PETROLEUM EXPLORATION AND DEVELOPMENT

Volume 43, Issue 6, December 2016

Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2016, 43(6): 925–940. RESEARCH PAPER

Assessment of global unconventional oil and gas resources

WANG Hongjun*, MA Feng, TONG Xiaoguang, LIU Zuodong, ZHANG Xinshun, WU Zhenzhen,
LI Denghua, WANG Bo, XIE Yinfu, YANG Liuyan
PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China

Abstract: This paper evaluates the recoverable unconventional oil and gas resources around the world, reveals main controlling factors
and potential regions for the rich accumulation of unconventional oil and gas, and standardizes the classification of seven types of re-
sources (i.e., heavy oil, oil sand, tight oil, oil shale, shale gas, tight gas, and coalbed methane). By virtue of commercial databases for
global petroliferous basins, together with single-well data packages in North America and basic data of exploration and development of
Chinese companies in unconventional oil and gas resources blocks around the world, contour maps of abundance for global recoverable
resources are formed through spatial graphic interpolation of key assessment parameters of seven types of unconventional oil and gas re-
sources on the Geographic Information System (GIS) platform, which systematically evaluate the potential of seven types of unconven-
tional oil and gas resources. The assessment reveals: (1) These seven types of resources around the world are distributed predominantly in
476 formations in 363 petroliferous basins. (2) Total recoverable unconventional oil and gas resources in the world are respectively 442.1
billion tons and 227 trillion cubic meters. (3) Unconventional oil and gas resources can be divided into “source-bound type” and “stra-
ta-bound type”. The “source-bound type” resources are mainly controlled by 6 groups of high-quality source rock around the world,
among which, the tight oil and gas resources are featured by the “integration of reservoir and source”, presenting the best prospect for the
development and application, and the “strata-bound type” oil sand and heavy oil resources, controlled by the transformation of the late
structure, are mainly distributed in the slope belt of the Mesozoic-Cenozoic basins, presenting a good prospect for the resource develop-
ment and application in the shallow layers. (4) Besides hot spots in North America, tight oil in the West Siberia Basin and the Neuquen
Basin as well as heavy oil in the Arab Basin will become potential targets for the development of unconventional oil and gas resources in
the future.

Key words: unconventional oil and gas; resources assessment; assessment parameters; assessment methods; recoverable resources;
main control factors of enrichment

Introduction oil and gas supply and demand structure[2], and are now
Currently, unconventional oil and gas resources which have gradually becoming alternatives of conventional oil and gas
been in commercial development and application around the resources[45].
world can be classified into seven types, i.e. heavy oil, oil To evaluate the global unconventional oil and gas resources,
sand, tight oil, oil shale, shale gas, coalbed methane, and tight 3 issues need to be addressed: (1) geological conditions for
gas. In recent years, the production of unconventional oil and the formation of resources and their distribution; (2) selection
gas has increased continuously. In 2015, the annual produc- of assessment parameters and methods; and (3) selection of
tion of unconventional oil amounted to 3.7×108 t, accounting favorable areas based on the recoverability of resources.
for 9% of the global annual production of oil, and the annual Since 2000, with the exploration and development of un-
production of unconventional gas was 9 273×108 m3, ac- conventional oil and gas, several world famous energy as-
counting for 27% of the global annual production of gas[1]. In sessment agencies such as the United States Geological Sur-
the same year, the US annual production of tight oil amounted vey (USGS), Energy Information Administration (EIA), and
to 2.59×108 t, accounting for 45% of its annual production of Hart Energy have successively evaluated potential unconven-
oil, and the unconventional gas was 4 500×108 m3, accounting tional oil and gas resources around the world[613]. However,
for 50% of its annual production of gas[23]. Unconventional these assessment agencies only released their assessment re-
oil and gas resources have made significant impact on global sults without specific assessment methods and key parameters,

Received date: 20 May 2016; Revised date: 21 Sep. 2016.

* Corresponding author. E-mail: whj@petrochina.com.cn
Foundation item: Supported by China National Science and Technology Major Project (2011ZX05028-002); CNPC Key Science and Technology Project
(2013E-050102).
Copyright © 2016, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
 WANG Hongjun et al. / Petroleum Exploration and Development, 2016, 43(6): 925–940

so the results are not comparative and summarizable, and it is assessment parameters on the geographic information system
difficult to tell their accuracy. For example, EIA released that (GIS) platform, and calculating the contour map of recover-
the total recoverable shale gas resources in China was 32×1012 able resource abundance of the assessment units. The assess-
m3, many Chinese researchers question its assessment basis ment results of resources for one assessment unit change from
and reliability because they didn’t provide any proof[2]. one figure to one contour map of resource abundance, which
Based on the assessment technique of global unconven- makes the selection of favorable zones much easier. For ba-
tional oil and gas resources and favorable zone selection of sins with high development degree of tight oil and shale gas,
China National Petroleum Corporation (CNPC), the global estimated ultimate recovery (EUR) is used to calculate the
unconventional oil and gas resources have been evaluated in distribution of recoverable reserve abundance of assessment
this study. The assessment involves four steps: (1) based on units. Based on recoverable resource abundance and reserve
international universality and operability of geological as- distribution obtained, potential zones can be ranked based on
sessment, reviewing the definitions and screening standards of their abundance values, and accordingly the favorable zones
7 types of unconventional oil and gas resources; (2) examin- of global unconventional oil and gas resources can be se-
ing the resource distribution in global petroliferous basins in lected.
details, including the strata combination of every basin; (3)
1. Assessment of unconventional oil and gas
dividing evaluation objects into two types, i.e. detailed as-
resources
sessment and statistical assessment, according to screening
results and data details. The former need to make contour 1.1. Resource classification
maps according to assessment parameters, while the later only
need to make maps of key parameters (for other parameters, In order to simplify assessment process and make assess-
only probability distribution values are needed); (4) selecting ment results more practical, this study establishes the defini-
assessment method, improving currently prevailing volumet- tions and classification criteria for every type of unconven-
ric method, realizing spatial interpolation operation of several tional resource (Table 1) based on applicable Chinese

Table 1. Definitions and classification criteria of unconventional oil and gas resources
Criteria
Resource Overburden pres- Calorific Refer-
Definition Viscosity/ Oil Methane
type sure matrix perme- value/ ences
(mPa·s) 3 content/% content/%
ability/10 μm 2
(MJ·kg1)
Refers to the crude oil that is difficult to or 50
Heavy oil [4,6,13]
cannot flow at reservoir temperature. 10 000
Or called tar sand, specially refers to sandstone
or other rocks containing natural asphalt,
Oil sand >10 000 [4,14,19]
which is composed of asphalt, sand,
water, clay, and other minerals.
Refers to a kind of oil accumulating in tight sand-
stone, tight carbonatite, and other reservoirs; tight
[1,4,9,
oil wells generally have no natural production
Tight oil ≤0.200 11,15,
capacity, but can obtain industrial oil production
2023]
by taking some technical measures under
certain economic conditions.
Refers to combustible shale with high ash content
Oil shale and high organic matter content;shale oil can be >3.5 >4.18 [4,19,24]
obtained through low temperature carbonization.
Refers to natural gas occurring in rich organic shale
reservoir in free and absorbed states; shale gas wells
[12,4,9,
Shale gas generally have no natural production capacity, but ≤0.001
12,16]
can obtain industrial oil production with some tech-
nical measures under certain economic conditions.
Refers to natural gas accumulating in tight sand-
stone and other reservoirs; tight gas wells generally
Tight gas have no natural production capacity, but can obtain ≤0.100 >85 [4,17,23]
industrial oil production under certain economic
conditions and technical measures.
Refers to hydrocarbon gas occurring in coal seam,
Coalbed which mainly absorbs on the surface of coal matrix
[78,18]
methane grains, but part of which exists in free state in coal
pores or dissolves in coalbed water.

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National Standards and China’s oil and gas industry standards, In addition to hot spots like Williston, Gulf, Fort Worth, and
by investigating the standards adopted by global agencies and Permian Basins in North America[3031], the assessment of tight
oil companies[14, 624] and considering the geological charac- oil and shale gas put stress on evaluating resource potential of
teristics of every type of resource, international generality of the following areas: Jurassic-lower Cretaceous Vaca Muerta
their concepts and operability of resource assessment. Fm and Jurassic Los Molles Fm of the Neuquen Basin in
South America, Silurian Tannezuft Fm and Devonian Frasnian
1.2. Selection of assessment parameters
Fm of the Triassic Ghadames Basin as well as Cretaceous
After over one hundred years of exploration and develop- Sirte Fm and Etel Fm of the Sirte Basin in Africa, Bazhenov
ment, conventional oil and gas in most petroliferous basins Fm of the West Siberia Basin in Russia, Silurian Llandovery
and formations have already been discovered and studied. But Fm of the Baltic Sea in Europe, and Ordovician Goldwyer Fm
the distribution of unconventional oil and gas resources in the of the Canning Basin in Australia.
global petroliferous basins has not yet been systematically The assessment of tight gas resources focuses on the basins
examined. In this study, we collected data from IHS, USGS, under commercial development, such as Clinton Fm of the
EIA, C&C and other world’s large databases as well as explo- Appalachian Basin, Milk River Fm in Alberta, and Cotton
ration, development and production data acquired in conven- Valley of the Gulf Basin. Considering the applicability of ex-
tional blocks by Chinese oil companies in North America, istent mining technologies, in the assessment of coalbed
South America and Australia, and also purchased the data methane, many basins with abundant resources but not suit-
packets of unconventional oil and gas of North America, Eu- able for recovery due to large burial depth are not included.
rope, South America, Central Asia, and Russia. According to
1.3. Assessment method
the above definitions and standards (Table 1), we have
completed the selection, forms, and map compilation of geo- Based on the available data of global unconventional oil
logical parameters for the assessment of global unconven- and gas enrichment basins, the improved volumetric method,
tional oil and gas resources. For the first time, it is figured out EUR abundance analogy method, and parameter probabilistic
that the seven types of unconventional oil and gas resources method are adopted to assess the 7 types of unconventional oil
are mainly distributed in 476 formations in 363 basins around and gas resources. For basins with high exploration and de-
the world (Tables 2, 3, 4, 5, and 6). velopment degree, production wells, production well data,
Based on the above study on the geological features of the basic geological data are used for mapping, after confirming
enrichment series of global unconventional oil and gas re- the effective assessment areas, EUR abundance method is
sources, we have figured out the distribution and enrichment adopted for assessment. In this paper, EUR abundance method
regularity of the global unconventional oil and gas resources, is used to calculate tight oil and shale gas. For developed and
which ensures the accuracy of the assessment parameters and explored basins with rich basic geological data, but lack of
improves the reliability of assessment results. For example, oil production data, the improved volumetric method is adopted.
sand resource in the East Siberian Basin has long been a hot In this study, this method is used to calculate heavy oil, oil
research topic, but quantity of its resources estimated by many sands, oil shale, tight gas and coalbed methane. The calcula-
institutions vary greatly[6,13,2628]. According to the USGS tion principle, software and computing platform used for all
evaluation in 1987, its geological reserves were 886×108 t and the 7 types of resources are the same, but the assessment pa-
revised to 85×108 t in 2006[26]. In 1989, Medaisko evaluated rameters and variables (Table 2 and Table 6) are different. For
its geological resources at (9601 098)×10 8 t [27] , and basins with lower degree of exploration and development and
Starosel’tsev gave the assessment result of 820×108 t[28]. Ac- lack of basic data and basic parameter maps, the parameter
cording to CNPC’s evaluation during the “11th Five-Year probabilistic method is used for assessment. In this study, this
Plan”, its recoverable resources were 367×108 t, almost equiv- method is adopted to evaluate the basins of 7 types of re-
alent to that in the Alberta Basin (383×108 t). source with low degree of exploration.
In the heavy oil resource assessment, this paper focuses on
1.3.1. Improved volumetric method
new areas outside South America, such as the Cretaceous in
the Zagros Basin in the Middle East, the Cretaceous and Tri- Unconventional oil and gas is characterized by large-scale
assic in the West Arabian Basin, and the Cretaceous and Ju- continuous accumulation and distribution, so the volumetric
rassic in the Middle Arabian basin. Based on previous re- method is the most effective resource assessment method[3840].
searches, the assessment of oil sands focuses on three large But reservoirs in most areas containing unconventional oil and
paleo-uplift areas (i.e., Olenyok, Anabar and Aldan)[28, 29] in gas are strongly heterogeneous and greatly different in oil and
the East Siberia Basin, through careful geological analysis, the gas abundance, calculation results from traditional methods
oil sands zones, lithology, oil content, and mining way of dif- can only reflect total quantity but not the difference in re-
ferent accumulation belts have been confirmed. The assess- source distribution. In this study, the volumetric method has
ment result indicates that the recoverable resources are 61×108 t been improved, and based on the GIS platform, organic matter
(Table 7). abundance, maturity assessment, thickness, buried depth,

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Table 7. Geological parameters of oil sand resources in the East Siberian Basin[6,13,2628 3337]
Name of Accumulation Area/ Lithology of Net thick- Oil Quantity of Recovery Recoverable
Reservoir
zone zone km2 the reservoir ness/m content/% resources/108 t method resources/108 t
Olenyok mesozone Permian 1 750 Dolomite 5 2.0 5.2 Steam-
Cambrian- Limestone, assisted
Olenyok Olenyok zone 5 850 15 3.0 62.0
Vendian sandstone gravity 37
uplift
Riphean, Oolitic drainage
Cueca zone 1 750 5 3.0 7.0
Cambrian limestone (SAGD)
Southern Middle Cambrian-
9 100 5 2.5 25.0 10
Anabar uplift Upper Cambrian
Oolitic
Anabar Eastern Lower
2 400 limestone, 12 3.0 25.0 10
uplift Anabar uplift Cambria Steam
dolomite
Northern Riphean, flooding
600 10 2.0 2.5 1
Anabar uplift Cambrian
Aldan Lower Cambrian,
Tuloba zone 3 650 dolomite 5 2.0 7.8 3
uplift Vendian

porosity and other key parameters have been gridded by re-

gion. Each parameter is spatial interpolated on a grid basis,
and in this way, the total resources of each grid are calculated,
then through accumulating all grids, resources of a whole
assessment area are calculated. Meanwhile, recoverable coef-
ficient under different geological conditions of the assessment
unit are analyzed and mapped. After calculation, the distribu-
tion map of “recoverable resource abundance” of unconven-
tional oil and gas in the assessment unit is obtained. Total
recoverable resources can be automatically obtained by ac-
cumulating area of the map.
Taking the assessment of tight oil of the Cretaceous Vaca
Muerta Fm in the Neuquen Basin as an example, we plotted
contour maps of some key factors in volumetric method for-
mula, such as oil saturation, thickness, porosity and oil recov-
ery, and then gridded them respectively. We used the Inverse
Distance Weighted method for grid interpolation and calcu-
lated the recoverable resources of each grid using the volu-
metric method formula, i.e., based on the confirmed
oil-bearing area of tight oil and gas-bearing area of shale gas, Fig. 1. Assessment parameters and recoverable resources of
superimposed the oil/gas bearing area, conversion coefficient tight oil and shale gas of the Vaca Muerta Fm in the Neuquen
Basin.
and oil/gas saturation with the contour maps of new reservoir
thickness, porosity, and recovery rate. In this way, we ob- time limit and economic capacity limit are determined. When
tained the recoverable resources abundance distribution and the annual production decline rate is less than 10%, the hy-
calculated the final recoverable resources through integration perbolic decline trend is transformed into exponential decline
(Fig. 1). trend, thus avoiding the problem that hyperbolic decline
model still shows a constant capacity after several years of
1.3.2. EUR abundance method
production. According to EUR and control area of a well, the
For the tight oil production blocks with lots of production EUR abundance of this well position can be obtained. Then
wells in North America, hyperbolic decline and exponential after space grid interpolation and integration, recoverable
decline methods are used to calculate single-well EUR[4145] reserve abundance map and recoverable reserves within the
and EUR abundance. Then the EUR abundance is grid inter- block can be obtained (Fig. 2).
polated to get the recoverable reserve abundance and the total
1.3.3. Parameter probabilistic method
reserves, which is equivalent to the highest level of recover-
able resources. In the study, a composite pattern of hyperbolic For basins with low degree of exploration and development
decline and exponential decline method is used to calculate and little data, the parameter probabilistic method is directly
the single-well EUR[46, 47]. According to the conditions of the used to calculate the resources[48]. The method, based on the
existing production wells in this block, reasonable production probability theory and using volumetric method, takes
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in 363 basins around the world has been evaluated. The as-
sessment reveals that global recoverable unconventional oil
resources are 4 421×108 t, including heavy oil of 1 267×108 t,
oil sands of 641×108 t, tight oil of 414×108 t, and oil shale of
2 099×108 t; global recoverable unconventional gas resources
are 227×1012 m3, including shale gas of 161×1012 m3, tight gas
of 17×1012 m3, and coalbed methane of 49×1012 m3 (Tables 8
and 9).

2.2. Distribution of recoverable unconventional oil and

gas

2.2.1. Distribution by country

Recoverable unconventional oil resource mainly concen-

trates in 54 countries, the top 10 of them are the United States,
Russia, Canada, Venezuela, Brazil, China, Belarus, Saudi
Arabia, France, and Mexico, and the combined recoverable
unconventional oil resources of them account for 82.4% of the
global total. The recoverable unconventional oil resources in
Fig. 2. EUR abundance of tight oil of the Bakken Fm in the US the United States, mainly oil shale, heavy oil and tight oil, are
Williston Basin. (Data is sourced from the North American drill- 926×108 t, accounting for 21% of the global total; those in
ing database of IHS). Russia, mainly oil shale, oil sands and tight oil, are 892×108 t,
accounting for 20.2%; those in Canada, mainly oil sands and
parameters for resource assessment as random variables, and tight oil, are 397×108 t, accounting for 9%; those in Venezuela,
requires that the parameters are independent from each other. mainly heavy oil, are 353×108 t, accounting for 8%; those in
Through uncertainty analysis, finally it uses the Monte-Carlo China, mainly oil shale and tight oil, are 212×108 t, accounting
method and volumetric method to obtain the probability dis- for 4.8%. The recoverable unconventional oil resources in the
tribution of resources[4648]. Its estimation results are a prob- United States, Russia and Canada combined account for 50%
ability distribution curve of resources and the large, medium of the global total. Oil shale accounts for the largest percent-
and small resource values estimated according to specified age of 47.5% in global recoverable unconventional oil re-
probability values. sources, followed by heavy oil, 28.7% of the global total. Oil
2. Resource assessments sands and tight oil account for 14.5% and 14.5% of the global
total, respectively (Fig. 3).
2.1. Assessments of global recoverable
Recoverable unconventional gas resources mainly concen-
unconventional oil and gas resources
trate in 37 countries, the top 10 of them are the United States,
Using the improved volumetric method, EUR abundance China, Russia, Canada, Australia, Iran, Saudi Arabia, Argentina,
method and parameter probabilistic method, the recoverable Libya and Brazil, with their recoverable resources combined
resource potential of unconventional oil and gas of 476 series accounting for 76.8% of the global total. The recoverable

Table 8. Assessment of geological and recoverable resources of global unconventional oil

Resources/108 t
Region Heavy oil Oil sand Tight oil Oil shale Unconventional oil
Recoverable Geological Recoverable Geological Recoverable Geological Recoverable Geological Recoverable Geological
North
318 3 177 395 3947 91 2 540 699 3 279 1 503 12 943
America
Russia 88 449 156 599 77 1 555 570 1 927 891 4 530
South
409 4 092 0 0 68 1 954 150 280 627 6 326
America
Europe 82 224 18 54 26 700 354 2 334 480 3 312
Asia 130 502 48 273 79 2 050 120 137 377 2 962
Middle
177 1 208 0 0 13 357 102 176 292 1 741
East
Africa 63 186 24 140 42 1 191 68 115 197 1 632
Oceania 0 0 0 0 18 871 36 97 54 968
Total 1 267 9 838 641 5013 414 11 218 2 099 8 345 4 421 34 414

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Table 9. Assessment of geological and recoverable resources of global unconventional natural gas
Resources/1012 m3
Region Shale gas Tight gas Coalbed methane Unconventional gas
Recoverable Geological Recoverable Geological Recoverable Geological Recoverable Geological
North America 34 136 5 40 17 28 56 204
Asia 26 108 9 42 14 21 49 171
Russia 15 53 0 3 15 24 30 80
Middle East 21 94 0 2 0 0 21 96
Africa 19 73 0 0 0 0 19 73
South America 19 75 0 1 0 1 19 77
Europe 16 67 1 3 0 1 17 74
Oceania 11 44 2 4 3 6 16 51
Total 161 650 17 95 49 81 227 826

Fig. 3. Unconventional oil recoverable resource potential of the Fig. 4. Unconventional gas recoverable resource potential of the
top 20 countries. top 20 countries.

gas resources in the United States are 39×1012 m3, account- Volga-Ural Basin are 305×108 t, accounting for 6.9%, mainly
ing for 17.4% of the global total, mainly shale gas; those in oil shale and tight oil sands; those in the Piceance Basin are
China are 31×1012 m3, accounting for 13.9%, mainly shale 301×108 t, accounting for 6.8% of the global resources, mainly
gas, coalbed methane and tight gas; those in Russia are oil shale; those in the East Venezuela Basin are 262×108 t,
29×1012 m3, accounting for 12.6%, mainly shale gas and accounting for 5.9%, mainly heavy oil (Fig. 5).
coalbed methane; those in Canada are 16×1012 m3, account- Recoverable reserves of unconventional natural gas are
ing for 7%, mainly coalbed methane and shale gas; those in mainly discovered in 147 basins around the world, among
Australia are 16×1012 m3, accounting for 6.4% of the global which the top 10 basins are Alberta, Zagros, Appalachia, East
total. Recoverable unconventional gas resources in the Siberia, Gulf, Mid-Arab, Triassic-Ghadames, Kuznetsk, Can-
United States, China, Canada and Australia combined ac- ning and Parana basins, combined accounting for 43.3% of
count for 57.2% of the global total. The shale gas accounts the global resources. The recoverable unconventional natural
for the largest percentage of 71.1% in global recoverable gas reserves in the Alberta Basin are 16×1012 m3, accounting
unconventional gas resources, followed by coalbed methane for 7% of the global total, mainly coalbed gas and shale gas;
21.7%, tight gas 7% (Fig. 4). those in the Zagros Basin are 12×1012 m3, accounting for 5.2%,
mainly shale gas; those in the Appalachia Basin are 11.5×1012
2.2.2. Distribution and types of basins
m3, accounting for 4.5%, mainly shale gas; those in the East
Recoverable unconventional oil resources mainly exist in Siberia Basin are 10.3×1012 m3, accounting for 4.5%, mainly
216 basins around the world, among which the top 10 basins shale gas and coalbed methane (Fig. 6).
are the Alberta, West Siberia, Volga-Ural, Piceance, East Ven- The statistics on the types of basins rich in unconventional
ezuela, Uintah, Dnepr-Donetsk, East Siberia and Mid-Arab oil and gas reveal that unconventional oil resources are mainly
basins, with 57.2% of the global total. The recoverable re- distributed in foreland basins, craton basins, passive conti-
serves of unconventional oil in the Alberta Basin are 405×108 t, nental margins, rift basins, fore-arc basins and back-arc basins.
accounting for 9.2% of the global total, mainly oil sands; Foreland basins, richest in unconventional oil, have recover-
those in the West Siberia Basins are 312×108 t, accounting for able resources of 2,556×108 t, accounting for 58% of the
7.1%, mainly oil shale and tight oil; those in the global total, craton basins, passive continental margin and rift
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are 58×1012 m3, 26×1012 m3, 16×1012 m3 and 1×1012 m3, ac-
counting for 26%, 11%, 7% and 1%, respectively. Shale gas is
mainly discovered in the Zagros foreland basin, Appalachia
foreland basin, Gulf foreland basin, Triassic-Ghadames craton
basin, Cunning craton basin, West Siberia rift basin and mid-
Arab passive continental margin basin. Coalbed gas is mainly
distributed in the Alberta foreland basin, East Siberia craton
basin and Kuznetsk rift basin. Tight gas is mainly distributed in
the Appalachia foreland basin and Alberta foreland basin (Fig. 8).

2.2.3. Unconventional oil and gas distribution in

formations

Recoverable unconventional oil resources are mainly ac-

cumulated in the Mesozoic and Cenozoic. The recoverable
Fig. 5. Recoverable unconventional oil potential in the top 20 reserves of the Paleogene-Neogene, Cretaceous and Jurassic
basins. unconventional oil resources are 3 418×108 t, accounting for
77.3% of the global total. The recoverable reserves of the
Paleogene-Neogene resources are 1 433×108 t, accounting for
32.4% of the global total, which is the most potential, mainly
heavy oil and oil shale; those of the Cretaceous resources are
1 120×108 t, accounting for 25.3%, mainly oil sands, heavy oil
and oil shale; those of the Jurassic resources are 865×108 t,
accounting for 19.6%, mainly oil shale and heavy oil; those of
the Devonian resources are 379×108 t, accounting for 8.6%,
mainly oil shale and tight oil; those of the Precambrian re-
sources are 164×108 t, accounting for 3.7%, mainly oil shale
(Fig. 9).

Fig. 6. Recoverable unconventional gas potential in the top 20

basins.

basins, and fore-arc and back-arc basins in sequence contain

720×108 t, 481×108 t, 474×108 t, 128×108 t and 63×108 t of
unconventional oil, accounting for 16%, 11%, 11%, 3% and
1% respectively. Heavy oil is mainly distributed in the East
Venezuela foreland basin, Maracaibo foreland basin, Tampico Fig. 7. Types of basins rich in recoverable unconventional oil
basin and Arab passive continental margin basin. Oil sands are resources.
mainly distributed in the Alberta foreland basin and East Si-
beria craton basin. Tight oil is mainly distributed in the Neu-
quen foreland basin, Williston craton basin, and West Siberia
rift basin. Oil shale is mainly distributed in Piceance foreland
basin, Volga-Ural foreland basin, Uintah foreland basin,
Dnepr-Donetsk foreland basin, Paris craton basin, West Sibe-
ria rift basin and Arab passive continental basin (Fig. 7).
Unconventional natural gas resources are mainly distrib-
uted in foreland, craton, rift, passive continental margin and
back-arc basins. Foreland basins with the highest degree of
enrichment, have an unconventional natural gas recoverable
resources of up to 125×1012 m3, accounting for 55% of the
global total; the unconventional gas resources in craton basins, Fig. 8. Types of basins rich in recoverable unconventional gas
rift basins, passive continental margins, and back-arc basins resources.

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Fig. 9. Recoverable unconventional oil potential in the forma- Fig. 10. Recoverable unconventional gas potential in the forma-
tions. tions.

sands belong to “strata-bound type” resources.

Unconventional natural gas is extensively distributed in
Mesozoic and Paleozoic, mainly in Jurassic, Cretaceous, Silu- 3.1. “Source-bound type” resources
rian, Carboniferous and Permian systems, in the form of shale
Comparing accumulation formations of “source-bound
gas and coalbed methane. The recoverable reserves in the Ju-
type” resources such as coalbed methane, shale gas and tight
rassic are 44×1012 m3, accounting for 19.6% of the global total,
oil, and formations of source rocks, reservoirs, and caprocks
mainly shale gas, followed by coalbed methane and tight gas;
of conventional oil and gas shows that the enrichment and dis-
those in the Cretaceous are 36×1012 m3, accounting for 15.8%,
tribution of “source-bound type” resources have good cor-
mainly shale gas and coalbed methane; those in the Silurian
responding relationship with the main source rocks of global
are 30×1012 m3, accounting for 13.1%, mainly shale gas and
conventional oil and gas. The regional high-quality source
tight gas; those in both Permian and Devonian are 18×1012 m3,
rocks around the world are mainly formed in six geological
accounting for 8% respectively, mainly shale gas (Fig. 10).
periods[49]: (1) Silurian (409430 Ma); (2) the Late Devo-
3. Primary factors controlling enrichment of nian (362374 Ma); (3) Pennsylvania (Carboniferous) to the
unconventional oil and gas resources late Permian (299318 Ma); (4) the Late Jurassic (161175
Based on the characteristics that unconventional oil and gas Ma); (5) the Cretaceous (99145 Ma); and (6) the Oligo-
are usually enriched in source rock and later damaged reser- cene-Miocene (534 Ma) (Fig. 11). The global sea-level rise
voir rock formations[25], they are divided into “source-bound and fall control the sequence distribution, and the sequence
type” and “strata-bound type”. “Source-bound type” resources type determines the organic matter content and reservoir
are oil and gas mainly controlled by one or more sets of property of source rocks. The top of the largest marine
(Carbonaceous) shale, and accumulate in dense layers with flooding surface corresponds to the condensed section (CS)
low porosity and permeability (the porosity of less than 12%, and transgressive systems tract (TST), rich in organic matter,
and the matrix permeability under overburden pressure of less there develop 6 sets of source rocks of this kind. These 6 sets
than 0.1×103 μm2) such as shale, coal seam, siltstone, sand- of source rocks not only control the generation of conven-
stone and carbonate rock, in other words, they are retained tional oil and gas, but also are the main formations of the
within source rock or accumulated in reservoirs between “source-bound type” unconventional oil and gas enrichment
source rock. This type of unconventional oil and gas resources (Fig. 11).
feature large-scale continuous accumulation, no trap boundary, Taking tight oil, shale gas and coalbed methane as exam-
and almost no natural capacity, etc. Tight oil, shale gas, coal- ples, the total recoverable reserves of these three types of un-
bed methane, tight gas and oil shale are all “source-bound conventional oil and gas combined in the 6 sets of source
type”, which are mainly controlled by high-quality source rocks are 1 972×108 t, accounting for 92% of the global total,
rock, and characterized by “integration of reservoir and in which tight oil is mainly distributed in the Devonian, Juras-
source” or near-source enrichment. “Strata-bound type” re- sic and Cretaceous strata, shale gas in the Silurian, Devonian,
sources are mainly oil that enriches in reservoirs with poor Carboniferous, Permian, Jurassic and Cretaceous strata, and
capping conditions, and thickens after suffering washing and coalbed methane in the Carboniferous, Jurassic and Creta-
biodegradation, featuring migration and accumulation from ceous strata (Fig. 11).
source sag to slope or high spots of the structure. In addition,
3.2. “Strata-bound type” resources
conventional oil and gas, intermediate and heavy oil, and oil
sands are distributed in layers, among which heavy oil and oil Since the formation of heavy oil and oil sands resources is
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Fig. 11. Analysis of the proportion of 6 sets of source rocks in “source-bound type” unconventional and conventional oil and gas re-
sources[4]. The oil-gas ratio is the ratio of the oil to the gas generated by the source rock.

mainly controlled by geotectonic background, basin type, grated upward to form the Paleogene-Neogene carbonate
source rock and reservoir scale as well as later thickening and heavy-oil belt[52]. In the ancient cratonic basin of East Siberia,
preservation conditions, the resources are extensively en- biodegradation of oil and gas with the tectonic uplift is the
riched in the Jurassic, Cretaceous, Paleogene and Neogene main genesis of heavy oil, where the oil and gas generated in
strata. In gentle structure areas of large sedimentary basins, Riphean Supergroup source rocks migrated to Vendian and
where source and reservoir formations are in planar contact, Cambrian carbonate reservoirs, forming the early conven-
oil and gas after long-distance migration, shallow burial and tional oil and gas reservoirs covered by thick Cambrian shale
strong late thickening effect, are likely to form large heavy oil caprock; then since the Late Paleozoic, with regional tectonic
and oil sands accumulation, especially since the Cenozoic. uplifting continuously, the original oil and gas reservoirs were
More than 95% of the global oil sands resources are distrib- destroyed, giving rise to oil sands in extensive distribution.
uted in and above the Cretaceous and only the oil sands in the The buried depth of oil sands and heavy oil in large-scale ac-
East Siberia Basin accumulate in the Riphean Supergroup, cumulation is usually less than 3 000 m. The buried depth of
Vendian and Cambrian. Although the recoverable resources heavy oil in eastern Venezuelan Basin is 1002 000 m. The
reach 61×108 t there, the overall oil content is low (2%3%) buried depth of oil sands in the Alberta Basin is much smaller,
and the recoverability is poor. generally less than 200 m, and most part is directly exposed to
Second, long-distance migration is a typical feature of the surface. Shallow burial makes heavy oil and oil sands
large-scale oil sands accumulation[5051]. In eastern Venezuela resources easy to produce.
and the Alberta Basin, oil migrated about 150200 km from
4. Assessment of potential unconventional oil
the foredeep hydrocarbon-generating sags to the slope belts,
and gas regions
during which light components gradually lost, biodegradation
led to gradual thickening of the crude oil, sealing condition Based on the above evaluation results and findings, com-
needed gradually lowered, oil accumulation and dissipation bining with the prospect analysis of development and utiliza-
were balanced, the whole accumulation process tended to be tion of unconventional oil and gas resources, potential zones
stable, and a large oil sand accumulation zone was formed in of tight oil, shale gas and heavy oil around the world have
the slope zone. In passive continental margins and rift basins, been selected and evaluated.
heavy oil and oil sands are mainly transported through faults. 4.1. Potential regions
For example, oil generated by the Triassic, Jurassic and Cre-
4.1.1. Tight oil
taceous source rocks in the Western Arabian Basin migrated
1 000 m vertically but less than 20 km horizontally, and mi- The global top 10 potential regions of tight oil are the Wil-
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liston Basin, the Permian Basin and the Alberta Basin in sin, the Sichuan Basin, the Parana Basin, the Neuquen Basin
North America; the Russian West Siberian Basin; the Neuquen and the Alberta Basin. At present, there is no commercial dis-
Basin and the Chaco-Parana Basin in South America; the Sirte covery of shale gas in the basins outside the Americas. There-
Basin and the Triassic-Ghadamès Basin in Africa; the Indus fore, selection of shale gas enrichment areas still focuses on
Basin in Asia; and the Canning Basin in Oceania. The Siirt the areas with high degree of exploration and development in
Basin in North Africa (tight oil formations evaluated having a the United States, and the grade I and II zones of the Barnett
distribution area of 4.8×104 km2 and thickness of 31 m), the Shale, Fayetteville Shale, Woodford Shale, Eagle Ford Shale
Triassic-Ghadames Basin (with a tight oil distribution area of a n d H a y n e s v i l l e S h a l e a r e s e l e c t e d ( Ta b l e 1 2 ) .
4.1×104 km2 and thickness of 51 m), the Chaco-Parana Basin
4.1.3. Heavy oil
in South America (with a tight oil distribution area of 3.4×104
km2 and thickness of 91 m), the Canning Basin in the Oceania The top 10 regions of heavy oil resources in the world are
(with a tight oil distribution area of 4.5×104 km2 and thickness the Venezuelan Basin, the Arabian Basin, the Tampico Basin,
of 76 m) and the Indus Basin in Asia (with a tight oil distribu- the San Joaquin Basin, the Maracaibo Basin, the Yucatan Ba-
tion area of 7.1×104 km2 and thickness of 61 m), are large in sin, the Ventura Basin, the Northwest Germany Basin, the
distribution area (greater than 1×104 km2) and thickness (more Morondava Basin and the Sumatran Basin. Eastern Venezue-
than 30 m) of tight oil formations, and huge in tight oil re- lan Basin is still in the first place, where the heavy oil is
sources, however, in these areas, important geological pa- mainly concentrated in the shallow Paleogene-Neogene Ofi-
rameters such as porosity, brittle mineral content and crude oil cina Fm.; with undiscovered recoverable heavy oil resources
density are limited and the data cannot effectively cover the of 260×108 t, it is the first choice for future exploration. The
basin area, so the estimated resources have a high uncertainty, Arabian Basin is rich in heavy oil resources, in the West Ara-
and need to be further confirmed. For key basins in the Amer- bian Basin, the Cretaceous heavy oil is distributed in the Sina
ica and Russia and the Muglad Basin in Africa, grade I, II and Graben and some Euphrates grabens[52], where the reservoirs
III zones in each basin are defined (Tables 10 and 11) accord- have a buried depth of about 1 500 m, thickness of 11 m, po-
ing to main geological parameters of evaluation resources rosity of 15%, high oil saturation of up to 81%; with recover-
such as abundance and recoverability, in which grade I zones able reserves of 29.2×108 t, the heavy oil there is suitable to
are the “sweet spots” of the tight oil-rich basins. be produced by steam huff and puff technology; in the Central
Arabian Basin, the Jurassic heavy oil is distributed in the
4.1.2. Shale gas
Saman uplift, with the buried reservoir depth of up to 2,400 m,
The top 10 regions of shale gas in the world are the Zagros reservoir thickness of 41.8 m, porosity of 22.5%, oil satura-
Basin, the Gulf Basin, the Appalachian Basin, the Middle tion of 81%, recoverable reserves of 83.5×108t, the heavy oil
Arabian Basin, the Triassic-Gudamis Basin, the Canning Ba- there has bright development prospect in the future.
Table 10. Resource grading standard of tight oil enrichment areas
Source rock parameters Reservoir stratum parameters Reservoir parameters
Grading
Buried Thickness/ Brittle mineral Water Pressure Fracture
standards TOC/% Ro/%
depth/m m content/% saturation/% coefficient
Fairly
I >2.0 0.81.2 <3000 >15 >65 <50 ≥1.30
developed
II 1.52.0 0.71.3 <3500 1015 >55 <65 ≥1.15 Developed
III 1.01.5 0.61.4 <4500 510 >45 <80 ≥1.00 Undeveloped

Table 11. Examples of grading data of global tight oil enrichment formations
Resource grading
Typical I II III
formations Recoverable Recoverable Recoverable
Area/km2 Area/km2 Area/km2
resources/108 t resources/108 t resources/108 t
Bakken Fm of the Williston Basin 31 880 6.75 38 690 2.52 195 090 3.62
Eagle Ford Fm of the Gulf Basin 4 690 2.36 7 360 2.22 16 930 2.71
Wolfcamp Fm of the Permian Basin 10 630 3.20 22 350 4.70 38 930 2.91
Niobrara Fm of the Denver Basin 5 520 1.45 15 090 2.64 76 870 4.45
Bazhenov Fm of the West Siberia Basin 71 200 9.20 176 200 12.20 992 600 54.60
Vaca Muerta Fm of the Neuquen Basin 3 920 5.15 7 830 8.14 10 050 8.16
Los Molles Fm of the Neuquen Basin 1 110 1.18 3 910 1.79 6 400 2.98
AG2 Fm of the Muglad Basin 13 0.01 180 0.11 1 660 0.40

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Table 12. Ranking of shale gas resources in the foreland basins of western United States
Favorable area of recoverable resources/108 m3 Recoverable resources/
Shale Favorable exploration area Ranking
I II III 108 m3
Central
Haynesville 12 011 22 421 17 056 51 488 1
eastern region
Eagle Ford 11 900 11 894 11 153 34 947 West part 2
Barnett 8 517 4 636 2 134 15 287 Northeastern part 3
Woodford 4 764 4 531 5 517 14 812 Southwest corner 4
Fayetteville 2 809 5 870 4 094 12 773 Southwest part and northeast corner 5
Total 40 001 49 352 39 954 129 307

4.2. Examples sand, tight oil, oil shale, shale gas, tight gas, and coalbed me-
thane) are distributed predominantly in 476 formations in 363
4.2.1. Tight oil in the West Siberia Basin
petroliferous basins. Generally unconventional oil and gas
The recoverable tight oil in the West Siberian Basin ranks feature continuous accumulation in large area. Using the im-
the first in the world. The major source rock, Bazhenov Fm proved volumetric method for computing resources, evalua-
shale in the basin is characterized by large distribution area tion of the abundance of recoverable reserves has been done
(124×104 km2), shallow buried depth (2 2003 500 m), large based on interpolation of key evaluation parameters in gridded
effective thickness (2550 m), high organic matter abundance space, which can not only evaluate the resource potential but
(TOC value, 5%17%), moderate maturity (Ro value, also intuitively select the target area based on the resource
0.6%1.1%), high brittle mineral content (quartz content of abundance contour map.
70%) and high oil saturation (50%80%), similar to the condi- The global recoverable unconventional oil reserves are
tions of tight oil reservoirs in the Bakken Fm and Eagle Ford 4 421×108 t, and recoverable unconventional gas reserves are
Fm. that have been in industrial development in North America, 227×1012 m3. Controlled by large-scale oil and gas-rich basins,
and tight oil layer in Member-7 of Yanchang Fm in the Ordos
the resource distribution is not even. North America, South
Basin of China[53]. This evaluation shows the recoverable Juras-
America and Russia are regions with the richest unconven-
sic tight oil reserves in the West Siberia Basin are 76×108 t,
tional oil and gas resources, and the potential resources in the
ranking the first in the world. There, Grade I area with an area
Middle East, Africa and Asia are also substantial.
of 7×104 km2, has recoverable light oil reserves of 9×108 t, a
Unconventional oil and gas resources can be divided into
buried depth of less than 3 000 m, reservoir thickness of more
than 25 m, high organic matter content (TOC>7%), moderate
maturity (Ro value, 0.7%1.1%) and high resource abundance;
close to the conventional oil and gas accumulation area, it is the
favorable block for tight oil development (Figs. 12 and 13).
4.2.2. Tight oil in the Neuquen Basin
High-quality shale formations in the Upper Jurassic-Lower
Cretaceous Vaca Muerta Fm of the Neuquen Basin, have a
burial depth of less than 3 000 m, thickness of 2001 000 m,
type I and II organic matter, organic matter abundance of
6%14% and Ro value of 0.6%1.5%, being in mature - high
mature stage. With thin interbeds of grey mudstone and dolo-
mite, the shale formations have quartz and carbonate contents
of more than 20%, porosity of 7%12%, and permeability of
(0.050.20)×103 μm2. Comprehensive evaluation shows that
the recoverable tight shale oil reserves in the Upper Juras-
sic-Lower Cretaceous Vaca Muerta Fm of the Neuquen Basin
are 21.44×108 t, in which the recoverable reserves in grade I,
II and III areas are 5.15×108 t, 8.14×108 t and 8.15×108 t,
respectively. The assessment results reveal that tight oil in the
Neuquen Basin is the unconventional resource with the largest
development potential in South America.

5. Conclusions
Fig. 12. Shale thickness and TOC contours of the Bazhenov Fm
Seven types of unconventional resources (i.e., heavy oil, oil in the Western Siberian Basin.

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Fig. 13. Maturity of organic matter and distribution of recoverable resource potential in the Bazhenov Fm of the Western Siberian Basin.

“source-bound type” and “strata-bound type”. The “source- Shenyan, Song Jianguo, Gu Jiayu, Lu Minggang and other
bound type” resources are mainly controlled by six sets of experts for their guidance and help in the research.
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