Journal of Earth Science, 2016 online ISSN 1674-487X Printed in China DOI: 10.1007/s12583-016-0920-0

Classification of hydrocarbon-bearing fine-grained sedimentary rocks

Jiang Zaixing1, Duan Hongjie1, Liang Chao2, Wu Jing3, Zhang Wenzhao4, Zhang Jianguo1

1. College of Energy, China University of Geosciences, Beijing 100083, China;

2. School of Geosciences, China University of Petroleum, Qingdao 266000, China;

3. Exploration and Production Research Institute, SINOPEC, Beijing 100083, China;

4. Research Center of China National Offshore Oil Corporation, Beijing 100027, China

Abstract

Fine-grained sedimentary rocks are defined as rocks which mainly composed by fine grains

(<62.5 μm). The detailed studies on these rocks has revealed the need of a more unified,

comprehensive and inclusive classification. The study focus on fine-grained rocks has turned

from the differences of inorganic mineral components to the significance of organic matter

and microorganisms. The proposed classification is based on mineral composition, and it is

noted that organic matters has been took as a very important parameter in this classification

scheme. Thus, four parameters, the TOC content, silica (quartz plus feldspars), clay minerals

and carbonate minerals, are considered to divide the fine-grained sedimentary rocks into eight

categories, and the further classification within every category is refined depending on

subordinate mineral composition. The nomenclature consists of a root name preceded by a

primary adjective. The root names reflect mineral constituent of the rock, including low

organic (TOC<2%), middle organic (2%<TOC<4%), high organic (TOC>4%) claystone,

siliceous mudstone, limestone, and mixed mudstone. Primary adjectives convey structure and

organic content information, including massive or limanited. The lithofacies is closely related

to the reservoir storage space, porosity, permeability, hydrocarbon potential and shale oil/gas

sweet spot, and is the key factor for the shale oil and gas exploration. The classification helps

to systematically and practicably describe variability within fine-grained sedimentary rocks,

what’s more, helps to guide the hydrocarbon exploration.

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Key words: Fine-grained sedimentary rocks; Classification; Mineral composition; TOC

content; Shale oil and gas

1 Introduction

Fine grained sedimentary rocks mostly comprise grains smaller than 62.5μm, accounting

for approximately two thirds of the stratigraphic record (Aplin et al. 1999; Tucker, 2001;

Macquaker and Adams, 2003).  However, it has been often overlooked because of its

seemingly simple appearance. Meanwhile, due to the limitation of ultra-microcosmic

experimental conditions, deposition and diagenesis of fine-grained sediments remains a

relatively weak research field in sedimentology (Arthur and Sagenman, 1994; Schieber et al.,

2000; Tripsanas et al., 2004; Potter et al., 2005; Peltonen et al., 2009; Jiang et al., 2013). With

the exploration and development of shale oil and gas, the study of fine-grained sedimentary

rocks becomes increasingly urgent (Aplin and Macquaker, 2011).  

Fine grained sedimentary rocks include a range of rock types whose mineralogies vary

from pure carbonates and siliceous to siliciclastic muds which primarily composed of clay

minerals and silts (Pickard, 1971, Kranck et al., 1996; Macquaker and Adams, 2003). Over

past decades, many scholars have put forward various classification schemes of fine-grained

sedimentary rocks, among them some are suitable for fieldwork and some are based on

laboratory analysis. Many authors have attempted to systematically describe either all or a

subset of these diverse sediments variously according to their: corlor, grain size, texture,

heavy-mineral composition, bulk composition, presence or absence of lamination, fossil

content, fissility, and organinc richness (Potter et al., 1980; Weaver, 1989;Wignall, 1994;

Aplin et al., 1999). Recent years, due to shale gas exploration and production, there has been

an increase in the researches on shale lithofacies analyses (Loucks et al, 2007; James and Bo,

2007; Liu et al., 2011; Liang et al., 2012; Abouelresh and Slatt, 2012). On the basis of

mineralogy, fabric, biota, and texture, Loucks et al (2007) identified three general lithofacies

in Barnett shale: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone

(marl); and (3) skeletal, argillaceous lime packstone. Through petrographic study of

conventional core samples, James and Bo (2007) recognized the lower part of the Barnettof

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the following rock types: organic-rich black shale, fossiliferous shale, dolomite rhomb shale,

dolomitic shale, phosphatic shale, and concretionary carbonate.

All above classifications are easily measured and quite useful, while a broad applicable

classification of fine-grained sedimentary rocks has not yet formed. Meanwhile, the existed

classification mostly focused on composition of inorganic minerals, and the organic matters

has been often overlooked (Loucks and Ruppel, 2007; Liu et al., 2011). In fact, the organic

matters have great significance on the deposition process, diagenesis and reservoir formation

of fine-grained sedimentary rocks, especially these rocks rich in carbonate minerals (Jiang et

al., 2013). Therefor a more comprehensive, informative and unified classification is needed to

describe and compare all fine-grained sedimentary rocks.

In the paper, we propose an effective, broad applicable classification for gas/oil bearing

fine-grained sedimentary rocks placing emphasis on organic matter as well as mineral

composition. We take the TOC content as an important parameter in this classification. The

rock type classified is concerned with its genesis, reservoir property and hydrocarbon

potential.

2. Methodology and data

In the study, five cored wells from different intervals, three basins are used. The basic

data in this study include 853 m of cores, 1206 thin sections, SEM observations from 47

samples, X-ray diffraction data from 1391 samples, source rock data (vitrinite reflectance,

TOC, maceral compositions) from 515 samples (Tab. 1). Core samples were studied at the

hand-specimen, thin-section, and scanning electron microscope (SEM) scales. A

centimeter-scale core description of sedimentary structures and textures forms the primary

basis of petrology characterization. Lots of thin sections were prepared from samples trimmed

from core and analyzed for rock fabric, texture, biotic content and mineralogy. The

quantitative mineral composition analyses were conducted by X-ray diffraction XRD. These

analyses coupled with measuring total organic carbon (TOC) content by combustion, provide

the basis for identifying and classifying fine-grained sedimentary rocks based on fabric and

compositional features. Also the related datas of some top foreign shales are obtained through

literature research for reference and comparison.

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Table 1. The basic datas used in the study.

Cored wells Locations Cores

length /m Strata

Thin

section

SEM

analysis

XRD

data

TOC

data

BY1 Biyang Depression 36 Eh3s 62 10 35 35

CH2 Biyang Depression 59 Eh3s 69 2 64 13

L69 Zhanhua Depression 230 Es3x 876 15 435 245

NY1 Dongying Depression 204 Es4s 117 18 765 194

YY1 Sichuan Basin 324 S1lx 82 2 92 28

3 Results

Clay minerals, quartz, feldspars and carbonate are the most abundant minerals in

fine-grained sedimentary rocks. A variety of other minerals may occur in these rocks in minor

quantities, including zeolites, iron oxides, heavy minerals, sulfates and sulfides, as well as

fine-size organic matter (Tab. 2). The statistics show that the average content of silica (quartz

plus feldspar) in the fine-grained sedimentary rocks ranges from about 15 to 60%, average

clay-mineral content ranges between about 15% and 38%, average carbonate (calcite plus

dolomite) content ranges from less than 5% to more than 63%. The abundance of siderite and

pyrite, which are secondary minerals are relatively low. The average content of organic

carbon present in fine-grained sedimentary rocks ranges from about 1.95 to 5.8%.

Table 2. The mineral composition, TOC and Ro of different mudstones

Area Silica/wt.% Clay/% Carbonate/% Ro/% TOCpd/%

Marcellus 37 35 25 1.5 4.01

Haynesville 30 30 20 1.5 3.01

Barnett 45 25 15 1.6 3.74

Fayetteville 35 38 12 2.5 3.77

Muskwa 60 20 10 2 2.16

Woodford 55 20 5 1.5 5.34

Montney 40 15 30 1.6 1.95

Eagleford 15 15 60 1.2 2.76

Green river 19 16 63 / /

NewAlbany 47 26 15 0.82 5.8

Es3x Zhanhua

Depression19.4(3~50) 18.6(2~48) 57.9(11~94) / (0.27~12.8)

Es3x& Es4s Dongying

Depression26.6(4~57) 22.1(2~59) 47.8(3~97) / 2.95(0.27~12.8)

Eh3s Biyang 38.3(11~61) 27.8(3~46) 28.9(4~51) 0.71(0.52~0.87) 2.89(1.08~4.96)

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Depression

S1l Sichuan

Basin 42.7(28-51) 38.2(6-56) 5.6(1-15) 2.82(1.6-3.78) 1.86(0.4-6.8)

Caption: “Silica” refers to quartz plus feldspars

3.1 The significance of organic matter – why the TOC content is taken into the

classification scheme?

Most of the organic matters in fine-grained sedimentary rocks are fine-size sapropel,

which consists largely of the remains of phytoplankton, zooplankton, spores, pollen, and the

macerated fragments of higher plants. Organic matter is one indispensable part of fine-grained

sedimentary rocks.

(1) Supplement of sediments

As mentioned above, the fine-grained sedimentary rocks in the Zhanhua and Dongying

Depression are rich in carbonate minerals. In fact, the deposition of these carbonate and

carbonate-rich fine-grained rocks are closely related to the organic matters. Photosynthesis by

planktonic algae and microbial processes can absorbs the CO2 and reduce waterbody pH

value, thereby promoting the precipitation of CaCO3 (Reid et al., 2006; Wang et al., 2011).

(2) Influences on the morphology of calcite crystals

The observation of a large number of thin section shows that the calcite in the shale

mainly occurs as three manners, micrite, microspar, and sparry occurrence. Statistics data

reveals that these occurrences are closely related to TOC content. As the TOC is less than 2%,

calcite exists in micrite, when TOC is more than 2%, calcite begins to recrystal as microspar

or granular sparry occurrence, and when TOC is greater than 4%, calcite exists as needle-like

sparry occurrence (Fig.1). That is to say, the calcite’s crystallization degree goes further with

the increase of total organic carbon content of the shale. This phenomenon can be explained

related to the thermal evolution and content of organic matter. As organic matter matures and

hydrocarbon expulses, organic acid also be released, which can dissolve the micritic calcite,

and promote the recrystallization process (Jiang et al., 2013). The higher TOC content means

more released organic acid, which means higher recrystallization degree.

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Figure 1. Carbonate mineral crystal morphology VS TOC. (a) micritic calcite, TOC=1.74%, Well

BYHF1, 2425.4m; (b) granular microspar calcite, TOC=3.67%, Well BYHF1,2435.6m; (c) needle sparry

calcite, TOC=4.05%, Well BYHF1, 2440.7m.

(3) Hydrocarbon generation potential

Organic matter is the material basis for hydrocarbon, its richness decides the

hydrocarbon- generation potential and in-situ oil content. And total organic carbon (TOC) is a

measure of the abundance of organic matter present in a sediment sample. Researches prove

that when the TOCo (original organic carbon) value is low, the hydrocarbon generated from

source rock is mainly adsorbed in organic matters and minerals themselves; As the TOCo

increases, the hydrocarbon generated can be expelled out and fill in the matrix pore or

proceed secondary migration in a large number. Chloroform asphalt “A” extracted from

source rock and rock geochemical pyrolysis analysis parameter “S1” (volatile hydrocarbon

content) are good indications of oil content. The positive correlation relationship between “A”

and “S1” with TOC shown in Fig.2 reveals that organic matter content determines the oil

content of shale.

Figure 2. Oil content VS TOC of Eh3s Formation in Biyang Depression

(4) Effect on the storage capacity for shale gas/oil.

In addition, high organic carbon content can provide extra high storage capacity for gas

and oil (Fig. 3). Organic pores in gas shales has been well documented since first identified in

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the Barnett Shale (Loucks et al., 2009). These pores are generated during burial and

maturation of organic material. When the Ro level of approximately 0.6% or higher, the OM

pores occur, and the amount of porosity within an OM particle in a single sample ranges from

0 to 40%, (Loucks et al., 2009; 2012). In the process of OM evolution, a shale of which TOC

is 7%, consumes 35% of the organic carbon will lead to a 4. 9% increase of its porosity

(Jarvie et al., 2007). The relationship of porosity with TOC shows that the higher the organic

matter content is, the better the storage capacity it has (Fig.3), provided similar mineral

composition, kerogen type and its maturity (Liang et al., 2014).

The pores related to organic matters evolution is not limited to organic pores. As

mentioned above, organic acid expelled during the organic matters evolution dissolve

carbonate minerals and feldspar, generating dissolved pores. Meanwhile, organic matters

evolution prone to cause recrystallization, accompanied by a large intercrystal pores, which

common occur in the recrystallization calcite and dolomite. The dissolution and

recrystallization caused by the hydrocarbon generation, on the one hand provides

recrystallization intercrystal pores, dissolution pores and inter-layer storage space, improving

the porosity to a certain extent, what’s more, changes mechanical properties of rocks and

increase rock brittleness, which is very beneficial for shale reservoir fracturing (Fig. 3A).

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Figure 3. The relationship between fractures density, porosity and TOC content   

(5) Effect on the shale oil production.

Statistics show that, shale oil production (or test oil production) is closely related to the

TOC content(Tab. 2). The relationship can be generally described that (1) it is invalid

reservoir and basically does not produce shale oil even after artificial fracturing when TOC

content is lower than 2.0 %, (2) when 2.0 %<TOC<4.0 %, it can be low abundance reservoir

with natural low yield potential, and can reach industrial oil flow after fracturing, (3) it acts as

high abundant reservoir with certain natural capacity, even up to industrial oil flow, and can

gain stable high shale oil production when the TOC content if greater than about 4.0 %.

Table 2.The lacustrine shale oil yield of different depression in China. 

Depressions  Intervals  Wells  Depth (m) 

Oil yield (m3/d) TOC  of  target 

interval (%) Lithology Before 

Fracturing 

After 

Fracturing 

Zhanhua 

Depression 

Es3x‐Es4s, 

Paleogene 

L67  3287‐3310 0.69  2.09  2.74 (1.63‐3.85) 

Shale, 

Calcareous 

shale 

L42  2828‐2861 79.9  /  5.02 (3.27‐7.73) 

L20  2870‐2880 2.3  9.2  4.62 (3.48‐5.50) 

L19  3061‐3070 1.8  1.98  / 

XYS9  3370‐3566 24  /  2.18 (0.5‐4.02) 

Biyang 

Depression 

Eh3x, 

Paleogene BYHF1  2431‐2441

2.5 (Fracturing) 

Max: 23.6 3.40 (2.16‐4.96) 

Silty  shale, 

Calcareous 

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AS1  2488‐24980.23 (Fracturing) 

Max: 4.68 3.03 

shale 

Shulu 

Depression 

Es3x‐Es4s, 

Paleogene 

J97  3623‐3747 /  17.7  3.96 Shale, Marl 

J116x  3857‐3944 0.89  12.1  2.17 

MalangDepr

ession 

LucaogouFor

mation, 

Permian 

   Max: 22.2 

(Fracturing) 

3.0‐6.0 

(1.38‐11.9) 

Limestone, 

dolomitic 

shale 

3.2 Classification Principle

Several aspects are considered in the paper to choose parameters for classification of

fine-grained rocks. Firstly, the parameters need to be objective, easy to identify or acquire,

and can reflect the genesis of the rock. Secondly, the classification should be suitable for both

field work and laboratory research. Besides, as here we’re discussing the gas/oil bearing

fine-grained sedimentary rock, the classification need throw some light on the petroleum

prospecting, especially reservoirs.

As a result, we propose a classification scheme taking TOC, silica (quartz plus feldspars),

clay minerals and carbonate minerals as the four end-members.

1) The majority of clay minerals and silt-sized quartz in fine-grained sedimentary rocks

are terrigenous siliciclastic particles generated through the disintegration of pre-existing rocks

surrounding the basin. Thus the silts&clay content represents terrigenous clasts input intensity.

While carbonate are mostly autochthonous through chemical precipitation or biochemical

process within the basin, which is related to the climate and water conditions. Thus the

carbonate mineral content can reflect the climate and water depth.

2) The TOC content not only relies on the original productivity of organic matter, but

also depends on preservation conditions. Fast deposition rate and strong reducibility are good

for preservation of organic carbon. The original productivity is closely related to the climate

and nutrients, and the TOC content can reflect the physical and chemical conditions. (Lu et al.,

2004). As mentioned above, the TOC content is closely related to the shale oil yield and the

reservoir properties (including calcite crystallinity, porosity, permeability and fracture

development), and the sharp boundaries are 2% and 4%. Therefore, the TOC content is

considered and TOC being 2% and 4% are used in the mudstone classification.

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3.3 Classification scheme and nomenclature

According to Fig.4A, first on the basis of TOC content, taking 2% and 4% as the

boundary value, the fine-grained rocks are divided into three broad class, that are low organic

(TOC<2%), middle organic (2%<TOC<4%) and high organic (TOC>4%) ones. In samples

where clay, silica or carbonate exceed 50%, the rocks should be given the appropriate root

name of the dominant constituent, including argillaceous mudstone/claystone, siliceous

mudstone/siltstone, calcareous rock/limestone, and mixed fine-grained mudstone. Within

every class, further classification can refer to the Fig.1.B, which depends on subordinate

mineral taking the frequently-used 25% as a limit value. This above classification is based on

the main components, without considering the secondary minerals and special mineral, if

fine-grained rock contains some, additional name can be appended. Also sedimentary

structure information should be incorporated into this scheme by prefixing the rock name with

descriptions such as “massive” or ‘‘laminated ’’.

As to a mudstone in which there is no dominant mineral, that is all the three

compositions (clay, silt or carbonate) are less than 50%, what is called “mixed mudstone”, we

suggest incorporate the quartz, feldspars and clay as the terrigenous siliceous clastic fraction,

together with the carbonate to form a binary classification. Here we adopt siliceous clastic

fraction exceeding 65% or carbonate fraction exceeding 35% as the boundary, rocks in which

clay and silt together exceed 65% can be named siliciclast-type mixed mudstone, otherwise

named carbonate-type mixed mudstone. Appropriate descriptive terms would be included

with the rock name. Moreover, the total organic carbon evaluation is also involved. For

example, a blocky rock of which the clay content is 27%, quartz and feldspars content is 34%,

carbonate content is 39% and TOC content is 1.8% can be named as a massive organic-poor

carbonate-type mixed mudstone. Unedited

Figure 4. Proposed classification scheme for fine-grained sedimentary rocks. (A) I-claystone;

II-siliceous mudstone/siltstone; III-calcareous rock/limestone; IV-mixed mudstone. (B) a. claystone; b.

silty claystone; c. calcareous claystone; d. siltstone; e. clayey siltstone; f. calcareous siltstone; g. limestone;

h. argillaceous limestone; i. silty limestone; j. mixed fine-grained sedimentary rock.

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3.4 Practical applications - descriptions of fine-grained sedimentary rocks

To illustrate the above nomenclature scheme based on TOC and mineral abundance, we

provide a number of examples from five wells cores. Ternary diagrams of mineralogical

constituents of these four well cores show relative proportions of clay, carbonate and

terrigenous silica (Fig.5). These three depressions generally contain less than 50% clay

minerals and Biyang depression contains less calcite and more silica than the other twos.

Table 3 shows that calcareous mudstone is the predominant lithofacies in Zhanhua and

Dongying depression and mixed mudstone take the second place, while in Biyang depression,

the mixed mudstone is the predominant and siliceous mudstone second. These lithofacies in

the lithology classification (Fig.4) may not well developed in one strata. Therefore, the

description is mainly based on the typical lithofacies in different shale formation.

Well Luo69

Clay

Carbonate(calcite,dolomite, ankerite,siderite)

Quartz,feldspar,pyrite

50 50

50

Well NY1

Clay

Carbonate(calcite,dolomite, ankerite,siderite)

Quartz,feldspar,pyrite

50 50

50

Well BYHF1 Well Cheng2

Clay

Carbonate(calcite,dolomite, ankerite,siderite)

Quartz,feldsparpyrite

50 50

50

(A) (B)

(C) (D)

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Figure 5. Ternary diagrams of mineral composition. (A) Well L69, Zhanhua depression; (B) Well NY1,

Dongying depression; (C) Well BYHF1and CH2, Biyang depression; (D) Well YY1, Chongqing City,

Sichuan Basin;

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Table 3. Mineralogical Analysis of Four Key Well Cores Based on XRD Data

Zhanhua Depression (L69) Dongying Depression (NY1) Biyang Depression (BYHF1&C2)

M Coun Mini Maximu M Co Mini Maxim M Co Minim Maxi

Total calcite (%) 5 435 9 91 36 762 1 80 14. 119 0 79.1 argillaceous mudstone: calcite (%) 2 18 9 39 7. 64 1 26 5.4 18 0 17.4

siliceous mudstone: calcite (%) 1 15 9 48 11 79 1 36 6.1 31 0 25.35

calcareous mudstone: calcite (%) 5 303 13 91 52 349 5 80 42. 17 7.1 79.1

mixed fine-grained sedimentary rock: calcite (%) 3 99 19 46 29 265 1 50 12. 53 0 38.4

Total dolomite (%) 6 435 1 78 11 762 1 81 10. 119 0 42.6 argillaceous mudstone: dolomite (%) 3 18 2 12 6. 64 1 20 4.6 18 0 16.1

siliceous mudstone: dolomite (%) 4 15 2 12 5. 79 1 27 8.2 31 0 25.83

calcareous mudstone: dolomite (%) 6 303 1 78 12 349 1 81 12. 17 0 42.6

mixed fine-grained sedimentary rock: dolomite (%) 5 99 2 18 10 265 1 49 13 53 0 35.8

Total quartz (%) 1 435 3 48 22 762 1 54 20. 119 5.8 52.6 argillaceous mudstone: quartz (%) 2 18 16 35 25 64 14 44 20. 18 14.4 27.3

siliceous mudstone: quartz (%) 3 15 33 48 39 79 21 54 24. 31 9.65 52.6

calcareous mudstone: quartz (%) 1 303 3 27 18 349 4 33 13. 17 5.8 33.2

mixed fine-grained sedimentary rock: quartz (%) 2 99 16 31 24 265 5 36 19. 53 10.1 35.4

Total feldspar (%) 1 435 0 12 4. 762 1 22 17. 119 4.72 50.45 argillaceous mudstone: feldspar (%) 3 18 2 9 8. 64 4 14 16. 18 6.8 24.7

siliceous mudstone: feldspar (%) 3 15 0 12 6. 79 2 21 27. 31 6.3 50.45

calcareous mudstone: feldspar (%) 0 303 0 4 2. 349 1 18 10. 17 4.72 22.21

mixed fine-grained sedimentary rock: feldspar (%) 2 99 0 4 5 265 1 22 14. 53 5.7 26

Total clay (%) 1 435 1 48 22 762 2 59 32 119 3.31 55.6 argillaceous mudstone: clay (%) 4 18 39 48 46 64 41 59 47. 18 40.8 55.6

siliceous mudstone: clay (%) 2 15 10 38 32 79 16 51 27. 31 8.64 46.43

calcareous mudstone: clay (%) 1 303 1 28 14 349 2 30 18. 17 3.31 33.7

mixed fine-grained sedimentary rock: clay (%) 2 99 18 35 26 265 5 40 34. 53 13.6 44.9

Total pyrite (%) 3 435 1 16 2. 762 1 15 3 119 0 19.4 argillaceous mudstone: pyrite (%) 6 18 2 16 3. 64 2 11 2.5 18 0 9.6

siliceous mudstone: pyrite (%) 6 15 2 13 3. 79 1 12 3.4 31 0 9.2

calcareous mudstone: pyrite (%) 3 303 1 8 2. 349 1 15 2.1 17 0 7.6

mixed fine-grained sedimentary rock: pyrite (%) 4 99 2 11 3. 265 1 12 3.1 53 0 19.4 Uned

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3.4.1 Claystone

1) Low organic claystone

The lithology is mainly gray/blue-gray and massive in well cores (Fig. 3A, B and C),

occasionally developing indistinct horizontal bedding. Massive claystone abruptly contracts with

overlying laminated shale or siltstone (Fig. 5A). The mineral composition is mainly clay minerals,

more than 50%, calcium, ranging from 10% to 30%, terrigenous debris, ranging from 10% to 20%.

Also fine-grained framboidal pyrite is present in most samples. In addition, a little ostracods debris

and orientated carbon dust can be found. The massive claystone is poor organic matters, with TOC

content ranging from 0.4 % to 1.2 %. All minerals are disorganized and chaotic in optical light. The

quartzes are always angular, and range from several to a dozen of micrometers (Fig. 5D, E).

Organic type is mainly Type II-III, indicating the leading role of terrigenous organic matter.

Figure 5. A. The massive low organic claystone abruptly contacts with overlying laminated shale, Well FY1,

3394.24 m, Es4s formation, Eocene; B. Massive claystone, Well NY1, 3473.7 m, Es4s formation, Eocene; C. Gray

massive claystone, Well YY1, 17.6 m, S1l formation, Lower Silurian; D. Massive low organicr claystone with

detrital angular quartz grains and ostracode fossil fragments. Well L69, 2937.6m, Es3x formation, Eocene. E.

Massive low organic claystone with detrital silt quartz grains and pyrite dispersed distributed in the matrix, Well

L69, 3019.3m, Es3x formation, Eocene. F. Clay minerals are dominated in the lithology, and fractures, Well NY1,

3474.55 m, Es4s formation, Eocene.

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The massive claystone is characterized by massive structure, disorganized and chaotic detrital

grains and low TOC content, which different from the laminated mudstone obviously. Previous

study shows that the long axis of the detrital grains will be horizontally arranged in the slow

suspension settling (Potter et al., 2005). The massive structure and disorganized detrital grains

suggest a rapid depositional process of massive mudstone, which is different from the suspension

settling. Here, the massive claystone is interpreted to be deposited by turbidity current. The low

TOC content of the massive mudstone can be interpreted as the result of the turbidity current

carrying a large amount of oxygen into the ocean bottom (Potter et al., 2005).

2) Middle-High organic claystone

Compared with the low organic massive claystone, the high organic claystone is dark colored,

mainly black and dark gray. These rocks contain clay minerals ranging from 47% to 59% and up to

30% silt with minor calcite (<20%). Also fine-grained framboidal pyrite is present in most samples.

The lithology is mostly laminated, and the laminas boundaries can be sharp or blurring (Fig. 6A, B,

C). The laminas can be silt laminas, clay laminas, organic laminas, carbonate laminas, etc. and the

clay laminas are dominated in this lithology. Organic type is mainly Type I, indicating the leading

role of planktonic organic matters, which can be confirmed by the blue-green algae, dinoflagellates.

The formation of the laminas is related to the water stratification and cyclical climate change. The

development of laminas, rich in clay minerals, a scarcity of large debris and a composition rich in

pyrite and organic matters. These characteristics indicate that they were formed by suspension

deposition in a quiet deep-water region with a low deposition rate. As the differences of minerals

composition, high organic silty claystone (Fig. 6B) and calcareous claystone (Fig. 6C) can be

further classified. Massive high organic claystone can be seen (Fig. 6D, E), and different from

organic laminated claystone and low organic massive claystone, they deposited with a low

deposition rate and high TOC content. Previous scholars suggested they formed related to tsunami

(McHugh et al., 2006). These rocks characterized as high TOC content and Type-I prone kerogen,

as other parameters (Ro, thickness, etc.) meet the requirements, these rocks can act as good source

rock.

 

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Figure 6. A. Laminated high organic claystone with sharp laminas boundary, Well CH2, 2772.96m, Eh3s

formation, Eocene; B. laminated high organic claystone with blurring laminas, Well NY1, 3387.8m, Es4s

formation, Eocene; C. Laminated high organic calcareous claystone with recrystallized sparry calcaite layer, Well

CH2, 2813.08m, Eh3s formation, Eocene; D. Massive high organic claystone with few quartz and feldspar, Well

NY1, 3494.1m, Es4s formation, Eocene; E. Massive low organic claystone with detrital silt quartz grains and

pyrite dispersed distributed in the matrix, Well L69, 3019.3m, Es3x formation, Eocene; F. Laminated high organic

claystone with sharp laminas boundary, Well BY1, 2428.6m, Eh3s formation, Eocene.

3.4.2 Siliceous mudstone

1) Low organic siliceous mudstone

The low organic siliceous mudstone are well developed in the lake basin (Biyang, Zhanhua

and Dongying Depression) and as the turbidity in marine shale in the Sichuan Basin. Organic poor

siliceous mudstone is mainly composed of silt-size quartz and feldspar (41%~74%) with some clay

(average 26.0%) and carbonate (average 14.5%), in addition to minor organic matter (average TOC

is 1.57%) and pyrite. Detrital quartz and feldspar silt is a major component of the siliceous

mudstone (Fig. 7A, B). Microcrystalline silica is also present (Fig.7C, D) which is probably a

diagenetic product, but it’s far less abundant than detrital silica. Siliceous mudstones range from

calcareous to nearly total noncalcareous. The majority of this rock type presents absence of

lamination, while a few show graded bedding, which exhibits upward-fining couplets on a

millimeter scale that are composed of silt-rich mudstones at their bases and clay-rich mudstones

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towards their tops (Fig.7E, F).

 

Figure 7. Photographs of siltstone. A. Massive low organic siltstone. The size of quartzs are mostly under 70

microns, Well YY1, 178.6m, S1l formation, Silurian; B. Massive low organic calcareous siltstone, Well L69,

3135.95m, Es3x formation, Eocene; C. Massive low organic muddy siltstone, Well L69, 2942.89m, Es3x

formation, Eocene; D. Massive low organic siliceous mudstone, Well NY1, 3387.8m, Es4s formation, Eocene; E.

Laminated low organic muddy siltstone. Detrital silt grains half orientated in layers upword-fining., Well CH2,

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2820.57m, Eh3s formation, Eocene; F. Laminated low organic muddy siltstone. Silt laminas and clay laminas

alternate frequently. Well CH2, 2820.57m, Eh3s formation, Eocene. G. Laminated middle organic argillaceous

siltstone, Well YY1, 201.6m, S1l formation, Silurian; H. Middle organic argillaceous siltstone with silt grains

ranging from 10-30um, in which carbonized organic matters (black in the photo) are abundant, Well YY1, 201.6m,

S1l formation, Silurian.

2) Middle organic siliceous mudstone

However, not all siliceous mudstone are organic poor. In the Sichuan Basin, The Longmaxi

shale, Silurian, are mainly middle organic siliceous mudstone. The organic rich siliceous mudstone

are dark colored and laminated (Fig. 7G). In this lithology, quartz is dominant, with minor clay

minerals. The quartz grains sizes are very small, mainly ranging from 10-20μm, partly up to 40 μm

(Fig. 7H). These small quartz grains are from terrestrial transport and autogenous, while the ratio of

the two origin quartz is uncertain. Studies suggest that the organic matters are mainlyfrom

planktonic algae and are carbonized because of strong diagenesis and thermal evolution (Ro>2%).

Middle organic siliceous mudstone (main Longmaxi shale) characterizes as high TOC content and

high quartz, which means high brittleness. These rocks act as an important source rockand gas shale

interval (Li et al., 2009; Liang et al., 2014). What’s more, SINOPEC has gained industrial shale gas

flow in Longmaxi shale of the Jiaoshiba experiment area (Gou et al., 2014; Wang, 2014). The

middle organic siliceous mudstone has considerable industrial value for hydrocarbon exploration

and development.

3.4.3 Limestone

Calcareous mudstone is the predominant rock type within Dongying and Zhanhua depression

and is highly variable in character. These rocks are complex and studies show that their

characteristics (calcite crystal size and conformation) are closely related to the TOC content.

Following we detailed describe these rocks.

1) Low organic limestone

The low organic limestone is light colored, mainly light gray and gray. The laminas are

continuous or wavelike, with relative blurred laminas boundaries (Fig. 8A, B and C). Carbonate

minerals are mainly micritic calcite (Fig. 8G, I), accounting for 50%- 70%. Laminas are well

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developed and distributed horizontal or wavy (Fig. 8D). The light laminas are mainly micritic

calcite with subordinate silts, while the dark laminas mainly consist of clay and organic matters (Fig.

8E, F). The organic rich laminas are thin and the organic matters are dispersed (Fig. 8H). The test

data show that TOC content is relative low, mainly lower than 2.0%. The cores and thin sections

show that the calcite laminas are dominant with great thickness. Some calcite laminas are lenticular

(Fig. 8E), suggesting it maybe received a certain degree of water disturbance.

 

Figure 8. Photographs of low organic limestone. A. Light gray low organic laminated limestone, Well L69,

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3102.85m, Es3x formation, Eocene; B. Gray low organic limestone with blurred laminas boundaries, Well FY1,

3211.78m, Es3x formation, Eocene; C. Gray low organic limestone with blurred laminas boundaries, Well LY1,

3631.5m, Es3x formation, Eocene; D. Laminated and lenticular micritic calcite, Well L69, 3112.9m, Es3x

formation, Eocene; E. Lenticular micritic calcite with a small amount of quartz, Well FY1, 3211.78m, Es3x

formation, Eocene; F. Very thin organic and clay laminas (black laminas), Well LY1, 3631.5m, Es3x formation,

Eocene; G. Micritic calcite of the (F), Well LY1, 3631.5m, Es3x formation, Eocene; H. Dispersed organic matters

in the fluorescent thin section, Well L69, 3100.9m, Es3x formation, Eocene; I. Very small automorphic calcite

crystals showed in the SEM photo, with crystals size about 2-4μm, Well L69, 3117.65m, Es3x formation, Eocene.

2) Middle organic limestone

In these rocks the calcite laminas may recrystallize partly along the calcite laminas boundaries

(Fig. 9A). As the increasing of the clay minerals and silt, decreasing of calcite and TOC, the

layering becomes weakened. The middle organic argillaceous limestone (Fig. 9B, C) and silty

limestone can be furtherly classified according to the clay minerals and silt content. Massive

limestone also can be seen occasionally, in which, sharply angular quartz grains are common and

organic matters are dispersed distributed (Fig.9, D-F). The organic matters are mainly planktonic.

The disorganized detrital grains suggest a rapid depositional process deposited in the agitated

waterbody, which is different from the laminated limestone.

3) High organic limestone

Organic rich laminated limestone characterizes by dark colored, high carbonate content (up to

80 wt. %) and high TOC content (2.0 wt. %-9.8 wt. %), while clay minerals and terrigenous silts are

rare. The cores and thin sections show the clear laminar boundaries, and laminas have pure

components (Fig. 9, G-I). The light laminas are mainly composed by recrystallization calcite, which

mainly occur as "needle" or grain crystal closely packed (Fig. 9J and K). The dark laminas are rich

in organic matters and pyrite, and with strong fluorescence (Fig.9, L).

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Figure 9. A. Well layering micritic calcite and organic rich laminas, Well FY1, 3385.54 m, Es3x formation,

Eocene; B. Laminated organic rich argillaceous limestone. Micrite calcite layers are interbeded with OM laminas

and pyrite framboids are enriched distributed in OM laminas, Es4s formation, Well NY1, 3390.1m, Es4s

formation, Eocene; C. Laminated organic silty limestone. Micrite calcite laminas are interbeded with OM laminas

and pyrite framboids are dispersedly distributed in OM laminas, Well BYHF1, 2425.4m, Eh3s formation, Eocene;

D. Massive organic rich limestone, Well L69, 2996.71 m, Es3x formation, Eocene; E. Massive organic rich

limestone with sharply angular quartz, Well L69, 2992.50m, Es3x formation, Eocene; F. The fluorescent thin

section show the dispersed organic matters in massive limestone, Well L69, 2983.94 m, Es3x formation, Eocene.

G. Organic rich laminated limestone, lenticular calcite, Well LY1, 3661.96 m, Es3x formation, Eocene; H. Organic

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rich laminated limestone, Well L67, 3347.8 m, Es3x formation, Eocene; I. Organic rich laminated argillaceous

limestone, Well L69, 3056.81m, Es3x formation, Eocene; J. Granular calcite with high euhedral crystals as a result

of recrystallization, Well LY1, 3662.1 m, Es3x formation, Eocene. K Columnar calcite crystals with clear laminas

boundaries, interlayer fractures well developed, Well FY1, 3325.49 m, Es4s formation, Eocene; L. The fluorescent

thin section of (K) show the organic matters laminas with fluorescence.

3.4.4 Mixed mudstone

Mixed mudstone are those do not contain 50 percent clay, silt or carbonate, in which the

mineral abundance is kind of homogeneous and none is predominant. These rocks are dominant in

the Eh3 shale of Biyang Depression. In order to study these rocks well, we classify these rocks in to

two types: 1) siliciclastic mixed mudstone, in which siliciclastic content (including quartz, feldspar

and clay minerals) is greater than 65% (Fig. 10, A-D); and 2) carbonate mixed mudstone, in which 

carbonate content is greater than 35% (Fig. 10, E-F).

 

Figure 10. Photographs of mixed fine-grained sedimentary rock. A. Massive organic-poor carbonate-type mixed

fine-grained rock, Well L69, 2939.6m, Es3x formation, Eocene; B. Massive organic-poor siliciclast-type mixed

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fine-grained rock with striped organic matter scattered in the matrix, Well BY1, 2442.5m, Eh3s formation, Eocene;

C. Laminated organic rich carbonate-type mixed fine-grained rock, Well CH2, 2787.89m, Eh3s formation, Eocene;

D. Laminated organic rich siliciclast-type mixed fine-grained rock, Well BY1, 2421.6m, Eh3s formation, Eocene;

E. Massive organic rich carbonate-type mixed fine-grained rock. Rhombhedral calcite recrystallize within organic

matter fragment,Well NY1, 3370.15m, Es4s formation, Eocene; F. Massive organic-rich siliciclast-type mixed

fine-grained rock, Well L69, 2934.63m, Es3x formation, Eocene.

4 Discussion

Shale oil and gas have now become important exploration targets (Jarvie et al., 2007; Liang et

al., 2014). In North America, the discovery of Bakken shale play, Eagle Ford shale paly,

Haynesville shale play et al., have proven that fine-grained sedimentary rocks have a huge

hydrocarbon potential to secure world energy in the future. In these organic rich fine-grained

sedimentary rocks, a porosity network is well interconnected, and the porosity and permeability are

high. Additionally, these rocks have high brittle minerals content (quartz/calcite), especially the

organic rich siliceous mudstones and limestone, which is conductive to artificial fracturing. In fact,

these fine-grained sedimentary rocks have gained industrial oil/gas flow, such as shale oil and gas.

Here we discuss the lithofacies and the shale oil/gas exploration, taking the Es4s-Es3x shale in

Dongying Depression as an example. In the study of Es4s-Es3x shale in Dongying Depression,

further lithofacies dividing has been made in the aforementioned classification system: high organic

laminated limestone (LL-1), middle organic laminated limestone (LL-2), low organic limestone

(LL-3), middle organic laminated marl (LM), middle organic laminated calcareous claystone

(LCM), low organic laminated dolomite mudstone (LDM), low organic laminated gypsum

mudstone (LGM), low organic massive mudstone (MM). We discuss the storage space, hydrocarbon

generating potential, sweet spot and shale oil exploration with the main lithofacies.

The fine-grained reservoir is closely associated with lithofacies, which is mainly reflected in

the storage space types and abundance, porosity and permeability. Structural fractures mainly occur

in these lithofacies with high brittleness, and statistics show that the structural fractures density has

a positive correlation with the brittle minerals (refer to the calcite in Dongying Depression

Es4s-Es3x shale and quartz in the Sichuan Basin Longmaxi shale) content. Interlaminated fractures

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mainly develop in organic-rich laminated lithofacies. Laminated shale shows much heterogeneity in

sedimentary structure, organic matters, mineral composition, and so on, which lead to aeolotropism

in physical properties. Organic pores are abundant in the organic-rich laminated mudstone, while

rare in these organic-poor lithofacies. Floccules pores are rich in the clay-rich lithofacies, such as

laminated calcareous claystone, laminated claystone and massive mudstone. Recrystallization

intercrystal pores are rich in these lithofacies with recrystallization, especially the high organic

laminated limestone, in which the calcite with strong recrystallization. The development of pyrite

intercrystal pores is related to the pyrite content. The inter-particle pores are rich in these lithofacies

with high debris grains content.

Statistics suggest the big differences of reservoir space type and abundance in different

lithofacies (Fig.11). The organic-rich laminated limestone contains abundant reservoir space, such

as recrystallization intercrystal pores, organic pores, interlaminated fractures, etc. and higher

porosity and permeability. Meanwhile, LL-1 has high TOC content (average being 4.68 wt. %),

chloroform bitumen “A” (average being 2.6 wt. %, Fig. 12) and brittle minerals content (average

being 70 wt. %). For high-quality shale reservoirs, abundant reservoir space and good connectivity,

high organic abundance and hydrocarbon potential, high brittleness and other factors are

indispensable. It is no doubt that, the LL-1 has these characteristics and maybe act as the shale oil

exploration dessert. The cumulative thickness of organic-rich laminated limestone can be 36 m in

the Es4s of Well NY1 and single thickness is ca 2m- 6m (Fig. 13). In addition, the LL-1 is always

associated with those lithofacies which has relative good reservoir properties and hydrocarbon

potential, such as laminated calcareous mudstone (LCM) and laminated marl (LM).The favorable

lithofacies assemblages can form great thickness vertically.

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Figure 11. The relative content of different storage space of different lithofacies in Es4s-Es3x shale, Dongying

Depression. SF, structural fractures; APF, abnormal pressure fractures; MCF, mineral contraction fractures; IF,

interlaminated fractures; OP, organic pores; FP, floccules pores; RIP, recrystallization intercrystal pores; PIP,

pyrite intercrystal pores; IP, interparticle pores. LL-1, high organic laminated limestone; LM, laminated marl;

LCM, laminated calcareous mudstone; LL-2, low organic laminated limestone; LGM, laminated gypsum

mudstone; LDM, laminated dolomite mudstone; MM, massive mudstone.

 

 

Figure 12.The chloroform bitumen “A” of different lithofacies in Es4s-Es3x shale, Dongying Depression.

The “sweet spot” lithofacies (LL-1) and the assemblages with LCM and LM occur as certain

regularity in the vertical. Evidence from element geochemistry suggests that the lithofacies are

always corresponds to once lake level rise, that is the flooding surface (Wu et al., 2014, 2015). The

different thickness reflects the difference of flooding degrees. The sequence and parasequence

groups division of Well NY1(the detaileddivision principle and basis will be discussedin another

paper, and the results was directly used here) shows that the organic-rich laminated limestone is

concentrated in the transgressive systems tract (TST) and mainly developed in the top of the

parasequence groups, such as PS4, PS5 and PS6 (Fig. 13). Therefore, on the basis of regional

stratigraphic framework, it is easy to find out the developed interval of the shale oil reservoir “sweet

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spot” and its plane distribution characteristics.

While selecting favorable zone of shale oil exploration, the main controlling factors of shale

reservoir mentioned above should be taken into account. Overall, there is little difference in

diagenesis degree of the study area (little variety of Ro). Therefore, the diagenesis has little impact

on the favorable zone prediction and can be ignored here. The control of tectonic activity is mainly

reflected in its impact on the development of natural fractures, which are very important for the

storage and immigration of shale oil. Therefore, the preferred exploration zone should be near the

fault belts. The preferred zone of the TOC content needs more work. Firstly, based on the detailed

the TOC content test data of key well cored wells (here Well NY1 and Well FY1 are used), the well

log interpretation of the TOC content was analyzed. The calibration of the interpretation results was

constructed to establish the accurate interpretation model of the TOC content. Then, the TOC

content of non-coring wells can be calculated by the model. A mass of wells analysis of the TOC

content helps to achieve the high TOC content zones. The dessert lithoface (LL-1) has been selected

after comprehensive analysis, and the dessert interval will be analyzed in the wells. Then, it is easy

to predict the favorable lithofacies zones based on the stratigraphic framework of the study area.

Under the guidance of the idea, the favorable zones can be predicted based on the comprehensive

analysis.

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Figure 13. The vertical lithofacies distribution of Well NY1 

5 Conclusion

The proposed classification is based on mineral composition of the rocks, which takes TOC

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content, silica (quartz plus feldspars), clay minerals and carbonate minerals as parameters, therefore

divides fine-grained sedimentary rocks into four categories based on the dominant minerals, and

within every category further classification is refined depending on subordinate mineral. The

nomenclature consists of a root name preceded by a primary adjective. The root names reflect

mineral constituent of the rock, including argillaceous mudstone/claystone, siliceous

mudstone/siltstone, calcareous rock/limestone, and mixed fine-grained sedimentary rock. Primary

adjectives convey structure and organic content information, including massive or limanited and

organic-poor, organic-rich. Such a classification helps to systematically and practicably describe

variability within fine-grained sedimentary rocks. What’ more, the classification provide an

important method to help us study the hydrocarbon exploration, especially shale oil and gas.

Acknowledgement

The work presented in this paper was supported by the National Science and Technology

Special (Grant No. 2016ZX05009-002) and the Certificate of China Postdoctoral Science

Foundation Grant (2015M582165). We are grateful to the Geoscience Institute of the Shengli

Oilfield, Henan Oilfield, SINOPEC, for permission to access their in-house database.

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