Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Depositional characteristics and petroleum geological significance of wetland

Depositional characteristics and petroleum geological significance of wetland Pet.Sci.(2009)6:347-353 347 347 DOI 10.1007/s12182-009-0054-4 Depositional characteristics and petroleum geological signifi cance of wetland 1 1 1 2 2 Jin Zhenkui , Gu Junfeng , Su Nina , Wang Zhaofeng and Huang Xiaoping State Key Laboratory of Hydrocarbon Resources and Exploration, China University of Petroleum, Beijing 102249, China Research Institute of Petroleum Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay, Xinjiang 834000, China Abstract: Wetland is a new type of sedimentary facies proposed for the fi rst time in this paper. It is a type of sedimentary environment, transitional between land facies and water (sea or lake) facies. In this paper, wetland is redefi ned as “a type of sedimentary environments whose ground surface is fl at and wet throughout the year, which may be covered with very shallow water (less than 2 m deep), and is covered with lush plant growth”. Wetland is reclassifi ed into two types, swamp wetland (swamp in brief) and wet plain wetland (wet plain in brief). Swamp deposits are coal, while wet plain deposits are dark colored mudstones or silty mudstones rich in plant fossils and carbon debris. The deposits of wet plain are different from those of lake and fl oodplain in the abundance of plant fossils, color, resistivity well logging curve, and other sedimentary characteristics. In boreholes, resistivity logging can be used to distinguish between wet plain mudstones and lake mudstones. Understanding the sedimentary characteristics of wet plain wetland can help to identify wet plain deposits which were formerly classifi ed as fl oodplain or lake deposits. This will help to reconstruct the palaeogeography and to understand the history of basin and climate evolution accurately, and is especially important in evaluation of the hydrocarbon generating potential of a basin. With the Jurassic Sangonghe Formation of the Yanqi Basin as an example, the identifi cation characteristics of wetland are described. Key words: Wetland, wet plain, Yanqi Basin, Sangonghe Formation of Jurassic, braided river delta depth less than 6 m at ebb tide” (Wilen and Tiner, 1993). 1 Introduction According to this definition, lakes, rivers, ponds, estuaries, Although wetland is well studied from the viewpoint mangroves, and marine coastal zones less than 6 m deep of ecology and environment protection (Brockmeyer et al, at ebb tide are all grouped into wetland. Obviously, this 1996; Hostedde et al, 2007; Lian, 2006; Lü and Huang, definition of wetland is too broad, and is not suitable for 1998; Meng, 1999; Peng et al, 2008; Yu, 2000; Zhang et al, sedimentary facies study. 2008), it has never been mentioned as a type of sedimentary In this paper, “wetland” is redefi ned from the viewpoint facies in papers or books on sedimentology and lithofacies of sedimentology for the fi rst time, as “a type of sedimentary paleogeography (Tian, 1997; Wang, 1995; Zeng and Xia, environments whose ground surface is fl at and wet throughout 1986; Zhao and Zhu, 2001). In fact, wetland is developed the year, which may be covered with very shallow water in many areas on the earth and is a very important type of (less than 2 m deep), and is covered with lush plant growth” sedimentary environment, transitional between land facies (Fig. 1). “Being covered with lush plant growth” is the most and water (sea or lake) facies. important characteristic of wetland defi ned in this paper. “Flat In 1971, the “Ramsar Convention on Wetlands” and wet ground surface which may be covered with very was passed officially in Ramsar in Iran, which provided shallow water” is the cause of lush plant growth. Wetland a framework for wetland protection and international defined in this paper may be called “wetland in a narrow cooperation for each contracting state. In November 1980, the sense”, and is different from the former one, which may be fi rst contracting states congress was held in Cagliari in Italy, called “wetland in a broad sense”. and the basic definition of wetland was given. According Wetland in a narrow sense can be further classified into to the definition, wetland refers to “swamp, wet plain, peat two types: swamp wetland (or swamp in brief) and wet land, or other water covered areas, which may be permanent plain wetland (or wet plain in brief). Swamp deposits are or temporary, natural or artifi cial, and may be covered with coal, while wet plain deposits are dark colored mudstones stagnant or flowing, fresh or salty water including sea with or silty mudstones rich in plant fossils and carbon debris. The deposits of these two types of wetland are completely different. *Corresponding author. email: jinzhenkui@188.com Received February 16, 2009 348 Pet.Sci.(2009)6:347-353 as follows: 1) The deposits are fine, mainly mudstones and silty mudstones. Because of the fl at topography and the lush plants acting as baffles, the velocity of floods flowing through wetlands is low. So wetland is a low energy environment. The sediments are mainly mud and fine silt transported in suspension during fl ooding. 2) The deposits are dark, mainly dark grey, greyish black, and black. In the wet plain environment, the ground water table is near or higher than the ground surface, so the soil Fig. 1 Modern wet plain wetland is full of water, and both ground water and surface water is stagnant. Thus wet plain is a reducing environment, organic matter can be preserved in mudstone, and hence the color According to the characteristics of modern narrow sense of the mudstone is generally dark. The higher the level of wetlands, wetlands may be developed from filling up of organic matter, the darker the color (Fig. 2). seashores, bays and lakes, from delta plains, and from flat and low land (such as fl uvial fl ood plain, and plain between and before alluvial fans). For example, the wetland developed on the Huanghai coast of northern Jiangsu province is the largest marine wetland in China. It is 582 kilometers long and 243,000 hectares in area. It is formed from mud fi lling of seashores. The wetlands around the Poyang Lake and Dongting Lake are developed from filling up of the lakes. The wetlands at the Yellow River mouth are formed on the Yellow River delta plain. The wetlands in the Momoge Nature Reserve in Jilin province are developed from fl ooding and deposition of the Nenjiang River and its tributaries. The wetlands in the Kekexili area result from mountain meltwater. In China, wetlands are mainly distributed in northeast China, east China, northwest China, and the Qinghai-Tibet plateau. The size of the wetlands ranges from hundreds meters or several kilometers to tens of kilometers or hundreds of kilometers in length and width. Wet plain dark mudstone rich in plant fossil, Jurassic, Baiyanghe Fig. 2 Wetlands are mainly developed in wet climate, but can succession in southern margin of Junggar Basin also form in arid climate, such as the reed wetlands developed around the Bositeng Lake in Yanqi County, Xinjiang, and 3) In wet plain deposits, plant fossils are abundant, and those developed in front of alluvial fans. However, generally organic matter content is high. Because of the dense plant speaking, wetlands developed in arid climate are relatively coverage and the reducing environment, plant fossil remains small in area. Wetlands of different origin and developed in are abundant in wet plain deposits, and plant roots, stems, different climate have different plant community. leaves, and carbon debris are common. Thus, the content of It is easy to identify ancient swamp, because its deposits organic matter in wet plain deposits is high, commonly over are coal. The deposits of wet plain are dark mudstones 5%, and may reach about 20%, even forming carbonaceous and silty mudstones, which are easily confused with lake mudstone. However, the organic matter is humic, and can not and floodplain deposits. In this paper, the identification form good petroleum source rocks. characteristics of ancient wet plain, its difference from 4) Wet plain deposits are massive in structure, and do not other similar environments, and the signifi cance of accurate show stratification, but root fossils are common. As plants identifi cation of ancient wet plain deposits are summarized. are dense in a wet plain environment, the root systems are Finally, taking the Sangonghe Formation of the lower well developed, which can disturb the deposits and destroy Jurassic in the Yanqi Basin, Xinjiang as an example, methods stratification. Thus stratification is not developed, and root for identifying wet plain deposits and their application in fossils are common. analysis of sedimentary facies are discussed. 5) Because of the abundant plant fossils and the high content of organic carbon, wet plain deposits can be easily 2 Sedimentary characteristics of wet plain identifi ed on resistivity logging curves. The resistivity curve wetland is sawtooth shaped and shows higher resistivity than pure According to the study of the Jurassic deposits in the mudstone, but lower than coal and sandstone. Junggar Basin and Yanqi Basin in Xinjiang, combined with In conclusion, the identifi cation characteristics of ancient the investigation of the modern wet plain wetland of Poyang wet plain deposits are as follows: the lithology is mudstone, Lake in east China and Bositeng Lake in northwest China, the silty mudstone, and carbonaceous mudstone, rich in plant characteristics of wet plain wetland deposits are summarized fossils and high in organic carbon content; the color of the Pet.Sci.(2009)6:347-353 349 deposits is mainly dark grey to black; the resistivity logging plain could be called “interdistributary wet plain wetland”. curve of the deposits is serrate in shape, and the resistivity The sedimentary characteristics of flood wet plain wetland value is between that of pure mudstone and sandstone. are similar to those of interdistributary wet plain wetland, We think that the reason why wet plain can not form and just the depositional systems in which they occur are coal but swamp can is that although plants are commonly different. developed in wet plain wetland environment, the deposition 4 Differences between wet plain deposits and rate of terrigenous mud and silt is still higher than that of plants, and the content of mud and silt in wet plain deposits lacustrine deposits is over 50%. While in swamp wetland, plants are dense, the A lake is an underwater environment, and thus mudstone deposits mainly consist of organic matter (Jin et al, 2007; deposited in lake usually shows reducing color, similar to 2008), and there is little terrigenous matter being deposited. that of wet plain deposits. Although showing reducing color, For wetlands large in area, swamp wetland may develop mudstone deposited in a lake is usually pure and does not in the central part of the wetland, while wet plain wetland contain plant fossils and carbon debris. However, mudstone develops in the marginal areas. In the Jurassic outcrop at deposited in wet plain wetland contains abundant plant fossils the southern margin of the Junggar Basin, dark mudstone and carbon debris. This is the most important difference deposited in wet plain wetland is interbedded with coal between wet plain and lake deposits. deposited in swamp wetland. This shows that wet plain In addition, in mudstone deposited in lakes, the organic wetland and swamp wetland are closely associated (both matter type is sapropelic and the organic carbon content is geographically and in time). Of course, these two types of usually less than 5%. However, in mudstone deposited in sedimentary environments may occur independently in some wet plain wetland, the organic matter type is humic and the cases. organic carbon content is usually over 5%, reaching 10%- 15% or even higher. 3 Differences between wet plain deposits and Therefore, the major differences between wet plain fl oodplain deposits deposits and lacustrine deposits are whether plant fossils and Wet plain deposits are different from fl oodplain deposits. carbon debris are abundant and what type the organic matter In previous literature, floodplain is defined as flat land is. environment located between river channels, inundated only during fl ooding. It is a sedimentary microfacies belonging to 5 Signifi cance of correct identifi cation of wet fl ood basin subfacies in fl uvial facies. plain deposits The floodplain defined in previous literature in fact It has substantial theoretical and practical significance includes two types of environments. The first type is a to identify wet plain deposits correctly and differentiate wet relatively dry oxidizing environment in which plants are plain wetland from swamp wetland, fl oodplain, and lake. rare, its deposits are mudstone and silty mudstone in which Firstly, with knowledge of sedimentary characteristics of plant fossils are rare, and the color of the deposits, usually wet plains, we can identify the wet plain deposits which were red, purplish red, or yellowish brown, refl ects their oxidation previously believed to be floodplain or lake deposits, then (Jin et al, 2002; 2006). The second type is wet reducing the palaeogeography can be identified correctly, and basin environment in which plants are dense, its deposits are evolution and climate evolution can be understood more mudstone and silty mudstone in which plant fossils are accurately. This is important. abundant, and the color of the deposits, usually gray, dark Secondly, present determination of lithology and gray, and black, refl ects the reducing environment. sedimentary facies of underground strata in oilfield mainly Obviously, these two types of environments are very relies on borehole rock cutting logging and electrical logging. different. They should be distinguished from each other and All dark colored mudstones, as the color is reducing color, are could not be both called fl oodplain. We suggest that the fi rst believed to be lake deposits. Almost nobody expects to fi nd type of environment should be called fl oodplain, representing plant fossils in the rock cuttings because they are too small in dry oxidizing environment with scarce plants and a relatively size. If these dark colored mudstones are not lake deposits, but deep ground water table. It is mainly developed in arid to from a wet plain wetland environment, serious mistakes will semiarid climates. The second type of environment should be made in exploration: 1) A lake may be believed to develop be called wet plain wetland, representing a wet reducing extensively in the basin, and as a lake is a good petroleum- environment with dense plants and a very shallow ground generating environment, the exploration potential of the basin water table. It is mainly developed in wet climates. may be believed to be great. This false evaluation might Therefore, the major differences between wet plain result in great waste in manpower and material resources. 2) wetland deposits and floodplain deposits are whether the Sandstone intercalated in the dark mudstones, which in fact deposits contain abundant plant fossils and whether the color are fluvial deposits, will be believed to be deltaic or even of the deposits is a reducing color or an oxidizing color. gravity fl ow deposits, resulting in incorrect interpretation of Wet plain wetland may develop both on floodplain of the palaeogeography and basin evolution history. This may fluvial depositional systems and on delta plains. Wet plain also have an adverse infl uence on the petroleum exploration wetland developed on fl oodplain could be called “fl ood wet in this basin. plain wetland”, and wet plain wetland developed on delta Bohu South Tianshan Mountain Kuluke Mountain Yanqi Uplift Hejing Depression Depression 350 Pet.Sci.(2009)6:347-353 On outcrop, plant fossils can be easily seen if present, and Mongolian Autonomous Prefecture in Xinjiang, and at thus it is easy to distinguish between mudstones deposited in the eastern end of the southern Tianshan fold belt. It is a wet plain wetland and those deposited in lake. But how can Mesozoic and Cenozoic petroleum-bearing and coal-bearing the distinction be made in boreholes? The following methods basin developed from Hercynian fold basement. Its area is can be used: 13,000 km . The basin consists of two depressions and one 1) Resistivity logging can be used to distinguish them. central uplift (Yuan, 2003), which are the Bohu Depression, Mudstone deposited in wet plain contains abundant plant Yanqi Uplift, and Hejing Depression from south to north. In fossils and is high in organic matter content, so the resistivity the basin, the Triassic, Jurassic, Paleogene, and Neogene are of wet plain mudstone is usually higher than that of pure developed. The Jurassic is the major exploration target. mudstone deposited in lake. Based on the study of Jurassic The study area is located in the northern part of the cores and electrical logging of several tens of wells in Junggar Baolangsumu anticline in the Bohu Depression (Long et al, Basin and Yanqi Basin in Xinjiang, the resistivity curves 2007) (Fig. 3), and its area is about 6 km . At present, there of wet plain mudstone are usually serrated in shape and the are 25 wells including fi ve cored wells and three-dimensional resistivity ranges between 6 Ωm and 10 Ωm, while that of seismic data. The lower Jurassic Sangonghe Formation is the pure mudstone deposited in lake is usually 1-4 Ωm. producing horizon (Table 1), and is also the horizon studied 2) The abundance of carbon debris in borehole rock in this paper. cuttings can be used to distinguish them. In mudstone The Sangonghe Formation of the study area is mainly deposited in wet plain, there is abundant carbon debris in composed of gray sandstone, fine conglomerate, dark addition to plant fossils. In rock cuttings, it is diffi cult to fi nd mudstone, and coal. The formation is deposited in braided large plant fossils, but easy to fi nd carbon debris if present. river deltas under a humid climate, and the evidence includes: Therefore, if carbon debris is common in rock cuttings, then 1) The lithology is coarse on the whole. Fine conglomerate is the mudstone is wet plain deposits. If there is no carbon common, and the total content of sandstone and conglomerate debris, then the mudstone is lake deposits. is high, reaching 50%-60%, which is a typical characteristic With the above two methods, it is easy to determine of a braided river delta (Wu et al, 1999); 2) Parallel bedding whether the dark mudstone in boreholes is deposited in wet plain or lake. and trough cross bedding are common in sandstone and conglomerate. This reflects high current velocities, also 6 Case study a typical characteristic of a braided river delta; 3) The Taking the Jurassic Sangonghe Formation in the Yanqi roundness of grains is poor, mostly subangular, which refl ects Basin, Xinjiang as an example, the depositional characteristics short transportation distance and near-provenance deposition; of wet plain and lake facies are discussed. 4) Sandstone and conglomerate are frequently interbedded with lacustrine mudstone and coal and mudstone deposited 6.1 Geological setting on land, showing upward-coarsening sedimentary succession, The Yanqi Basin is located in the Bayinguoleng which is another characteristic of delta deposition. 0 10 20 km Hejing Heshuo Wushitaw Yanqi Study area Outcrop Oilfield Ku’erle Place name Structural division line Fig. 3 Structural units of the Yanqi Basin and the location of the study area Pet.Sci.(2009)6:347-353 351 Table 1 Stratigraphy and petrology of the Jurassic of the study area Stratigraphic unit Thickness, Tectonic movement and Rock characteristics m movement property Erathem System Series Formation grey mudstone, silty mudstone, coal Middle Xisanyao Formation J x 0-300 seam interbedded with light grey sandy series conglomerate and fi ne sandstone grey fine sandstone, fine conglomerate, Indo-China movement, Mesozoic Jurassic Sangonghe Formation J s 0-600 and sandy conglomerate intercalated with uplifting charcoal grey mudstone and coal seam Lower series grey pebble sandstone interbedded with charcoal grey mudstone and grey black Badaowan Formation J b 500-1200 carbonaceous mudstone intercalated with coal seam, siltstone, and fi ne conglomerate 6.2 Differences between wet plain deposition and lake deposition In the Sangonghe Formation of the study area, the color of Plant root the mudstone interbedded with sandstone and conglomerate is mainly dark gray and grayish black. Which dark colored mudstones was deposited in a lake and which on a wet plain? How can delta front and delta plain deposits be distinguished? In cores from wells Bao101, Bao103, and Bao108, it is easy to distinguish between the mudstone deposited on a wet plain and that deposited in lake facies. The former generally Bao 108 contains stem and leaf fossils of plant and carbon debris, 2191.1 m and does not show bedding (Figs. 4 and 5). Carbonized plant root fossils are common. Mudstone deposited in a lake is Fig. 5 Silty mudstone deposited in a wet plain. Plant pure, and does not contain plant fossils and carbon debris, root fossils are common. Well Bao108 (2,191.1 m), and horizontal bedding is well developed (Fig. 6). Ostracod Jurassic Sangonghe Formation in Yanqi Basin, Xinjiang fossils are common. Bao 101 2227.65 m Fig. 4 Mudstone deposited in a wet plain, rich in plant Fig. 6 Pure lake mudstone containing no plant fossil or carbon debris, fossils and carbon debris, Well Bao101 (2,227.65 m), Jurassic Sangonghe Formation, Well Bao101, Yanqi Basin, Xinjiang Jurassic Sangonghe Formation in Yanqi Basin, Xinjiang 352 Pet.Sci.(2009)6:347-353 Based on the logging curve characteristics of different Ωm – 10 Ωm, and that of lake mudstone is usually less types of rock, it is found that the resistivity curve can be than 4 Ωm, around 3 Ωm (Figs. 7 and 8). According to used to distinguish wet plain mudstone and lake mudstone, the logging characteristics of the two types of mudstone, but other logging curves can not. In the study area, because it is easy to distinguish wet plain mudstone from lake of the higher organic matter content, the resistivity of wet mudstone in intervals that are not cored, so it is also easy plain mudstone is usually higher than that of lake mudstone. to distinguish between delta plain and delta front. This is of The organic matter content of coal deposited in swamp great significance to reconstruction of palaeogeographical is almost 100%, so its resistivity is the highest (Figs. 7 environments, correct evaluation of the scope of the lake, and 8). The resistivity of wet plain mudstone is usually 6 and the distribution of the source rock. Fig. 7 Electrical logging characteristics of wet plain wetland, swamp wetland, and underwater interdistributary bay (lake) of delta front deposits in Well Bao108, Yanqi Basin, Xinjiang Fig. 8 Electrical logging characteristics of underwater interdistributary bay (lake) of delta front deposits in Well Bao103, Yanqi Basin, Xinjiang plain deposits are dark colored mudstones or silty mudstones 7 Conclusions rich in plant fossils and carbon debris. The deposits of wet Wetland can be divided into two types, swamp wetland plain are different from those of lake and floodplain in the and wet plain wetland. Swamp deposits are coal, while wet abundance of plant fossils, color, resistivity well logging Pet.Sci.(2009)6:347-353 353 2002. 4(4): 99-107 (in Chinese) curve, and other sedimentary characteristics. Correct Lia n J G. Main wetlands in China and protective suggestions. Resources identifi cation of wet plain deposits is helpful in reconstructing Environment and Development. 2006. (1): 30-32 (in Chinese) the palaeogeography and understanding the history of basin Lon g G Q, Deng H W, Liu B, et al. Base level cycle and reservoir and climate evolution accurately. Correct differentiation macroscopic heterogeneity of the Sangonghe Formation in Baolang between wet plain deposits and lake deposits is especially Oilfi eld. Petroleum Geology and Engineering. 2007. 21(2): 13-17 (in important in evaluation of the hydrocarbon generating Chinese) potential of a basin. Lü X G and Huang X C. Progress in wetland research in China. Scientia Geographica Sinica. 1998. 18(4): 294-300 (in Chinese) Acknowledgements Men g X M. Wetlands and Global Environmental Change. Scientia Geographica Sinica. 1999. 19(5): 385-391 (in Chinese) This study is fi nancially supported by the “973” National Pen g Y S, Zhou Y W and Chen G Z. The restoration of mangrove Major Fundamental Research & Development Project wetland: a review. Acta Ecologica Sinica. 2008. 28(2): 786-792 (in “Reservoir-forming mechanism and distribution law of oil Chinese) and gas in superimposed basins of China” (2006C13202300). Tia n Z Y. Lithofacies Palaeogeography and Hydrocarbon of Petroliferous Basin in China. Beijing: Geological Publishing House. 1997 (in References Chinese) Wan g Y H. New Progress of Sedimentology and Lithofacies Bro ckmeyer R E, Rey J R, Virnstein R W, et al. Rehabilitation of Palaeogeography. Beijing: Petroleum Industry Press. 1995 (in impounded estuarine wetlands by hydrologic reconnection to Chinese) the Indian River Lagoon, Florida (USA). Wetlands Ecology and Wil en B O and Tiner R W. Wetlands of the United States. In: Wetlands Management. 1996. 4(2): 93-109 of the World I. Netherlands: Kluver Academic Publishers. 1993 Hos tedde B S, Walters D, Powell C, et al. Wetland management: An Wu S H, Jin Z K, Huang C D, et al. Reservoir Modeling. Beijing: analysis of past practice and recent policy changes in Ontario. Petroleum Industry Press. 1999 (in Chinese) Journal of Environmental Management. 2007. 82(1): 83-94 Yua n Z W. Analysis on structural evolution in Yanqi Basin. Journal of Jin Z K, Qi C W, Xue J Q, et al. Sedimentary facies of the Jurassic in the Jianghan Petroleum Institute. 2003. 25(4): 33-35 (in Chinese) northern margin of Qaidam Basin. Journal of Palaeogeography. 2006. Yu G Y. Respect and prospect of the wetland research in China. World 8(2): 199-209 (in Chinese) Sci-Tech R & D. 2000. 22(3): 61-66 (in Chinese) Jin Z K, Su N N and Wang C S. Controlling factors of reservoir of Zen g Y F and Xia W J. Sedimentary Petrology. Beijing: Geological coal rocks in Carboniferous-Permian, North China. Acta Geologica Publishing House. 1986 (in Chinese) Sinica. 2008. 82(10): 1323-1329 (in Chinese) Zha ng Y M, Zhao S D and Guo R C. Global wetlands: Status and trends, Jin Z K, Wang C S, Wang P, et al. Controlling factors of reservoir ability scenarios and response options. Advances in Earth Science. 2008. of Carboniferous and Permian coal rocks, North China. Petroleum 23(4): 415-420 (in Chinese) Exploration and Development. 2007. 34(5): 534-540 (in Chinese) Zha o C L and Zhu X M. Sedimentary Petrology (Third Edition). Beijing: Jin Z K, Zhang X X, Zou Y R, et al. Quantitative study of sedimentary Petroleum Industry Press. 2001. 229-233 (in Chinese) facies of the lower part of Xiaganchaigou Formation of Paleogene of Shaxi Oilfi eld in Qinghai Province. Journal of Palaeogeography. (Edited by Hao Jie) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

Depositional characteristics and petroleum geological significance of wetland

Download PDF
7 pages

Loading next page...
 
/lp/springer-journals/depositional-characteristics-and-petroleum-geological-significance-of-hgQIE8vbfC

References (20)

Publisher
Springer Journals
Copyright
Copyright © 2009 by China University of Petroleum (Beijing) and Springer Berlin Heidelberg
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
ISSN
1672-5107
eISSN
1995-8226
DOI
10.1007/s12182-009-0054-4
Publisher site
See Article on Publisher Site

Abstract

Pet.Sci.(2009)6:347-353 347 347 DOI 10.1007/s12182-009-0054-4 Depositional characteristics and petroleum geological signifi cance of wetland 1 1 1 2 2 Jin Zhenkui , Gu Junfeng , Su Nina , Wang Zhaofeng and Huang Xiaoping State Key Laboratory of Hydrocarbon Resources and Exploration, China University of Petroleum, Beijing 102249, China Research Institute of Petroleum Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay, Xinjiang 834000, China Abstract: Wetland is a new type of sedimentary facies proposed for the fi rst time in this paper. It is a type of sedimentary environment, transitional between land facies and water (sea or lake) facies. In this paper, wetland is redefi ned as “a type of sedimentary environments whose ground surface is fl at and wet throughout the year, which may be covered with very shallow water (less than 2 m deep), and is covered with lush plant growth”. Wetland is reclassifi ed into two types, swamp wetland (swamp in brief) and wet plain wetland (wet plain in brief). Swamp deposits are coal, while wet plain deposits are dark colored mudstones or silty mudstones rich in plant fossils and carbon debris. The deposits of wet plain are different from those of lake and fl oodplain in the abundance of plant fossils, color, resistivity well logging curve, and other sedimentary characteristics. In boreholes, resistivity logging can be used to distinguish between wet plain mudstones and lake mudstones. Understanding the sedimentary characteristics of wet plain wetland can help to identify wet plain deposits which were formerly classifi ed as fl oodplain or lake deposits. This will help to reconstruct the palaeogeography and to understand the history of basin and climate evolution accurately, and is especially important in evaluation of the hydrocarbon generating potential of a basin. With the Jurassic Sangonghe Formation of the Yanqi Basin as an example, the identifi cation characteristics of wetland are described. Key words: Wetland, wet plain, Yanqi Basin, Sangonghe Formation of Jurassic, braided river delta depth less than 6 m at ebb tide” (Wilen and Tiner, 1993). 1 Introduction According to this definition, lakes, rivers, ponds, estuaries, Although wetland is well studied from the viewpoint mangroves, and marine coastal zones less than 6 m deep of ecology and environment protection (Brockmeyer et al, at ebb tide are all grouped into wetland. Obviously, this 1996; Hostedde et al, 2007; Lian, 2006; Lü and Huang, definition of wetland is too broad, and is not suitable for 1998; Meng, 1999; Peng et al, 2008; Yu, 2000; Zhang et al, sedimentary facies study. 2008), it has never been mentioned as a type of sedimentary In this paper, “wetland” is redefi ned from the viewpoint facies in papers or books on sedimentology and lithofacies of sedimentology for the fi rst time, as “a type of sedimentary paleogeography (Tian, 1997; Wang, 1995; Zeng and Xia, environments whose ground surface is fl at and wet throughout 1986; Zhao and Zhu, 2001). In fact, wetland is developed the year, which may be covered with very shallow water in many areas on the earth and is a very important type of (less than 2 m deep), and is covered with lush plant growth” sedimentary environment, transitional between land facies (Fig. 1). “Being covered with lush plant growth” is the most and water (sea or lake) facies. important characteristic of wetland defi ned in this paper. “Flat In 1971, the “Ramsar Convention on Wetlands” and wet ground surface which may be covered with very was passed officially in Ramsar in Iran, which provided shallow water” is the cause of lush plant growth. Wetland a framework for wetland protection and international defined in this paper may be called “wetland in a narrow cooperation for each contracting state. In November 1980, the sense”, and is different from the former one, which may be fi rst contracting states congress was held in Cagliari in Italy, called “wetland in a broad sense”. and the basic definition of wetland was given. According Wetland in a narrow sense can be further classified into to the definition, wetland refers to “swamp, wet plain, peat two types: swamp wetland (or swamp in brief) and wet land, or other water covered areas, which may be permanent plain wetland (or wet plain in brief). Swamp deposits are or temporary, natural or artifi cial, and may be covered with coal, while wet plain deposits are dark colored mudstones stagnant or flowing, fresh or salty water including sea with or silty mudstones rich in plant fossils and carbon debris. The deposits of these two types of wetland are completely different. *Corresponding author. email: jinzhenkui@188.com Received February 16, 2009 348 Pet.Sci.(2009)6:347-353 as follows: 1) The deposits are fine, mainly mudstones and silty mudstones. Because of the fl at topography and the lush plants acting as baffles, the velocity of floods flowing through wetlands is low. So wetland is a low energy environment. The sediments are mainly mud and fine silt transported in suspension during fl ooding. 2) The deposits are dark, mainly dark grey, greyish black, and black. In the wet plain environment, the ground water table is near or higher than the ground surface, so the soil Fig. 1 Modern wet plain wetland is full of water, and both ground water and surface water is stagnant. Thus wet plain is a reducing environment, organic matter can be preserved in mudstone, and hence the color According to the characteristics of modern narrow sense of the mudstone is generally dark. The higher the level of wetlands, wetlands may be developed from filling up of organic matter, the darker the color (Fig. 2). seashores, bays and lakes, from delta plains, and from flat and low land (such as fl uvial fl ood plain, and plain between and before alluvial fans). For example, the wetland developed on the Huanghai coast of northern Jiangsu province is the largest marine wetland in China. It is 582 kilometers long and 243,000 hectares in area. It is formed from mud fi lling of seashores. The wetlands around the Poyang Lake and Dongting Lake are developed from filling up of the lakes. The wetlands at the Yellow River mouth are formed on the Yellow River delta plain. The wetlands in the Momoge Nature Reserve in Jilin province are developed from fl ooding and deposition of the Nenjiang River and its tributaries. The wetlands in the Kekexili area result from mountain meltwater. In China, wetlands are mainly distributed in northeast China, east China, northwest China, and the Qinghai-Tibet plateau. The size of the wetlands ranges from hundreds meters or several kilometers to tens of kilometers or hundreds of kilometers in length and width. Wet plain dark mudstone rich in plant fossil, Jurassic, Baiyanghe Fig. 2 Wetlands are mainly developed in wet climate, but can succession in southern margin of Junggar Basin also form in arid climate, such as the reed wetlands developed around the Bositeng Lake in Yanqi County, Xinjiang, and 3) In wet plain deposits, plant fossils are abundant, and those developed in front of alluvial fans. However, generally organic matter content is high. Because of the dense plant speaking, wetlands developed in arid climate are relatively coverage and the reducing environment, plant fossil remains small in area. Wetlands of different origin and developed in are abundant in wet plain deposits, and plant roots, stems, different climate have different plant community. leaves, and carbon debris are common. Thus, the content of It is easy to identify ancient swamp, because its deposits organic matter in wet plain deposits is high, commonly over are coal. The deposits of wet plain are dark mudstones 5%, and may reach about 20%, even forming carbonaceous and silty mudstones, which are easily confused with lake mudstone. However, the organic matter is humic, and can not and floodplain deposits. In this paper, the identification form good petroleum source rocks. characteristics of ancient wet plain, its difference from 4) Wet plain deposits are massive in structure, and do not other similar environments, and the signifi cance of accurate show stratification, but root fossils are common. As plants identifi cation of ancient wet plain deposits are summarized. are dense in a wet plain environment, the root systems are Finally, taking the Sangonghe Formation of the lower well developed, which can disturb the deposits and destroy Jurassic in the Yanqi Basin, Xinjiang as an example, methods stratification. Thus stratification is not developed, and root for identifying wet plain deposits and their application in fossils are common. analysis of sedimentary facies are discussed. 5) Because of the abundant plant fossils and the high content of organic carbon, wet plain deposits can be easily 2 Sedimentary characteristics of wet plain identifi ed on resistivity logging curves. The resistivity curve wetland is sawtooth shaped and shows higher resistivity than pure According to the study of the Jurassic deposits in the mudstone, but lower than coal and sandstone. Junggar Basin and Yanqi Basin in Xinjiang, combined with In conclusion, the identifi cation characteristics of ancient the investigation of the modern wet plain wetland of Poyang wet plain deposits are as follows: the lithology is mudstone, Lake in east China and Bositeng Lake in northwest China, the silty mudstone, and carbonaceous mudstone, rich in plant characteristics of wet plain wetland deposits are summarized fossils and high in organic carbon content; the color of the Pet.Sci.(2009)6:347-353 349 deposits is mainly dark grey to black; the resistivity logging plain could be called “interdistributary wet plain wetland”. curve of the deposits is serrate in shape, and the resistivity The sedimentary characteristics of flood wet plain wetland value is between that of pure mudstone and sandstone. are similar to those of interdistributary wet plain wetland, We think that the reason why wet plain can not form and just the depositional systems in which they occur are coal but swamp can is that although plants are commonly different. developed in wet plain wetland environment, the deposition 4 Differences between wet plain deposits and rate of terrigenous mud and silt is still higher than that of plants, and the content of mud and silt in wet plain deposits lacustrine deposits is over 50%. While in swamp wetland, plants are dense, the A lake is an underwater environment, and thus mudstone deposits mainly consist of organic matter (Jin et al, 2007; deposited in lake usually shows reducing color, similar to 2008), and there is little terrigenous matter being deposited. that of wet plain deposits. Although showing reducing color, For wetlands large in area, swamp wetland may develop mudstone deposited in a lake is usually pure and does not in the central part of the wetland, while wet plain wetland contain plant fossils and carbon debris. However, mudstone develops in the marginal areas. In the Jurassic outcrop at deposited in wet plain wetland contains abundant plant fossils the southern margin of the Junggar Basin, dark mudstone and carbon debris. This is the most important difference deposited in wet plain wetland is interbedded with coal between wet plain and lake deposits. deposited in swamp wetland. This shows that wet plain In addition, in mudstone deposited in lakes, the organic wetland and swamp wetland are closely associated (both matter type is sapropelic and the organic carbon content is geographically and in time). Of course, these two types of usually less than 5%. However, in mudstone deposited in sedimentary environments may occur independently in some wet plain wetland, the organic matter type is humic and the cases. organic carbon content is usually over 5%, reaching 10%- 15% or even higher. 3 Differences between wet plain deposits and Therefore, the major differences between wet plain fl oodplain deposits deposits and lacustrine deposits are whether plant fossils and Wet plain deposits are different from fl oodplain deposits. carbon debris are abundant and what type the organic matter In previous literature, floodplain is defined as flat land is. environment located between river channels, inundated only during fl ooding. It is a sedimentary microfacies belonging to 5 Signifi cance of correct identifi cation of wet fl ood basin subfacies in fl uvial facies. plain deposits The floodplain defined in previous literature in fact It has substantial theoretical and practical significance includes two types of environments. The first type is a to identify wet plain deposits correctly and differentiate wet relatively dry oxidizing environment in which plants are plain wetland from swamp wetland, fl oodplain, and lake. rare, its deposits are mudstone and silty mudstone in which Firstly, with knowledge of sedimentary characteristics of plant fossils are rare, and the color of the deposits, usually wet plains, we can identify the wet plain deposits which were red, purplish red, or yellowish brown, refl ects their oxidation previously believed to be floodplain or lake deposits, then (Jin et al, 2002; 2006). The second type is wet reducing the palaeogeography can be identified correctly, and basin environment in which plants are dense, its deposits are evolution and climate evolution can be understood more mudstone and silty mudstone in which plant fossils are accurately. This is important. abundant, and the color of the deposits, usually gray, dark Secondly, present determination of lithology and gray, and black, refl ects the reducing environment. sedimentary facies of underground strata in oilfield mainly Obviously, these two types of environments are very relies on borehole rock cutting logging and electrical logging. different. They should be distinguished from each other and All dark colored mudstones, as the color is reducing color, are could not be both called fl oodplain. We suggest that the fi rst believed to be lake deposits. Almost nobody expects to fi nd type of environment should be called fl oodplain, representing plant fossils in the rock cuttings because they are too small in dry oxidizing environment with scarce plants and a relatively size. If these dark colored mudstones are not lake deposits, but deep ground water table. It is mainly developed in arid to from a wet plain wetland environment, serious mistakes will semiarid climates. The second type of environment should be made in exploration: 1) A lake may be believed to develop be called wet plain wetland, representing a wet reducing extensively in the basin, and as a lake is a good petroleum- environment with dense plants and a very shallow ground generating environment, the exploration potential of the basin water table. It is mainly developed in wet climates. may be believed to be great. This false evaluation might Therefore, the major differences between wet plain result in great waste in manpower and material resources. 2) wetland deposits and floodplain deposits are whether the Sandstone intercalated in the dark mudstones, which in fact deposits contain abundant plant fossils and whether the color are fluvial deposits, will be believed to be deltaic or even of the deposits is a reducing color or an oxidizing color. gravity fl ow deposits, resulting in incorrect interpretation of Wet plain wetland may develop both on floodplain of the palaeogeography and basin evolution history. This may fluvial depositional systems and on delta plains. Wet plain also have an adverse infl uence on the petroleum exploration wetland developed on fl oodplain could be called “fl ood wet in this basin. plain wetland”, and wet plain wetland developed on delta Bohu South Tianshan Mountain Kuluke Mountain Yanqi Uplift Hejing Depression Depression 350 Pet.Sci.(2009)6:347-353 On outcrop, plant fossils can be easily seen if present, and Mongolian Autonomous Prefecture in Xinjiang, and at thus it is easy to distinguish between mudstones deposited in the eastern end of the southern Tianshan fold belt. It is a wet plain wetland and those deposited in lake. But how can Mesozoic and Cenozoic petroleum-bearing and coal-bearing the distinction be made in boreholes? The following methods basin developed from Hercynian fold basement. Its area is can be used: 13,000 km . The basin consists of two depressions and one 1) Resistivity logging can be used to distinguish them. central uplift (Yuan, 2003), which are the Bohu Depression, Mudstone deposited in wet plain contains abundant plant Yanqi Uplift, and Hejing Depression from south to north. In fossils and is high in organic matter content, so the resistivity the basin, the Triassic, Jurassic, Paleogene, and Neogene are of wet plain mudstone is usually higher than that of pure developed. The Jurassic is the major exploration target. mudstone deposited in lake. Based on the study of Jurassic The study area is located in the northern part of the cores and electrical logging of several tens of wells in Junggar Baolangsumu anticline in the Bohu Depression (Long et al, Basin and Yanqi Basin in Xinjiang, the resistivity curves 2007) (Fig. 3), and its area is about 6 km . At present, there of wet plain mudstone are usually serrated in shape and the are 25 wells including fi ve cored wells and three-dimensional resistivity ranges between 6 Ωm and 10 Ωm, while that of seismic data. The lower Jurassic Sangonghe Formation is the pure mudstone deposited in lake is usually 1-4 Ωm. producing horizon (Table 1), and is also the horizon studied 2) The abundance of carbon debris in borehole rock in this paper. cuttings can be used to distinguish them. In mudstone The Sangonghe Formation of the study area is mainly deposited in wet plain, there is abundant carbon debris in composed of gray sandstone, fine conglomerate, dark addition to plant fossils. In rock cuttings, it is diffi cult to fi nd mudstone, and coal. The formation is deposited in braided large plant fossils, but easy to fi nd carbon debris if present. river deltas under a humid climate, and the evidence includes: Therefore, if carbon debris is common in rock cuttings, then 1) The lithology is coarse on the whole. Fine conglomerate is the mudstone is wet plain deposits. If there is no carbon common, and the total content of sandstone and conglomerate debris, then the mudstone is lake deposits. is high, reaching 50%-60%, which is a typical characteristic With the above two methods, it is easy to determine of a braided river delta (Wu et al, 1999); 2) Parallel bedding whether the dark mudstone in boreholes is deposited in wet plain or lake. and trough cross bedding are common in sandstone and conglomerate. This reflects high current velocities, also 6 Case study a typical characteristic of a braided river delta; 3) The Taking the Jurassic Sangonghe Formation in the Yanqi roundness of grains is poor, mostly subangular, which refl ects Basin, Xinjiang as an example, the depositional characteristics short transportation distance and near-provenance deposition; of wet plain and lake facies are discussed. 4) Sandstone and conglomerate are frequently interbedded with lacustrine mudstone and coal and mudstone deposited 6.1 Geological setting on land, showing upward-coarsening sedimentary succession, The Yanqi Basin is located in the Bayinguoleng which is another characteristic of delta deposition. 0 10 20 km Hejing Heshuo Wushitaw Yanqi Study area Outcrop Oilfield Ku’erle Place name Structural division line Fig. 3 Structural units of the Yanqi Basin and the location of the study area Pet.Sci.(2009)6:347-353 351 Table 1 Stratigraphy and petrology of the Jurassic of the study area Stratigraphic unit Thickness, Tectonic movement and Rock characteristics m movement property Erathem System Series Formation grey mudstone, silty mudstone, coal Middle Xisanyao Formation J x 0-300 seam interbedded with light grey sandy series conglomerate and fi ne sandstone grey fine sandstone, fine conglomerate, Indo-China movement, Mesozoic Jurassic Sangonghe Formation J s 0-600 and sandy conglomerate intercalated with uplifting charcoal grey mudstone and coal seam Lower series grey pebble sandstone interbedded with charcoal grey mudstone and grey black Badaowan Formation J b 500-1200 carbonaceous mudstone intercalated with coal seam, siltstone, and fi ne conglomerate 6.2 Differences between wet plain deposition and lake deposition In the Sangonghe Formation of the study area, the color of Plant root the mudstone interbedded with sandstone and conglomerate is mainly dark gray and grayish black. Which dark colored mudstones was deposited in a lake and which on a wet plain? How can delta front and delta plain deposits be distinguished? In cores from wells Bao101, Bao103, and Bao108, it is easy to distinguish between the mudstone deposited on a wet plain and that deposited in lake facies. The former generally Bao 108 contains stem and leaf fossils of plant and carbon debris, 2191.1 m and does not show bedding (Figs. 4 and 5). Carbonized plant root fossils are common. Mudstone deposited in a lake is Fig. 5 Silty mudstone deposited in a wet plain. Plant pure, and does not contain plant fossils and carbon debris, root fossils are common. Well Bao108 (2,191.1 m), and horizontal bedding is well developed (Fig. 6). Ostracod Jurassic Sangonghe Formation in Yanqi Basin, Xinjiang fossils are common. Bao 101 2227.65 m Fig. 4 Mudstone deposited in a wet plain, rich in plant Fig. 6 Pure lake mudstone containing no plant fossil or carbon debris, fossils and carbon debris, Well Bao101 (2,227.65 m), Jurassic Sangonghe Formation, Well Bao101, Yanqi Basin, Xinjiang Jurassic Sangonghe Formation in Yanqi Basin, Xinjiang 352 Pet.Sci.(2009)6:347-353 Based on the logging curve characteristics of different Ωm – 10 Ωm, and that of lake mudstone is usually less types of rock, it is found that the resistivity curve can be than 4 Ωm, around 3 Ωm (Figs. 7 and 8). According to used to distinguish wet plain mudstone and lake mudstone, the logging characteristics of the two types of mudstone, but other logging curves can not. In the study area, because it is easy to distinguish wet plain mudstone from lake of the higher organic matter content, the resistivity of wet mudstone in intervals that are not cored, so it is also easy plain mudstone is usually higher than that of lake mudstone. to distinguish between delta plain and delta front. This is of The organic matter content of coal deposited in swamp great significance to reconstruction of palaeogeographical is almost 100%, so its resistivity is the highest (Figs. 7 environments, correct evaluation of the scope of the lake, and 8). The resistivity of wet plain mudstone is usually 6 and the distribution of the source rock. Fig. 7 Electrical logging characteristics of wet plain wetland, swamp wetland, and underwater interdistributary bay (lake) of delta front deposits in Well Bao108, Yanqi Basin, Xinjiang Fig. 8 Electrical logging characteristics of underwater interdistributary bay (lake) of delta front deposits in Well Bao103, Yanqi Basin, Xinjiang plain deposits are dark colored mudstones or silty mudstones 7 Conclusions rich in plant fossils and carbon debris. The deposits of wet Wetland can be divided into two types, swamp wetland plain are different from those of lake and floodplain in the and wet plain wetland. Swamp deposits are coal, while wet abundance of plant fossils, color, resistivity well logging Pet.Sci.(2009)6:347-353 353 2002. 4(4): 99-107 (in Chinese) curve, and other sedimentary characteristics. Correct Lia n J G. Main wetlands in China and protective suggestions. Resources identifi cation of wet plain deposits is helpful in reconstructing Environment and Development. 2006. (1): 30-32 (in Chinese) the palaeogeography and understanding the history of basin Lon g G Q, Deng H W, Liu B, et al. Base level cycle and reservoir and climate evolution accurately. Correct differentiation macroscopic heterogeneity of the Sangonghe Formation in Baolang between wet plain deposits and lake deposits is especially Oilfi eld. Petroleum Geology and Engineering. 2007. 21(2): 13-17 (in important in evaluation of the hydrocarbon generating Chinese) potential of a basin. Lü X G and Huang X C. Progress in wetland research in China. Scientia Geographica Sinica. 1998. 18(4): 294-300 (in Chinese) Acknowledgements Men g X M. Wetlands and Global Environmental Change. Scientia Geographica Sinica. 1999. 19(5): 385-391 (in Chinese) This study is fi nancially supported by the “973” National Pen g Y S, Zhou Y W and Chen G Z. The restoration of mangrove Major Fundamental Research & Development Project wetland: a review. Acta Ecologica Sinica. 2008. 28(2): 786-792 (in “Reservoir-forming mechanism and distribution law of oil Chinese) and gas in superimposed basins of China” (2006C13202300). Tia n Z Y. Lithofacies Palaeogeography and Hydrocarbon of Petroliferous Basin in China. Beijing: Geological Publishing House. 1997 (in References Chinese) Wan g Y H. New Progress of Sedimentology and Lithofacies Bro ckmeyer R E, Rey J R, Virnstein R W, et al. Rehabilitation of Palaeogeography. Beijing: Petroleum Industry Press. 1995 (in impounded estuarine wetlands by hydrologic reconnection to Chinese) the Indian River Lagoon, Florida (USA). Wetlands Ecology and Wil en B O and Tiner R W. Wetlands of the United States. In: Wetlands Management. 1996. 4(2): 93-109 of the World I. Netherlands: Kluver Academic Publishers. 1993 Hos tedde B S, Walters D, Powell C, et al. Wetland management: An Wu S H, Jin Z K, Huang C D, et al. Reservoir Modeling. Beijing: analysis of past practice and recent policy changes in Ontario. Petroleum Industry Press. 1999 (in Chinese) Journal of Environmental Management. 2007. 82(1): 83-94 Yua n Z W. Analysis on structural evolution in Yanqi Basin. Journal of Jin Z K, Qi C W, Xue J Q, et al. Sedimentary facies of the Jurassic in the Jianghan Petroleum Institute. 2003. 25(4): 33-35 (in Chinese) northern margin of Qaidam Basin. Journal of Palaeogeography. 2006. Yu G Y. Respect and prospect of the wetland research in China. World 8(2): 199-209 (in Chinese) Sci-Tech R & D. 2000. 22(3): 61-66 (in Chinese) Jin Z K, Su N N and Wang C S. Controlling factors of reservoir of Zen g Y F and Xia W J. Sedimentary Petrology. Beijing: Geological coal rocks in Carboniferous-Permian, North China. Acta Geologica Publishing House. 1986 (in Chinese) Sinica. 2008. 82(10): 1323-1329 (in Chinese) Zha ng Y M, Zhao S D and Guo R C. Global wetlands: Status and trends, Jin Z K, Wang C S, Wang P, et al. Controlling factors of reservoir ability scenarios and response options. Advances in Earth Science. 2008. of Carboniferous and Permian coal rocks, North China. Petroleum 23(4): 415-420 (in Chinese) Exploration and Development. 2007. 34(5): 534-540 (in Chinese) Zha o C L and Zhu X M. Sedimentary Petrology (Third Edition). Beijing: Jin Z K, Zhang X X, Zou Y R, et al. Quantitative study of sedimentary Petroleum Industry Press. 2001. 229-233 (in Chinese) facies of the lower part of Xiaganchaigou Formation of Paleogene of Shaxi Oilfi eld in Qinghai Province. Journal of Palaeogeography. (Edited by Hao Jie)

Journal

Petroleum Science – Springer Journals

Published: Nov 26, 2009

There are no references for this article.