formed. The lower members of Eh3 source rocks were mainly deposited in lacustrine facies, and the effective source rocks in Eh3 mainly consist of dark mudstone, oil shale and argillaceous dolomite that were formed in a semi-deep to deep lake environment. The basis of the rich oil and gas resources in the Biyang Depression is the great potential and high efficiency of Eh3 source rocks for hydrocarbon generation as well as their wide distribution and large thickness. 3.2 Distribution and development characteristics of the lower members of Eh3 source rocks The lower members of Eh3 source rocks mainly deposited in a semi-deep to deep lake sedimentary environment. Fig. 2 shows that the depocenter is in the Anpeng-Andian area with a thickness of over 120 m. Generally, the location of the depocenter remained unchanged, and the source rocks are widespread and have a large thickness. Each sand Lq5 Bq44 L11 L6Jinglou South Fq13 Fq5 Bq40 Fq8 B27 Bq24 B272 GuchengB127 Gz1 Bq41 Fq7 Fq12 Bq36 B145 Yq12 Yq13 Yq6 Yq7 40Yq4 B175 Yq350 Yq14 Yq18 B171 area 60 B137 B1Z8h1angchdanogwnslope W7106W5W20WB4104WB11233W11 9800 Wangji 100 120 L3L8 BQ14ia3ndulou B224 B248 B46 B107 Nort hB122 B125 B124 B227 B184 B144 B208B50 B47BBiMd58ianiddlBe168 deep depreAsndsianioB9n5 aBSr1u9en9gaang BC20h2enBBgB11d9191i9a07n B123 B142 B44 B93 B189 B188 B176B177 B32 Houzhuang Lishuao B63 B180 Xiaermen B29 B57 BB3438 BB5391 B32 Jianghe Gulan B42 B34abrBu51pBt283 Shuanghe B21 group in the lower members of Eh3 is thicker than 100 m, and the distribution areas of pelitic source rocks from Eh3-8 to upper Eh3-5 show a variation from small to large, which agrees with the change of sedimentary facies of the Biyang Depression. The Biyang Depression started to subside in the Eh3-8 sedimentary period, and developed a deep lake in the northwest of the Anpeng-Andian area due to humid climate and increased rainfall. The source rocks which are thicker than 100 m relatively occurred in a small area. In Eh3-7 sedimentary period, the sedimentary center was still in Anpeng-Andian with a lower subsidence rate. The lake area reduced and the pelitic source rocks have the same characteristics as the Eh3-8 source rocks. In the Eh3- sedimentary period, the subsidence rate of the Biyang Depression became higher again, and the lake area became larger. Widespread thick pelitic source rocks developed, and the depocenter extended to the Bidian-Zhaoao area. In the Eh3-5 sedimentary period, abundant and thick pelitic source rocks developed due to the larger and deeper lake, and this was also the biggest lake erosion period in the lower Eh3 sedimentary history. In summary, the pelitic source rocks were affected by the subsidence rate of the depression and the change of sedimentary facies, which also determined the size and quality of source rocks. Eh3-6 source rocks’ thickness map 3.3 Organic matter types and abundance of the lower members of Eh3 source rocks The Eh3 source rocks are mainly dominated by algae, Type organic matter. According to the H/C and O/C atomic comparison chart (Fig. 3), the source rocks are mainly dominated by Type II1 and secondarily by the Types and II 2 kerogen. In addition, the kerogen maceral results show that the organic matter in the lower members of Eh3 source rocks was mainly amorphous sapropelinite and amorphous huminite of sapropelinite. Vitrinite accounts for less than 10%, and inertinite accounts for less than 5%. So both the organic matter maceral analysis and the organic matter type index agree with the above conclusion (Table 1). Furthermore, HI indexes of the lower Eh3 source rocks in well BQ31, YQ3, B145, B224 and B227 are all more than 470 mg/g.TOC and lower than 70 mg/g.TOC, and all of the evidence indicates that the source rocks belong to the Type I and Type II1. Fig. 4 is a total organic carbon contour map of the lower members of Eh3- source rocks. We can find that the high organic abundance areas mainly occur in the semi-deep to deep lake area and the edge of the lake basin. The highest /C 1.00 H 2.00 1.80 1.60 1.40 1.20 0.80 0.60 0.40 0.20 0.00 Well Depth Layer m BQ31 310.4 Eh3-5 organic abundance areas concentrate around the edge of the lake basin, and the abundance reduces from the edge to the center and falls to 1.0% in deep lake area due to the high maturity of source rocks in the deep lake. There are many explanations for this observation. The lake (lacustrine facies) in the Biyang Depression was mainly dominated by algae, and it is true that the delta front and prodelta had a great amount of algae, but the deep lake area was lacking in algae. The delta front and prodelta were close to the estuary so the lake brought abundant nutrient and luxuriant organisms, resulting in a high sedimentary rate so that the organic matter can be well preserved. The deep lake reducing environment is favorable for the preservation of organic matter, whereas the deep lake source rocks occur widely, algae of the deep lake is good for hydrocarbon generation. The Wangji-Zhangchang-Qiandulou-Shuanghe areas have high organic abundance with an average around 2.0%, and these areas were all in the delta front. The abundance of organic matter in the Biyang Depression is mostly above 0.5%, but this index is low in the eastern Xinzhuang-Houzhuang area, which reveals that the whole depression has a high level of organic abundance which is good for hydrocarbon generation. 3.4 Maturity of the lower members of Eh3 source rocks The lower members of Eh3 source rocks are all mature due to the high geothermal gradient (4.1°C/100m) and deep burial depth (Fig. 5), with RO over 1.2% in the Anpeng-Andian area that is located in the middle deep depression, indicating that this area reached the mature stage and is entering gas condensate and natural gas phase, which is consistent with exploration results. RO is about 0.5% in Gucheng-Xinzhuang which is located in the north shelf, showing that most of the lower members of Eh3 source rocks are mature. It is discovered that RO is between 0.5% and 1.0% in the north shelf by contrasting each group of the lower Eh3, because this shelf was exposed to strong uplift and denudation in the Liaozhuang group period, and the biggest denudation thickness was about 1,000 m. The source rocks in this area became mature because of the thermal evolution before denudation, so the maturity remained the same as the period before denudation, which can be testified by the thermal evolution history of the source rocks. According to the RO contour map, the Eh3 source rocks are all mature. The source rocks which are at the oil peak stage (RO=1.0%) and the gas condensate wet gas stage (RO>1.2%) enlarged with the increasing depth, but always in the Anpeng-Andian area. 4 Potential of the Eh3 source rocks 4.1 Hydrocarbon-generation uantity contrast between the upper members and the lower members of Eh3 source rocks Based on the first order chemical reaction kinetics and the Arrhenius law, we select geothermal gradient parameters (Qiu et al, 2005a) , denuded thickness of each layer, chemical kinetics parameters and hydrocarbon accumulation coef cient in the Biyang Depression, according to the chemical dynamics hydrocarbon-generating history model (Shi, 2004; Pepper and Corvi, 1995a; 1995b; Pepper and Dodd, 1995; Pang et al, 2010) , the oil-generating quantity of the Eh3 source rocks is 2.33 billion tonne, and the gas-generating quantity is 199.48 billion m3 and the total hydrocarbon-generation quantity is 2.53 billion tonne. For the lower members of Eh3 source rocks, the oil-generating quantity is 1.17 billion tonne, the gas-generating quantity is 13.0 billion m 3 and the total hydrocarbon-generation quantity is 1.30 billion tonne. For the upper members of Eh3 source rocks, the oil-generating quantity is 1.1 billion tonne, the gas-generating quantity is 2.88 billion m3, and the total hydrocarbon-generation quantity is 1.22 billion tonne. Eh3-6 source rocks’ TOC isoline map 0.5 Fq6 (0.70) North Bq20 (0.68) According to Fig. , the Eh3-2 sand group has the highest oil-generating quantity of all the Eh3 source rocks; the Eh3sand group has the highest oil-generating quantity of all the lower members of Eh3 source rocks, and the Eh3-7 sand group has the lowest oil-generating quantity. Every sand group’s oil-generating quantity is similar to each other. The gasgenerating quantity rises from the Eh3-1 sand group to the Eh3-8 sand group, but the gas-generating quantity plays little part in the total Eh3 resource quantity, so the Eh3-2 sand group still has the highest total resources, and the Eh3- sand group has the most resource quantity of all the sand groups of the lower members of Eh3. From Eh3-5 to Eh3-8, the sand groups account for 13%, 14%, 12%, 12% of the total hydrocarbon-generation quantities, respectively, collectively 51% of the total. 4.2 Comparison of hydrocarbon-generation uantity of Eh3 of every structural belt The Biyang Depression is divided into six structural belts, namely Shuanghe-Anpeng, Jinglou-Gucheng, Xiaermen, Wangji-Xinzhuang, Fuwan and Dawuzhuang. Fig. 7 is a comparison chart of the estimated oil & gas resources of Eh3 in each of the structural belts. From this gure, the Eh3 source rocks of the Shuanghe-Anpeng structural belt are estimated to have produced 0.21 billion tonne of oil and gas in situ, the highest of all the belts, so it has the best prospect for petroleum exploration. The Eh3 source rocks of the Jinglou-Gucheng structural belt have produced estimated 0.09 billion tonne of oil and gas, the E h3 source rocks of the Xiaermen structural belt 0.080 billion tonne of oil and gas, and the Eh3 source rocks of the Wangji-Xinzhuang structural belt 0.087 billion tonne of oil and gas. The three structural belts’ total oil and gas resources are similar, so they all have good prospect for petroleum exploration. The Fuwan and Dawuzhuang structural belts are likely to have produced few resources, so they have little prospect for petroleum exploration. s e nn2.5 o t million21..05 d e dr 1 n u eh0.5 n O 0 4 3.5 3 2.5 2 1.5 1 0.5 0 2.105 Eh3 source rocks’ oil & gas generating quantity of every structural belt 5 Conclusions The lower members of Eh3 source rocks of the Biyang Depression are typically continental lacustrine facies deposits. The climate was warm and wet, so lacustrine aquatic algae were luxuriant, and the source rocks are mainly of the Type II1, secondarily the Type I and II2, which is good for hydrocarbon generation. Due to the lacustrine transgressionregression cycle, dark mudstone and oil shale of the semideep to deep lake facies developed, and these efcient source rocks were mainly distributed in the Anpeng-Andian and Bidian-Zhaoao areas. The river brought nutrient materials, and the delta front and prodelta located in Jinglou-ShuangheGucheng-Zhangchang were full of aquatic organisms, so the abundance of organic matter was high. Most of the source rocks in the Biyang Depression are mature, especially those of the semi-deep to deep lake facies that are at the peak of hydrocarbon generation. In conclusion, the systematic study of the geochemical characteristics of the lower members of Eh3 source rocks confirms that the lower members of Eh3 source rocks are widespread, and their kerogen type is better and in the mature stage, so they have good material basis to produce oil and gas. Comparing the lower members of Eh3 source rocks with the upper members of Eh3, the former have higher hydrocarbon-generation potential. The hydrocarbongeneration quantity accounts for 51% of the total, specically, the oil-generating quantity accounts for 50% and the gasgenerating quantity accounts for two thirds. In addition, from the comparison among Eh3 oil & gas resources of every structural belt, Shuanghe-Anpeng, Xiaermen and WangjiXinzhuang structure belts will be the key exploration areas, and Fuwan and Dawuzhuang structure belts have little hydrocarbon-generation potential. To sum up, comprehensive geological study and geochemical analysis, qualitative analysis and quantitative analysis, have together proved that the source rocks of the lower members of Eh3 are high-quality hydrocarbon source rocks and have a good potential of hydrocarbon resource. Recently, the lower members of Eh3 have shown commercial oil in the well of E-wang 25, conrming that the lower members of Eh3 source rocks are good prospects for exploration for primary hydrocarbon reservoirs and deepbasin gas. Luo J Q. Reservoiring patterns of oil and gas in the deep layer of the Biyang Depression . Oil & Gas Geology . 2003 . 24 ( 1 ) 55 - 57 (in Chinese) Luo J Q , Gan H J , Cai J , et al. 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Wenzhe Gang, Qingyu Wang, Jiaqun Luo. Oil and gas resource potential of the lower members of Eh3 in the lower Tertiary of the Biyang Depression, China, Petroleum Science, 2012, 18-24, DOI: 10.1007/s12182-012-0177-x