Scholarly article on topic 'The sedimentological characteristics of microbialites of the Cambrian in the vicinity of Beijing, China'

The sedimentological characteristics of microbialites of the Cambrian in the vicinity of Beijing, China Academic research paper on "Earth and related environmental sciences"

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Journal of Palaeogeography
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{Cambrian / Stromatolite / Bioherm / Microbialite / "Oil and gas reservoir" / "Marine sedimentary environment" / Beijing}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Yin-Ye Wu, Tian-Shu Zhang, Jia-Lei Lü, Yan Liu

Abstract With oil and gas exploration transferring to deeper and more ancient marine strata, more researches have been conducted about the Meso–Neoproterozoic and Cambrian microbial carbonate rocks by petroleum geologists. The Cambrian deposits experienced the first transgression of the Paleozoic, with shallow marine facies depositing in most areas, which are favorable for different kinds of biological reproduction. The Lower Cambrian in Beijing area is lithologically dominated by purple red shales interbedded with limestones, the Middle Cambrian is mainly composed of thick oolitic limestones, and the Upper Cambrian consists of thin limestones and flat-pebble conglomerates. Two beds of microbial carbonate rocks were discovered in the Cambrian outcrops in the vicinity of Beijing. One is from the Zhangxia Formation of Middle Cambrian, and the other is from the Gushan Formation of Upper Cambrian. The microbialites are characterized by combination of multiple stromatolites forming different bioherms. The bioherms are mostly in oval shape and with different sizes, which are 3–4 m long, and 1–3 m high. The surrounding strata beneath the bioherms are oolitic limestones. A central core of flat-pebble conglomerates occurred within each bioherm. Wavy or columnar stromatolites grow on the basis of flat-pebble conglomerates, with dentate erosional surfaces. The bioherm carbonate rocks are interpreted as products from a deep ramp sedimentary environment where potential oil and gas reservoirs can be found. The analysis of sedimentological characteristics of bioherm carbonate rocks and its lithofacies palaeogeography has significant implication for petroleum exploration. Research on geological record of microbialites is beneficial to investigating the Earth evolution, biodiversity, palaeoenvironment and palaeoclimate change, as well as biological extinction event during geological transitions. It also gives warning to human beings of modern biological crisis.

Academic research paper on topic "The sedimentological characteristics of microbialites of the Cambrian in the vicinity of Beijing, China"

Journal of Palaeogeography, 2017, 6(2): 117-131 (00118)

Available online at www.sciencedirect.com

journal homepage: http://www.journals.elsevier.com/journal-of-palaeogeography/

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JOURNAL OF

PALAEOGEOGRAPHY

Lithofacies palaeogeography and sedimentology

The sedimentological characteristics of microbialites of the Cambrian in the vicinity of Beijing, China

Yin-Ye Wu a'*, Tian-Shu Zhang a, Jia-Lei Lu a, Yan Liu b

CrossMark

a Department of Petroleum Geology, Research Institute of Petroleum Exploration and Development,

PetroChina, Beijing 100083, China b School of Earth Sciences, Zhejiang University, Hangzhou 310027, Zhejiang Province, China

Abstract With oil and gas exploration transferring to deeper and more ancient marine strata, more researches have been conducted about the Meso—Neoproterozoic and Cambrian microbial carbonate rocks by petroleum geologists. The Cambrian deposits experienced the first transgression of the Paleozoic, with shallow marine facies depositing in most areas, which are favorable for different kinds of biological reproduction. The Lower Cambrian in Beijing area is lithologically dominated by purple red shales interbedded with limestones, the Middle Cambrian is mainly composed of thick oolitic limestones, and the Upper Cambrian consists of thin limestones and flat-pebble conglomerates. Two beds of microbial carbonate rocks were discovered in the Cambrian outcrops in the vicinity of Beijing. One is from the Zhangxia Formation of Middle Cambrian, and the other is from the Gushan Formation of Upper Cambrian. The microbialites are characterized by combination of multiple stromatolites forming different bioherms. The bioherms are mostly in oval shape and with different sizes, which are 3—4 m long, and 1—3 m high. The surrounding strata beneath the bioherms are oolitic limestones. A central core of flat-pebble conglomerates occurred within each bioherm. Wavy or columnar stromatolites grow on the basis of flat-pebble conglomerates, with dentate erosional surfaces. The bioherm carbonate rocks are interpreted as products from a deep ramp sedimentary environment where potential oil and gas reservoirs can be found. The analysis of sedimentological characteristics of bioherm carbonate rocks and its lithofacies palaeogeography has significant implication for petroleum exploration. Research on geological record of microbialites is beneficial to investigating the Earth evolution, biodiversity, palae-oenvironment and palaeoclimate change, as well as biological extinction event during geological transitions. It also gives warning to human beings of modern biological crisis.

Keywords Cambrian, Stromatolite, Bioherm, Microbialite, Oil and gas reservoir, Marine sedimentary environment, Beijing

©2017 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Received 23 December 2016; accepted 6 March 2017; available online 30 March 2017

* Corresponding author.

E-mail address: wyy@petrochina.com.cn (Y.-Y. Wu).

Peer review under responsibility of China University of Petroleum (Beijing).

http://dx.doi.org/10.1016/j.jop.2017.03.003

2095-3836/© 2017 China University of Petroleum (Beijing). Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The marine Cambrian is important for oil and gas exploration in China. Many new discoveries in Ordos Basin, Tarim Basin and Sichuan Basin, especially in oil and gas reservoirs of microbialites attracted strong interests of petroleum geologists. Stromatolites were widely distributed in ancient marine environments (Bhat et al., 2012; Chang and Zheng, 2012). The complicated process of metabolism in microbial community forms special and complex microfabrics of carbonate rocks grains including benthic radial ooids, oncolites and thrombolites etc. (Mei et al., 2015). Previous studies indicated that stromatolites were related to microbial mat. For example, the carbonate ramp preserved microbial fabrics in Carawine Formation, Hamersley Group of Archean in western Australia, which represented the special environment of subtidal zone (Murphy and Sumner, 2008). Dupraz et al. (2009) summarized the microbial composition of main element circulation in microbial mat which indicated that the integrating activity of key microbial combination with similar metabolism controlled main element circulation such as oxygen, nitrogen, sulfur and carbon circulation. These microbial combinations included: (1) photosynthetic petrification autotrophs (i.e. cyanobacteria); (2) aerobic (chemically organic) heterotrophic organisms; (3) fermenting organisms; (4) anaerobic heterotrophic organisms (mainly sulfate-reducing bacteria); (5) sulfide oxidation (sulfide-oxidizing bacteria); (6) anaerobic photoautotrophic microorganisms (i.e. purple and green sulfur bacteria); and (7) methane bacteria. The stromatolite biostromes were discovered from the Fengshan Formation of the Cambrian in Xishan area of Beijing (Mei et al., 2015). On the basis of geological survey, two beds of microbial carbonate rocks were discovered from the Cambrian outcrops in the vicinity of Beijing. One is the Zhangxia Formation of Middle Cambrian, and the other is the Gushan Formation of Upper Cambrian. The microbialites were characterized by combination of multiple stromatolites forming different bioherms (Gao et al., 2009; Mei, 2014).

2. Geological setting and methods

In North China, the Cambrian deposited in areas including Beijing, Yinchuan, Xi'an, Hefei, Qingdao and Shenyang cities. The Cambrian is distributed at 42°-30° N and 106°-126° E (Fig. 1) (Pan etal., 2013).

It is divided into 10 formations in North China (Table 1). Among these formations, Zhangxia Formation, equivalent to Drumian Stage of international stratigraphic division, belonged to the Middle Cambrian; and Gushan Formation, equivalent to Guzhangian Stage, belonged to the Upper Cambrian. The Lower Cambrian uncon-formably overlay the Meso-Neoproterozoic (commonly Jixian System or Qingbaikou System) in the vicinity of Beijing (Wu et al., 2016). The boundary between the Cambrian and the Ordovician is conformable mostly in North China, occasionally disconformity occurred in the western North China. During the Xinji depositional period of the Early Cambrian, the first marine transgression began in North China. When sea water flew into the eastern and southern part, land occurred on the north-western area which is named giant North China Palaeoland. It is a peneplanation palaeoland which supplied fine clastic materials to marine area (Li et al., 2014; Scotese, 2009). The deposits from adjacent area were sandy mudstone and muddy dolostone interpreted as tidal flat. The outer is open sea and broad carbonate platform.

The transgression reached its maximum at Zhang-xia Age, Middle Cambrian. Consequently, there were oolitic beaches all over the carbonate platform. During the Late Cambrian, carbonate platform was covered by flat-pebble conglomerate beaches. The land shrunk, mud flats vanished, and dolomite flats appeared which indicated the peneplanation went further (Feng etal., 2004; He, 2010).

The purpose of this research is to explain the distribution of the microbialite and its significance for petroleum exploration. Detailed information has been acquired by research on 40 published papers, observation of 12 outcrop profiles (in total about 2400 m long), which locate in Beijing, Shanxi and Shanxi Provinces respectively, and analyses of 35 rock thin sections. The photographs of outcrops were taken by Nikon D700 camera, and interpreted with Microsoft PowerPoint 2010. While the analyses of thin sections were conducted by polarizing microscope of Leica DM4500P YQ08008. The maps and graphs were compiled by CorelDRAW X4.

3. The sedimentary characteristics of the Cambrian bioherm

The studied Cambrian strata are distributed in Xishan area and Northern Mountains (Fig. 1). The strata are exposed particularly well in Yongding River

Fig. 1 Cambrian geological profiles and lithofacies palaeogeography of the surrounding areas of Beijing, China (modified after Feng et al., 2004).

basin (Qingshui River is included), and Dashi River basin (at the southern foot of Baihua Mountains). While in Northern Mountains, the outcrops are located in the vicinity of the Ming Tombs Reservoir (Luo et al., 2013). The Cambrian strata are divided into respectively the Lower, Middle, and Upper series. From top to bottom, the strata were subdivided into Fengshan Formation, Changshan Formation, Gushan Formation, Zhangxia Formation, Xuzhuang Formation, Maozhuang Formation, Mantou Formation and Changping Formation. The lithologic features were described in Table 2 (Su et al., 2013).

Two sets of Cambrian bioherms were discovered from the Zhangxia Formation of Middle Cambrian and Gushan Formation of Upper Cambrian, respectively (Figs. 2 and 3). The clear boundary between the Cambrian and the underlying Neoproterozoic Qing-baikou System indicates unconformity and exposure (Majid et al., 2012). Argillaceous zebra limestone dominated the Qingbaikou System. While the micro-bialite of Zhangxia Formation is oval in shape, 3.5 m in length, and 0.8 m in height (Fig. 2D). The bioherms of

the Gushan Formation are characterized by larger size and arrangement in rows.

3.1. The bioherms of the Zhangxia Formation

As a rule, thin layers of micritic limestones inter-bedded with oolitic limestones and/or calcarenites (Fig. 4) at the geological profiles of Zhangxia Formation. Micrite accounts for a large proportion of the matrix in the oolitic limestones and calcarenites. The bioherms of the Zhangxia Formation are oval in shape, 2.5—3 m long in diameter, and 2 m in height, with a basement being composed of flat-pebble conglomerates; while the surrounding rock is made up of thick-layer limestones. Especially, contemporaneous strata with bioherms consisted of marlites (depositing aside the bioherms), which reveal deep-water environment. Columnar stromatolites were discovered inside the bioherms, with crenate dissolution surfaces (Mei and Meng, 2016).

We compared the stromatolites of the Precambrian Wumishan Formation with those of the Cambrian

Table 1 Stratigraphic correlation of the Cambrian of North China (modified after Feng et al., 2004; Mei et al., 2015).

System Series Stage Trilobiteszone Conodontszone Small shelly fossil zone Formation Remarks

Cambrian Upper Fengshan 26 Mictosaukia Cordylodus proavus Proconodontus Fengshan Corresponding to Jiangshanian Stage

25 Changia

24 Ptychaspis-Tsinania

Changshan 23 Kaolishania Westergaardodina aff. fossa Changshan Corresponding to Paibian Stage

22 Changshania Muellerodus? erectus

21 Chuangia

Gushan 20 Drepanura Westergaardodina matsushitai Gushan Corresponding to Guzhangian Stage

19 Blackwelderia Westergaardodina moessebergensis

Middle Zhangxia 18 Damesella- Yabeia Shandongodus priscus Zhangxia Corresponding to Drumian Stage

17 Amphoton-Taitzuia Laiwugnathus laiwuensis

16 Crepicephalina

Xuzhuang 15 Bailiella Xuzhuang Corresponding to Stage 5

14 Poriagraulos

13 Sunaspus

12 Hsuchuangia

Maozhuang 11 Shantungaspis Maozhuang Corresponding to Stage 4

Lower Longwang miao 10 Hoffetella-Redlichina murakamii Mantou

Canglangpu 9 Megapalaeolenus Xinji

8 Palaeolenus

7 Drepanuroides

6 Yiliangella-Yunnanaspis

Qiongzhusi 5 Eoredlichia Heigouzi

4 Parabadiella

Meishucun 3 Sinosachites Shuidong

2 Siphogonuchites Paragloborilus

1 Anabarites Protohertzina

Zhangxia Formation. It indicated that the former occurred in groups with sheet-like shape. The enlarged photograph of the top of an oncolite showed a circular structure, 20-30 cm long in diameter. However, some stromatolites were irregularly shaped, iron-containing, and brown in color. By contrast, the latter appeared isolatedly, dome in shape, building up bioherms (Fig. 5).

3.2. The bioherms of the Gushan Formation

There are a great variety of lithologic types in the Gushan Formation, Xishan area, including grain limestone, micritic limestone, wackestone, and calcareous mudstone (Fig. 6). The grain limestones (oolitic

limestones and flat-pebble conglomerates) generally contain terrigenous sand-strips and calcite intraclasts, in addition to ooids (Ji et al., 2001). The bioherms of the Gushan Formation are characterized by many bioherms deposited in the same layer, semi-ellipse in shape, 3-5 m long in diameter, and 2-3 m in height. The basement of the bioherms is composed of flat-pebble conglomerates, and surrounding rock is made up of thick-layer limestones. The contemporaneous strata with the bioherms consist of marlites (depositing aside the bioherms), which indicated deep-water environment. Inside the bioherms, there are wavy stromatolites with crenate dissolution surfaces (Fig. 7). Oolitic limestones deposited at the bottom of the bioherms with uniform-sized ooids.

Table 2 Lithological and sedimentary features of the Middle and Upper Cambrian strata of surrounding areas of Beijing, China (modified after Su etal., 2013).

Formation

Lithological and sedimentary features

Remarks

Yeli (O1y)

Changshan and Fengshan (23c+ f)

Gushan ( 23g)

Zhangxia and Xuzhuang (e2z + x)

Maozhuang and Mantou (e2m)

Muddy strip limestones, limestones, and dolomites

Muddy strip limestones, calcareous siltstones, edgewise conglomerates. Changshan Formation was deposited during the maximum flooding period. Therefore, deep-water deposits (containing glauconite), 35 m thick, played a leading role. Marlites strips and edgewise conglomerates, 85-90 m thick, dominated Fengshan Formation. The dolomites at the top indicated an environment of intertidal and supralittoral zone.

Purple-red edgewise conglomerates, muddy strip limestones, interbedded with oolites limestone; 70 m thick. It is generally accepted that storm surge controls the most part of the area (from the slope of epicontinental sea to basin margin)

At bottom, there are oolitic limestones with calcareous siltstone interbeds, thin-layer muddy strip limestones, and thick-layer oolites limestones. At top, there are huge thick oolites. The total thickness of Zhangxia and Xuzhuang Formations is 200-250 m. At the stage of Xuzhuang Formation, the transgression began to expand, and the water got deeper. In that case, several cycles of mudstone-oolites limestone formed, and content of carbonate and fossils increased, which constituted the thickest strata of the Cambrian. Oolites shoals dominated the depositional period of Zhangxia Formation. At top there are purple-red shales with mudstones interbeds. In the middle there are dolomites and dolomitic limestones. At bottom there are conglomerates. The strata are 50-146 m thick in Beijing area, and 110 m thick in Shanxi area. The dolomites flats and mud flats deposit alternately (lagoon).

Conformable contact with underlying strata

China Commission of Stratigraphy (2012) combined the two formations as Chaomidian Formation.

2-3 sets of slump deformation deposits developed at bottom.

China Commission of Stratigraphy (2012) combined the two formations as Zhangxia Formation.

China Commission of Stratigraphy (2012) combined the two as Mantou Formation, which overlies Changping Formation with a parallel unconformity.

Fig. 2 The geological characteristics of the Cambrian profiles, Xishan area, Beijing, China. A—The boundary between the Cambrian and the underlying Neoproterozoic Qingbaikou System; B—Unconformity and exposure on the boundary; C—Argillaceous zebra limestone of the Qingbaikou System; D—Bioherms of the Zhangxia Formation (arrow) are oval in shape, 3.5 m in length, and 0.8 m in height.

;. 3 Distribution of

the bioherms in Xishan area, Beijing, China (modified after Zhu et al., 2009).

Formation Member Layer No. Lithology Description Depositional environment

Gushan • • J-rJ- ! ) Outer ramp

i i i £ Zhangxia 3 Upper Limestone 22 « I p I k ir k Limestone and storm calcirudite

21 -!-!—o_ > > 1 o Marlite Inner / Outer ramp \ ramp

20 0 I 0 1 O Oolitic limestone

19 0 1 O 1 i Algal limestone

18 17 o I 0 1 0 Shale (mudstone, calcareous shale) Limestone and storm calcirudite

16 6 16 1 1 1 1 Marlite

15 0 1 0 / / -I— Algal limestone Inner / Outer ramp \ ramp

14 1 0 | 0 o loi Oolitic limestone

> ) Algal limestone

Panchegou 13 12 0 0 ) ) 1 0 1 0 1- k Limestone and storm calcirudite Marlite Inner ramp

11 0 Iii > ' 1 Oolitic limestone / Outer \ ramp

10 0 1 0 / / 1 Algal limestone

9 Limestone and storm calcirudite

H -fl ° 8| r- < ' 1 Bioclastic limestone

7 / ' Shale (mudstone, calcareous shale)

H o 1 o 1 —L-1—\ Limestone and storm calcirudite / \

5 0 1 o 1 Marlite

4 Ô 1 a 1 Oolitic limestone

3 J M ? Algal limestone Outer ramp

Lower Limestone 2 j-f-t-r 1 0 1 0 0 ,1, \ J, Oolitic limestone Inner ramp

nn Marlite

•l oi Oolitic limestone

Fig. 4 Lithology of the Upper Zhangxia Formation in Xishan area, Beijing, China. Black solid stars refer to the stratigraphic horizon of the bioherms (Chen et al., 1998).

Fig. 5 The characteristics of bioherms of the Zhangxia Formation, Xishan area, Beijing, China. A—An oval-shaped bioherm, 3—3.5 m long in diameter, 2 m in height. The basement of the bioherm is composed of flat-pebble conglomerates, and surrounding rock is made up of thick-layer limestones. The contemporaneous strata with the bioherm consist of marlites (deposited in the left of the bioherm), which indicated deep-water environment. The scale is the umbrella at bottom, 85 cm long; B—Columnar stromatolites were discovered inside the bioherms, with crenate dissolution surfaces (arrow). The diameter of the camera lens cover = 10 cm.

Comparison analysis indicated that ooids are of different sizes in the oolitic limestones of the Jixian System. The biggest ones are 3—5 mm long in diameter (Fig. 8) and radiating in shape, with cruciform symmetry extinction under perpendicular polarized light. The oolitic limestones developed basal cementation by crystal calcites at the late diagenesis stage with development of a great number of cracks.

4. The sedimentary model of bioherms and its significance in hydrocarbon accumulations

4.1. The sedimentary model of bioherms

In summary, lithofacies palaeogeography of the Cambrian in Beijing area indicated deep ramp environment of carbonate platforms (Qi et al., 2012). Detailed observation on a single bioherm showed that during the same period the bioherms appeared in rows. From bottom to top, there are oolitic limestones, marlites, oolitic limestones, bioherms, limestone with mud strips, and limestones (Fig. 9). The sedimentary model illustrated that at the stage of lowstand systems tract and transgressive systems tract, the flat-pebble conglomerates which deposited below storm wave base, overlay the oolitic limestones. Subsequently, the flat-pebble conglomerates became the basement of stromatolites, and many columnar stromatolite combinations formed bioherms. Consequently, at the stage of highstand systems tract, the bioherms were covered by muddy strip limestones, and oolitic limestones developed in high-energy zones (Fig. 10).

4.2. Hydrocarbon accumulation significance of bioherm sediments

Formation of hydrocarbon accumulations includes all geological elements for forming hydrocarbon accumulations, i.e. source rocks, reservoirs, seals, traps, overlying strata for maturation and migration pathway (Chen etal., 2013; Zhao etal., 2011). All key elements should occur on appropriate depositional sequence (Liu et al., 2011; Wu et al., 2015). The multiple columnar stromatolite bioherms commonly develop in transgressive systems tract with good source rock conditions, and bioherms themselves being easy to form relatively good oil and gas reservoirs due to development of dissolution pores and fractures. The oolitic limestones in high-energy grainstone shoal of lowstand systems tract are good oil and gas reservoirs (Ghoriet al., 2009; He et al., 2009). Marlites and mudstones in highstand systems tract are high-quality seals (Hu et al., 2009). Therefore, the stromatolite bioherms section of the Cambrian has good conditions for gas accumulation, which also was confirmed by oil and gas exploration practice. For example, gas accumulations in reservoirs of grainstone shoal were discovered in shallow marine ramp in Moxi area of Sichuan Basin had (Fig. 11). Well Moxi-8 drilled in 2012 in the Cambrian acquired high gas production of one million cubic meters per day (Du et al., 2015; Gu et al., 2016; Xie et al. ,2013; Zhao et al., 2012).

The microscopic observation shows that oolitic limestones beneath bioherm of the Cambrian in the vicinity of Beijing are composed of sparite calcite and partly light brown muddy cements (Fig. 12A). The light brown yellow oolitic limestones represent

Lithology Sedimentary structure Description Depositional environment

o O o ■^y-a^ "A-j2m -L 0 Oolitic limestone, scouring surface Inner/shallow ramp to oolitic shoal

o O o

o O o

o 0 1 o

Calcareous mudstone

o O o Oolitic limestone, scouring surface

1 ni l<~> 1 Oolitic limestone, scouring surface, stromatolitic bioherm

o O o

o 1 1 1 o o Oolitic limestone, scouring surface

o 1 o 1 o

wormkalks

O 1 o O Oolitic limestone Inner/shallow ramp

O 1 o Calcareous mudstone

o o o Oolitic limestone

Calcareous mudstone

lili Micrite, wormkalks Mid to outer/deep ramp

Ie-} 1 1C-3 1 lili

11111111 Lens of micrite, wormkalks

lili <— i —»

Micrite, lens of micrite, wormkalks

lili 1 1

I ! I ! I ! I !

l<=l l<=>l lól k==l Oolitic limestone, wormkalks, micrite, lens of micrite

i i —> Outer/deep ramp

1 i 1 i 1 i 1 i

1 1 -=»

l<=l l<=>l

«=" 1 1 "=>

Calcareous mudstone

lili Micrite, lens of micrite, calcareous mudstone Outer/deep ramp to shelf

Fig. 6 Lithology and depositional characteristics of the Gushan Formation at Xiaweidian section, Beijing, China. Black solid star refers to the stratigraphic horizon of the stromatolitic bioherm (Mei et al., 2011).

Fig. 7 The characteristics of bioherms of the Cambrian Gushan Formation, Xishan area, Beijing, China. A-Many bioherms deposited in the same layer with semi-ellipse in shape, 3-5 m long in diameter, and 2-3 m in height. The basement of the bioherms is composed of cal-cirudites, and surrounding rock is made up of thick-layer limestones. The contemporaneous strata with bioherms consist of marlites (depositing aside the bioherms), which indicated deep-water environment. The scale is the umbrella at bottom, 85 cm long; B and C-Inside the bioherms, there are wavy stromatolites with crenate dissolution surfaces (arrow); D-Oolitic limestones deposited at the bottom of the bioherms with uniform-sized ooids.

high-content organic matters. The early stage (first generation) of micrite limestones occurred in the layer, the late stage was sparite calcite, and the latter fully replaced oolitic limestones into oolitic

sparite limestones. There are partially blocky micrite limestones with both sparite and muddy cements (Fig. 12B). The microscopic features of bioherms in the Cambrian of Xishan area indicated that pellet

Fig. 8 The oolitic limestones under polarizing microscope from the Neoproterozoic Jixian System, Yanqing, Beijing. A—The ooids are of different sizes. The biggest ones are 3—5 mm long in diameter, and with basal cementation by crystal calcites at the late diagenesis stage with development of a great number of cracks (-); B—The ooids are radiating in shape, with cruciform symmetry extinction (+).

Fig. 9 The detailed dissection and interpretation of stromatolite bioherms in Xishan area, Beijing, China. The microbialites mounds are interpreted to be formed in deep ramp environment which could be potential oil and gas reservoirs. Person at the bottom is taken as a scale, 1.7 m high. 1 = Oolitic limestone; 2 = Marlite; 3 = Stromatolite bioherm; 4 = Flat-pebble conglomerate; 5 = Limestone with mud strips; 6 = Limestone.

Fig. 10 The bioherm sedimentary model of the Cambrian in the vicinity of Beijing, China. A-During lowstand and transgressive systems tracts depositional periods, flat-pebble conglomerates below the storm wave base were deposited on the layer of previously deposited oolitic limestones. Then stromatolites grew on flat-pebble conglomerates. Multiple columnar stromatolites were combined into a bioherm; B-During highstand systems tract, limestones with mud strips were deposited on bioherms. Occasionally oolitic limestones were deposited in high-energy belts.

Fig. 11 Gas accumulation discoveries in typical high-energy oolitic limestones in Moxi area of Sichuan Basin (revised from Du et al., 2015).

Fig. 12 The microscopic features of underlying strata beneath bioherms in the Cambrian of Xishan area, Beijing, China. A-The oolitic limestones are composed of sparite calcite and partly light brown muddy cements (-); B-The light brown yellow oolitic limestones represent high content of organic matters (-).

micrite limestones existed within bioherm, which are dominated by grayish-brown micrite and micro-crystalline carbonate with high content of organic matter, locally with sparite cements. The margin of pellets was replaced by micrite, and surrounding circular fracture was filled with organic matter. Limestones were partly in grayish black and brown color with irregular distribution of mud and micrites (Fig. 13). These characteristics suggest further that bioherm layers could be good source rocks, and bioherms themselves and oolitic limestones could be oil and gas reservoirs under certain geological conditions.

5. Conclusions

1) Two beds of microbial carbonate rocks were developed in the Cambrian in the vicinity of Beijing. One was from the Zhangxia Formation of Middle Cambrian, and the other was from the Gushan Formation of Upper Cambrian. The micro-bialites were characterized by combination of multiple stromatolites which formed different bio-herms. Commonly multiple bioherms appeared in the same layer, and oolitic limestone, marlite, oolite, bioherm, marlite, limestone and marlite developed from bottom to top orderly.

Fig. 13 The microscopic features of bioherms in the Cambrian of Xishan area, Beijing, China. A—Pellet micrite limestones occurred within bioherm, which were grayish-brown micrite and microcrystalline carbonate with high content of organic matter, locally with sparite cement (-); B—The margin of pellets was replaced by micrite and surrounding circular fracture was filled with organic matter (-); C—Circular fracture was filled with organic matter (+); D—Part of the limestones showed grayish black and brown color with irregular distribution of mud and micrite (-).

2) Although red strata occurred in the Cambrian in Beijing area during early stage, the palae-ogeography of the Middle and Late Cambrian was characterized by deep water ramp of carbonate platform. The depositional model of the Cambrian bioherm in the vicinity of Beijing was characterized by multiple columnar stromatolite combination in mound shape based on flat-pebble conglomerates beneath storm wave base, and the flat-pebble conglomerates were developed overlying the oolitic limestones of lowstand and transgressive systems tracts. The overlying politic zebra limestones were deposited on the bioherms in highstand systems tract.

3) The multiple columnar stromatolite bioherms appear commonly in transgressive systems tract with good source rock conditions, and bioherms themselves are easy to form better oil and gas reservoirs due to development of dissolution pores and fractures. The oolitic limestones in high-energy

grainstone shoal of lowstand systems tract are high-quality oil and gas reservoirs. Marlite and mudstone of highstand systems tract are high-quality seals. Therefore, the stromatolite bioherms section and adjacent oolitic limestones in the Cambrian have favorable conditions for gas accumulation which will be potential oil and gas exploration targets.

Acknowledgements

This work was financed by the China National Science Foundation (research project No. 2016ZX05004-001). We would like to thank team members Bin-Chen Guo, Ping Luo et al., and postgraduates Kui Ma, Ting Yue, and Rong-Chang Feng for joining field excursion in Beijing and Tianjing areas. We are also grateful to arrangement of field excursion by Prof. Zeng-Zhao Feng and the Organizing

Committee of the 2nd International Palaeogeography Conference, sample analysis by lab center of Zhejiang University, compilation of some figures by Ms. Hong Shu, as well as support from RIPED and related oilfield companies. Finally, our sincere thanks are given to Prof. Stephen Kershaw and another reviewer who helped us improve the original paper with helpful comments.

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