1. college of energy, china university of geosciences...
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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
ited
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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