He Youbin, Luo Jinxiong, Wen Zhan

School of Geosciences, Yangtze University, Jingzhou 434023, China

1 Regional and geological setting*

The Middle and Upper Yangtze Region includes Yunnan, Guizhou, Sichuan, Chongqing, Shaanxi, Hubei, Hunan, and Jiangxi Provinces, and is namely, the geographical position of 102°-116° E, 26°-33° N, a total area of about 50×104km2.The southwest boundary of the study region is Baoxing-Yingjing-Zhaojue-Liupanshui, the northwest boundary is Baoxing-Guangyuan-Hanzhong,the north boundary is Hanzhong-Xiangfan-Jiujiang, and the southeast boundary is Xiushui-Yueyang-Changde-Jishou-Guiyang.The range of the study region is almost the same as the Yangtze block (Maet al., 2004).

The distribution of the Permian System of South China is extensive, and the thickness ranges from 400 m to 800 m.Based on detailed investigation and study, the Middle Permian Qixia Formation, Maokou Formation and the Upper Permian Wujiaping Stage and Changxing Stage can be found in the Middle and Upper Yangtze Region, while the distribution of the Lower Permian is rather limited to Majiaba of the Jiangyou area in northwest Sichuan (Liet al.,2005).The thickness of the Changxing Stage in the study region varies from 7.5 m to 503 m.The Changxing Stage conformably overlies older strata, whereas its contact with overlying strata is marked by a parallel unconformity.

The Changxing Stage in the study region mainly includes the upper part of the Xuanwei Formation, the Wangjiazai Formation (Tian and Zhang, 1980), the Xingwen Formation (Yanget al., 1986), the Changxing Formation, the Dalong Formation and others.Different viewpoints about the Wangjiazai Formation and the Xingwen Formation have been proposed: (1)the two formations should not be adopted because they are not lithostratigraphic units (Guizhou Bureau of Geological and Mineral, 1997; Sichuan Bureau of Geological and Mineral, 1997).(2)their sedimentary environments are typical coal-bearing deposits of the alternating marine and continental facies whose characteristics are neither different from the Changxing Formation,the Dalong Formation nor the Upper Xuanwei Formation.The Changxing and Dalong Formations were deposited in marine sedimentary environments, and the Upper Xuanwei Formation belongs to the coal-bearing continental facies.The Wangjiazai Formation is similar to the Xingwen Formation in respect of lithology, lithofacies and biota,therefore, both formations should be classified as the same lithofacies.Moreover, the Wangjiazai Formation should be adopted as the formation name according to the naming priority principle (Zhu and Wu, 1988), and thus the term Wangjiazai Formation is used in this paper.The subdivision and correlation of the Upper Permian Changxing Stage in the study region are shown in Table 1.

2 Methodology

Based on petrological study, according to single factor analysis and multifactor comprehensive mapping methods(Feng, 1977, 1992, 2004, 2009), the quantitative lithofacies palaeogeography of the Upper Permian Changxing Stage in the Middle and Upper Yangtze Region was studied.

Single factor analysis means the factor can independently reflect some characteristics of sedimentary environments of a stratigraphic interval of a section in an area.In this research, eight single factors were chosen, including thickness, content of marine rocks, content of shallow water carbonate rocks, content of bioclasts with limemud matrix, content of bioclasts with sparry cement, distribution of reefs, content of thin-bedded siliceous rocks and content of deepwater sedimentary rocks.The presence or absence as well as the high or low percentage of single factors can all reflect some characteristics of sedimentary environments of a stratigraphic interval of a section in an area, both independently and quantitatively (Feng, 2009).

This method includes three steps: First, in-depth petrological and lithofacies study of each section, especially the fundamental sections, were made in order to obtain various reliable firsthand qualitative and quantitative data, and to understand the characteristics of sedimentary environments of each interval of each section.

In the course of the research, the outcrop sections were divided into three classes, and the drilling profiles were divided into two classes.The first class of outcrop sections, namely the fundamental sections, are the sections measured by the authors.The petrological study of these sections is detailed.The second class outcrop sections refer to the sections of partial quantitative single factors; the third class outcrop sections refer to the sections whose data only include thickness and brief lithological data.The first class drilling profiles are the profiles whosedata are rather complete; the second class drilling profiles refer to the profiles whose data only include thickness and brief lithological data.The third class outcrop sections and the second class profiles mainly act as reference sections.In this research, 12 first class outcrop sections,14 second class outcrop sections, 262 third class outcrop sections, 23 first class drilling profiles and 88 second class drilling profiles were chosen.

Table 1 Correlation of the Changxing Stage in the Middle and Upper Yangtze Region

Second, according to the mapping unit and the quantitative data of each section, the percent content of various single factors of each mapping interval of each section in the area was made.Based on these data, various quantitative single factor maps, mainly various isoline maps, were composed.

Third, through the superimposition and synthesis of these quantitative single factor maps, combined with other quantitative and qualitative data, with the dross discarded and the essential selected processes, with the false eliminated and the true retained quantitative lithofacies palaeogeographic maps of the interval of the area were composed.The core of this methodology is quantitative, or, based on the quantitative single factor data of each section (such as the Daxiakou section of Xingshan in western Hubei, Luoet al., 2009).Starting from the analysis of each quantitative single factor maps, through the superimposition and comprehensive analysis and judgment of these single factor maps, the quantitative lithofacies palaeogeographic maps can be composed.In such palaeogeographic maps,the division and determination of each palaeogeographic unit is based upon accurate quantitative data and single factor analysis.

3 Petrological and palaeontological characteristics

3.1 Petrological characteristics

The Changxing Stage is widely developed and well outcropped in the study region, and is mainly composed of carbonate rocks; clastic rocks and siliceous rocks also occur, along with rare coal and pyroclastic rocks.Carbonate rocks are mostly limestones.According to structure, composition and genesis, several types of the Changxing Stage rocks in the study region can be distinguished (Table 2).

Lithofacies of the Changxing Stage mainly includes:clastic rock assemblage, clastic rock-carbonate assemblage, limestone assemblage, limestone-siliceous rock assemblage, and siliceous rock-clastic rock assemblage.Clastic rocks include the upper part of the Xuanwei Formation dominated by continental coal-bearing clastic rocks(Fig.1a), and the Dalong Formation dominated by marine clastic rocks (Fig.1g).Clastic-carbonate consists of the Wangjiazai Formation, which is dominated by oil-bearing clastic rocks of alternating marine and continental facies(Fig.1b, Fig.1c).Limestone consists of the Changxing Formation, which is dominated by medium- to thick beddedblocky limestone (Fig.1d).Carbonate-siliceous rock type is the Dalong Formation, which is dominated by medium- to thick-bedded limestone and thin-bedded siliceous rock(Fig.1e).Siliceous rock-clastic rock type is the Dalong Formation, which is dominated by thin-bedded siliceous rock and shale (Fig.1f).

The Xuanwei Formation is mainly developed in the Yingjing-Yanjin-Yiliang area from the eastern Yunnan Province to the western Sichuan Province, and the upper part of the formation is equivalent to the Changxing Stage.The lithology is dominated by sandstones and mudstones intercalated with coal of the continental facies.The number of coal layers is more than 30, but the coal layers are generally thin.Some conglomerates and siltstones also can be found.The plant fossilGigantopterisoccurs, and marine animal fossils can be found occasionally.

The Wangjiazai Formation occurs in the Baoxing-Leshan-Zhenxiong-Liupanshui area.It represents the strata of alternating marine and continental facies, which is dominated by clastic rock intercalated with coal and limestone.The plant and animal fossils, such asGigantopteris,Ullmannia,Palaeofusulina, are found in this formation.The thickness of the unit is stable, generally ranging from a few tens of meters to 100 m.From west to east, the clastic rock content and the coal layers content decreases, whereas the limestone content increases gradually.

The Dalong Formation is found in the northern study region and western Hubei Province.This formation is composed of grey to dark grey thin-bedded siliceous rock,siliceous shale, mudstone (shale), calcareous mudstone(shale), and is intercalated with thin- to medium-bedded micritic limestone locally.In the west of the Xingshan-Xianning area, the thin-bedded siliceous rock is rarely developed, whereas the black calcareous mudstones and shales are developed abundantly (Fig.1f), and is intercalated with the thin-bedded dark grey micritic limestone.Fossils such as ammonites, brachiopods, bivalves, radiolaria and foraminifera occur in the formation, and the ammonites and radiolaria can be found commonly.The ammonites in this formation are mainlyPseudotirolitesandPseudogastrioceras.The thickness of the formation varies from 10 m to 70 m, and the average is about 30 m.

Fig.1 Lithologic sections of the Changxing Stage in the Middle and Upper Yangtze Region.a-f-Field outcrop profiles; a, c-after Yang et al.(1986), b-after Tian et al.(1980); d-f-from the measured prof iles in this study; g-Drilling profile.

The Changxing Formation is mainly composed of grey and dark grey medium- to thick-bedded or massive bioclastic-micritic limestone, biohermal limestone, fine to medium crystalline dolomite and limey dolomite.In addition, nodular, lump-shaped and banded siliceous rocks can also be found.The Changxing Formation contains abundant fossils such as algae, foraminifers, brachiopods,echinoderms, sponges, corals, gastropods, bivalves and bryozoans.In the eastern Chongqing, western Hubei, and eastern Guizhou Province, the bottom of the Changxing Formation consists of grey to black thin-bedded siliceous rock and siliceous limestone, with several meters to more than ten meters in thickness;Pseudotirolites,Pseudogastrioceras,Leptoduscan be observed.The stratum thickness of the Changxing Formation ranges from tens of meters to more than 500 m, and is usually tens of meters to over 100 m.

3.2 Palaeontological characteristics

The study of fossils plays an important role in analyzing sedimentary microfacies, especially for carbonate rocks, because the formation of the carbonate rock is af-fected by biology directly or indirectly (Flügel, 2004).The Changxing Stage in the study region contains a rich fossil assemblage, including algae, foraminifera, brachiopods,sponges, corals, echinoderms, gastropods, bivalves, ostracods, bryozoans, trilobites, ammonites and higher plants,most of which accumulated in situ.The higher plant fossils, which belong to theGigantopterisFlora, are the important evidence of continental deposits.Four fundamental ecological types in the study region are recognized:marine-benthos, swimming, zooplankton, and terrestrial plant; as well as five fossil assemblages, including plant assemblage, plant andPalaeofusulinaassemblage, algalforaminiferal-brachiopod assemblage, sponge assemblage and ammonite-radiolarian-sponge spicule assemblage.Each assemblage represents a different sedimentary environment (Table 3).

Table 2 Rock types and distribution of the Changxing Stage in the Middle and Upper Yangtze Region

Table 3 The fossil assemblages of the Changxing Stage in the Middle and Upper Yangtze Region

4 Single factor maps

In this study, single factors include thickness, contents of marine rocks, shallow water carbonate rocks, content of bioclasts with limemud matrix, content of bioclasts with sparry cement, distribution of reefs, content of thin-bedded siliceous rocks and content of deep water sedimentary rocks.According to the comprehensive analysis of these single factors, corresponding isoline maps were completed.

4.1 Isoline map of thickness

The thickness of sedimentary layers in some sedimentary areas is mainly controlled by deposition and subsidence rates.The isoline maps of thickness can reflect the distribution and geometrical morphology of the strata during the deposition, as well as the palaeotectonic setting such as relative uplift and sag in the structure.According to the thickness data of 12 first class outcrop profiles, 14 second class outcrop profiles, 262 third class outcrop profiles, 23 first class drilling profiles and 88 second class drilling profiles, the isoline map of thickness of the Changxing Stage in the Middle and Upper Yangtze Region has been completed (Fig.2).The representative section and points are marked on the map (Fig.2; Table 4), others are omitted.

As shown in Figure 2 and Table 4, the thickness of the Changxing Stage in the study area has a large and varying range between 0-503 m, generally between 50-200 m;only two sections are absent (Rikujiao section of Jinyang in Sichuan Province and Haizitou section of Weining in Guizhou Province).The general trend of thickness isolines is northwest-southeast, and from northwest to southeast there exist three thick zones alternating with two thin zones,with distributions as follows: (1)the area of Yiliang-Hezhang-Liupanshui in the utmost southwestern part of the study region, with thickness ranging from 50 m to 100 m, is an area of terrigenous clastic rocks, which may be related to the supply of terrigenous clastics, the relatively high deposition rate and larger subsidence amplitude.(2)the area of Yingjing-Yanjin-Weixin-Xifeng in the southwestern part of the study region, with relatively thin thickness (less than 50 m), is a area dominated by terrigenous clastic rocks, where the deposition rate may be lower.(3)the thickness of the central part of the study region exceeds 50 m in the Cangxi-Dazhu-Dianjiang area and is greater than 200 m in the Tongjiang-Kaixian-Lichuan area, with a thickness of 503 m, reflecting a higher deposition rate and larger subsidence amplitude.The southwest area is thickening slowly while the northeast is thinning rapidly,which may be related to the development of reefs.(4)The northern part, namely, the area of Guangyuan-Zhenba-Chengkou-Wuxi-Nanzhang-Huangshi, whose thicknessis less than 30 m, is an area with low deposition rate; the thickness decreases from over 100 m (and even 200 m in the adjacent area), to less than 30 m, reflecting the possibility of non-compensative starved deposition with lower deposition rate and larger subsidence amplitude.(5)the thickness of the Hanzhong-Xixiang area in the utmost northern part of the study region is generally more than 150 m, and the Liangshan section in Hanzhong of Shaanxi Province, with a thickness of 180 m, may have a relatively high deposition rate and larger subsidence amplitude.

Fig.2 Isoline map of thickness of the Changxing Stage in the Middle and Upper Yangtze Region.

Table 4 Thickness data of the Changxing Stage in the Middle and Upper Yangtze Region

4.2 Isoline map of content (%)of marine rocks

The limestones, dolomites and siliceous rocks of the Changxing Stage are marine in origin while the siliciclastic rocks are either continental or marine rocks.According to the content (%)data of marine rocks in the 12 first class outcrop profiles, 14 second class outcrop profiles, 146 third class outcrop profiles, 20 first class drilling profiles and 8 second class drilling profiles, the isoline map of content of the Changxing Stage marine rocks in Middle and Upper Yangtze Region has been completed (Fig.3).Some representative data are shown in Table 5.

As shown in Figure 3 and Table 5, the area of zero content of marine rocks is distributed in a nearly N-S trending narrow zone in Yingjing-Yanjin-Yiliang area in the western part of the map, where the zero isoline is along the line of Baoxing-Leshan-Muchuan-Hezhang.The content of marine rocks in the western area from the line is zero,while the content of marine rocks increases to 75% quickly and then increases to 100% about 30 km eastward of the line.This phenomenon indicates that the transgression was very rapid from east to west, and the transition zone between the land and sea was narrow.The zero content of marine rocks is used to define the boundary between continent and sea of the Changxing Stage in the study area.

4.3 Isoline map of content (%)of shallow water carbonate rocks

There are two types of carbonate rocks in Changxing Stage in the study region.One is developed in the shallow water carbonate platform, and the other is in the deep water basin and slope; their lithological characteristics and biological fossils are inconsistent with each other.Shallow water carbonate rocks include the medium- to thick-bedded massive limestone and reef limestone.Bioclasts abound in these limestones and fossil assemblages are composed ofthe algal-foraminiferal-brachiopod assemblage and the sponge assemblage respectively (Table 3).The area where the content of the shallow water carbonate rocks is not less than 50% is defined as the carbonate platform in the study.According to the content data of shallow water carbonate rocks of 12 first class outcrop profiles, 14 second class outcrop profiles, 166 third class outcrop profiles, 23 first class drilling profiles and 35 second class drilling profiles, the isoline map of content of shallow water carbonate rocks of the Changxing Stage in Middle and Upper Yangtze Region was completed (Fig.4).Some representative section data are shown in Table 6.

Fig.3 Isoline map of marine rock content (%)of the Changxing Stage in the Middle and Upper Yangtze Region.

Table 5 Content (%)data of marine rocks of the Changxing Stage in the Middle and Upper Yangtze Region

Figure 4 and Table 6 show that the shallow water carbonate rocks are mainly distributed in the Sichuan-Guizhou-Chongqing area and Xingshan-Yichang-Yueyang-Xiushui area as well as the Hanzhong-Xixiang area.Thecontent of the shallow water carbonate rocks in the central-western part of the study region, whose eastern boundary is along the Baoxing-Renshou-Weiyuan-Xingwen-Weixin-Bijie-Nayong-Zhijin, and the western boundary is along the Jiange-Yilong-Liangping-Wuxi-Lichuan-Laifeng, is not less than 50%.In addition, there are two areas, Xingshan-Yichang-Yueyang-Xiushui area and Hanzhong-Xixiang area, whose content of the shallow water carbonate rocks is more than 50%.This phenomenon indicates the characteristics of the shallow water carbonate platform in these areas.

Fig.4 Isoline map of content (%)of shallow water carbonate rocks of the Changxing Stage in the Middle and Upper Yangtze Region.

4.4 Isoline map of content (%)of bioclasts with limemud matrix

The bioclasts whose pores are filled completely with limemud matrix or mostly with limemud matrix are called bioclasts with limemud matrix.Miscellaneous grain size,no erosion phenomena and autochthonous burial are the main characteristics of the bioclasts of the Changxing Stage; and the limemud matrix filled in the bioclasts indicates that the hydrodynamic energy was too weak to winnow the limemud during deposition.According to the content data of bioclasts with limemud matrix of the 11 first class outcrop profiles, 10 second class outcrop profiles, 15 third class outcrop profiles, 6 first class drilling profiles and 1 second class drilling profiles (Table 7), the isoline map of content of bioclasts with limemud matrix of the Changxing Stage in the Middle and Upper Yangtze Region was constructed (Fig.5).The isoline ranges are amplified in order to display the contents of bioclasts with limemud matrix and contents of bioclasts with sparry cement more clearly.

As seen in Figure 5 and Table 7, the area where the content of bioclasts with limemud matrix is more than 50% includes Tongkou of Beichuan County, Well Rong 3 of Hejiang county, Songkan of Tongzi County, Jianshan of Wuxi County, Daxiakou of Xingshan County, Well Liangxiang 1 of Liangping County and Tuheba of Xishui County; the content reaches up to 63.8% in Songkan of Tongzi County;the content lies between 30% and 49% in the area of Liziya of Huayingshan, Yuelaichang of Puling County, Pengshui, Honghua of Kaixian County, Lishihe of Xuanhan County and Huangnitang of Lichuan County; except for the above-mentioned areas, the contents of the bioclasts with limemud matrix are almost all less than 30% in the study region.The area with high content indicates that itssedimentary environment is shallow water open platform where the sea had normal salinity, good water circulation and high biological productivity; however, because of the relatively lower hydrodynamic energy, its intergranular space is filled with limemud.The environments where the bioclasts were enriched should be organic banks, these areas were of the low energy, so they are named low energy organic bank.In this paper, the areas which contain no less than 30% of bioclasts with limemud matrix in the shallow water carbonate platform are defined as the low energy organic bank.

Table 6 Content (%)data of shallow water carbonate rocks of the Changxing Stage in the Middle and Upper Yangtze Region

Table 7 Content (%)data of bioclasts with limemud matrix, and of bioclasts with sparry cement of the Changxing Stage in the Middle and Upper Yangtze Region

Fig.5 Isoline map of content (%)of bioclasts with limemud matrix, content (%)of bioclasts with sparry cement of the Changxing Stage in the Middle and Upper Yangtze Region.

4.5 Isoline map of content (%)of bioclasts with sparry cement

The bioclasts whose pores are filled completely or mostly with sparry cement are termed bioclasts with sparry cement.The high content of bioclasts with sparry cement indicates that the hydrodynamic energy during deposition was strong enough to winnow the limemud between the bioclasts, which provides the premise for the later precipitation of sparry cement.According to the content data of bioclasts with sparry cement of 12 first class outcrop profiles, 9 second class outcrop profiles, 2 third class outcrop profiles and 5 first class drilling profiles (Table 7), the isoline map of content of bioclasts with sparry cement of the Changxing Stage in the Middle and Upper Yangtze Region was complied and is shown in Figure 5.

From Figure 5 and Table 7, it can be seen that the area with more than 30% contents of bioclasts with sparry cement only occurs in the Xintan of Zigui County, and indicates shallow water and the high energy during deposition.The content is lower in other areas, generally about 10%, or none.This phenomena shows that hydrodynamic conditions are weak, and the energy is low in most of the study region except the Xintan area in Zigui County.The area where the content of bioclasts with sparry cement is not less than 30% is defined as the high energy organic bank in the shallow water carbonate platform.

4.6 Distribution map of reefs

As an important type of carbonate deposition, reefs are the product of the influence of the reef-building organism and the specific environment.Reefs are developed in the tropical shallow sea where there is good seawater transpar-ency, good water circulation, sufficient sunshine, enough oxygen, normal salinity and moderate wind waves.Reefs can be generated in the margin and inner part of the carbonate platform.Based on field observations, and combined with some corresponding geological data (Table 8)(Qianget al., 1985; Liu, 1987; Zhanget al., 1990; Wang and Qiang, 1992; Shen and Yang, 1995; Fan, 1996; Wanget al., 1998, 2008; Zhuet al., 1999; Yanget al., 2002; Mouet al., 2004; Maet al., 2006), the distribution map of reefs of the Changxing Stage in the Middle and Upper Yangtze Region was completed (Fig.6).

As shown in Figure 6 and Table 8, the reefs are distributed in at least 56 places, among which 19 places are in the outcrop and 37 places are in the down well.Reefs are mainly distributed in the geographical position of 106°-110° east longitude and 29°-33° north latitude, including eastern Sichuan, eastern Chongqing and western Hubei,where the strata are all thick.The reefs distribution exhibits regularity, and can be divided into four zones: (1)the first zone is located in Jichang-Kaixian area; (2)the second is the area of Nanjiang-Tongjiang-Kaijiang-Wanzhou-Lichuan; (3)the third is the area of eastern Jiange-northern Cangxi-Yilong-Liangping; (4)the last one is Huaying-Dianjiang-Pengshui area.The four zones are approximately arranged in parallel in a N-W direction, and the second and the third zones converge into one in Lichuan.In addition, a few reefs are also distributed in Changping of Cili County of Hunan Province, Yushan of Chongyang County of Hubei Province, and Qingshuiyan of Xiushui County of Jiangxi Province.Moreover, the reefs in Changping, Yushan, Qingshuiyan and reefs in the first to third zones are distributed next to the area where the strata thickness decreases rapidly.This study proves that the reefs migrated from east to west gradually with the Changxing Age transgression (Wanget al., 1998, 2008).

4.7 Isoline map of content (%)of deep water sedimentary rocks

Deep water deposits in this paper refers to sediments deposited on the slope and in basin environments, which can be judged by the comprehensive analysis of lithology, sedimentary structures and palaeoecology.The deep water sedimentary rocks of the Changxing Stage are mainly composed of thin-bedded siliceous rock, siliceous mudstone (shale), dark medium- to thin-bedded micritic limestone and mudstone (shale).The fossils consist of ammonites, radiolarians, brachiopods, bivalves, sponges and so on; the brachiopods and bivalves are small.The fossil assemblage is an ammonite-radiolarian-sponge assemblage.According to the content data of the deep water sedimentary rocks of 12 first class outcrop profiles,14 second class outcrop profiles, 158 third class outcrop profiles, 23 first class drilling profiles, and 25 second class drilling profiles, the isoline map of content of deep watersedimentary rocks of the Changxing Stage in Middle and Upper Yangtze Region was completed (Fig.7).Some data of representative profiles is shown in Table 9.

Fig.6 Distribution of reefs of the Changxing Stage in the Middle and Upper Yangtze Region.

Table 8 Distribution of reefs of the Changxing Stage in the Middle and Upper Yangtze Region

As shown in Figure 7, the deep water sedimentary rocks are mainly distributed in the area of Guangyuan-Wangcang-Kaijiang-Liangping located in the north of the study region, the area from the north of Nanjiang-Zhenba to the south of Hanzhong-Xixiang, the area of Chengkou-Wuxi-Xingshan-Jingzhou-Xianning-Ruichang,and the area of Wuxi-Jianshi-Enshi-Sangzhi.The content of deep water sedimentary rocks increases progressively from zero to 100% in these areas, while in the area of Sichuan-Guizhou-Chongqing and the eastern Yunan Province, as well as the area of Xingshan-Yichang-Shimen-Yueyang-Xiushui, the content is lower than 10% or even zero.The content of deep water sedimentary rocks increases with the decrease of the content of shallow water sedimentary rocks (including carbonate and siliciclastic rocks).The area where the content of deep water sedimentary rocks is not less than 65% is defined as the basin; the area in which the content of deep water rocks is not more than 35% is defined as areas of shallow deposi-tion.The area where the content is between 35%-65% is defined as the slope if evidence supports this interpretation, such as debris-flow deposits and turbidity current deposits; otherwise, the area is defined as the transition zone if evidence does not.

Table 9 Content (%)data of deep water sedimentary rocks of the Changxing Stage in the Middle and Upper Yangtze Region

4.8 Isoline map of content (%)of thin-bedded siliceous rocks

Thin-bedded dark grey-grey black-black siliceous rocks are mainly distributed in the Dalong Formation and the Changxing Formation.The thin-bedded siliceous rocks generally coexist with mudstone (shale),and siliceous mudstone (shale), forming rhythmic stratification, in which the micro-horizontal laminations are well developed.Sometimes thin-bedded siliceous rocks also coexist with the thin-to medium-bedded limestone or lenticular limestone.Ammonites and radiolarians are the main fossils found in the thin-bedded siliceous rocks,with small quantities of brachiopods, bivalves and ostracods, with small bodies and thin shells.Thus, the siliceous rocks were the products of a deeper-water anoxic sedimentary environment (Luo and He, 2011).The area with high contents of thin-bedded siliceous rocks reflects the deep water sedimentary environment.According to the content data of thin-bedded siliceous rocks of 12 first class outcrop profiles, 14 second class outcrop profiles, 163 third class outcrop profiles, 23 first class drilling profiles, and 13 second class drilling profiles, the isoline map of content of thin-bedded siliceous rocks of the Changxing Stage in Middle and Upper Yangtze Region was completed (Fig.8).Some representative profile data are shown in Table 10.

As shown in Figure 8 and Table 10, thin-bedded siliceous rocks in the study region are mainly distributed in the area where the thickness is typically less than 30 m,including the area from the north of Guangyuan-Zhenba to the south of Hanzhong-Xixiang, the area of Chengkou-Nanzhuang-Wuhan-Ruichang, the area of Jianshi-Wufeng and the area of Enshi-Sangzhi.The content of thin-bedded siliceous rocks is usually between 10%-40%,while it is more than 50% locally, such as the content of rocks in Zhaolaihe in Changyang of Hubei Province, Zoumaping in Sangzhi of Hunan Province, Fenghuang Mountain in Daye of Hubei Province and Ruicang of Jiangxi Province.Their distribution is similar to the distribution of deep water sedimentary rocks, only with smaller ranges,reflecting these areas that were deposited in the deeper water sedimentary environments where thin-bedded rocks were deposited.Besides, the content of thin-bedded siliceous rocks is between 10%-30% in the areas of Shaziin Yanhe, Magou in Zunyi, Gamu in Dafang and Jiaozishan in Anshun of Guizhou Province, indicating that the deep water deposition was well developed in these areas of Changxing Stage.

Fig.8 Isoline map of content (%)of thin-bedded siliceous rocks of the Changxing Stage in the Middle and Upper Yangtze Region.

5 Lithofacies palaeogeography of Changxing Stage

5.1 The compilation of lithofacies palaeogeographical maps

Through the superimposition of quantitative single factor maps, combined with other responding data of the Changxing Stage, the quantitative lithofacies palaeogeographical maps of the Upper Permian Changxing Stage were accomplished.First, according to the isoline map of content of marine rocks, two palaeogeographic units (land and sea)are distinguished by taking the zero line as the boundary.Second, in the continental facies, the area with the zero isoline of the thickness is defined as the erosion area, and the area with the isoline of a thickness of more than zero is defined as the alluvial plain.Thirdly, in the marine facies area, the areas where the content of the shallow water carbonate rocks is not less than 50% are defined as the carbonate platform.The areas where the content of deep water sedimentary rocks is less than 35%, where the content of shallow water carbonate rocks is less than 50%,and where the content is mainly composed of siliciclastic rocks are all defined as the siliciclastic platform.The area where the content of deep water sedimentary rocks is less than 65% is defined as basin, whereas the area with the content between 35%-65% is defined as slope area (or transition region).Fourthly, on the carbonate platform, the area where the content of bioclasts with sparry cement is not less than 30% is defined as the high energy organic bank, while the area where the content of bioclasts with limemud matrix is not less than 30% is defined as the low energy organic bank.According to their distribution, reefs can be divided into the platform marginal reef and the intraplatform point reef.Fifthly, the basin can be defined as siliceous rock basin and mudstone basin according to the content of the thin-bedded siliceous rocks and mudstones in the basin.Therefore, the lithofacies palaeogeographical maps of the Upper Permian Changxing Stage in the Middle and Upper Yangtze Region can be completed (Fig.9).

The distribution of palaeogeographic units shows that some of them appear in the following forms from west to east (Fig.9): Kangdian land (including eastern Yunan denudation area and western Sichuan-eastern Yunnan alluvial plain), Sichuan-Yunnan-Guizhou clastic platform, Mid-dle and Upper Yangtze carbonate platform, northern slope area, Guangyuan-Liangping basin, western Hubei basin,northern basin, southern Shaanxi slope and Hanzhong carbonate platform.In addition, some secondary palaeogeographic units such as low energy organic bank, high energy organic bank and reefs developed in the carbonate platform are also defined.

Table 10 Content (%)data of thin-bedded siliceous rocks of the Changxing Stage in the Middle and Upper Yangtze Region

Fig.9 Palaeogeographical map of the Upper Permian Changxing Stage in the Middle and Upper Yangtze Region.

5.2 The characteristics of each palaeogeographic unit

5.2.1Kangdian land

Kangdian land includes the eastern Yunnan denudation area and the western Sichuan-eastern Yunnan alluvial plain.

The distribution of the eastern Yunnan denudation area is very limited during the Changxing Stage in the study region, and mainly located in Rikujiao of Sichuan Province and Haizitou of Guizhou Province, which are both in the northwest of the study region.If the study region extends westward, the distribution range of denudation area may be larger.

Western Sichuan-eastern Yunnan alluvial plain is located in Yingjing-Yanjin-Yiliang area, in an NNW direction, which is the main distribution of the Upper Xuanwei Formation.The alluvial plain usually developed deposits such as the meandering river and the alluvial fan.The main rock types of the alluvial plain include grey-yellow sandstone, siltstone, mudstone and the coal layers (Figs.1a,1b).The thickness of the coal layer is relatively thin or the layer appears as the coal steak.Sometimes conglomerates can also be found, which generally is the product of braided channels of fluvial fan and the fluvial-channel lag deposits of meandering rivers.Medium to fine cross-beddings and parallel-beddings can be found in sandstones and siltstones, while the scouring surface structure can be found commonly at the base of the sandstone, which was generated in the shallow water with strong hydrodynamic conditions.In addition, plant root fossils and siderite nodules can be found frequently in mudstones.

5.2.2Sichuan-Yunnan-Guizhou clastic platform

The Sichuan-Yunnan-Guizhou clastic platform is located in Baoxing-Leshan-Zhenxiong-Liupanshui area,extending in a long NNW strip.Clastic platform occurred under marine and continental deposition where sedimentary facies includes meandering river delta, onshore and tide, and the carbonate platform is developed locally.Their rock types are mostly composed of terrigenousclastic rocks, but the medium-to thick-bedded limestone can also be found (Figs.1b, 1c).Clastic rocks consist mainly of siltstones, mudstones, sandstones, coal layers and minor conglomerates.Rhythmically alternating thin layers of siltstones and mudstones can be recognised,in which the typical wavy- and lenticular-bedding, as well as the horizontal-bedding are well developed.Basalt lithic sandstones are generated in the subtidal zone,in which the flaser cross-bedding and the herringbone cross-bedding are developed.From west to east, the content of clastic rocks decreases gradually, while the content of carbonate rocks increases.The coal layer, whose depositional environment is the coastal swamp, is mainly located in Liupanshui of western Guizhou Province,Zhenxiong and Weixin of eastern Yunnan Province, as well as the southern Sichuan Province, and the thickness of coal layers thins from west to east.

5.2.3Carbonate platform

There are three carbonate platforms in the study region,the Sichuan-Guizhou-Chongqing carbonate platform,the Xingshan-Shimen-Xiushui carbonate platform and the Hanzhong carbonate platform.The first two carbonate platforms are integrated into single platform, which is named the Middle and Upper Yangtze carbonate platform.This platform is peninsula-shaped in northeastern Sichuan, eastern Chongqing and western Hubei Province separately.The carbonate platform is mainly composed of grey to dark-grey medium- to thick-bedded massive limestone (Figs.1d, 1e).Clastic rocks, whose content is less than 50%, can be found in the transitional zone of the west of Middle and Upper Yangtze carbonate platform and the Sichuan-Yunnan-Guizhou clastic rock platform.

Bioclasts are well developed in the carbonate platform,generating 13 low energy organic banks and one high energy organic bank.As shown in Figure 9, the organic banks are distributed in the parallel beach belts: the first belt is located in the line of Lishihe-Honghua-Jianshan-Daxiakou; the second belt traverses from Well Liangxiang 1 in Liangping to Huangnitang in Lichuan; the third belt is located in the line of Tongkou-Liziya-Yuelaichang-Pengshui; the fourth belt is located in the line from Well Hejiangrong 3 in Hejiang to Tuheba in Xishui.The parallel distribution of these beach belts reflects that the development sites of reef beach have a tendency of migrating toward land during the transgression of the Late Permian Changxing Stage.

There are at least 56 Changxingian reefs in the study region.Eastern Sichuan, eastern Yunnan and western Hubei are the most developed areas where the reefs spread parallel along the near NW direction, reflecting that reefs have a tendency of migrating toward land with the Upper Permian Changxing Stage transgression.Reefs can be divided into the marginal platform reef and the internal platform reef.The reef-building organisms mainly include calcareous sponges and sphinctozoans, with minor inozoans.The ratio of reef-building organisms in eastern Sichuan and western Hubei is about 5:1 (Zhuet al., 1999).The sponge reef limestones can be divided into the reef limestones cemented by sparite and the reef limestones filled with limemud.In the sponge reef limestones cemented by sparite,the reef-building organisms have cyanobacterial coatings and are filled with the sparry calcite.In the sponge reef limestones filled with limemud, the reef-building organisms have almost no blue-green algal coatings, and are filled with fossils of organisms attached to the reef and the micritic calcite.In Cili area of Hunan Province, the reefbuilding organisms are mainly the calcareous sponges and corals, which can build the reefs separately.The corals are typicallyWaagenophyllumandLiangshaophyllum, and mostly preserved in situ.The coral bundle starts up with a fan-shape in the appearance.The height of the large one is more than 1 m, while the small is 30-50 cm.The cores of the coral reefs are mostly dolomitized.The rocks covered by the coral bundle generate the coral reefs with thicknesses of more than 50 m (Shen and Yang, 1995).Besides, the common reef-building organisms include tabulozoans and bryozoans; and the organisms attached to the reef mainly include algae, foraminifera, brachiopods, gastropods, bivalves and echinoderms.

5.2.4Basin

The basin is a siliceous mudstone basin located in the northern part of the study region, including Guangyuan-Liangping basin, western Hubei basin and northern basin.The rocks are mainly composed of mudstone, siliceous mudstone, shale, siliceous shale and thin-bedded siliceous rock, intercalated with the dark grey-grey black thin- to medium-bedded micritic limestones (Figs.1e, 1f).The Xingshan-Jingzhou-Xianning area is dominated by black calcareous mudstone and shale intercalated with dark grey thin-bedded micritic limestone, while the thin-bedded siliceous rock is not developed (Fig.1g).Two types of biological fossils are common in the basin: one is planktonic or nektonic large fossils, including ammonites, small, thin-shelled brachiopods, ostracods,bivalves and foraminifera, which are more common in the study region; the other one is plankton microfossilsinclude ammonite and radiolarian, with radiolarians the main type.There are many differences between the trace element content of rocks deposited in basin and the content of rocks from the carbonate platform.The content of Sr, Ba, Cd, Co, Cr, Ni, V, Zn, Ti in the basin is higher than the ones in the carbonate platform, indicating the anoxic character of the sedimentary environment (Luo and He, 2011).

5.2.5Slope (transition zone)

The slopes located along the north and the south sides of the basin are called the northern slope and the southern Shaanxi slope, respectively.Debris flow deposits and turbidity current deposits are developed to the east of the reefs in Lichuan of western Hubei and to the northwest of the reefs in Cili of northwestern Hunan.The slope deposits can be found near Guangyuan (Wanget al., 1994); and they are developed in the transition zone between carbonate platform and basin of Changxing Stage in the study region.The slope surrounds the platform, and extends from Jiange of northwestern Sichuan to De’an of Jiangxi.The rock type is dominated by thin- to medium-bedded micritic limestone, siliceous mudstone and shale, medium-to thick-bedded limestone, thin-bedded siliceous rock and mudstone.The slope may be also developed between the Hanzhong carbonate platform and the northern basin.

6 Relationship between the lithofacies palaeogeography and the oil and gas

There are important marine hydrocarbon-bearing strata in the Permian of South China, in which a quantity of hydrocarbons and hydrocarbon reservoirs can still be found at present.A preliminary analysis of the source and reservoir condition was accomplished on the basis of the lithofacies palaeogeography.

6.1 Hydrocarbon source rock conditions

Carbonate rocks, mudstones and coal are recognized as the hydrocarbon source rocks in the Permian of South China.Hydrocarbon source rocks of Changxing Stage in the study region are mainly composed of the dark carbonate rocks of the Changxing Formation, and mudstones,shales, micritic limestones, thin-bedded siliceous rocks of the Dalong Formation.

The content of organic carbon is one of the important parameters to analyze hydrocarbon source rocks, though some scholars do not approve that the content is a good index for measuring the hydrocarbon generating quantity(Dembicki, 2009).The analysis of the hydrocarbon contents of 35 samples from the Changxing Stage in the study region shows that (1)the hydrocarbon samples in the basin are good-quality source rocks, whose contents are 0.12%-12.84%, with a mean value of 4.30%; (2)the contents of the hydrocarbon samples in the carbonate platform are 0.07%-3.21%, with a mean value of 0.48%.The hydrocarbon content in the basin is one order of magnitude higher than it in carbonate platform, indicating that the environment indeed controls the development of the hydrocarbon source rock.

There has been less study of the hydrocarbon source rock conditions of the thin-bedded siliceous rocks in the Dalong Formation before.The content of three samples from the Jingshan area of Hubei varies from 0.66% to 0.86%, and the average value reaches 2.78% according to the analysis of six samples from the Dalong Formation in Shangshi of Guangyuan of Sichuan Province (Liet al.,2009).These contents reflect that the thin-bedded siliceous rocks of the Dalong Formation have quite high hydrocarbon generation potential.Therefore, more attention should be paid to these kind of rocks in the future.

6.2 Reservoir conditions

The reef gas reservoirs have been found gradually in the Changxing Formation in Jiannan, Bandong, Shuanglong,Shibaozai and Puguang areas in the study region, and great discoveries and achievements have been obtained in Yuanba and Longgang areas recently, illustrating that reservoirs of the Changxing Stage are dominated by dolostone reefs and reef limestone, and the main reservoir spaces include karst dissolution, intercrystalline solution, intercrystalline and intergranular solution pores.

Prospecting practices state that the Puguang gas field,Longgang area, Yuanba area are located in the platform margin, and the breakthrough has been obtained recently in Longgang and Yuanba areas.The platform margin surrounding the Guangyuan-Liangping Basin rim area is the most favourable region to search for hidden reefs in the Changxing Formation.

7 Conclusions

1)The Upper Permian Changxing Stage is mainly composed of carbonate rocks, while clastic rocks and siliceous rocks are also developed, with a small quantity of coal and pyroclastic rocks.

2)Four fundamental ecological types in the study region,i.e., marine benthos, swimming organisms, zooplank-ton, and terrestrial plant; as well as five fossil assemblages,namely: plant assemblage, plant andPalaeofusulinaassemblages, algal-foraminiferal-brachiopod assemblage,sponge assemblage and ammonite-radiolarian-sponge assemblage are recognized.All of them represent the various sedimentary environments.

3)The distribution of palaeogeographic units is regular.From west to east, they are land (denudation area and alluvial plain), siliciclastic platform, carbonate platform,slope and basin.Some secondary palaeogeographic units such as low energy organic bank, high energy organic bank and reefs are distinguished in the carbonate platform.

4)The sedimentary environment had obvious controls on the development of the resource rocks, and the hydrocarbon condition of the basin environment is far superior to that of the carbonate platform environment; and the high quality source rocks are developed in the Dalong Formation in the study region.The reservoir rock is dominated by the reef facies of the Changxing Stage in the study region, and the platform margin surrounding the Guangyuan-Liangping Basin rim is the most favourable area for the reef reservoirs of the Changxing Formation.

Acknowledgements

This research was supported by the marine petroleum exploration project “Study of the quantitative lithofacies, palaeogeography and petroleum predication of Permian in South China” from China Petroleum & Chemical Corporation (YPH08019)and the twelfth Five-Year Plan of major national science and technology project “Study on accumulation conditions and favorable exploration area evaluation of marine carbonate rocks in South China” (2011ZX05004-001-004).We express our sincerest gratitude to Li Xiangdong, Wang Dan, Liu Na, Wen Jing, Li Weiyang, Zhou Xinping, Du Hongquan, Tian Yu,Li Hua for their help.Senior Engineers Li Tiansheng, Zhu Yonggang, Li Yuegang and Zhang Yinben of the Sichuan Oilfield Company of PetroChina are also acknowledged.Finally we are grateful to Prof.Feng Zengzhao of China University of Petroleum (Beijing)for his suggestion.

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