Unconventional oil and gass

1. RESEARCH PAPER
PETROLEUM EXPLORATION AND DEVELOPMENT
Volume 46, Issue 5, October 2019
Online English edition of the Chinese language journal
Cite this article as: PETROL. EXPLOR. DEVELOP., 2019, 46(5): 847–855.
Received date: 05 Apr. 2019; Revised date: 21 Jun. 2019.
* Corresponding author. E-mail: jfz@petrochina.com.cn
Foundation item: Supported by National Science and Technology Major Project (2017ZX05035).
https://doi.org/10.1016/S1876-3804(19)60244-2
Copyright © 2019, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Com-
munications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Re-recognition of “unconventional” in unconventional oil
and gas
JIAO Fangzheng*
China National Petroleum Corporation, Beijing 100007, China
Abstract: Taking the Wufeng–Longmaxi shale gas in the Sichuan Basin as a typical example, based on the new progress in exploration
and development, this study re-examines the “unconventional” of unconventional oil and gas from two aspects: oil and gas formation and
accumulation mechanisms, and main features of oil and gas layers. The oil and gas of continuous accumulation and distribution from in-
tegrated source and reservoir is unconventional oil and gas, and the study focusing on shale oil and gas in comparison with conventional
oil and gas has made progress in five aspects: (1) Unconventional oil and gas have source-reservoir-in-one and in-situ accumulation; ac-
cording to the theory of continuous oil and gas accumulation, the accumulation power of oil and gas is overpressure and diffusion; for con-
ventional oil and gas, the source and reservoir are different formations, the trapping accumulation is its theoretical foundation, and the ac-
cumulation power is characterized by buoyancy and capillary force. (2) The unconventional oil and gas reservoirs are mainly formed in the
low-energy oxygen-anaerobic environment, dominantly semi-deep to deep shelf facies and the semi-deep to deep lake facies, simple in lithol-
ogy, rich in organic matter and clay minerals; conventional oil and gas mainly occur in coarse-grained sedimentary rocks formed in
high-energy waters with complex lithology. (3) The unconventional oil and gas reservoirs have mainly nano-scale pores, of which organic
matter pores take a considerable proportion; conventional oil and gas reservoirs mainly have micron-millimeter pores and no organic matter
pores. (4) Unconventional shale oil and gas reservoirs have oil and gas in uniform distribution, high oil and gas saturation, low or no water
content, and no obvious oil and gas water boundary; conventional oil and gas reservoirs have oil and gas of complex properties, moderate
oil and gas saturation, slightly higher water content, and obvious oil, gas and water boundaries. (5) Organic-rich shale is the main target of
unconventional oil and gas exploration; the sedimentary environment controls high organic matter abundance zone and organic matter
content controls oil and gas abundance; positive structure and high porosity control the yields of shale wells; bedding and fracture devel-
opment are important factors deciding high yield.
Key words: unconventional oil and gas; theoretical connotation; shale oil and gas; hydrocarbon accumulation dynamics; organic matter;
fine grain deposition; Wufeng–Longmaxi Formations; Sichuan Basin
Introduction
The development of petroleum geology theory experienced
a process of "searching oil by oil and gas seepages", "anti-
cline" theory, "trap" theory and "continuous oil and gas accu-
mulation" theory[13]
. Since the beginning of this century, the
development of unconventional oil and gas geology has fur-
ther propelled the shift of global oil and gas exploration and
development from conventional oil and gas to unconventional
oil and gas[411]
. In fact, the discovery and exploration of un-
conventional oil and gas is associated with the development of
the entire oil and gas industry. In the United States, the first
shale gas well was drilled in 1821[3, 12]
, but in the following
over one hundred years, shale gas had not received theoretical
and practical attention due to technical conditions. With the
improvements of oil and gas engineering technology, tech-
nologies such as horizontal drilling, staged fracturing and
platform-based “factory” operation have been widely ap-
plied[1315]
. The development of technology has driven the
gradual change of oil and gas exploration and development
objects from traditional oil sands to tight oil and gas, and then
to shale oil and gas, bringing about the major transformation
from “outside source” to “within source” in oil and gas ex-
ploration and development, and the revolution from “conven-
tional oil and gas” to “unconventional oil and gas”. The oil
production “within source” has grown rapidly. In 2018, the
production of shale oil and shale gas in the US reached
3.29108
t and 6 072108
m3
respectively, while the shale gas
production in China amounted to 108108
m3
.

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 848 
Although there are many classification schemes for uncon-
ventional oil and gas resources in academia, most of the cur-
rent standard definitions for unconventional oil and gas are
still based on the technical difficulty of development engi-
neering and the commercial value of oil and gas exploitation.
Such defining standards have promoted oil and gas explora-
tion and development at a certain stage, but they are bound to
be affected by technological innovation and oil price fluctua-
tions, and cannot maintain the relative stability and durability
of the theory. In addition, the unconventional oil and gas geo-
logic theory is a great breakthrough of the traditional petro-
leum geologic theory, and defining from technical and eco-
nomic aspects can’t clarify the difference between the two. In
summary, it is believed that the re-discovering the "unconven-
tional" in unconventional oil and gas will be of great signifi-
cance for deepening the understanding of oil and gas geology
theory and exploration practice.
As shale oil and gas has become an important strategic re-
placement field[1623]
, theoretical studies on the formation
mechanism and enrichment law of typical unconventional oil
and gas (shale oil and gas) are of strategic significance for the
development of unconventional oil and gas theories. There-
fore, taking the main shale oil and gas producing areas of Si-
chuan Basin and Ordos Basin in China, the Permian Basin and
the Western Gulf Coast Basin in the US as research objects,
our research team have continuously tracked and compared
the previous research results[2, 16, 24]
, "re-understand" uncon-
ventional oil and gas from theoretical connotation, reservoir
dynamics, reservoir lithology, pore type, and fluid characteris-
tics, and clarified the geological definition and characteristics
of unconventional oil and gas, in the hope to provide a scien-
tific basis for the study of shale oil and gas formation mecha-
nism, “sweet spot” evaluation and rapid development of ex-
ploration and development.
1. Re-recognition of unconventional oil and gas
theory
1.1. Concept of unconventional oil and gas
In the past, unconventional oil and gas refer to oil and gas
that cannot reach natural industrial production with traditional
technologies and can only be economically exploited by im-
proving reservoir permeability or fluid viscosity with new
technologies. Unconventional oil and gas include oil sands, oil
shale, tight oil and gas, shale oil and gas, coalbed methane,
gas hydrate and so on[23]
. According to the classical petro-
leum geological theory and the latest theoretical and explora-
tion progress, the above-mentioned unconventional oil and
gas can be divided into two categories according to their ac-
cumulation mechanism: (1) Oil and gas generated from source
rocks accumulate or suffer damages in traps after a certain
distance of migration under the action of buoyancy and capil-
lary pressure difference just like traditional conventional oil
and gas with different locations of source and reservoir. They
could occur in any kinds of reservoirs, and they include heavy
oil, oil sands, tight oil and gas, and gas hydrates; this kind of
oil and gas still follow the trap formation mechanism and
process described by traditional petroleum geologic theory. (2)
Oil and gas accumulate within source rocks under overpres-
sure and diffusion, including shale oil and gas and coalbed
methane, which are completely different from conventional
oil and gas. Although oil and gas in the first category are dif-
ficult to exploit economically, it is not different from conven-
tional oil and gas in hydrocarbon accumulation mechanism.
Therefore, the unconventional oil and gas defined in this pa-
per belong to the second category, that is, oil and gas of in-situ
accumulation and continuous distribution inside source,
including shale oil, shale gas and coalbed methane (Fig. 1).
This paper mainly discusses shale oil and gas, and analyzes
the shale gas in the Sichuan Basin as an example. For
comparison, the oil and gas reservoirs discussed in this paper
are mainly sedimentary rocks.
1.2. Unconventional oil and gas formation mechanism
After deposition, the organic-rich shale undergoes compac-
tion, thermal evolution, and water drainage under geological
conditions and then enters the hydrocarbon generation process.
A part of the oil and gas generated from kerogen migrates to
favorable reservoirs such as sand bodies and carbonate rocks
through the transport system. This kind of reservoir has res-
ervoir space dominated by micron-sized pore throats of above
0.3 m largely, in which, affected by buoyancy and capillary
force, the hydrocarbon flow is mainly Darcy flow (Table 1).
The fluid in the free fluid dynamic field would move upwards
Fig. 1. Classification of conventional and unconventional oil
and gas (modified according to reference [2]).

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Table 1. Types and formation mechanisms of oil and gas accumulations (modified according to references [1-2]).
Oil and gas accumu-
lation type
Lithology Hydrocarbon type
Pore throat
size
Driving force
Knudsen
number
Flow mechanism
Mainly
conventional oil
Darcy flow (pipe
flow+seepage flow)
Conventional oil and
gas far from source
Carbonate rock, sandstone
and conglomerate etc. Mainly
conventional gas
>100 m
Capillary force +
buoyancy
<0.01
Darcy flow
(seepage flow)
Conventional oil and
gas near source
Carbonate rock,
sandstone, etc.
Tight oil and gas 0.10.5 m
Capillary force +
buoyancy
0.010.10
Mainly Darcy flow
(seepage flow)
Organic-rich shale Shale oil 2050 nm 0.1010.00
Non-Darcy flow
(transitional flow)
Unconventional oil
and gas integrated
source and reservoir Organic-rich shale Shale gas 520 nm
Overpressure +
diffusion
>10.00
Non-Darcy flow
(Knudsen flow)
to structural highs or low potential areas, forming conven-
tional oil and gas reservoirs under trap conditions, including
tight oil and gas reservoirs, which have obvious oil, gas and
water interfaces after stabilization[12]
. Heavy oil, oil sand or
hydrate could be formed when the reservoirs were destroyed
and modified in later stage.
The other part of oil and gas generated from kerogen is re-
tained and reach equilibrium inside the source rock under
overpressure and binding of capillary force and molecular
force, forming “in-source in-situ oil and gas accumulation”,
i.e. shale oil and gas. The majority of shale oil and gas exists
in the nano-scale pores with the pore throat ranging from 5 to
50 nm. Lack of buoyancy and hydrodynamic force, the shale
oil and gas is in a bound fluid dynamic field and flow mainly
in the non-Darcy pattern. The main driving force of this kind
of oil and gas accumulation is internal overpressure, including
pressurization caused by hydrocarbon generation, overpres-
sure formed as a result of undercompaction and pressurization
by tectonic stress. After the accumulation of a large amount of
oil and gas, diffusion also becomes the main accumulation
mode[13, 1214 ]
. The shale reservoir contains large amount of
oil and gas, with no water or only a small amount of water
(mainly bound water), and the oil and gas enrichment bound-
ary are jointly controlled by the internal overpressure and
capillary forces.
2. New understandings on geological
characteristics of unconventional oil and gas
Shale oil and gas are retained and accumulated in the
source rock driven by overpressure. The organic-rich shale is
both the source rock and hydrocarbon reservoir. Based on the
exploration and development practice of shale gas in the Up-
per Ordovician Wufeng Formation–Lower Silurian Longmaxi
Formation of the Sichuan Basin, shale oil and gas is compared
with conventional oil and gas. Shale oil and gas is different
from conventional oil and gas in lithofacies, reservoir space,
saturation, and occurrence state.
2.1. Lithofacies characteristics
Shale oil and gas reservoirs are dominated by fine-grained
shale rich in organic matters. The "shale" here is not pure
shale considered by most researchers in China, that is, shale
rich in clay minerals or siliceous minerals. The North Ameri-
can shale oil and gas reservoirs include organic-rich shale,
argillaceous carbonate rock or argillaceous siltstone. With
well-developed bedding or lamellation and rich organic matter,
they are commonly referred to as "shale". Therefore, the
lithofacies of a shale oil and gas reservoir is a set of
fine-grained sedimentary rocks rich in organic matter, com-
plex in lithofacies and lithologic combination. The Barnett
Shale in North America is a combination of organic-rich bio-
clastic, carbonate and siliceous shale. The Eagle Ford shale is
a combination of organic-rich argillaceous carbonate and cal-
careous shale[25]
. The Niobrara shale consists of chalk layers
poor in organic matter and argillaceous limestone rich in or-
ganic matter. The Wufeng–Longmaxi Formation in the Si-
chuan Basin in China is a combination of organic-rich sili-
ceous and calcareous shale, clayey shale, argillaceous shell
limestone, and argillaceous siltstone.
Conventional oil and gas reservoirs of marine facies are
mainly distributed in high-energy facies such as onshore and
platform marginal slopes[2627]
(Fig. 2). In these strong hydro-
dynamic environments, coarse clastic rock and reef flat car-
bonate often deposit, which are good reservoirs. Similar to the
marine facies, conventional oil and gas reservoirs of conti-
nental facies are mainly high-energy water bodies such as
rivers, deltas, and shore-shallow lakes, where coarse clastic
rocks deposited, providing good reservoir space for conven-
tional oil and gas accumulation. Conventional oil and gas
reservoirs are mainly controlled by the sedimentary environ-
ment, epigenesis, and tectonism.
In contrast, the organic-rich shale of marine and continental
Fig. 2. Sedimentary environment model of marine organic-rich
shale.

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Table 2. Statistics on types and diameters of pores in organic-rich shale formations in the world.
Foreign examples
Pore origin Pore type Connectivity
Development
degree
Pore
diameter/nm
Sichuan Basin
Country Basin
Pore di-
ameter/nm
Intergranular pore
Isolated or
connected
Relatively
developed
8610(230) Canada
Western
Canada
817
Primary
pore Intercrystalline
pore
Isolated or
connected
Developed 60100(30) America Appalachia 745
Intragranular pore Isolated A few 10610(270) America Anadarko 20160
Secondary
pore Dissolution pore Isolated A few 104 080(1 200)
Wufeng Formation–
Longmaxi Formation,
Yanchang Formation,
Xujiahe Formation[2]
America Fort Worth 5100
Organic
matter pore
Kerogen pore
Asphalt pore
Connected Developed 152 000(200)
Wufeng Formation–
Longmaxi Formation
Canada Alberta 745
Note: the values in brackets are the average values.
facies are mainly formed in low-energy environments in
semi-closed to closed waters[2829]
. During the transgression of
marine facies, the deep-water shelf would become an oxygen-
anaerobic environment, where planktonic organisms such as
algae would boom, giving rise to "marine snow" sedimenta-
tion phenomenon, making it the depocenter of the marine
basin, highly organic shale in depocenter doesn’t develop. For
example, the maximum total organic carbon content (TOC) of
the organic-rich shale in the Wufeng Formation–Longmaxi For-
mation in the Sichuan Basin is 25.73%, and layers with TOC
value greater than 2% accounts for 30% to 45%. Analysis of
paleoenvironment and paleogeography based on trace element
data shows that the organic-rich shale deposited in the semi-deep
to deep shelf environment of the continental slope (Fig. 2).
2.2. Reservoir space
Conventional oil and gas reservoirs mainly include clastic
reservoirs and carbonate reservoirs. The reservoir space can
be divided into primary pore, secondary pore, and fracture.
Primary pore includes intergranular pore and intercrystalline
pore. Secondary pore includes dissolution pore, and mold pore,
etc.[1]
. All the pore pores are larger in size, mostly in mi-
cron-millimeter scale, and simple in pore structure. Shale oil
and gas reservoirs also have such inorganic pores (Table 2),
such as pores between quartz or feldspar particles, intercrys-
talline pores of clay minerals, dissolution pores of carbonates,
etc. However, all these pores in shale are smaller in size,
mostly nanoscale, and very complicated in pore structure[2]
.
Besides, the shale oil and gas reservoir has a unique kind of
reservoir space, organic matter pore, which is more developed
in the “sweet spot” section with main nano-pores.
During the evolution of organic matter, the hydrocarbon
generation material not only produces oil and gas, but also
generates nano-scale reservoir space in the organic matter,
forming a three-dimensional coupling space of mineral
pore-organic matter pore-microfracture. As unconventional oil
and gas can be generated almost over the entire organic hy-
drocarbon evolution process (Fig. 3), almost all unconven-
tional oil and gas reservoirs have organic matter pores. More-
over, the thermal evolution degree at vitrinite reflectance (Ro)
of 0.8%-3.5% is more favorable for the development of such
pores. Organic matter pores are mainly divided into two types:
organic matter pores in kerogen and organic matter pores in
solid asphalt. The organic matter pores in kerogen are honey-
comb-like or sporadic, with a pore diameter of 10 to 200 nm.
The organic matter pores in solid asphalt are elliptical and
arranged in bead strings, even with boundary completely
fused, and 300 nm to 2 μm in diameter. With a Ro value of
shale organic matter of 1.8%3.1%, the Wufeng Forma-
tion–Longmaxi Formation shale in the Sichuan Basin is in the
stage of thermal pyrolysis and dry gas generation. The organic
matter such as kerogen and asphalt generated groups of
“honeycomb-like” organic matter pores during primary deg-
radation and secondary cracking (Fig. 4a), which are good
storage space for oil and gas. Through petrophysical models
and a large number of SEM images (Fig. 4a), it is found that
the surface porosity of organic matter pores in the high-yield
interval is 30% to 50%, accounting for 1/3 to 1/2 of the porosity.
2.3. Reservoir temperature
It is found that shale oil and gas reservoirs are generally
higher in temperature than conventional oil and gas reservoirs
Fig. 3. Schematic diagram of development stages of organic
matter pores in unconventional oil and gas reservoirs.

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Fig. 4. Development characteristics of organic matter pores in Wufeng Formation–Longmaxi Formation shale of Well in Sichuan Basin.
(a) FIB-HIM imaging, pores developed in organic matter, with pore boundary dissolved and connected, pore diameter of up to 2 μm and
good connectivity; (b) Surface porosity of the organic matter in graph (a) is 43.6%; (c) FIB-HIM imaging, organic matter pores in shale
matrix, with clear pore boundaries, medium connectivity, and pore diameter of 10-400 nm; (d) Surface porosity of the organic matter in
graph (c) is 30.6%.
Table 3. Statistics on temperatures of conventional gas reservoirs and shale gas reservoirs in Sichuan Basin.
Target stratum Reservoir lithology Gas source rock
Pressure
coefficient
Temperature of
gas reservoir/C
Carboniferous Huanglong Formation in Eastern
Sichuan Basin
Dolomite Longmaxi Formation 1.21.3 80110
Longmaxi Formation in Fuling block Siliceous shale Self-generation 1.55 90120
Longmaxi Formation in Changning block Siliceous/calcareous shale Self-generation 1.22.1 110140
Longmaxi Formation in Weiyuan block Siliceous/calcareous shale Self-generation 1.32.3 100134
with oil and gas supplied by shale (Table 3). Conventional oil
and gas reservoirs are mainly in the form of "source below
reservoir", and are much shallower in burial depth than shale
oil and gas reservoirs. Moreover, organic-rich shale in marine
and continental facies mainly deposit in the relatively lower
part of basins, while conventional oil and gas reservoirs are
formed in the high-energy zones of the higher part of basins
(Fig. 2), and shallower in burial depth, so under the same ge-
othermal gradient, the conventional reservoirs are lower in
temperature compared with shale gas layers. The downhole
temperatures of the North American shale gas reservoirs are
80100 C and 110130 C[25]
; the temperatures of the shale
gas reservoirs in the Wufeng Formation–Longmaxi Formation
in the Sichuan Basin of China are 100140 °C; while the
temperatures of the structure gas reservoirs in the overlying
Upper Carboniferous Huanglong Formation are 80110 °C,
much below the temperatures of the shale gas reservoirs sup-
plying oil and gas for them.
On the one hand, the high-temperature and high-pressure
characteristics of the shale oil and gas reservoirs squeeze out
and consume a large amount of free water and bound water
inside the reservoirs, forming production intervals with high
oil and gas saturation; on the other hand, these characteristics
bring about great challenges to engineering, thus increase in
cost, so in shale oil and gas reservoirs, the instruments must
withstand higher forces, and the temperature and pressure
deformation of the casing during completion is far more com-
plicated than in conventional oil and gas reservoirs.
2.4. Oil and gas properties
Retained in-situ, with short-distance or no migration, the
fluid in shale oil and gas reservoir is uniform in nature and
relatively simple in composition and does not contain H2S.
For example, the shale gas in the Wufeng Formation–Long-
maxi Formation of the Sichuan Basin is mainly crude oil
cracking gas with 95% to 99% of CH4 and less than 5% of
non-hydrocarbon gases such as CO2 and N2, and does not
contain H2S[30]
. The shale gas in North America is similar in
gas composition, but different in CH4 content due to low
thermal evolution degree (Table 4). The oil of shale oil en-
richment area is lighter (0.700.85 g/cm3
) and high in gas-oil
ratio, which makes it easy to flow and exploit.
Table 4. Composition data of shale gas in Wufeng–Longmaxi Formation of the Sichuan Basin.
Block/Basin Formation CH4/% CO2/% N2/% H2S/%
Changning-Zhaotong, Sichuan 97.1199.45 0.010.91 0.031.79 0
Weiyuan, Sichuan 95.5299.27 0.021.07 0.012.95 0
Fushun-Yongchuan, Sichuan 95.3299.59 0.061.74 0.014.05 0
Fuling, Sichuan
Wufeng-
Longmaxi
97.6798.95 0.021.16 0.321.36 0
Eastern Sichuan Basin, Sichuan Huanglong 94.3699.63 0.202.68 0.303.26 0.120.79
Appalachian Marcellus >95.00 0
Fort Worth Barnett 77.0293.05 0.312.68 0.987.56 0
Michigan Antrim 64.9990.81 0.015.76 0.5114.33 0
Illinois New Albany 54.2192.42 5.5510.32 0

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Table 5. Statistics on gas saturation of typical shale gas and tight gas reservoirs in China and abroad[2, 3032]
.
Country Gas field Basin Epoch Lithology Gas saturation/% Water saturation/%
ChangningWeiyuan Sichuan Late OrdovicianEarly Silurian Siliceous/calcareous shale 6074 2035
FushunYongchuan Sichuan Late OrdovicianEarly Silurian Siliceous/calcareous shale 6067 2030
North Ordos Ordos Middle Permain Tight sandstone 4450 5056
North Ordos Ordos Late Carboniferous Tight sandstone 5255 4548
China
East Ordos Ordos Middle Permain Tight sandstone 4050 5060
Barnett Fort Worth Mississippian Siliceous/calcareous shale 6575 2535
Haynesville Texas-Louisiana Salt Late Jurassic Siliceous/calcareous shale 6585 1535
America
Marcellus Appalachian Middle Devonian Siliceous/calcareous shale 6588 1235
However, all of the conventional gas reservoirs in carbonate
rock of the Sichuan Basin contain H2S as the carbonate rock is
prone to TSR reaction with hydrocarbon to form H2S. The
H2S content in the natural gas of the Huanglong Formation in
the eastern Sichuan Basin is 0.12%0.79%. In comparison,
shale gas exploitation is safe and environmentally friendly
with less corrosion of equipment, and the shale gas can be
used directly after a small amount of processing, so shale gas
exploitation can save costs.
2.5. Oil and gas saturation
Organic-rich shale is characterized by high oil and gas sat-
uration and ultra-low water content. Table 5 shows that the
shale gas reservoirs in the United States and the Sichuan Basin
of China have a gas saturation of 65% to 88% and 60% to 74%
respectively. And all of them with contain low water content
of 12%35%[31]
. According to statistics, tight sandstone res-
ervoirs and sandstone oil and gas reservoirs have a gas satura-
tion of 30% to 55% and 50% to 75% respectively[32]
.
In the process of hydrocarbon generation and expulsion,
organic-rich shale is the first reservoir saturated with oil and
gas. Affected by overpressure, molecular force, and high
temperature, the oil and gas will displace the free water and
some bound water in the shale, resulting in relatively high gas
saturation. In conventional reservoirs, oil and gas accumulate
under the driving of buoyancy and capillary pressure differ-
ence, but these forces are limited in effective scope, and only
the free water in the dominant migration path is driven away,
and the oil and gas saturation is relatively low under the re-
striction of relative permeability.
Therefore, shale gas reservoirs often produce less water,
while conventional oil and gas reservoirs often have water
channeling and waterflooding, which cause troubles for oil
and gas production.
3. New understandings of enrichment and high
production of unconventional oil and gas
The recoverable resources potential of shale oil and gas
reservoir depends on the amount of hydrocarbons that have
been generated and retained by organic-rich shale, and the
amount of hydrocarbons that can be produced after shale res-
ervoir stimulation[33]
. At present, techniques such as horizontal
drilling and completion and staged volume fracturing are
commonly used, which can realize the effective and scale
development of shale oil and gas. In order to reduce the risk
of exploration and development, it is necessary to identify the
“sweet spot area” on the plane and the “sweet spot interval”
on the section for the shale oil and gas in a large area and con-
tinuous distribution[3436]
. Based on the exploration practice of
the Wufeng Formation–Longmaxi Formation, we have some
new understandings on shale gas enrichment regularities and
high production.
3.1. Shale oil and gas enrichment
The material basis of shale oil and gas is the shale zone
with high organic matter abundance, which is dependent on
the lithofacies and paleogeography, paleoproductivity and
water environment at the time the shale deposited. Most ma-
rine organic-rich shale formations deposit in the semi-deep to
deep shelf; due to the connection with the open sea and the
influence of upwelling of ocean currents, plankton was pros-
perous and thick organic-rich shale deposited, forming the
sedimentary center of the basin, which is the favorable area
for shale oil and gas exploration. Continental organic-rich
shale formations mainly deposit in the semi-deep to deep lake
environment with weak hydrodynamic effect and low dis-
solved oxygen, where the sedimentary center is relatively
consistent with the subsidence center, with massive or-
ganic-rich shale built, so is often the shale oil and gas enrich-
ment center too.
The key to the success of shale gas exploitation in North
America and China is the development of high-quality or-
ganic-rich shale. The TOC content of the “sweet spot interval”
in shale gas fields discovered in North America is generally
greater than 4%, mostly from 5% to 10%, and the TOC con-
tent of the lower member of the Marcellus shale, the gas-pro-
ducing shale largest in area, is 10% to 20%[2425]
. The Wufeng
Formation–Longmaxi Formation shale in the Sichuan Basin
of China has a TOC content of greater than 2% in general, and
the high-quality shale interval has a TOC content of above
3.5% and gas content of 4-8 m3
/t (Table 6). High TOC content
is an important material basis for the formation of shale gas
“sweet spot”.
The TOC content of shale is not only related to the amount
of gas generated, but also positively related to the develop-
ment degree of organic matter pores. The organic matter pores
in shale provide the main space for the accumulation and
storage of shale gas (Fig. 5). When the thermal evolution

7. JIAO Fangzheng / Petroleum Exploration and Development, 2019, 46(5): 847–855
 853 
Table 6. Main parameters of shale gas reservoirs in the Sichuan Basin.
Gas field TOC/%
Gas content/
(m3
t1
)
Free gas
content/%
Porosity/
%
Microfracture
development degree
Local structure
Pressure
coefficient
Original aver-
age daily pro-
duction/104
m3
Fuling 0.36.8(3.6) 0.359.63(4.21) 6080 5.08.6(6.4) Developed Box anticline 1.55 32.3
Weiyuan 1.96.4(2.7) 2.745.01(2.92) 6070 3.96.7(5.3) Relatively developed High part of the slope 1.001.70 16.8
Changing 1.97.3(4.0) 1.706.50(4.10) 6070 3.48.2(5.4) Relatively developed High part of the slope 1.352.03 18.6
Zhaotong 1.64.9(3.2) 0.605.80(2.30) 6070 2.67.9(5.0) Relatively developed High part of the slope 1.00 18.0
Evaluation crite-
rion of sweet spot
>3.0% >3 >60 >4.0 Relatively developed Positive structure >1.20 >10
Note: the values in brackets are the average values.
Fig. 5. Gas occurrence pattern in the organic matter pores of
shale in the Wufeng Formation–Longmaxi Formation of Sichuan
Basin. 3 145 m of Well W205, TOC content is 3.2%, and the di-
ameter of organic matter pores is 10 nm2 µm.
degree of shale organic matter is moderate, the TOC content
and the porosity of organic matter are positively correlated.
According to the petrophysical model used to characterize
porosity, the contribution of each kind of pore to the total po-
rosity was calculated[37]
. The organic matter pores in the
“sweet spot intervals” of the shale gas of Wufeng Forma-
tion–Longmaxi Formation in the Sichuan Basin are up to 30%
to 50%, providing abundant and effective reservoir space for
shale gas enrichment.
3.2. New understandings of shale oil and gas production
The gas in organic-rich shale is mainly composed of free
gas and adsorbed gas. The ratio of free gas to adsorbed gas is
controlled by the current temperature and pressure of the shale
gas reservoir, and the free gas content is positively correlated
with the shale gas production. The development practice of
shale gas reservoir in the Wufeng Formation–Longmaxi For-
mation of the Sichuan Basin shows that the higher the free gas
content of the shale gas layer, the higher the daily production
of a single well will be, the higher the EUR of a single well
will be, and positive structures are conducive to the accumu-
lation of shale gas and high production. The Fuling shale gas
field is a broad anticline (Fig. 6) where the strata relief at the
core is no more than 10; the internal free gas content is 60%
to 80%, and the average production of a single well is
32.3104
m3
/d[38]
. In Changning shale gas field, the free gas
content is 55%65%, and the original average production of a
single well test is 18.6104
m3
/d. The difference in original
production is proportional to the free gas content.
The porosity of shale is composed of matrix porosity and
fracture porosity. The pores with large volume and small spe-
cific surface area in the reservoir are the main space storing
free gas (Fig. 5), while the development of bigger pores and
microfractures is the key for the high production of shale gas.
The high production of shale gas layer is the external mani-
festation of high porosity and high permeability. From the
initial production distribution of shale gas wells at different
depths in the Changning and Fuling blocks (Fig. 7)[39]
, it is
found that although the two gas fields have different tectonic
backgrounds, the intervals of high-production are located in
the Wufeng Formation and the lower part of the Longmaxi
Fig. 6. Geological profile of shale in Wufeng Formation–Longmaxi Formation in the Fuling gas field of the Sichuan Basin (modified
according to reference [38]).

8. JIAO Fangzheng / Petroleum Exploration and Development, 2019, 46(5): 847–855
 854 
Fig. 7. Relationship between initial production of shale gas well
and microfracture development of the Wufeng Formation-
Longmaxi Formation in the Sichuan Basin.
Formation where the porosity is higher and microfractures are
more developed. The high-quality shale gas reservoirs in wells
JY1, JY2, JY3 and JY4 of Fuling shale gas field have a poros-
ity of 4.65%6.20% and permeability of (0.131.27)103
μm2
;
moreover, the reticular fractures formed by slippage effect of
the reverse faults on both sides of the gas reservoir greatly
increase the storage space and flow efficiency of the free
gas[40]
. In the Changning shale gas field, the main production
interval is comparable to that of the Fuling shale gas field in
porosity, but is slightly lower in permeability (the average
value is 2 orders of magnitude lower than that of the Fuling
gas field); lamellation seams and a small number of structural
fractures are the main seepage channels, and the overall pro-
duction is significantly lower than that of the Fuling gas field
(Fig. 7). According to the petrophysical model of shale in the
Wufeng Formation–Longmaxi Formation established be-
fore[38]
, the porosity of the shale gas layers of the Fuling and
Changning gas fields is about 4.3%5.4%, and microfractures
are generally developed. Combined with the statistics, it is
confirmed that the porosity of the high-production interval is
above 4.0%.
4. Conclusions
According to the accumulation mechanism, heavy oil, oil
sands, tight oil and gas, and gas hydrates are classified as
conventional oil and gas; unconventional oil and gas include
shale oil, shale gas, and coalbed methane. The formation and
accumulation mechanism of unconventional oil and gas is
further clarified in this paper, and it is pointed out that uncon-
ventional oil and gas reservoirs feature source-reservoir-in-
one, continuous in-situ accumulation, and accumulation under
the driving forces of overpressure and diffusion.
Unconventional oil and gas reservoirs are mainly formed in
the low-energy oxygen-anaerobic environment. They are sim-
ple in lithology, and rich in organic matter and clay minerals.
Their main reservoir space is nano-scale and complex pore
structure. Organic matter pores constitute a major part of res-
ervoir space in them. In addition, unconventional oil and gas
reservoirs have oil and gas in a uniform distribution, high oil
and gas saturation, low water content, and no obvious boun-
daries between oil, gas, and water.
The sedimentary environment controls high organic matter
abundance zone, organic matter content controls oil and gas
abundance; positive structure, high porosity, bedding (lamel-
lation) and the development of fractures control the produc-
tion of shale wells. The TOC content greater than 3.0%, po-
rosity of more than 4.0%, and densely developed microfrac-
tures are important indicators for identifying “sweet spot” of
shale gas.
Re-recognition of “the unconventional” in unconventional
oil and gas further enriches the unconventional oil and gas
geologic theory and will promote new progress in unconven-
tional oil and gas exploration and development.
Acknowledgment
During the research process, this study has received support
from CNPC and related enterprises and the National Science
and Technology Major Project 2017ZX05035. In the process
of writing this article, the authors received help from Guan
Quanzhong from China Petroleum University (Beijing), and
Sun Shasha, Zhang Surong, Jiang Shan, Guo Wen, Shi Zhen-
sheng, Ma Chao, Qiu Zhen and Yu Rongze from Research
institute of China Petroleum Exploration and Development,
and Zhang Hualing from University of Houston. Thanks for
their help here.
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