Types and distribution of the shale sedimentary facies of the Lower Cambrian in Upper Yangtze area, South China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 1, February 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(1): 20–31.

RESEARCH PAPER

Types and distribution of the shale sedimentary facies of the Lower Cambrian in Upper Yangtze area, South China LIU Zhongbao1, 2, 3, *, GAO Bo1, 2, 3, ZHANG Yuying4, DU Wei1, FENG Dongjun1, NIE Haikuan1 1. State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China; 2. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China; 3. Key Laboratory of Shale Oil/Gas Exploration and Production, SINOPEC, Beijing 100083, China; 4. China University of Geosciences, Beijing 100083, China

Abstract: Based on comprehensive analysis of outcrops, cores, rock thin sections, mineral X-ray diffraction and Argon ion-milling scanning electron microscopy, nine lithofacies types and five facies marks in the Lower Cambrian Meishucun Formation and Qiongzhusi Formation in the Upper Yangtze are identified, a classification scheme of the shale sedimentary facies is proposed, and the deposition and evolution model of the shale in the Lower Cambrian is figured out. This research shows there are difference in sedimentation, types and distribution of the sedimentary facies in the Meishucun Formation and Qiongzhusi Formation. The main sedimentation modes in the Meishucun Stage were mechanical-chemical and biological sedimentation, the sedimentary facies (from west to east) were carbonate ramp, shelf, and slope and bathyal basin. The main sedimentation of the Qiongzhusi Stage was clastic mechanical, argillaceous flocculation and biological deposition, and the sedimentary facies were shore, shelf, and ramp and bathyal basin. There are two depositional centers of organic-rich shale in the Upper Yangtze which are the prospective areas for shale gas exploration. The first one is in the Ziyang-Changning area in nearly north-south strike, which is characterized by multiple thin layers. The other one is in the Western Hubei-Eastern Chongqing-Middle Guizhou and Yichang-Jianshi-Fangxian, which is characterized by the thick-layer shale. The shale gas exploration in these areas should take pertinent strategies in line with their differences in the future. Key words: shale; lithofacies; sedimentary facies; Lower Cambrian Qiongzhusi Formation; Lower Cambrian Meishucun Formation; Upper Yangtze area; Sichuan Basin

Introduction In recent years, with the success of exploration and exploitation of shale gas in America, Canada and China etc., the traditional understandings on shale from petroleum geology have been changed, and the new understandings on shale are: shale can not only generate and seal hydrocarbon, but also can be reservoir for oil-gas, capable of forming source-reservoir-cap in one reservoir, which had overthrown the conventional oil and gas exploration, meanwhile, made exploration of unconventional shale oil and gas a hot research spot in the world. Analysis of shale sedimentary facies is an indispensable foundational work of shale oil and gas exploration, and it is believed shallow shelf is the main sedimentary environment for marine organic-rich shale deposition[1, 2]. Sediments in ancient shallow shelf (epicontinental sea) and modern shallow shelf (marginal sea) have some different features, the former have a longer depositional time, bigger thickness and wider distribution, while the latter have shorter depositional time,

smaller thickness and limited distribution. Foreign experts consider that marine organic-rich shale deposited in black sea pattern and/or coastal ascending current pattern[3]. The research on Lower Silurian Longmaxi Formation in Sichuan basin shows that the deep shelf if the favorable facies for shale gas exploration[4]. In conclusion, researchers have realized that finding out favorable sedimentary facies through fine sedimentary facies division can provide important geological basis for shale oil and gas exploration. A set of marine black shale developed in Early Cambrian in the Upper Yangtze area, large in thickness, high in TOC (total organic carbon), and wide in distribution range, it is one of the important exploration strata for shale gas in southern China (Fig. 1). Former researchers have carried out several rounds of paleogeographical mapping of multiple series[58], and reached the important finding that Lower Cambrian shale was shallow shelf deposition in cratonic basin, but due to large mapping scope and few wells drilled, the maps drawn then can’t meet

Received date: 24 Mar. 2016; Revised date: 19 Dec. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (41202103); SINOPEC Ministry of Science and Technology (P15114). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

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Fig. 1.

Distribution of sedimentary facies in Upper Yangtze area in Meishucun stage of Early Cambrian.

the requirements for shale gas exploration, so it is urgent to study sedimentary facies carefully based on the existing study results. Some shale gas discoveries have been achieved in Lower Cambrian on account of abundant investigation and drilling: shale gas flow was obtained in fracturing test from Qiongzhusi Formation in Well W201 and Well JY1 in southeastern Sichuan, Jiumenchong Formation in Well HY1 in southeastern Guizhou and Niutitang Formation in Well TX1 in southern Guizhou, which have proved high exploration potential of shale gas in Lower Cambrian and provided abundant research data. The lithofacies, symbols, types, and characteristics of sedimentary facies have been studied based on observation of typical outcrops, cores from shale gas wells, mineral analysis, and analysis of logging and drilling data; and sedimentary and evolutionary model of shale in Lower Cambrian has been built according to features of structure-paleogeo-

morphy. These results have been combined with lithofacies and formation thickness from drilled wells to characterize the distribution of sedimentary facies in Meishucunian and Qiongzhusian Formations of Early Cambrian, and the favorable facies of organic-rich shale development has been pointed out, which has great theoretical and practical significance in developing fine-grained sedimentology and guiding the selection of shale gas exploration areas in Lower Cambrian shale.

2.

Stratigraphy

Lower Cambrian can be divided into 2 series (Terreneuvian and Series 2) and 4 stages internationally based on the dividing scheme proposed by Peng (2006)[910], with multiple stratigraphic divisions and names. Stratigraphic divisions and their corresponding formations are shown in Table 1. Stratigraphic correlation shows strata in all 4 stages can be compared

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Table 1. Comparison between main stratigraphic division and classification of the Terreneuvian to the Series 2 of the Lower Cambrian, Upper Yangtze System

Series

Formation

Southwestern Sichuan

Central-northern Guizhou

Longwangmiao

Longwangmiao

Qingxudong

Canglangpu

Canglangpu

Nanjiang Sichuan Kongmingdong

Huangping Southern Guizhou

Western Hubei Eastern Chongqing

Qiongxudong

Shilongdong

Jindingshan Cambrian

Series 2

Terreneuvian Sinian

Mingxinsi Qiongzhusi

Qiongzhusi

Meishucun

Maidiping

Upper Sinian

Niutitang

Dengying

Dengying

Types of the lithofacies

Mineral components of shale in Meishucun Formation and Qiongzhusi Formation mainly consist of quartz and clay minerals, then of feldspar, carbonate, pyrite and collophanite. Based on mineral composition, microscopic structures, organic content and fossils, nine types of shale lithofacies have been identified (Fig. 2). The phosphorus carbonaceous shale with small shell fossils and silicious shale with small shell fossils mainly deposited in Meishuncun stage; the silicious shale and carbonaceous shale deposited in both Meishuncun and Qiongzhusi stages; and the carbonaceous shale with calcareous laminae, calcareous shale with micrite or crystalline, laminated grapholith and (laminated) silty shale mainly deposited in Qiongzhusi stage. Lithofacies (a)-(e) in Fig. 2 are formed in oxygenic shallow environment with relative weak hydrodynamics; and lithofacies (f)-(i) in Fig. 2 are formed in anoxia deep environment with (relative) weak hydrodynamics.

4.

Types and features of sedimentary facies

The typical symbols of sedimetary facies are important basis for sedimentary facies classification. Through observing outcrops, drilling cores, rock slices and Argon ion-scanning electron microscopy (SEM), five kinds of sedimentary facies symbols, including bedding, pyrite, nodule, product of ascending current and fossil have been identified. The characteristics of each symbol are described as follows. 4.1.

Bedding

There are eight types of beddings: (1) horizontal bedding (Fig. 3a) or lamina; (2) parallel bedding; (3) ripple bedding; (4)

Balang-Bianmachong Shipai

Xiannvdong Guojiaba Kuanchuanpu

laterally well: the Meishucun sediments are marked by abundant small shell fossils, high gamma value and high resistivity; the sediments in Qiongzhusi stage are marked by straight gamma curve and resistivity curve; the sediments in Canglangpu stage are marked by serrated gamma curve and resistivity curve; and the sediments in Longwangmiao stage are carbonate with low gamma and high resistivity. In this study, the strata of Meishucun-Qiongzhusi are the study object.

3.

Tianheban Yanwangbian

Dengying

Jiumenchong

Shuijingtuo

Dengying

Dengying

small cross bedding; (5) limy stripe; (6) small deformed bedding or disturbance structure (Fig. 3b); (7) lenticular bedding (Fig. 3c); and (8) slump structure (Fig. 3f, 3g). Types (1) and (2) are most common in outcrops and drilling cores, and are formed in quiet sedimentary environment. Types (3)-(7) can be only found in laminated sand-shale in local areas, e. g. lower part of Shipai Formation in Well CS1, which reflect turbulent sedimentary environment with varying sediment supply and water body. 4.2.

Pyrite

Pyrite is a typical mineral in organic-rich sediments, and of great significance for indicating sedimentary environment. Its crystal shape and granularity can be used to distinguish origins and the differences of depositional environment[11]. Through macroscopic core observation and SEM, four types of pyrite have been identified: (1) thunderstone, with max individual size of up to 5 cm × 8 cm but very fine single grain; (2) automorphic granulous pyrite aggregate, 0.2 cm1.0 cm in diameter of single crystal (Fig. 3i); (3) striped pyrite and pyrite lens, the former parallel to bedding and 0.3 cm2.0 cm width, the later distributing intermittently along beddings; (4) pyrite framboids, spherical aggregate like strawberry in shape, composed of massive micron-sized pyrite microlites almost in the same shape and size, and the size of aggregates can indicate oxygenic or anoxic environment[11]. Identification of massive shale images by SEM shows that pyrite framboids were mostly 25 μm (Fig. 3l), pyrite framboids greater than 10 μm are found in only a few samples. In general, multiple types of pyrite, especially massive pyrite framboids in the shale indicate that the shale was mainly formed in anoxic environment. 4.3.

Concretion

Concretion, a kind of sedimentary structure, generally occurs in outer shelf to basin facies, which is formed in a low depositional rate, and stopped growing after buried[12]. Observation of outcrops and drilling cores showed concretions are quite common in the study area and can be divided into two

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Fig. 2. The shale lithofacies in Upper Yangtze area in Early Cambrian Meishucun-Qiongzhusi stages. (a) phosphorous carbonaceous shale with small shell fossils, uniform phosphorous single-layer shell, filled with granular or taxitic materials, Well JY1, plane-polarized light; (b) siliceous shale with small shell fossils, conch shell being siliceous single-layer shell, small in size (several to dozens of microns), and siliceous sponge spicule observed, Well HY1, plane-polarized light; (c) siliceous shale, schistose quartz, tiny quartz grains aggregating into irregular polygon during diagenetic process, Longbizui Outcrop, plane-polarized light; (d) carbonaceous shale, quartz grains <5 μm in size, well rounded, with massive flocculent organic matter, Well HY1, plane-polarized light; (e) calcareous lamina carbonaceous shale, calcareous laminae made of fine granular calcite, in discontinous distribution, Longbizui outcrop, plane-polarized light; (f) micrite (powder crystal) calcareous shale, fine granular calcite aggregations in patches and flocculents, Well L1, plane-polarized light; (g) laminated clay shale, a little organic matter distributed along lamina, Longbizui outcrop, plane-polarized light; (h) laminated silty shale, composed of clay minerals, quartz grains, and organic matter, organic matter in granular and porphyritic forms, Well JY1, plane-polarized light; (i) silty shale, composed of mainly clay minerals and quartz, and debris and feldspar, debris grains being poorly rounded, angular-subangular, Well JY1, plane-polarized light.

types according to shape and composition: (1) elliptic concretion with compact inner structure of calcareous mudstone or argillaceous limestone, larger in size (0.10.5 m in diameter), for example, those in large number in upper member of Guojiaba Formation in Qiaoting outcrop in Nanjiang, northern Sichuan Basin (Fig. 3d), indicating quiet environment with low energy; (2)nodular concretion, composed of siliceous-rich concretion and organic-rich argillaceous shell, smaller in size (0.32.0 cm), which is formed by cementation under bottom flow or ocean current during early stage of diagenesis, and slow in growth rate, reflecting deep water and low energy environment. For example, the concretions in siliceous shale in lower member of Jiumenchong Formation (Fig. 3h).

tinental shelf and continental slope environment, which can promote deposition of organic-rich source rock through bringing nutrients such as phosphorous, silicon, iron group elements and bacteria[13]. Five kinds of sedimentary symbols created by ascending current have been found in the study area: (1) phosphorite or phosphatic shale (Fig. 3e); (2) siliceous shale- carbonaceous shale; (3) carbonaceous shale-siliceous shale-phosphatic shale; (4) stone coal- siliceous shale-metal sulfite-rich layer-black shale; and (5) carbonaceous siliceous shale-metal sulfite-rich layer. These 5 kinds of symbols formed in ascending current are commonly seen in the lower part of Niutitang Formation (southeastern Sichuan and northern Guizhou).

4.4.

4.5.

Products of ascending current

Ascending current is a kind of important deposition in con 23 

Fossils

Different organisms live in different environments, and

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Fig. 3. Symbols of sedimentary facies in Upper Yangtze area in Meishucun-Qiongzhusi stage of Early Cambrian. (a) horizontal bedding, silty shale in Shipai Formation, Well CS1, core; (b) perturbation structure, silty shale in Shipai Formation, Well CS1, core; (c) lenticular bedding, silty shale in Shipai Formation, Well CS1, core; (d) axiolitic plaster concretion, calcareous laminated shale in Guojiaba Formation, Qiaoting Outcrop in Nanjiang County; (e) phosphorous-rich layer, upwelling symbol in Niutitang Formation, Yonghe Outcrop in Weng’an county; (f) dolomitic slump breccia in slope, silicite in the bottom of Jiumenchong Formation Well HY1, core; (g) slump sandy turbidite, Qiongzhusi Formation, Well JY1, core; (h) siliceous-rich nodular concretion, with clayey shell, siliceous shale in Jiumenchong Formation, Well HY1, core; (i) granular pyrite aggregation, carbonaceous shale in Jiumenchong Formation, Well HY1, core; (j) siliceous radiolarian, carbonaceous shale in Niutitang Formation, Yonghe Outcrop in Weng’an county; (k) cellular siliceous organic grains, red algae sac, construction holes preserved well, shale in Niutitang Formation, picture under scanning electron microscope; (l) framboids pyrite aggregation, shale in Shuijingtuo Formation, Xiushan Rongxi outcrop, picture under scanning electron microscope.

the types of fossils in shale can indicate the deposition environment. Types of ancient organisms in the research area mainly include: (1) Algae, different kinds of algae lived in different water depth (generally 5060 m). The red algae lived in deep water (Niutitang Formation in Songtao), and the brown algae lived in deep water (Niutitang Formation in Yangtiao) too, while the cyanobacteria only lived in shallow water. Therefore, red algae (Fig. 3k) and brown algae indicate

deep shelf environment, while cyanobacteria indicate shallow shelf environment [14]; (2) radiolarian, the shell of polycystine (one of the radiolarain) is composed of undissolvable amorphous silicon which can be buried and preserved well in benthonic sediments. The radiolarian is an important kind of fossil for the research of deep ocean sedimentation[15]. This kind of siliceous radiolarian has been found in the lower part of Niutitang Formation carbonaceous shale on Yonghe outcrop in

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Weng’an (Fig. 3j), indicating the shale deposited in deep water sedimentary environment.

5.

Types s of the sedimentary facies

On the basis of lithofacies, sedimentary facies symbols, facies analysis if typical outcrops and wells (Fig. 4), with the consideration of both practicability and operability in production and by combining water depth and sediment types, a classification schedule of sedimentary facies has been proposed. Five types of sedimentary facies, eight types of subfacies and sixteen types of microfacies have been identified in Meishucun-Qiongzhusi depositional stages (Table 2). The characteristics of subfacies and microfacies in shelf are introduced in detail in the following section:

Fig. 4. Depositional structure and microfacies in upper part of Qiongzhusi Formation, Well JY1.

Table 2.

Types and characteristics of sedimentary facies in Meishucun and Qiongzhusi stages of Early Cambrian, Upper Yangtze

Facies

Subfacies

Onshore

Nearshore

Microfacies

Colour

Sand bar

Brown gray, gray

Dolomitic flat

Rock types

Signatures and features

Medium-fine grain Tabular-trough cross-bedding, wave mark, bioturbation quartz sandstone Phosphorous silty-fine Thin-laminate, deformed beddolomite, Light gray, ding, with abundant phosphoPhosphorous micrite gray rous small shell limestone

TOC/%

Representative section Qiongzhusi Fm. in Lianghong outcrop Maidiping Fm. in Fandian and Zhuazhuayan outcrop Shuijingtuo Fm.in Shizhu outcrop

<0.2 Carbonate Carbonate flat gentle Thick-laminae, massive bulk, Argillaceous carslope Argillaceous limeShallow Shuijingtuo Fm. in Well argillaceous limestone, carbonate gentle slope Gray, dark <0.2 stone, carbonate mudgentle L2 bonate mudstone, or their ingray Carbonate argillastone slope terbeds ceous gentle slope Sandy argillaceous Qiongzhusi Fm. in Well Low carbon, wavy bedding, 0.10.2 shelf JY1 small cross-bedding, Argillaceous shelf 0.11.0 Shipai Fm. in Well CS1 Clay shale, carbonate sandy-muddy interbeds, clastic Light Shallow mudstone, silty shale, Guojiaba Fm. in Qiaoting rock mixed carbonate, green 0.10.5 shelf Carbonate shelf gray, gray carbonaceous shale outcrop Nanjiang alga, cyanobacteria, few pyrite Mingxinsi Fm. in in local parts <0.2 Mixed shelf Well DS1 Phosphorous-siliceous-carb 0.52.5 Qiongzhusi Fm. in GS17 Shelf onaceous argillaHigh phosphorous and carbon ceous shelf content, abundant pyrite and Carbonaceous shale, Carbonaceous Niutitang Fm. in sediments of ascending cur1.58.5 phosphorous dolomite argillaceous shelf Yonghe outcrop rent. Lamina and horizontal Deep Black, shale, silty shale, calbedding developed in local shelf black gray Shuijingtuo Fm. in careous laminated Carbonate shelf 0.21.5 parts, with red alga, brown Well EY1 shale alga, sponge spicules, and low Sandy argillaceous Niutitang Fm. in 2.05.0 Th/U value. shelf Yankong outcrop Qiongzhusi Fm. in Turbidite sand Well JY1 Lower Jiumenchong Steep Abundant pyrite, slump strucSiliceous carbonaFm. in Well HY1, lower Black, Siliceous shale, slope Slope ture, sediments of ascending 2.010.0 ceous argillaceous Longbizui Fm. in Jishou black gray carbonaceous shale facies Gentle current, low Th/U value. slope Longbizui outcrop slope Xioayanxi Fm. in DongSilicalite, siliceous Formation thickness decreased Deep Siliceous argillaBlack, Basin 1.513.0 kou Yuexi and Xinhua shale, siliceous carbo- sharply, without sand and euxinic ceous basin black gray facies carbonate, with high silicon. Dashichong outcrop naceous shale basin Tidel flat

Fm.: Formation.

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5.1.

Shallow shelf

Shallow shelf is the shallow water and gentle slope area located between offshore wave base and storm wave base, where the sediments are mainly sand, mud and carbonate with features of traction current, which indicate high energy and intermittent turbulence[16]. It is further divided into four microfacies types, including sandy-argillaceous shallow shelf, argillaceous shallow shelf, carbonate shallow shelf and mixed depositional shallow shelf. 5.1.1.

Sandy argillaceous shallow shelf

This kind of microfacies is located near clast provenance of ancient land, where the sediments are light gray silty shale or argillaceous siltstone interbeded with gray clay shale and siltstone, with parallel or horizontal laminae in centimeter or millimeter order, as well as wave laminae, lenticular laminae and small cross laminae occasionally. For example, Qiongzhusi Formation in Well JY-1 (3 3503 550 m) and Shipai Formation in Well CS-1 (1 1811 876 m), mainly laminated silty shale and argillaceous siltstone of unequal thickness (Fig. 5), showed straight and low gamma value (90130 API) and obvious aggradation features, with the large thickness of up to hundreds meters. 5.1.2.

Sichuan (2 5032 550 m). The sediments are mainly shale with little change in lithology formed in quiet and stable water environment, and featured by straight gamma curve and middle-low gamma value (110150 API). 5.1.3.

This kind of microfacies is located in the surrounding region of carbonate ancient land or submarine uplift, and mainly contains gray lime mudstone, argillaceous limestone interbeded with thin-layer micrite limestone, e.g. Well CS-1 (2 253 2 256 m) in Pengshui, Shuijingtuo Formation in Wangzishi outcrop and Shipai outcrop around Changyang western E’zhong ancient land. 5.1.4.

This kind of microfacies is located far from ancient land provenance, developed gray clay shale with seldom silty laminae and low TOC (total carbon content), such as Niutitang Formation (TOC < 0.2 %) in Well L1 in Southeastern

Mixed depositional shallow shelf

This kind of microfacies is located between ancient land providing clast material and carbonate ancient land (or submarine uplift), and mainly contains silty shale, clay shale or grey mudstone interbeded with argillaceous limestone, e.g. the stratigraphic sequence of clay shale- thin-layer argillaceous limestone- sandstone interbeds with shale in Well DS-1 (3 2503 350 m), with low slight jagged gamma line, indicating turbulent shallow water. 5.2.

Argillaceous shallow shelf

Carbonate shallow shelf

Deep shelf

Deep shelf is located under storm wave base and in deep water area near continental slope, with low water power and low energy[1617]. The sediments are deeper in color, smaller in particle size and higher in TOC than the sediments in shallow shelf. It can be divided into five microfacies types, including phosphorous-siliceous-carbonaceous argillaceous deep shelf, carbonaceous argillaceous deep shelf, carbonate deep shelf, sandy argillaceous deep shelf, and deepwater turbidite sandstone. 5.2.1. Phosphorous-siliceous-carbonaceous argillaceous deep shelf This kind of microfacies is characterized by rich phosphoros-silicon-carbon, with common ascending current features such as thunderstone and phosphorous concretions. For example, 4 275.04 471.5 m in Well Z-4, mainly dark-gray siliceous-phosphorous dolomite and argillaceous dolomite interbeded with siliceous shale and grey shale; Well GS-17 (5 3255 465 m), mainly dolomite and limestone interbeded with silicite, siliceous shale and black shale with collophanite[17]; Well JY-1 (3 5803 640 m), mainly interbeds of phosphorous dolomite, dolomitic mudstone and carbonaceous shale with low Th/U (<2) in spectral gamma-ray log; this kind of microfacies indicates sealed and anoxic deep water environment. 5.2.2.

Fig. 5. Characteristics of sandy argillaceous shallow shelf microfacies in Shipai Foramtion, Well CS1.

Carbonaceous argillaceous deep shelf

This kind of microfacies is characterized by dark-gray carbonaceous shale with little or no fine silt. For example, the upper part of Jiumenchong Formation in Well HY-1 and lower

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part of Niutitang Formation on Yankong outcrop in Jinsha or Yonghe outcrop in Wengan (Fig. 6) are this kind of microfacies, which has pyrite framboids (24 mm in diameter) under the SEM, extremely high gamma value (200500 API in general, up to 1 100 API in local part), and extremely low value of Th/U (<0.5), indicating anoxic deep water environment. 5.2.3.

Carbonate deep shelf

This kind of microfacies is characterized by dark-grayblack calcareous mudstone and argillaceous limestone with calcareous lamina carbonaceous shale interlayer, for example, the upper part of Shuijingtuo Formation of Well CS-1 (2 253.0 2 261.5 m) in Pengshui, mainly calcareous mudstone with carbonaceous shale interlayer and marlstone band; Shuijingtuo Formation in Well L2 (3 1503 350 m), mainly

dark-gray calcareous mudstone with thin argillaceous limestone interlayer (25 m), which are both sediments of calcite and argillaceous deep shelf microfacies. 5.2.4.

Sandy argillaceous deep shelf

This kind of microfacies is characterized by dark-gray carbonaceous shale, silty shale and argillaceous siltstone, which can be found in middle-upper part of Qiongzhusi Formation in southwestern Sichuan, for example 5 0505 110 m in Well GS-17, because of variations in silt and mud contents, it features wide gamma range (from 115250 API), abrupt contact at top and bottom, lightly jagged or box-funnel shape composite deposition, and granularity of coarse-fine-coarse from bottom to top. 5.2.5.

Deepwater turbidite sandstone

This kind of microfacies can be found only in local intervals of few wells, e. g. Well JY-1 (3 295.03 303.9 m), turbidite sandstone (13 cm thick) with granularity changing from coarse to fine (lithology in turn: dark-gray conglomeratic gritstone, medium sandstone-gritstone, dark-gray silty carbonaceous shale), and bottom in abrupt mutation contact with underlying siltstone (Fig. 3g). Combining with the study on regional structure and depositional setting[17-20], it was formed under the graben-horst activity during extensional rifting in Early Cambrian, when water depth increased quickly, and coarse grain sandy sediments from western detrital provenance were slumped and transported under the gravity to deep water.

6.

Fig. 6. Microfacies characteristics of Niutitang Formation on Yonghe Outcrop in Weng’an county, Guizhou Province.

Comparative analysis of sedimentary facies

On the basis of sedimentary facies classification, facies analysis of typical outcrops and well sedimentary, the sedimentary facies in different regions have been compared, which shows that the sediments in Lower Cambrian consist of (from bottom to top): silicon-phosphorous-carbon rich shale, sandstone interbeded with mudstone, mudstone mixed with limestone or marlstone, carbonatite, which indicates rapid transgression-slow regression with water depth changing from deep to shallow. For example, Jiumenchong Formation in south Guizhou, consists of (from bottom to up, in turn) silicalite of slope-deep basin microfaices, siliceous shale, carbonaceous shale of deep shelf, silty shale of shallow shelf, micrite and argillaceous limestone of carbonate gentle slope microfacies. Lateral comparison of different regions show: there is mainly deposited sandy argillaceous shallow shelf facies of smaller thickness in Sichuan Basin, except the area of Ziyang-Changning where the sediments are mainly phosphorous-siliceous-carbonaceous argillaceous deep shelf facies with large thickness. From the perspective of the whole Yangtze region, water depth increased from west to east (southwestern Sichuan-eastern Sichuan-central Guizhou-western Hunan, in turn), and the microfacies in turn were: sandy onshore, sandy argillaceous shallow shelf, carbonaceous argilla-

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ceous deep shelf, continental slope, and siliceous argillaceous deep basin.

7. Sedimentary facies model and horizontal distribution 7.1.

Sedimentary facies model

On the basis of lithofacies classification, single well facies and well-tie facies analysis, combining with strata and tectonic geomorphology[1719], shale sedimentary evolution model in early stage of Early Cambrian in Upper Yangtze area has been established (Fig. 7). The results show that: the sediments in different regions in Meishuncun stage were controlled by large-altitude difference karst geomorphology on the top of Sinian, after rapid transgression, large thickness dark shale deposited in rifting area (southwestern Sichuan Basin) and southeastern depressed area out of the basin, including dark-gray phosphorous-siliceous -carbonaceous shale, dolomite mudstone, dark-gray-black siliceous shale, carbonaceous shale, and thin-laminated phosphorous dolomite or phosphorous limestone on some top part. After this rapid filling stage, latitude difference in different regions decreased in Qiongzhusi stage. The next rapid transgression increased in range significantly and the regression was slower in speed, resulting in retrogradation and aggradation, and rifting area in southwestern Sichuan Basin and southeastern area outside the basin were still the depocenters, the organic-rich shale increased in distribution range but decreased in thickness and TOC; during Canglangpu stage, with continuous regression, sediments increased in grain size, and decreased in thickness difference between different regions; to Longwangmiao stage,

Fig. 7. brian.

the whole region became stable shallow carbonate platform. 7.2.

Horizontal distribution of the sedimentary facies

The comprehensive study on sedimentary facies in different sediment stages of Meishucun Formation and Qiongzhusi Formation of Early Cambrian (Upper Yangtze) shows that: the two stages were quite different in sedimentation, sedimentary system types and sedimentary facies. The Ziyang-Changning area, trending north-south in southwestern Sichuan Basin and western Hubei-eastern Chongqing-central Guizhou area trending northeast-southwest southeast out of Sichuan basin were two depositional centers in early Cambrian, controlling organic-rich shale deposition in deep shelf, and were favorable areas for shale gas exploration (Fig. 8). The sediments in Meishucun stage were featured by phosphorous-siliceous-carbonaceous carbonatite and small shell fossils. The main deposition forms were mechanical-chemical or biological sedimentation, and carbonate gentle slope platform-shelf-slope-deep basin system developed. The microfacies of this stage included (from west to east) carbonate gentle slope, phosphorous-siliceous-carbonaceous argillaceous deep shelf, carbonate gentle slope, carbonate (or dolomite) argillaceous shallow shelf, carbonaceous argillaceous deep shelf, siliceous argillaceous continental slope, and siliceous deep basin. Most of the area was carbonate gentle slope platform, depositing phosphorous dolomite or phosphorous limestone of smaller thickness (530 m), while in the two depositional centers (southwestern Sichuan Basin and the southeast area out of Sichuan Basin), the shale formation was thicker (60150 m, 190 m at maximum) (Fig. 1). The former center was phosphorous-siliceous-carbonaceous argillaceous deep

Sedimentary evolution and development model of shale in Upper Yangtze area in Meishucun-Qiongzhusi stages of Early Cam-

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Fig. 8.

Sedimentary facies in Upper Yangtze area in Qiongzhusi stage of Early Cambrian.

shelf formed in carbonate gentle slope platform (TOC: 0.5% 2.5%) controlled by cratonic rifting occurring in Late Sinian[2023], while the later was siliceous-carbonaceous argillaceous deep shelf developing between carbonate gentle slope platform and slope-deep basin (TOC: 1.5%8.5%). In southeast Guizhou (Sandu-Tianzhu-Xupu) and eastern Guizhou, and northern Chengkou-Fangxian, it is deposited thin black silicalite and siliceous shale (1530 m) with obvious under compensation features, representing slope-deep basin facies. In general, the sediments in Meishucun stage were controlled by karst geomorphology (carbonate surface structure formed in the end of Sinian) and rapid transgression jointly, and organic-rich shale developed maximum thickness in the two deep shelf areas. Rocks deposited in Qiongzhusi stage were characterized by high sand, shale and carbon contents. Paleogeomorphology in Qiongzhusi stage was flater after filling sedimentation in Me-

ishucun stage, so the sedimentation range increased when transgression happened again with the rise of sea-level in Qiongzhusi stage. In this stage, west and north of the research region began to receive sandy debris from ancient land, while southeast and northeast of it still saw argillaceous flocculation and biological deposition, thus the depositional system was onshore-shelf-slope-deep basin. Microfacies developed from west to east were sandy onshore-sandy argillaceous shallow shelf-carbonaceous argillaceous deep shelf-argillaceous shallow shelf- carbonaceous argillaceous deep shelf-carbonaceous continental slope-siliceous carbonaceous deep basin. Most of the area was in (sandy) argillaceous shallow shelf environment, range of carbonaceous argillaceous deep shelf in depositional center in southwestern Sichuan Basin (Ziyang-Changning in north-south direction) kept unchanged basically, while the other depositional center, southeast out of the basin, ex-

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panded from western Hubei-eastern Chongqing-central Guizhou to northeastern Yichang-Fangxian. Meanwhile, sedimentary environment in Sandu (southern Guizhou)-Fenghuang-Langxi and north of the Chengkou-Fangxian were still slope-deep basin environment. In conclusion, the deposition in Qiongzhusi stage was controlled by debris provenances in the west and north, carbonate uplift in the northeast (central Hubei), and relative sea-level eustasy, forming a variety of sediments, including sandy, argillaceous, carbonate and mixed deposits. The two deep shelf areas were still the favorable areas of organic-rich shale deposition, but organic-rich shale in southwest Sichuan Basin (Ziyang-Changning) was characterized by multiple (at least 3) and thin layers, in contrast, shale in the southeast area out of the basin was characterized by continuous thick layer. Therefore, shale gas exploration should take pertinent study and deployment according to the differences of shale in the two depositional centers.

8.

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