Stratigraphic division and correlation and sedimentary characteristics of the Cambrian in central-southern Sichuan Basin

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 39, Issue 6, December 2012 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2012, 39(6): 725–735.

RESEARCH PAPER

Stratigraphic division and correlation and sedimentary characteristics of the Cambrian in central-southern Sichuan Basin LI Wei1,*, YU Huaqi1, DENG Hongbin2 1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 2. PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China

Abstract: Under the guidance of sequence stratigraphy and through analyses of large amounts of well and outcrop data, the correspondence of all formations in the Cambrian of southern and central Sichuan Basin is defined and a new scheme of stratigraphic division and correlation for the Cambrian in central-southern Sichuan Basin is proposed based on the characteristics of lithology, logging curves and fossils. The Jiulaodong Formation in southern Sichuan Basin is corresponding roughly to the Qiongzhusi Formation in central Sichuan Basin, the Yuxiansi Formation in southern Sichuan roughly to the Canglangpu, Longwangmiao and Gaotai formations in central Sichuan Basin, the Xixiangchi Group in southern Sichuan roughly to the same group in central Sichuan. The Cambrian in central-southern Sichuan Basin is divided into the Qiongzhusi Formation, Canglangpu Formation, Longwangmiao Formation, Douposi Formation and Xixiangchi Group from bottom to top. On the basis of that, sedimentary facies of the Cambrian was studied. Controlled and impacted by the paleo-uplifts formed by Caledonian Orogeny, the deposition of the Cambrian experienced three evolutionary stages: shore–continental shelf, restricted platform–evaporation flat–shelf lagoon, restricted platform. The sandy, oolitic and carst dolomites in the Longwangmiao Formation and the Xixiangchi Group are the targets of oil and gas exploration. Key words: Sichuan Basin; Cambrian; stratigraphic division and correlation; sedimentary facies; sedimentary evolution

The Cambrian System is completely developed in the Sichuan Basin and buried at depth from 2 000 to 5 000 m. The burial depth is up to more than 9 000 m in sags [1]. Its thickness is from 400 to 1 000 m [2]. Some Cambrian formations crop out at the basin periphery. The Cambrian System is an important target of natural gas expoitation in the Sichuan Basin, from which some wells drilled in Weiyuan area have tapped commercial gas flow, and some other wells drilled in Gaoshiti, Anpingdian, Moxi and Longnüsi of Central Sichuan Basin area also have discovered low gas yield [3]. In order to deepen the understanding to the Cambrian System, this paper mainly presents its sedimentary environment and evolution features based on updated stratigraphic correlation and division.

1

Stratigraphic division and correlation

Researchers have made great efforts on the Cambrian stratigraphic division and correlation in the Sichuan Basin due to its significance to sedimentary environment study, but they have failed to give a consistent solution [48], especially in

Southern and Central Sichuan Basin. There are great divergence not only in stratigraphic nomenclature but also in stratigraphic division. The Cambrian in Southern Sichuan Basin was divided, from bottom to top, into the lower Cambrian Jiulaodong Formation, the lower to middle Cambrian Yuxiansi Formation, and the middle to upper Cambrian Xixiangchi Group [45], whereas the Cambrian in Central Sichuan Basin was divided, from bottom to top, into Qiongzhusi Formation, Canglangpu Formation and Longwangmiao Formation of the lower Cambrian, the middle Cambrian Gaotai Formation, and the middle to upper Cambrian Xixiangchi Group [1, 56]. The authors try to figure out the stratigraphic correspondence between Central Sichuan and Southern Sichuan Basin by using GR (natural gamma-ray) log, Rt (resistivity) log, lithologic features, and palaeontologic features and put forward an updated version of the Cambrian stratigraphic correlation and division in Central Sichuan-Southern Sichuan Basin (Table 1 and Figure 1). In this new version, Jiulaodong Formation in Southern Sichuan Basin basically corresponds to Qiongzhusi Formation in Central Sichuan Basin, Yuxiansi

Received date: 27 Feb. 2012; Revised date: 08 Aug. 2012. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the Science Program of PetroChina Research Institute of Petroleum Exploration and Development “Study on the Formation Conditions and Play Selection of Deep Oil and Gas Reservoirs” (2008KT-01). Copyright © 2012, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

LI Wei et al. / Petroleum Exploration and Development, 2012, 39(6): 725–735

Table 1

Cambrian stratigraphic division and correlation in central and southern Sichuan Basin and its vicinities Stratigraphy

RefeThe middle Cambrian rences The lower Cambrian Series Series

Region The middle to upper Cambrian Series

Longwangmiao Formation Central-Southern Sichuan Basin

Xixiangchi Group

Douposi Formation

Canglangpu Formation

This article

Qiongzhusi Formation Longwangmiao Formation Central Sichuan Basin

Xixiangchi Group

Gaotai Formation

Canglangpu Formation

[5]

Qiongzhusi Formation Southern Sichuan Basin

Xixiangchi Group

Yuxiansi Formation

Southwestern Sichuan Basin

Emei

Jiulaodong Formation

[1]

Longwangmiao Formation

Xixiangchi Group Douposi Formation Erdaoshui Formation

Yuxiansi Formation

Xiwangmiao Formation

Canglangpu Formation Qiongzhusi Formation

[2]

Maidiping Formation Longwangmiao Formation

Xixiangchi Group

Douposi Formation

Xiwangmiao Formation

Yuxiansi Formation

[7]

Jiulaodong Formation Longwangmiao Formation

Xixiangchi Group Eastern Sichuan Basin

Douposi Formation Maotian For- Houba For- Pingjing For- Shilengshui mation mation mation Formation

Canglangpu Formation Qiongzhusi Formation Maidiping Formation Qingxüdong Formation

Loushanguan Group

[8]

Jindingshan Formation

Northern Guizhou Province Douposi Gaotai Zunyi Maotian For- Houba For- Pingjing For- Shilengshui Formation Formation mation mation mation Formation

Mingxinsi Formation Niutitang Formation Gechongwu Formation

Formation in Southern Sichuan Basin to Canglangpu Formation, Longwangmiao Formation and Gaotai Formation in Central Sichuan Basin, and Xixiangchi Group in Southern Sichuan Basin to the same Group in Central Sichuan Basin. In summary, we subdivide the Cambrian System in Central Sichuan-Southern Sichuan Basin into Qiongzhusi Formation, Canglangpu Formation, Longwangmiao Formation, Douposi Formation and Xixiangchi Group from bottom to top. Qiongzhusi Formation ( 1q): entitled by Lu Yanhao in 1941[6], the source section is located in Qiongzhusi at Mount Guan, western suburb of Kunming City, Yunnan Province. Lithology in the lower part of the formation is dark grey to gray black carbonaceous shale and shale with spherical marl concretions and transmits upward to grey to dark grey siltstone and finestone with distinct minute stripy bedding [12, 47]. The lower part contains fossils of mollusks and small shelly animals [910] such as sponge (e.g. Protospongia sp.), sponge spicule (e.g. Chancelloria), and hyolithes (e.g. Turcutheca, Circotheca). The upper part is rich in fossils of arthropoda including trilobite [12, 5, 11] (e.g. Eoredlichia intermedia, Pachyredlichia, Wutingaspos, Parabadielia and Redlichia) and bradoriida (e.g. Meishucunella and Gaoqiaoella). In light

of the high-resolution sequence stratigraphy founded by Cross T A et al [1213], there may be two short-term base level cycles in Qiongzhusi Formation at the palaeohigh in Leshan-Longnüsi area, three short-term cycles in the slope area and four to five short-term cycles in the sag. On electrical logs, GR log features smooth profile and high magnitude which drops down upwards from 100 to 280 API at the lower section to 75 to 100 API at the middle and upper section; Rt log features smooth profile and upward increase from 1 to 100 ·m at the lower section to 30 to 300 ·m at most of the middle and upper section. Canglangpu Formation ( 1c): named by Yu Wenjiang and Wang Yuelun in 1914 and published by Yin Zanxun in 1937, the source section is located between Canglangpu in Malong County and Huangtupo, Yunnan Province [2, 7]. The lithology is dark grey thin shale and alternate layers of silty mudstone, pelitic siltstone and siltstone sandwiched with thin limestone or magenta argillaceous dolomite and thin siltstone in local area [12, 56]. Grain size decreases and red beds disappear gradually from the west to the east. With similar palaeontologic features to the Qiongzhusi Formation, the formation mainly contains marine arthropoda such as trilobite e.g. Mega-

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Fig. 1

Cambrian stratigraphic division and correlation from well Anping1 to well Gongshen1 in central and southern Sichuan Basin

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palaeolenus fengyangensis, Redlichia sp., Megapalaeolenus deprati and M.cf. fengyangensis [1, 14]. There are two short-term base level cycles. GR log assumes sawtooth profile with high magnitude mostly from 75 to 150 API. Rt log also appears as sawtooth shape. Large in magnitude and variation, Rt value may change from 20 to 3 000 ·m. Longwangmiao Formation ( 1l): entitled by Lu Yanhao in 1941, the source section is located near Longwangmiao on the west bank of the Dian Lake, Xishan, Kunming City, Yunnan Province. Lithology mainly includes dolomite and limestone with interlayer of gypseous dolomite, gypsum rock and salt rock in the upper section in local area [2, 47]. Lithology in the palaeohigh from Leshan to Longnüsi is mainly light grey dolomite with dolarenite or oolitic dolomite and changes to grey limestone at the lower section and grey dolomite with gypsiferous dolomite, gypseous dolomite, gypsum rock and crystal salt rock at the upper section in the slope and sag. The formation mainly contains trilobite e.g. Redlichia Chinese, Redlichia Guizhouensis and Redlichia murakamii-Hofeteiia as well as a small amount of brachiopods [12, 56]. The formation involves one complete short-term base level cycle. Its GR value is low, generally centering around 25 API. The log takes on a smooth box shape with a small 50 API peak at the lower part. Rt log features high value and large variation in accordance with different lithologies in different regions, which mainly changes between 100 and 50,000 ·m. Douposi Formation ( 2d): named by Lu Yanhao and Wang Hongzhen in 1939 and published by Xie Jiarong in 1941 [2, 7, 15] , the source section is located near Douposi at Yiliang, Yunnan Province. The formation is mainly composed of greenish yellow siltstone, shale, limestone and argillaceous dolomite with gypsum rock [12, 45, 15] and is rich in trilobite fossils, e.g. Kunmingaspis Kuetsingocephalus, Manchuriella, Chittidilla, Probowmaniella, Paramecephalus, Ptychoparia Kaotaia and Sinoptychopa-ria [12, 56] and sporadic brachiopod (e.g. Lingulella sp.) and algae fossils [15]. There is limited pure carbonate rock in Central-Southern Sichuan Basin district and grey dolomite sandwiched with thin argillaceous dolomite and silty dolomite only appears in the well field from well Gaoke1 to well Baolong1. Combined sedimentation of carbonate rock and clastic rock is distributed widely in the district: alternate layers of dolomite, mudstone and siltstone are mainly distributed in the slope; dolomite and limestone with interlayer of gypsum rock and shale basically occur in Southern Sichuan sag with local magenta siltstone and gray purple dolomite [7]. The formation contains two complete short-term base level cycles. GR log demonstrates distinct double-peak and high staircase and peak like shape. The upper part takes on gentle box shape with small value and the middle and lower part assumes undulation with data value from 25 to 125 API. Rt log shows sawtooth shape with high magnitude mostly from 100 to 10 000 ·m. Xixiangchi Group ( 2+3x): entitled by Zhao Yazeng in 1929 at Xixiangchi in Emei Mountain, Emei County, Sichuan

Province, the source section lies in Yixiantian scenic resort in Fandianxiang, Leshan [2, 7]. The formation is composed of thick grey dolomite sandwiched with thin argillaceous and silty dolomite [12, 35] and contains conodonts (e.g. Eoconodontua notc-hpeakensia, Teridontus and Drepanodus), brachiopod (e.g. Lingula and Obonella sp.) and trilobite fossils (e.g. Paragraulos, Kunmingaspis and Chittidilla) [12, 57]. There are 2 short-term base level cycles [1213]. GR log demonstrates low magnitude from 25 to 50 API and slight sawtooth shape of small fluctuation. Rt log features sawtooth shape and high magnitude from 7 000 to 20 000 ·m. In general, the Cambrian System is composed of three midterm base level cycles [1213], which are clearly observed in well Gongshen1 (Figure 1), i.e. black carbonaceous shale formed in transgression at an early depositional stage of Qiongzhusi Formation and red beds formed in marine regression at the end stage of Canglangpu Formation, carbonate rock formed in transgression at an early stage of Longwangmiao Formation and evaporate formed in regression at the stage of late Longwangmiao Formation and Douposi Formation, and carbonate rock formed in transgression at an early stage of Xixiangchi Group and red beds formed in regression at the late stage of Xixiangchi Group.

2

Sedimentary evolution

Except for Central and Southern Sichuan Basin with a certain amount of wells, the Cambrian System is relatively unexplored in other regions in the Sichuan Basin and there is only one well, i.e. well Wuke1, reaching the Cambrian in northeastern Sichuan [4, 16], so research on sedimentary facies mainly focuses on the Cambrian in Central Sichuan and Southern Sichuan. As per the updated stratigraphic division presented in this paper, thickness estimation, thickness percentage estimation, single factor mapping and multi-factor analysis for 34 prospecting wells and some outcrops at Xishuiliangcun and Fandianxiang have been conducted. Based on these work together with the previous palaeontology research results, we map the sedimentary facies of different stages in the Cambrian System in Central and Southern Sichuan Basin, which demonstrates new deposition and evolution features. 2.1 Sedimentary features at the depositional stage of early Cambrian Qiongzhusi Formation The depositional stage of Qiongzhusi Formation corresponds to the middle period of the early Cambrian Epoch [12, 6], when water depth differed significantly in Central and Southern Sichuan Basin with sufficient terrigenous detrital material supply. Sediment of shoreland facies without barrier coast and open sea shelf facies mainly developed in the region. This conclusion is consistent with that from Liu Baojun, Xü Xiaosong et al [17]. The authors also found local turbidite and commingled shelf deposits. At the end of the Sinian Period, earth crust rose due to

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Tongwan movement, and Longmenshan area in the western part of the Basin uplifted with Kangding and Baoxing rock complex as a center, which led to eastward tilt of platform basement to form the ancient topography of high in the west and low in the east [1416]. Subsequently affected by early Caledonian movement [1821], palaeohighs of Gaoshiti-Longnüsi and Yaan-Leshan occurred in central and southwestern part of the Basin. From Qiongzhusi thickness in Figure 2, the two-palaeohighs, large in scale, had evident influence on deposition. Formation thickness is relatively small at the palaeohigh and large in the depression. There were mainly three kinds of lithologic combinations of different features at this depositional stage. The first combination including dark grey shale, siltstone, pelitic siltstone and black carbonaceous shale exists in southwestern Sichuan Basin. Terrigenous detrital material came from Kangdian ancient land in the west [8, 21] was found in well Woshen1, Laolong1 and Han1 and outcrop at Fandianxiang with sandstone content between 15% and 30%. The Qiongzhusi Formation drilled in well Laolong1 demonstrates typical features of low energy muddy coast [22], i.e. dark grey shale with offshore features in the lower section, pelitic siltstone and siltstone with features of sandy shoal in coastal corcass in the middle section, and green and brown mudstone and pelitic siltstone of onshore plain in the upper section, which indicates near shore deposits of the first combination [2223]. The second combination consists of dark carbonaceous shale and dark grey shale sandwiched with pelitic siltstone, distributing in most Central

and Southern Sichuan Basin, with the sandstone content below 15%. Fossils such as mollusks, small shelly animals and arthropoda [910] sponge, sponge spicule, hyolithes, trilobite and bradoriida [12, 5, 11] denote features of deep open sea shelf and shallow open sea shelf deposits [15, 17, 24]. Successive siltstone of 90 m was encountered in well Woshen1 at interval 4 2304 320 m; such thick siltstone in deep water with GR low (115 API) in the lower part and high (130 API) in the upper part is very likely deep turbidite deposit. The third combination is thin limestone, mudstone and siltstone distributed in such limited area as Nanchuan outcrop and well Dingshan1. For example, thin interbed of dark shale, grey marl, limestone, lime siltstone, pelitic siltstone and siltstone in well Dingshan1 at interval 3 1483 500 m, with thickness of 352 m, shows typical features of commingled shelf deposition [22]. 2.2 Sedimentary features at the depositional stage of early Cambrian Canglangpu Formation The depositional stage of Canglangpu Formation corresponds to the middle and late period of the early Cambrian Epoch [12, 6], when the water body in Central and Southern Sichuan Basin became shallower and terrigenous detrital material was supplied in plenty. There occurred mainly commingled shelf facies, with local shoal bar microfacies, and shoreland facies in this region (Figure 3). This view point is different from that of other researchers, in the opinion of Men Yupeng et al, there was clastic rock deposition at gentle slope at

Fig. 2 Sedimentary facies at the depositional stage of the early Cambrian Qiongzhusi Formation in the central and southern Sichuan Basin

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Fig. 3

Sedimentary facies at Canglangpu depositional stage of the early Cambrian Epoch in central and southern Sichuan Basin

that time[25]. At this depositional stage, deposition in Central and Southern Sichuan Basin were still affected by palaeohigh, thin at palaeohigh and thick in the depression. But formation thickness variation lessened due to Qiongzhusi depositional infilling. Fossils, similar to those of Qiongzhusi stage, are mainly composed of trilobite of marine arthropods [1, 14]. There are two kinds of lithologic combinations of different features. The first is grey and magenta lime siltstone, pelitic siltstone and sandstone distributed in the west Basin such as well Laolong1, Han1 and outcrop at Fandian, with a sandstone content mainly between 15% and 30%. Expansion of the first lithologic combination compared with that at Qiongzhusi stage indicates water shallowing and extensive terrigenous detrital sediments [26]. For example, 22.0 m-thick magenta lime siltstone, 10.5 m-thick magenta mudstone and 23.0 m-thick grey finestone are developed in well Laolong1 at interval 2 520.02 575.5 m from bottom to top, the grain size changes from coarse to fine and then fine to coarse, and the intervals are characterized by small river bed deposition at aggradational onshore plain and small estuarine deposition at front, which is typical shallow water near-shore deposition [22]. The second combination distributed in most of Central and Southern Sichuan Basin is thin carbonate rock and clastic rock, i.e. thin interbed of grey mudstone, pelitic siltstone and siltstone and limestone and dolomite. In some local areas there is magenta mudstone and brown yellow argillaceous dolomite and pelitic siltstone at the upper part, e.g. 11 m-thick brown yellow argillaceous dolomite and 1.5 m-thick brown yellow siltstone drilled in well Gongshen1 at interval 3 975.0

4 037.5 m, and 56 m-thick magenta mudstone in well Dingshan1 at interval 3 0113 112 m, which indicates commingled shallow shelf deposition in arid climate condition [22]. Local thick siltstone may be found in this deposition environment and there is shoal bar microfacies. For example, the thickness of cumulative siltstone and finestone reaches 30 to 60 m with oolitic carbonate rock and arene carbonate rock of 1.6 to 10.2 m in Gaoshiti and Longnüsi area. Shoal bar sediment of certain thickness also exist in Well Zi1, Woshen1 and Pan1. 2.3 Sedimentary features at the depositional stage of early Cambrian Longwangmiao Formation The depositional stage of Longwangmiao Formation corresponds to the late period of the early Cambrian Epoch [12, 6], when clastic deposit in marine basin converts into chemical deposit and there is mainly restricted carbonate platform facies and evaporite tidal-flat facies-shelf lagoonal facies. Shoal microfacies is only distributed in local platform and evaporite tidal-flat and shelf lagoon (Figure 4). Men Yupeng et al have classified deposition in this period as gentle carbonate slope-shoal-evaporite tidal-flat deposit [25]. Ancient landform at Longwangmiao depositional stage is in succession to that at Canglangpu stage and two palaeohigh still affect deposition to a certain extent with small thickness at palaeohigh and large thickness in the sag. There is mainly platform deposit in Longwangmiao period with a small amount of trilobite and brachiopod fossils. The formation contains two kinds of lithologic association. The first is simple carbonate rock association of dolomite and limestone dis-

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Fig. 4

Sedimentary facies at the depositional stage of early Cambrian Longwangmiao Formation in central and southern Sichuan Basin

tributed in central Sichuan Basin and its surrounding area, which denotes typical carbonate platform deposition. Shoal deposit in local area mainly occurs in the lower and middle part of the formation, which implies deeper water in Longwangmiao period compared with water body at the end of Canglangpu stage. Carbonate rock evolves into dolomite after deposition at the shoal, which becomes favorable reservoir rock [2728]. Thin oolitic dolomite and oolitic limestone or dolarenite and calcarenite from 8 to 15 m are drilled in well Zi4, Gaoke1, Anping1, Baolong1, Pan1, Zishen1, Laolong1 and Nüji, and 20 m-thick oolitic limestone is drilled at the interval 640660 m in well Ning2, all indicate local shoal deposit. The second association is gypseous carbonate rock, i.e. alternate layer of carbonate rock, gypsum rock and salt rock or alternate layer of carbonate rock and gypseous dolomite, distributed in southern Sichuan Basin and Zuo3 well field in eastern Sichuan Basin. The second association develops on the basis of original commingled shelf deposit, so its lower part still maintains those features of commingled shelf; the difference is carbonate rock occupies larger proportion from the original 30% to 50%, a feature of evaporite tidal-flat facies-shelf lagoonal facies occurring after water shallowing. For example, 60 m-thick gypsum rock is drilled in well Zuo3 at interval 5 9095 969 m, 16 m-thick gypsum rock and 6.2 m-thick gypseous dolomite at interval 2 8702 900 m in well Dingshan1, 11 m-thick gypsum rock and 12 m-thick gypseous dolomite at interval 590690 m in well Ning2, 11.8 m-thick gypseous dolomite at interval 3 8403 852 m in well Gongshen1, cumulative gypseous dolomite, gypseous siltstone and

gypsum rock of 60 m at interval 3 6603 800 m in well Woshen1, and 301 m-thick crystalline salt rock, 141.9 m-thick hyaline gypsum, and 278 m-thick argillaceous gypsum rock at whole Longwangmiao interval in well Lin7 [26]. 2.4 Sedimentary features at the depositional stage of middle Cambrian Douposi Formation The depositional stage of Douposi Formation corresponds to the early period of the middle Cambrian Epoch [12, 6] and there is mainly carbonate platform facies in this period. This viewpoint is basically consistent with that from Men Yueng et al [25]. In our opinion, water body further shallows with slight increase of terrigenous detrital sediments. Restricted carbonate platform shrinks and evaporite tidal flat expands. Shoal microfacies mainly occurs in evaporite tidal flat-shelf lagoonal facies (Figure 5). Restricted platform facies and evaporite tidal flat-shelf lagoonal facies coexists at this stage and complex platform composed of restricted carbonate platform and evaporite tidal flat-shelf lagoon converts gradually into carbonate platform. Gypsum rock and salt rock appear in evaporite tidal flat [25, 29]. The formation contains a small amount of arthropoda, such as trilobite [2, 56]. Depositional environment is similar to that at the late stage of Longwangmiao Formation, but the area of evaporite tidal flat facies and shelf lagoonal facies widely expands in the southeast of the basin. Microfacies of arene shoal and oolitic shoal in evaporite tidal flat-shelf lagoonal facies is also common in this period. For example, thin dolarenite and oolitic dolomite with thickness from 0.6 to 3.0 m

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Fig. 5

Sedimentary facies at the depositional stage of middle Cambrian Douposi Formation in central and southern Sichuan Basin

is drilled in well Laolong1, Zishen1, Woshen1, Pan1, Zuo3, Ning2 and Dingshan1. Maroon dolomite and magenta clastic rock is more common in local area, e.g. 61 m-thick magenta mudstone and 63 m-thick maroon dolomite at interval 3 6153 760 m in well Gongshen1. There are a few trilobite fossils [10] and a small amount of mud in carbonate rock, e.g. argillaceous dolomite and dolomitic argillaceous shale. The formation features high GR. 2.5 Sedimentary features at the depositional stage of middle and late Cambrian Xixiangchi Group The depositional stage of Xixiangchi Group corresponds to the middle Cambrian Xuzhuan Age to the late Cambrian Fengshanian Age [12, 6]. Water body deepens at an early stage due to transitory transgression and then shallows gradually [30]. Terrigenous detrital supply decreases to a great extent and restricted platform facies and shelf lagoonal facies-evaporite tidal flat facies at the depositional stage of Douposi converts into restricted carbonate platform facies at Xixiangchi stage (Figure 6). Xixiangchi Group is almost totally denuded in the region to the west of Chengdu, Ziyang and Leshan at the northwest margin of Leshan-Longnüsi palaeohigh due to uplift effect of the late Caledonian movement and is well preserved at the slope and the sag area of palaeohigh [31]. The formation mainly contains fossils of conodont, a small quantity of brachiopods and trilobita [12, 56]. The formation thickness of Xixiangchi Group in Figure 6 is residual thickness of denudation in late Caledonian, which could not demonstrate the influence of

palaeohigh on deposition; whereas onlap reflections of the middle and upper Cambrian on seismic profile [32] may reveal the effect of palaeohigh on deposition, i.e. small formation thickness at palaeohigh and large thickness in the sag. The formation is mainly composed of carbonate rock and granular shoal deposit occurs in succession in local platform, e.g. 24 m oolitic dolomite and dolarenite in well Zishen1, 80 m granular dolomite in well He12, 40 m dolarenite at interval 2 175 2 325 m in well Laolong1, and 142 m dolorudite at interval 5 3905 705 m in well Zuo3. Besides, granular dolomite of certain thickness is also drilled in well Yangshen2 and Woshen1. All these sediments present features of granular shoal microfacies with strong hydrodynamic force in restricted platform. Granular shoal sediment contains a certain amount of primary pore space and also provides prerequisites for late extensive development and distribution of karst reservoir in Xixiangchi Group [3334]. 2.6

Sedimentary evolution of the Cambrian Period

The ancient landform in the early Caledonian has great effect on the Cambrian deposition in the central and southern Sichuan Basin and palaeohighs affected both deposition thickness and sedimentary facies at all depositional stages [35] (Figure 7). Meanwhile distinctive in depositional stages, the Cambrian system has unique sedimentary environment and lithologic combination at different depositional stages. The Cambrian Period experienced three phases of sedimentary evolution, i.e. transgression at the early stage of Qiongzhusi Formation to regression at the depositional stage of Cang-

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Fig. 6 Sedimentary facies at the depositional stage of middle and late Cambrian Xixiangchi Group in central and southern Sichuan Basin

Fig. 7

Cambrian sedimentary evolution model of central and southern Sichuan Basin and its surrounding area

langpu Formation, transgression at the early stage of Longwangmiao Formation to evaporation at the depositional stage of Douposi Formation, and transgression at the early stage of Xixiangchi Group to regression at the middle and late stage of Xixiangchi. In the first phase, there was mainly open sea shelf to shoreland deposition and commingled shelf to shoreland deposition with argillaceous sediment at the early stage, and commingled sand and mud debris and carbonate rock at the late stage. Restricted carbonate platform and evaporite tidal flat to shelf lagoonal deposition mainly occurred in the second phase, which features carbonate rock with local gypsum rock

and salt as well as arene and oolitic carbonate rock. The scope of gypsum and salt rocks expanded at the late stage. The third phase represented restricted carbonate platform sedimentary facies featuring carbonate rock deposit and limited arene and oolitic carbonate rock, and no gypsum rock basically. In summary sedimentary evolution of the Cambrian Period suggests a conversion from continental shelf to platform.

3

Conclusions

There are distinct marker beds in stratigraphic division and correlation of the Cambrian System in the central and south-

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ern Sichuan Basin, especially on GR logs. As for stratigraphic correlation between Southern and Central Sichuan Basin, Jiulaodong Formation in Southern Sichuan Basin corresponds to Qiongzhusi Formation in Central Sichuan, Yuxiansi Formation in Southern Sichuan to Canglangpu, Longwangmiao and Gaotai Formation (i.e. Douposi Formation in this article) in Central Sichuan Basin, and Xixiangchi Group in Southern Sichuan Basin to the same Group in Central Sichuan Basin. The Cambrian System in Central-Southern Sichuan Basin is subdivided from bottom to top into Qiongzhusi Formation, Canglangpu Formation, Longwangmiao Formation, Douposi Formation and Xixiangchi Group. The Cambrian sedimentary evolution was greatly affected by two palaeohighs formed in the early Caledonian movement and experienced three phases in general, i.e. shoreland to open sea shelf, restricted platform to evaporite tidal flat to shelf lagoon, and restricted platform. High-quality source rock of the lower Paleozoic black carbonaceous shale occurred in the development phase of barrier-free shoreland to open sea shelf. Regional layered dolomite reservoirs and restricted oolitic and arene shoal reservoirs of Longwangmiao Formation formed in the development phase of restricted platform - evaporite tidal flat - shelf lagoon. Extensive carbonate rock appeared in the phase of restricted platform, and karst dolomite reservoirs are distributed widely in Leshan-Longnüsi region. Hence, the favorable source-reservoir assemblage of “bottom-up source reservoir” comes about in the Cambrian System. Arene dolomite, oolitic dolomite and karst dolomite in Longwangmiao Formation and Xixiangchi Group will be important potential targets of carbonate reservoir exploration, and shale gas in Qiongzhusi Formation and tight gas in Canglangpu Formation will be future targets of unconventional gas exploration.

[5]

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Acknowledgements

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We appreciate instructions from Professor Song Wenhai and Wang Shiqian. We are also grateful to the staff of Exploration Office, Research Institute of Exploration & Development, PetroChina Southwest Oil & Gasfield Company, for their support and help.

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