Conventional and unconventional petroleum “orderly accumulation”: Concept and practical significance

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 41, Issue 1, February 2014 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2014, 41(1): 14–30.

RESEARCH PAPER

Conventional and unconventional petroleum “orderly accumulation”: Concept and practical significance ZOU Caineng*, YANG Zhi, ZHANG Guosheng, HOU Lianhua, ZHU Rukai, TAO Shizhen,YUAN Xuanjun, DONG dazhong, WANG Yuman, GUO Qiulin, WANG Lan, BI Haibin, LI Denghua, WU Na PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China

Abstract: Based on the latest global conventional-unconventional petroleum development situation and the conclusion of petroleum geology theory and technology innovation in recent 10 years, the connotation of conventional and unconventional petroleum “orderly accumulation” connotation is formulated. This concept indicates that, unconventional petroleum occurs in the hydrocarbon supply direction of conventional petroleum, and conventional petroleum may appear in the outer space of unconventional petroleum. Proper evaluation methods and engineering technology are important to push the conventional-unconventional petroleum co-development, and the petroleum finding thought from outer-source into inner-source. Unconventional petroleum evaluation focuses on source rocks characteristics, lithology, physical property, brittleness, oil-gas possibility and stress anisotropy. Taking shale gas for examples, in China, these six properties are TOC>2%, laminated silicious calcareous shale or calcareous silicious shale, porosity 3%−8%, brittle minerals content 50%−80%, gas content 2.3−4.1 m3/t, pressure coefficient 1.0−2.3, natural fractures; in north America, these six properties are TOC>4%, silicious shale or calcareous shale or marl, porosity 4%−9%, brittle minerals content 40%−70%, gas content 2.8−9.9 m3/t, pressure coefficient 1.3−1.85, natural fractures. “Sweet spot area” assessment, “factory-like” operation pattern and other core evaluation methods and technologies are discussed. And 8 key elements of unconventional “sweet spot area” are proposed, 3 of them are TOC>2% (for shale oil S1>2 mg/g), higher porosity (for tight oil & gas >10%, shale oil & gas >3%), and microfractures. Multiple wells “factory-like” operation pattern is elaborated, and its implementation needs 4 elements, i.e. batch well spacing, standard design, flow process, and reutilization. Through horizontal well volume fractures in directions, “man-made reservoirs” with large-scale fracture systems can be formed underground. For “shale oil revolution” in future, non-water “gas in critical state” and etc. fracturing fluid and matching technology should be stressed to be industrially tested and encouraged to be low cost developed. Key words: unconventional petroleum; “orderly accumulation”; co-development; “sweet spot area” assessment; “factory-like” operation pattern; “man-made reservoir”; tight oil; shale oil; shale gas; tight gas; petroleum exploration into source rocks

Introduction The primary energy is entering an age of coexistence of petroleum, natural gas, coal and new energy in the world, but petroleum will still be the mainstay in energy consumption in the next 30 years [1−10]. Based on forecast of Energy Information Administration (EIA) in 2013, petroleum, natural gas, coal, nuclear energy and renewable energy will account for 28%, 23%, 27% and 22% in the global primary energy resource consumption structure in 2040 respectively [11]. The global petroleum resource potential is huge, conventional and unconventional hydrocarbon resources, at a ratio of about 2:8, are about 5×1012 t in total; for the moment, the recovery percentage of conventional hydrocarbon resource is only 25%, and that of unconventional hydrocarbon resource is nominal, the oil industry life can extend for more than 150 years [2]. The

scientific and technological revolution of petroleum industry triggered by the North American “unconventional hydrocarbon revolution” is propelling the global oil industry to step over conventional hydrocarbon to unconventional hydrocarbon, and the status and role of unconventional hydrocarbon will become increasingly important. It is expected that the percentage of unconventional hydrocarbon output in the total output will ascend from 10% at present to more than 20% in 2030 [6, 8, 11]. On the basis of systematically investigating and studying the global conventional and unconventional hydrocarbon exploration and development progress and relevant theoretical and technical innovation, this paper, based on the development trend of China oil industry in a long run, combined with the latest achievements and experience of domestic petroleum

Received date: 22 Sep. 2013; Revised date: 28 Nov. 2013. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Key Basic Research and Development Program (973 Program), China (2014CB239000); National Science and Technology Major Project (2011ZX05001). Copyright © 2014, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

exploration and development tests, the connotation of conventional-unconventional hydrocarbon “orderly accumulation” is elaborated systematically, the assessment methods and procedures, key technologies and exploitation modes of conventional and unconventional hydrocarbon are summed up, the assessment core elements of “6 characteristics” of unconventional hydrocarbon based on cases are figured out, the “sweet spot area” assessment criteria and processes and wellpad “factory-like” operation pattern of conventional-unconventional hydrocarbon are presented, non-water fracturing “shale oil revolution” is initiated, and a new thought for codevelopment of conventional-unconventional hydrocarbon is put forward in this paper.

1

Research background

Currently, the global oil industry has entered a stage of stable production enhancement of conventional hydrocarbon and rapid expansion of unconventional hydrocarbon, and a setup of two onshore large scale conventional oil and gas production areas, four deep water conventional oil and gas important discovery areas and two unconventional oil and gas strategic breakthrough areas is taking shape. The global petroleum exploration shows four trends: “hotspot for discovery in deep water, bright spot for development in unconventional, difficulty for breakthrough in deep strata, and focus for scramble in North Pole”, and the four exploration domains will also become the four commanding points for the scientific and technological innovation and development of petroleum in the future. Based on statistics of IHS, the proved oil and gas reserves increased by more than 1 100×108 t in the world from 2000 to 2012 in total, in which, about 16% came from the onshore deep strata mainly in the Middle East and the Central Asia - Russia regions, whereas in China, the deepest oil and gas discoveries are all in the Tarim Basin so far, the deepest oil well is Jinyue 102, with a well depth of 7 350 m and daily oil production of 65 m3, and the deepest gas well is Keshen9, with a well depth of 7 445 m and daily gas production of 46×104 m3; about 28% came from the offshore deep water areas mainly in Brazil, Australia, Western Africa and the Gulf of Mexico; 81 oil and gas fields were newly discovered in the Arctic province and mainly distribute in the Barents Sea of Norway, and oil and gas discovery was obtained for the first time in the Baffin Bay west to Greenland [12]. A situation of two large conventional hydrocarbon production areas, the Middle East and the Central Asia-Russia regions, has basically taken shape in the world. About 2/3 global remaining recoverable reserves and undiscovered recoverable resources of conventional hydrocarbon concentrate in the two regions, where oil and gas account for 63% and 67% of the world total respectively. The oil and gas output proportion of the two large conventional hydrocarbon production areas has been rising continuously, from 43% and 34% in 2000 to 50% and 43% in 2012 respectively [6,12]. Meanwhile, two large unconventional hydrocarbon strategic breakthrough areas, USA (Western Hemisphere) and

China (Eastern Hemisphere), are also coming into being in the world. Unconventional hydrocarbon exploration and development have made major breakthroughs successively in tight gas, CBM, shale gas and tight oil, and the proportion of unconventional hydrocarbon output in the total output has quickly ascended to more than 10% [6,8,11]. The theoretical and technical innovation represented by “shale gas revolution” is propelling a new scientific and technological revolution in the world oil industry. High resolution 3D seismic survey and horizontal well volume fracturing have become the two core technologies for hydrocarbon exploration and development, and the wellpad “factory-like” production has become the new management model for low cost exploitation of oil and gas. The recent ten years represent the “revolutionary development golden decade” of American shale gas and tight oil, during this period, major breakthroughs in shale gas came one after another, from the Barnett in the south to the Hainesville and then to the Marcellus in the east, becoming the hotspot for the development of unconventional oil and gas; the shale gas output was 2 710×108 m3 in 2012, accounting for about 40% of total gas output of America. Major breakthroughs in tight oil extend from the Bakken in the north to the Eagle Ford in the south, the Monterey in the west and then to the Utica in the east, becoming the bright spot for the development of unconventional oil and gas; the tight oil output was 0.97×108 t in 2012, accounting for about 22% total oil output of America [11−13]. The rapid expansion of unconventional hydrocarbon like shale gas and tight oil has largely reduced the dependence of America on foreign oil and gas, which were reduced to 40% and 6% in 2012 respectively [6]. The recent ten years are the “pioneering development probing decade” of Chinese tight gas and oil. Tight gas has become an important field for the reserve increase and production enhancement of natural gas; in recent 10 years, the proved tight gas in place newly added is 3 110×108 m3, representing about 52% of total proved natural gas reserves in the corresponding period; in 2012, the tight gas output was about 300×108 m3, accounting for 28% total gas output of China; the Sulige tight gas province is the largest one discovered in China up to now, with a proved and basically proved gas in place of 3.5×1012 m3 and annual gas output of 169×108 m3 in 2012 [13]. A number of (5−10)×108 t tight oil reserve blocks have been discovered in basins like Ordos and Junggar, and the major breakthrough in tight oil has also made in basins like Songliao, Bohai Bay and Sichuan. 2 CBM surface production bases have preliminarily been built in the south of Qinshui Basin and at eastern margin of Ordos Basin; whereas commercial shale gas flow has been obtained in many wells in the marine shale of southern Sichuan Basin, and important progress has also been achieved in the construction of industrial pilot [14−16]. Currently, the tight oil and gas has become the first priority for the development of unconventional oil and gas in China, and both CBM and marine shale gas will all realize large- scale production. Petroleum exploration of China will mainly concentrate in

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four fields in the future, that is, conventional sandstone oil and carbonate gas pools, and unconventional tight and shale oil and gas. Scholars of China have put great effort in conventional and unconventional hydrocarbon exploration theoretical and technical study in recent years, have achieved a series of new understandings in aspects like hydrocarbon source, oil accumulation and drilling, and their petroleum prospecting thoughts also have turned from “outside source” to “inside source” [1−2, 13−40] (Table 1).

2 Connotation of conventional-unconventional hydrocarbon “Orderly Accumulation” 2.1 Concept of conventional-unconventional hydrocarbon “Orderly Accumulation” Conventional-unconventional hydrocarbon “orderly accumulation” is defined as inside the hydrocarbon bearing units (basin, depression or sag), the thermal evolution, hydrocarbon generation and expulsion of organic rich source rock have a whole process of coupling with the depth-related evolution of different types of reservoir space (Fig. 1), oil and gas charge continuously in time domain and are distributed orderly in spatial domain, conventional hydrocarbon and unconventional hydrocarbon are associated in origin and paragenetic in space, forming an unified conventional-unconventional hydrocarbon accumulation system. The different types of oil and gas in space can be looked for based on this universal law, generally, if conventional hydrocarbon is discovered, it forebodes that there is distribution of unconventional oil and gas in the hydrocarbon supply direction; on the contrary, if unconventional hydrocarbon is discovered, it foreshows that there may be association of conventional hydrocarbon in the peripheral space (Fig. 2). “Orderly” embodies fivefold implications: time evolution, formation order, accumulation mechanism, spatial distribution and hydrocarbon prospecting thought: the evolution of source rock and reservoir in different phases is orderly, the sequence for the kinship formation of different unconventional and conventional hydrocarbon resources is orderly, the types of oil and gas controlled by different pore diameters of reservoir space is orderly, the spatial distribution of different types of conventional-unconventional hydrocarbon is orderly, and the development of hydrocarbon prospecting thought from “outside source” to “inside source” in different phases is orderly. In such a way, the traditional thought of only focusing on conventional or unconventional hydrocarbon research, exploration and development is broken through. “Hydrocarbon prospecting inside source” involve seeking for shale oil and gas retained inside source, tight oil and gas near source and immature oil shale oil and CBM inside or near source rock, which breaks the thought of looking for traps around source rock, and shakes off the traditional method of “hydrocarbon prospecting outside source” in traps where hydrocarbon has been accumulated after secondary migration. “Hydrocarbon prospecting inside (near) source”

Fig. 1 Thermal evolution of source rocks and evolution of reservoir space of different types of reservoirs

Fig. 2 Planar orderly distribution of conventional-unconventional hydrocarbon

has resulted in a series of changes in the hydrocarbon exploration research thoughts, techniques and development modes. The thermal evolution, hydrocarbon generation and discharge process of different types of organic rich source rocks is also different, and Fig.1a shows the evolution cases of type I and II1 kerogen; different types of reservoirs follow different reservoir space evolution pattern in the burial process, and Fig.1b shows an evolution case of clastic reservoir The distribution pattern of “orderly accumulation, orderly paragenesis” of conventional-unconventional hydrocarbon in spatial domain is shown in Fig. 2. The conventional-unconventional hydrocarbon “orderly accumulation” reveals the formation and distribution pattern of different types of hydrocarbon resources, indicates that “synchronous research, deployment and exploration” should be taken for the conventional and unconventional hydrocarbon resources inside the hydrocarbon bearing units (basin, depression or sag), and wellpad “factory-like” operation pattern

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Type

Conventional oil and gas reservoir

Unconventional hydrocarbon accumulation

Oil and gas volume reMarine and contained in source tinental organic rock formation rich shale depocan reach 30%− sitional mode, 50%, hydrofine grained carbon generasandstone and tion Ro of coal carbonate depomeasure exsition distributends from tion, etc. 2.5% to 5.5%, etc.

Nm-level pore throats account for 70%−90% Distribution tight reservoir along basin space, pore and slope and in fracture resercentral sag, voir developed etc. in volcanics and metamorphics, etc.

Deep water Double sandy mudflow, Large unconlayer salt uncontrolled formable fractectonics in flow shallow ture and vug foreland type carbonate water delta, thrust belt, carbonate steep rock, large-area active large slope and gentle pore type dolopalaeohigh, slope platform mite, etc. etc. margin zone, etc.

Structure

Marine mud shale is the main source rock, “relayed” hydrocarbon generation, large scale crude oil cracked into gas, etc.

Reservoir

Sedimentation

Resource assessment

Technically recoverable method of single well or “sweet spot area”, commercially recoverable analogy method, etc.

The upper temperature limit of crude oil cracking Scale region is increased from approach, 200 °C to 230°C, technically industrial liquid recoverable hydrocarbon is method, etc. developed below 7 300 m

Depth

“continuous-type” hydrocarbon accumulation, nm Industrial tight pore throat hydrogas is developed carbon accumulabelow 7 500 m tion, tight oil and gas, shale oil and gas, etc.

“oil filled in sag”, large-area hydrocarbon accumulation, large hydrocarbon accumulation province, large-sized hydrocarbon accumulation, etc.

Source-reservoir differential pressure power, efficient migration and accumulation of lithologic stratigraphic reservoir near hydrocarbon source

Oil and gas retained in situ inside source, proximal diffusion and differential pressure migration and accumulation, etc.

Hydrocarbon accumulation

Migration

Hydrocarbon prospecting inside (near) source bed

Hydrocarbon prospecting in distal trap (outside source)

Thought

Geophysical prosDrilling Tendency pecting Multiple fracMulticomponent turing of vertidigital and high cal well and density seismic multisectional acquisition, refracturing of Conventional verse-time migrahorizontal well, unconventional tion imaging, producing by hydrocarbon “orpre-stack structure derly accumulamultilateral and pre-stack tion”, synchronous well or exreservoir forecast, tended-reach research, synchroetc. nous deployment, well wellpad “factory-like” produc“10 m3 distion and placement, 1 000 m3 prop- co-development, Microseismic pant volume from “hydrocarbon monitoring, and 10 000 m3 prospecting outside “6-characteristic” fluid volume” source” to “hydroevaluation of horizontal well carbon prospecting lithology, brittlevolume frac- inside source” ness and so on turing, wellpad “factory-like” production

New progress of China conventional and unconventional hydrocarbon theory and technology in recent 10 years

Hydrocarbon source

Table 1

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

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ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

can be used to conduct “synchronous exploitation” of different series of strata and different types of oil and gas, so as to expedite exploration and development rhythm and improve resource utilization efficiency and economic benefit. For instance, the exploration and development of unconventional hydrocarbon like shale gas, tight gas, tight oil and CBM is in full swing in America in recent 10 years, realizing development of conventional-unconventional hydrocarbon at the same time: the proportion of conventional to unconventional gas output is about 1:3, and that of conventional to unconventional oil output is about 3:1 [6] (Fig. 3). Conventional-unconventional hydrocarbon “orderly accumulation” is quite different from “hydrocarbon system” [41] in the following aspects: (1) Source rock consists of not only effective source rock, but also potential source rock like oil shale, and focuses on all the organic rich rocks; (2) Reservoir consists of not only “sweet spot” in good reservoirs, but also reservoirs in hydrocarbon generation series and on migration path, including all the reservoir spaces within the hydrocarbon supply scope; (3) Hydrocarbon resource is not limited to oil and gas in conventional traps and some unconventional gas, but all types of conventional and unconventional hydrocarbon resources including shale gas, shale oil, tight oil and so on; (4)

It is not limited to the perspective of “from source rock to trap” (focusing on solving two key issues: assessment on favorable unit and forecast favorable accumulation zone of hydrocarbon resources), but rather starts from the new approach of “source-reservoir coupling, orderly accumulation”, to conduct whole process analysis of hydrocarbon source, migration and accumulation, to look for all types of hydrocarbon resources, to carry out whole integration of exploration, development, engineering and gathering, to perform overall study and evaluation and stereoscopic exploration and development, and ultimately to realize the uttermost economic recovery of conventional-unconventional hydrocarbon in the whole hydrocarbon bearing unit. 2.2 Comparison of core elements for assessment of conventional and unconventional oil and gas Conventional oil and gas is significantly different from unconventional oil and gas in distribution characteristic, reservoir characteristic, source-reservoir assemblage, accumulation unit, migration regime, accumulation mechanism, percolation characteristic, fluid characteristic and resource characteristic. In search of conventional oil and gas, traps are the targets, and production relies on natural permeability; whereas

Fig. 3 Distribution of main type of conventional and unconventional hydrocarbon resources evaluated in North America (modified from reference [11])

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ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

in search of unconventional hydrocarbon, “sweet spots” are the target, and are produced by hydraulic fractured permeability. With the deepening of theoretical understanding and the progress of engineering technology, unconventional oil and gas are propelled to convert to conventional hydrocarbon. 2.2.1 “Six-element” assessment of conventional hydrocarbon Conventional hydrocarbon focuses on studying “whether hydrocarbon is accumulated in the trap” and assessing the 6 elements: “source, reservoir, caprock, trap, migration and preservation” and their matching relations [1−2, 42−55], typically represented by placanticline oilfield in Daqing Oilfield and Kara2 gas field (Table 2). 2.2.2 “Six characteristics” assessment of unconventional oil and gas Unconventional oil and gas concentrates on studying “whether oil and gas is contained in the reservoir” and assessing six characteristics: “hydrocarbon source properties, lithology, physical property, brittleness, hydrocarbon-bearing property and stress anisotropy” and their matching relations [13] (Tables 3 and 4). Hydrocarbon source properties assessment aims to find high organic content area; lithology assessment aims to hunt for effective reservoirs; physical property assessment aims to screen sweet spots where the porosity and permeability (including fracture) are relatively good; brittleness assessment aims to choose highly brittle reservoirs that are favorable for scale fracturing; hydrocarbon-bearing property assessment aims to choose reservoir with good oil-bearing property; and stress anisotropy assessment aims to drill horizontal well along the minimum stress direction, so as to make reservoir stimulation easier. Lucaogou Formation in Jimsar sag of Junggar Basin shows favorable “six characteristics”, so it is the tight oil breakthrough direction of the basin: the source rock condition is good, with mean TOC of 5%−6%, Ro of 0.5%−1.0%, type II kerogen, and 800 km2 of source rock of more than 200 m thick; good reservoirs like dolomitic siltstone/packsand are

Reservoir property Reservoir lithology

Poros-

Permeabil-

ity/%

ity/10−3 µm2

Pore throat

Seal

Trap type

field Source rock

Major geologic elements of placanticline oilfield in Daqing Oilfield of Songliao Basin and Kara2 gas field in Tarim Basin

size/µm

Trap measurement parameter Crest eleva- Oil/gas/wate tion/m

Closure

Closed

r contact/m amplitude/m area/km2

Migration characteristic

Preservation condition

Re-

Single

serve

well

abundance

output

Sandstone 22−30

>100

>3

Regional mud shale

Anticline

Shale

Faulting communica−660

−1 050

140−200

1 777.7 tion, longitu- Favorable dinal conduc-

256.5× 108

30−50 t/d

t/km2

tion Faulting Sandstone 15−20

200−800

0.4− 16.0

Regional gypsum salt

Anticline

Coal measure

Kara2 gas field

Daqing placanticline oilfield

Oil and gas

Table 2

developed; the reservoir property is fairly good, with rich matrix pore, porosity of 6%−20%, permeability of less than 1×10−3 μm−2 as a whole, mainly fine pore throats, and good connectivity; the oil-bearing property is fairly good, with oil saturation generally exceeding 70%, oil density of 0.88−0.92 g/cm3, basically free of water; the reservoir is relatively brittle, with high brittle mineral content, brittleness index of over 50%, elastic modulus exceeding 1.0×104 Mpa, and Poisson's ratio of less than 0.35; the horizontal earth stress differential value is small, at generally less than 6 MPa, favorable for volume fracturing. Sweet spot areas of Mesozoic Chang7 tight oil and Neopaleozoic tight gas in Ordos Basin with good “six characteristics” are also potential areas for exploration and development breakthroughs (Table 5). Silurian Longmaxi Formation in the Weiyuan, Changning and Fushun —Yongchuan blocks of southern Sichuan showing promising “six characteristics” in assessment, are prospective areas for preferential exploration and development of shale gas (Table 3): the organic rich shale is developed, with TOC exceeding 2%, and GR more than 130API; favorable reservoirs like lamellar siliceous calcareous shale and lamellar calcareous siliceous shale are developed; the physical property of shale reservoir is preferable, with total porosity of 3%−8%, gas-bearing porosity of 2%−5%, and matrix permeability of (10−5−1.0)×10−3 μm2; the gas bearing property is fairly good, with an average total gas content of 2.3−4.1 m3/t; the reservoir is high in brittleness, with an brittleness index exceeding 40, elastic modulus of generally more than 1.3×104 MPa, and Poisson's ratio of less than 0.29; the horizontal earth stress differential value is small, generally less than 20 MPa, apt to form fracture network, and favorable for improving single well production. It is worth to note that the fundamental research of finesedimentology counts for much in the geological evaluation of unconventional oil and gas. Pulveryte refers to the sedimentary rock with more than 50% particles of less than 0.1 mm, in which the particles mainly composed of terrigenous detritus particles like clay and silt, and a small amount of carbonate,

communica−1 970

−2 468

510

49.6

57.3×

3 600×

tion, longitu- Favorable

108

108

dinal conduc-

3

tion

− 19 −

2

m /km

m3/d

Continental

Marine

Cambrian Qiongzhusi Formation

Silurian Longmaxi Formation (Fushun- Yongchuan-Jiangjin block))

Silurian Longmaxi Formation (Zhaotong block)

Member III of Xujiahe Formation, Triassic

Silurian Longmaxi Formation (Weiyuan block)

Member V of Xujiahe Formation, Triassic

Silurian Longmaxi Formation (Changning block)

Triassic Yanchang Formation (Yanchang oilfield)

Permo Carboniferous Shanxi Formation

Horizon

I, II 1

I, II1

Weiyuan: 40- 1.7-3.6(2.8); 2.3-4.1 100 Changning: 1.6-7.1(3.2)

I, II 1

I, II 1

2.5-3.0

2400

2 600 4 000

1.6-6.8 (3.8)

60100

26000

3 200 4 500

2.1-3.0

1.6-4.9 (3.2)

30-40

1 100

900 2 200

2.7

1.9-6.4 (2.7)

26-60

2500

1 300 3 700

I, II 1

2.71-3.2

2.71-3.2

0.5-4.3 (1.7) Quartz 16-55(31), feldspar 0-6(2), calcite + dolomite 0-68(17), clay 15-76(49) Quartz 12-53(30), feldspar 0-6(2), calcite + dolomite 0-67(18), clay 15-75(49)

Quartz 23-43(34), feldspar 8-31(19), calcite + dolomite 0-43(4), clay 23-56(43)

1.80 3.40

2.40 3.70

0.17 0.24

0.20 0.27

1.90 4.30

1.30 1.36

0.18 0.19

0.12 0.29

0.86 4.10

1.10 3.20

1.20 3.10

1.05 3.20

0.50 1.90

0.10 0.25

0.19 0.30

0.20 0.30

0.19 0.26

Quartz 24-56(33), 0.22 0.24-6.27 feldspar 0-3, calcite (1.65) + dolomite 0-3, 0.33 clay 44-76(64)

Brittle mineral content/%

Earth stress characteristics

Extremely ④ developed

Extremely ③ developed

Developed

②

Developed

Extremely ① developed

Relatively developed

Relatively developed

Relatively developed

Developed

1.00

2.00 2.25

1.00

0.92 1.77

1.35 2.03

1.10 2.30

1.10 2.20

0.75 0.85

0.75 1.00

22.2 24.8

7.5 9.0

16.6 18.3

21.4 22.3

5.0 60.0

5.0 55.0

4.8 11.4

4.0 5.8

BidirecNatural Fluid Pois- Young’s fracture pressure tional stress son's modulus/ developing coeffi- difference/ ratio 104 MPa cient MPa level

Brittleness

Mainly siliceous Quartz 26-68(41), shale and cal0.000 22feldspar 0-14(5), careous sili- 3.4-8.2 1.7-6.5 0.001 9 calcite + dolomite ceous shale, a (5.4) (4.1) (0.000 29) 0-43(21), clay few clayey 10-53(31) siliceous shale Siliceous shale, Quartz 17-58(33), 0.000 015 calcareous feldspar 3-18(7), 3.9-6.7 2.74-5.01 siliceous shale calcite + dolomite (5.3) 0.000 090 (2.92) and clayey 4-65(22), clay (0.000 042) siliceous shale 15-49(34) Mainly siliceous Quartz 24-54(40), shale and cal0.004 3-0.042 feldspar 4-5(4.5), careous sili- 2.6-7.9 0.6-5.8 0 calcite + dolomite ceous shale, a (5.0) (2.3) 0-44(22), clay (0.019) few clayey 23-42(32) siliceous shale Mainly siliceous Quartz 45-48(47), shale and cal0.000 187 3.0%feldspar 3-9(7), careous silicalcite + dolomite 7.0% ceous shale, a 0.000 273 (4.2%) 4-6(5), clay few clayey (0.000 233) 37-40(39) siliceous shale Quartz 20-58(41), 0.000 011 feldspar 12-36(24), Mainly siliceous 1.4-3.1 1.2-6.0 shale, a few calcite + dolomite (2.2) 0.000 955 (2.8) siliceous shale 0-12(5), clay (0.000 147) 14-47(26)

<0.001

II2, III

III

Silty shale, 1.0-8.0 calcareous (2.3) shale, grapholith

1.0-2.0

0.9-1.5

2.3-2.8 (2.5)

1.0-9.0

0.003 4 -0.374 (0.068 3)

<0.001

1.3-3.1 (2.1)

Silty shale, 1.0-8.0 calcareous (2.3) shale, grapholith

Silty shale, grapholith

Gas content/ (m3·t−1)

Poros- Permeability/ ity/% 10−3 µm2

Major rock type

<0.01

Mainly III, a few II 2

Gas bearig property

Physical property

Lithology

Chang7: Silty shale, 0.5-1.3; Mainly II 1 clayey siltstone, 2.0-3.0 Chang9: and II2 migmatitic shale 0.4-1.4

0.7-2.3

1.9-7.3 (4.0)

40-60

5500

2 000 4500

5-30

2 000

800 5 200

3 000

1.5-8.0

5-65

3 400

500 4 400

Chang7:1.8 6.8(4.26); Chang9:1.0 7.4(5.64)

Chang7 : 20-80; Chang9 : 15-50

500 1 800

1.0-4.0 (2.9)

15-40 (24)

2 000 4 000 1 850

Burial depth/m

Source rock characteristics Recoverable resources/ TOC/ Ro/ Organic 108 m3 Thickness/m % % type

“Six characteristics” assessment of marine and continental shale gas in North America and China

Note: Data in brackets are mean values: ① SDT anomaly ratio: 1.2–1.4; ② fracture density: 1.1–4.4 fractures/m; ③ SDT anomaly ratio: 1.3–2.0; ④ fracture density: 2.3–9.7 fractures/m.

China

Region Depositional setting Basin

Ordos

Sichuan

Sichuan

Depression

Foreland

Foreland

Craton

Craton

Craton

9 500

Depression Prototype basin Structural type of favorable zone

Craton

− 20 −

Craton

27 500

63 900

4 300

2800

1 500

13 500

2 800

Gentle slope

Gentle anti- Gentle syncline Gentle syncline Gentle antiDepression Depression Gentle syncline Gentle slope cline area area cline and slope area and slope area

Favorable area/km2

9 000

Table 3

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

Region

USA

Canada

− 21 −

Triassic Montney

Craton

1.0 1.1 14.0 3.0 3.0

37 67 73 122

3 228

1 3875

Gentle 900 syncline and slope 2 740

West Canada[42]

Foreland

Gentle 1 829 syncline and slope 3 353

Devonian Woodford

Arkoma[42,48,51] 1.5

1.0 4.0

4.0 9.8

6 61

11 781

Gentle 305 syncline and slope 2 134

ArkoCarboniferous Foreland ma[42,44,50] Fayetteville

3.0 1.5 12.0 3.0

15 61

74 340

Gentle 1 200 syncline and slope 2 590

Foreland

Devonian Marcellus

Appalachian[42,48-49]

0.5 4.0

61 91

2.2 3.2

I, II1

I, II1

I, II1

I, II1

I, II1

Silty shale

2-9

3-9

<0.001

<0.001

<0.001

Siliceous 2-8 shale, calcareous shale Siliceous shale

<0.001

<0.005

8

Siliceous shale, grapholith shale

Siliceous shale, cal8-9 careous shale

1.13.2

5.68.5

1.72.6

1.72.8

2.89.3

Brittle mineral: 0.10 45-70; clay: 30-55 0.23

Brittle mineral: 0.10 50-75; clay:25-50 0.25

Brittle mineral: 0.23 40-70; clay: 30-60

Brittle mineral: 0.22 30-60; clay: 40-70

Brittle mineral: 35-65; 0.24 clay:35-65

2.40 3.80

1.20 2.40

1.40 3.20

1.30 2.50

1.40 3.50

Developed

Developed

Developed

Developed

Relatively underdeveloped

1.35 1.85

1.38 1.84

0.92 1.38

1.61 2.07

Physical Gas bearing Source rock characteristics Lithology Brittleness Earth stress characteristics Recoverproperty property able reNatural Fluid Bidirecsources/ Thick- TOC/ Ro/ Organic Major rock Poros- Perme- Gas content/ Brittle mineral Pois- Young's fracture pressure tional stress ability/ son's modulus/ 3 8 10 m ness/m % developing coeffi- difference/ % type type ity/% (m3·t−1) content/% 10−3 µm2 ratio 104 MPa level cient MPa Argillaceous 0.5 Brittle mineral: 0.20 1.30 1.35 limestone, 5 900 61 4.25 II 1 9.0 <0.003 2.8-5.7 Developed 45-65; limy mud2.0 clay: 35-55 0.30 3.50 1.58 stone Siliceous 30 1.0 shale, calBrittle mineral: 0.12 1.37 0.90 3.7 12 461 4.5 II 1 careous 4-5 <0.001 8.5-9.9 Developed 40-60; 183 2.1 shale, silty clay: 40-60 0.22 2.12 1.01 4.7 shale 71 083

Gentle 3 200 syncline and slope 4 200

Jurassic Foreland Haynesville

Gentle 1 980 syncline and slope 2 590

Fort Carboniferous Foreland Worth[42,45-46] Barnett

North Louisiana Salt[42,46,48]

Gentle 1 220 syncline and slope 4 270

Cretaceous Eagle Ford

West Texas[42-44]

Craton

Horizon

Basin

Structural Burial Prototype type of depth/ favorable basin m zone

29 000 142 000

Depositional setting

Marine

Marine

Favorable area/km2 3 000 13 000 2 3000 2 40 000 23 000

Continued

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

− 22 −

520

520

615

2040

1060

3080

530

30100

Cretaceous Quantou Formation Fuyang reservoir (Daqing)

Cretaceous Qingshankou Formation Gaotaizi reservoir (Daqing)

Cretaceous Qingshankou Formation, Quantou Formation (Jilin)

Permian Lucaogou Formation

Jurassic Daanzhai Formation, Shaximiao Formation

Permian Lucaogou Formation

Palaeogene Shahejie Formation (Liaohe)

Palaeogene Shahejie Formation (Huabei)

Ordos

Songliao

Jung gar

Neopaleozoic tight gas

Mesozoic tight oil

Hydrocarbon Type

Favorable area/ km2

870

300

400

Lacustrine shale, 20-30m thick, mean value of TOC 5%-8%, Ro 0.7%-1.2%, type I and II1 kerogens Coal measure, carbargilite, 30-120m thick; TOC: coal of 60%-70%, carbargilite of 3%-5%; Ro 1.3% - 2.5%, type III kerogen

II 1

0.5-0 I, II1 .9

0.7-1 .3

II

II

0.5- 0.5-2 I, II1 4 .0

2-8

2-8

1- 0.9-1 3.9 .4

2- 0.5-1 14 .0

1- 0.7-1 I, II1 2.5 .0

0.7-8 0.7-1 I, II1 .6 .3

40160

200400

150250

2070

3050

60300

80300

2060

2.08.0

4.012.0

2.05.0

6.020.0

8.012.0

5.012.0

5.012.0

7.013.0

Marl, dolomitic 0.5rock, conglomerate 2.5

Micritic dolomite, analcime

Limestone, dolomite, tuff

Coquina, siltstone/packsand

Dolomitic siltstone/packsand

Lithic graywacke, feldspar lithic siltstone

Lithic graywacke, feldspar lithic siltstone

Lithic graywacke, feldspar lithic siltstone

Lithic graywacke packsand

Brittleness

0.041.00

0.010.10

0.301.00

0.030.65

<1.00

0.101.00

0.022.00

0.031.20

0.020.30

0.67-0.86

0.840.89

0.850.91

0.760.87

0.880.92

0.830.86

0.780.87

0.800.87

0.800.86

4050

4050

>70

2050

>45

>45

>60

>50

40

>50

40-50

>50

32-46

40-60

40-60

35-45

0.200.30

0.250.45

0.100.35

0.25

0.25

Quartz sandstone, lithic quartz sandstone

Porosity of 6%-14%, permeability of (0.03-1.00) × −3 10 µm2, poro throat radius of 0.01-0.7 µm

Porosity of 7%-13%, Lithic arkose Permeability less than 1×10−3 µm2, pore throat radius of 0.06-0.80 µm

Physical property

Earth stress characteristics Horizontal principal Brittleness index of 35%-45%, Poisson’s ratio of stress difference of 0.25, Young’s modulous of 5-7MPa, pressure coefficient of 0.70-0.85 (2-3) × 104 MPa Brittleness

Horizontal principal Brittleness index of Gas saturation of 40%-60%, Poisson’s ratio of stress difference of 50%-60%, methane con0.23, Young’s modulous of 7-8MPa, pressure coeffitent more than 93% (2.5-4.0) × 104 MPa cient of 0.70-0.95

Oil saturation of 60%-80%, density of 0.80-0.86 g/cm3

Hydrocarbon-bearing property

2.03.0

2.04.0

1.03.0

3.04.5

2.03.0

Earth stress characteristics

4

3

（1.1-1.3）×108 m3/km2

20×104 t/km2

Reserve abundance

Relatively developed

Developed 1.10-1.60

Developed 1.00-1.30

Developed 0.80-1.72

Developed 0.90-1.40

Developed 0.90-1.10

Developed 1.20-1.50

Developed 1.20-1.50

Relatively developed, 0.70-0.85 small fracture spacing

Hydrocarbon area/104 km2

“Six characteristics” evaluation parameters of Mesozoic tight oil and Neopaleozoic tight gas in Ordos Basin

0.8

1

2

4

12

6

3

13

20

3- 0.7-1 I, II1 15 .2

Oil-bearing property

（2-4）× 104 m3/d

2-3 t/d

Single well output

<6

<10

<10

5-7

Natural Oil denYoung's Fluid Bidirectional Oil satura- Brittleness Poisson's fracture Poros- Permeability/ sity/ modulus/ pressure stress differ−3 2 tion/% index/% ratio ity/% 10 µm developing (g·cm−3) 104 MPa coefficient ence/MPa level

Physical property

“Six characteristics” features of continental tight oil in China

Source rock characteristics Lithology Reservese TOC/ Ro/ Organic Thickness/ extent/ Major rock type type m % % 8 10 t

Lithology

Table 5

2 2004 300

1 8003 000

1 2002 100 4 500

1 4007 000 3 200

2 3004 500

1 7502 200 2 300

1 8001 700 2 360

1 8009 900 2 400

1 00030 000 2 600

Source rock characteristics

1030

Horizon

Oil layer Burial thick- depth/ ness/m m

Triassic Yanchang Formation

Basin

Sichuan

Santa nghu

Bohai Bay

Table 4

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

biological kiesel and phosphate. This kind of sedimentary rock accounts for more than 75% of all sedimentary rock in the world. “Fine sedimentology” is a discipline that reveals the main factors control the formation of organic rich shale and tight reservoirs as well as their distribution pattern based on the dissection of pulveryte composition and structural features, and then points out direction for the exploration of unconventional oil and gas (Fig. 4). On this basis, it is necessary to create “unconventional reservoir geology”, which concentrates on research of the forming mechanism, distribution characteristic, volume scale and assessment method of µm—nm-level pore throat system of unconventional reservoirs, so as to provide theoretical support for tight oil and gas, shale oil and gas and so on. 2.3 Case of conventional-unconventional oil and gas “Orderly Accumulation” In the Sichuan Basin, marine strata from the Sinian to Middle-Triassic and non-marine strata from Upper-Triassic to Eocene both developed, and 21 oil and gas formations have been found, mainly covered three types conventional and

Fig. 4

three types unconventional “orderly accumulation”. Three types conventional hydrocarbon includes carbonate fractured-vuggy gas in the Sinian Dengying Formation, porous dolomite gas in the Cambrian Longwangmiao Formation and the Carboniferous, and carbonate reef gas in the Permian and Triassic. Three types unconventional hydrocarbon includes shale gas in the Cambrian Qiongzhusi Formation and the Silurian Longmaxi Formation, tight gas in the Upper-Triassic Xujiahe Formation, and tight oil in the Jurassic. For the Sinian-Silurian combination in the Sichuan Basin, conventional-unconventional gas are “orderly accumulation and symbiosis distribution”, and trillion class reserves of Sinian-Cambrian conventional super-giant gas field and trillion class reserves of Cambrian and Silurian unconventional shale gas have been found (Fig. 5). The Sinian-Cambrian conventional gas are mainly controlled by five geological factors, i.e. paleo-taphrogenic trough, paleo-source rock, paleo-karst reservoir, paleo-crude oil pyrolysis, and palaeohigh trap. The Cambrian and Silurian unconventional shale gas are mainly controlled by shale facies with deepwater shelf organic-rich and high content silicon and calcium.

Distribution pattern of continental fine sediment

Fig. 5 Distribution of Sinian-Silurian conventional-unconventional hydrocarbon “orderly accumulation” in Sichuan Basin. C 1q—Qiongzhusi Formation; S1l—Longmaxi Formation. Z2do—Doushantuo Formation; Z2d—Dengyin Formation; —

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3

Assessment methodology and technology

3.1 3.1.1

Assessment methodology and key technologies Assessment methods and procedures

Conventional-unconventional oil and gas “orderly accumulation” reveal an unified oil and gas orderly paragenetic system, but the assessment methods and procedures for conventional and unconventional oil and gas are quite different, so they should be treated specifically according to actual situations. Conventional hydrocarbon usually occurs in positive elements like macrotectonic zone in downfaulted basin, macrotectonics in foreland thrust belt, passive continental margin and craton large uplift, and secondary structural units control the hydrocarbon distribution. Oil and gas accumulates in structural highs in isolated spots on the plane, or accumulates in lithological and stratigraphic traps, which occur in large scale clusters. Conventional oil and gas exploration focuses on finding out 6 elements, “source, reservoir, caprock, trap, migration and preservation” and their optimum configuration relations; the key is to look for effective oil accumulation traps, and the core is to obtain discovery through preliminary prospecting and then assess and determine the trap boundaries. Unconventional hydrocarbon mainly distributes in the negative elements like depression—slope in foreland basin, central depression basin and craton syncline; the hydrocarbon occurrence is not restricted in the second class of structural unit, but rather covers the basin centre and slope, and distributes continuously or quasi-continuously in a large area. Unconventional hydrocarbon exploration focuses on assessing six characteristics, “source rock characteristic, lithology, physical property, brittleness, hydrocarbon-bearing propertyand geostress anisotropy” and their matching relations, evaluating “hydrocarbon generation, accumulation and production capacities”, and looking for hydrocarbon continuous distribution boundary and “sweet spot area”. (1) Choosing “sweet spots” by regional evaluation includes selecting oil rich sags and major source intervals, defining hydrocarbon bearing boundaries, systematic core analysis and assessment of “six characteristics”. (2) Volume fracturing research of horizontal well includes selection of well trajectory, optimization of horizontal section length, optimization of fracturing scale and fracturing fluid and implementation of “L” curve production test. (3) Wellpad “factory-like” production test involves optimization of number of wells at wellsite, synchronous management and cross fracturing, cost reduction management and so on, so as to form an economic benefit development system. 3.1.2

Comparison of key technologies

In view of tightness, low reserve abundance, high difficulty in exploration and development of unconventional oil and gas, apart from the common technologies used in conventional

hydrocarbon exploration, some other special exploration and development technologies like high resolution 2D and 3D seismic survey, high resolution sequence stratigraphy, prestack seismic reservoir forecast, seismic pre-stack fluid detection, diagenetic facies quantification, microseismic monitoring and horizontal well volume fracturing stimulation are adopted in the exploration and development of unconventional hydrocarbon. 3.1.3

Exploitation mode

Unconventional hydrocarbon is mainly exploited by horizontal well scale fracturing, wellpad “factory-like” production and IOR by nano-technology. At present, the unconventional hydrocarbon development mainly consists of primary and secondary development. Generally, horizontal wells and multilateral wells are used to expose the reservoir as much as possible; the horizontal well multistage and multisection volume fracturing is used to improve the reservoir fracturing scope and scale and improve the single well production rate; and the wellpad factory-like exploitation technology is used to develop and utilize the underground resources to the maximum extent. Horizontal well volume fracturing innovation that centers on “artificial permeability” and the wellpad “factory-like” low cost development mode innovation make the large scale exploitation of unconventional hydrocarbon resources economic. Shale gas is produced mainly by slick water fracturing, while shale oil is produced mainly by gas fracturing. For shale oil where no breakthrough has made, it is necessary to strengthen research on technologies like non-aqueous gas fracturing, e.g., industrial test of critical gas (carbon dioxide, gaseous hydrocarbon, nitrogen, air, etc.) fracturing fluid, so as to change the temperature and pressure parameters, occurrence state, flow property and so on of hydrocarbon in shale reservoir. Consulting the successful technology and experience of shale gas, the basins like Junggar, Ordos and Songliao are selected to conduct shale oil industrial test research. For instance, at the time of fracturing, some gas is injected in the shale oil reservoir, which enables some shale oil to turn into condensate oil or light oil, increase the reservoir pressure, form high gas-oil ratio and natural flow capacity, and in such a way, the commercial scale exploitation of shale oil may possibly be realized. 3.2 3.2.1

Core methodology and technology Assessment methods of resources and reserves

Unconventional hydrocarbon is quite different from conventional hydrocarbon in geologic control factors, genetic mechanism and distribution pattern, and different oil and gas resource estimate methods [56−57] (Table 6) are needed for resource evaluation. 3 hierarchies of unconventional hydrocarbon resource assessment method systems have been built up

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ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

Table 6

Item

Geological characteristics

Difference in geological characteristics and assessment methods of conventional and unconventional hydrocarbon resources Driving force for migration and accumulation of hydrocarbon in reservoir Conventional hy- Unconventional hydrodrocarbon carbon

Mainly buoyancy driving

Hydrocarbon migration and accumulation numerical simResource ulation method: assessment adopting buoyancy method driving model, e.g., Darcy flow model, etc.

Mainly overpressure driving

Continuous-type hydrocarbon accumulation numerical simulation method: adopting overpressure driving model, e.g., kerogen generated gas pressurization model, etc.

Hydrocarbon accumulation Hydrocarbon distribution pattern control factor Conventional hy- Unconventional Conventional hy- Unconventional hydrodrocarbon hydrocarbon drocarbon carbon Large-area continuous None of the 6 eleaccumulation, trap is not Accumulated in ments: source, res- Depend mainly ervoir, caprock, trap, on 3 geologic separated traps, pos- required, oil, gas and migration and pres- conditions like sesses apparent oil, water boundary is unobervation for hydro- source, reservoir gas and water inter- vious, there is no oil and gas reservoir, but there faces carbon accumulation and preservation are “sweet spots ” is dispensable Analogy method: analog parameters consist of 5 types of geologic parameters like source, reservoir, caprock, trap and preservation.

to now in China. (1) Quick assessment: volumetric method is used for areas with few available data and low geologic understanding level, and can be used for quick assessment for seven kinds of unconventional resources. (2) Key assessment: EUR (estimated ultimate rate, ultimate cumulative production predicted by single well) analogy method, resource abundance analogy method and small bin method are used for areas with more available data and higher geologic understanding level, and can be used for key assessment on tight oil, tight gas and shale gas. (3) Scale region fine evaluation: spatial distribution prediction and numerical simulation methods of resources are used for areas with plenty of available data and high geologic understanding level, and can be used for key assessment on tight oil, tight gas and shale gas. Specifically, volumetric method, EUR analogy method and resource abundance analogy method can only be used to assess the resource extent, and are incapable of predicting the resource distribution; whereas the small bin method and the spatial distribution prediction and numerical simulation methods of resources can be used to predict “sweet spots”. Unconventional oil and gas, unique in reservoir features, production performance characteristics and classification of assessment units, cannot follow or simply apply mechanically the reserve evaluation criteria and processes of conventional hydrocarbon, and a set of reserve evaluation standard and method suitable for different types of unconventional oil and gas of China is urgently needed, which can consult unconventional hydrocarbon reserve evaluation experiences of America. 3.2.2 Assessment method for unconventional oil and gas “sweet spots” Unconventional oil and gas “sweet spots” refer to unconventional oil and gas enrichment targets in the source-reser-

Analogy method: analog parameters consist of 3 types of geologic parameters: source, reservoir and preservation.

Statistical method: hydrocarbon reservoir discovery process model, scale sequential method, generalized Pareto model etc.

Statistical method: small bin method, spatial continuous distribution prediction of resource, etc.

voir paragenetic shale layer series which has superior collocation relationship of source rock characteristic, reservoir characteristic, hydrocarbon bearing characteristic, brittle characteristic and earth stress characteristic, and can be selected to explore and develop in priority based on formation testing and production test output and oil and gas well production performance. The assessment and selection of “sweet spots” are also the core for unconventional hydrocarbon exploration research running through the whole exploration and development process. Unconventional hydrocarbon sweet spots can be divided into “geologic sweet spots, engineering sweet spots and economic sweet spots”. Eight indices for the oil and gas enriched “sweet spots” assessment are proposed, among them, three critical indices are: TOC of more than 2% (for shale oil S1>2 mg/g), fairly high porosity (more than 10% for tight oil and gas, more than 3% for shale oil and gas), and rich microfractures. Geologic sweet spots center on integrated evaluation of source rock, reservoir, fracture and so on; engineering sweet spot center on integrated evaluation of burial depth, rock compressibility and earth stress anisotropy; whereas economic sweet spots stress assessment of resource extent, surface condition and so on. For instance, the current “sweet spot” assessment of unconventional tight oil and gas as well as shale oil and gas mainly focuses on the assessment of geologic sweet spot element, including source bed, reservoir, fracture and local structure as well as assessment of engineering sweet spot elements like pressure coefficient, brittleness, earth stress characteristics and burial depth (Table 7). “Sweet spot” assessment consists of 5 key technologies: (1) Source rock “sweet spots” predicting technique: conduct integrated evaluation on vertical distribution of source rock sweet spots based on core sample test, acoustic wave/resistivity calculation, and nuclear magnetic resonance+densimetry, and clarify the horizontal distribution characteristics of source

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ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

rock sweet spot by combining well-tie correlation with sedimentary and seismic facies analysis. (2) Reservoir “sweet spots” predicting technique: by integrating core measured physical property data with sedimentary and diagenetic facies research of favorable objective intervals, and overlapping porosity and permeability distribution graphs, the reservoir sweet spots are found. (3) Brittleness assessment and forecasting technique: analyze mineral components by X-diffraction etc, determine favorable intervals by combining stress experiment with dynamic log, and use pre-stack seismic attributes inversion to figure out horizontal distribution. (4) Earth stress assessment technique: by combining lithomechanics experiment with log data like array acoustic, and calculating elastic modulus of rock, parameters like pore pressure, overburden pressure, and maximum/minimum horizontal stress are obtained to direct well trajectory design, and determine fracturing mode and scale. (5) Comprehensive “sweet spots” seismic attributes predicting technology: utilize multiparameter crossplot analysis and pre-stack elastic inversion to determine the horizontal distribution of key parameters like lithology, porosity and brittleness; use post-stack multiattribute fracture prediction technology to forecast and interpret fracture development zone; and integrate multiple parameter analysis on lithology, physical property and brittleness to forecast sweet spot distribution. Table 7

4 Co-development of conventionalunconventional oil and gas 4.1 Co-development of conventional-unconventional oil and gas Co-development of conventional-unconventional oil and gas refers to explore and develop different types of hydrocarbon resources in different layer series of hydrocarbon bearing units (basin, depression or sag) in “stereoscopic exploration, collaborative development” mode according to the conventional-unconventional hydrocarbon “orderly accumulation” rule, and by making use of the difference and complementarity of their production characteristics. The geotectonic evolution of China is characterized by small craton and multicycle patch; marine, continental and coal measure source rocks as well as continental sandstone, marine carbonate, volcanics and metamorphics reservoirs are developed; hydrocarbon distribution spans over the Archean Neogene; and both conventional and unconventional hydrocarbon resources are very abundant. Meso-cenozoic continental middle-shallow clastic strata predominate in the hydrocarbon exploration of China, where reserves and output all take absolutely majority; whereas the Cambrian—Sinian gypsum salt bearing marine deep and ultra-deep carbonate series of strata are very low in exploration degree: with an oil discov-

Assessment criteria for “sweet spots ” of tight and shale oil and gas Geologic sweet spot

Hydrocarbon type

Tight oil and gas

Assessment criteria Tight oil of Lucaogou Formation in Jimsar sag, Junggar Basin Case Tight gas of Member He8 in Sulige gas field, Ordos Basin

Shale oil and gas

Assessment criteria Shale oil of Member Chang7 in Huaqing region, Ordos Basin Shale oil of Member Qing1 in Case Songliao Basin Shale gas of Longmaxi Formation in Shu'nan region, Sichuan Basin

Source bed

Engineering sweet spot Horizontal Burial Pressure Brittleprincipal Local strucdepth/ ness coeffiFracture stress difture m cient index/% ference/MPa Microfracture Relatively >1 >40 <6 <4 500 developed high position

Reservoir

Porosity of 10%−15% 25−50 m thick, Relatively 100−130 m thick, Microfracture porosity of high position 1.2−1.5 TOC of 5%−6%, developed 12%−20%, oil of slope Ro of 0.8%−1.0% saturation > 70% Relatively 6−20 m thick, TOC: coal 20−40 m thick, Microfracture high position 0.70− of 60%−70%, carbarporosity of 0.95 of gentle developed gilite of 3%−5%; Ro of 10%−14%, gas slope saturation > 50% 1.3%−2.5% TOC>2% (for shale oil Microfracture Relatively Porosity > 3% >1.2 S1>2 mg/g) developed high position 10-30 m thick, TOC of Relatively Porosity of Microfracture 3%−25%, Ro of high position 0.8−1.0 2%−5%, oil developed 0.8%−1.2%，S1 of 1−8 of slope saturation > 80% mg/g 40−60 m thick, TOC of Relatively Porosity of Microfracture 2%−8%, Ro of high position 1.2−1.6 2%−5%, oil developed 0.7%−1.4%, S1 of 1−7 of slope saturation > 80% mg/g TOC>2%

30−100 m thick, TOC>2%, Ro of 2.0%−3.0%

Porosity of 3%−8%, gas content > 3 m3/t

Microfracture Stable slope 1.3−2.0 developed area

− 26 −

>50

<6

1 000− 4 500

40−60

<8

1 500− 4 500

>40

<10

<4 500

30−50

<15

1 000− 3 000

30−60

<10

1 300− 2 000

>40

<20

1 500− 4 000

ZOU Caineng et al. / Petroleum Exploration and Development, 2014, 41(1): 14–30

ery rate of only 7.4% and the gas discovery rate of only 6.5% [32, 58−66], these strata will be the important replacement strata of hydrocarbon exploration in the future. Forming in complex geologic conditions, marine carbonate rocks in China feature wide distribution, old age，high thermal evolution level of source rock and big burial depth [32, 58−66]. Compared with America, the marine shale gas of southern China has more unique characteristics, and the differences between them are mainly as follows: (1) Marine shale structures in China experienced intense rework, resulting in rich faults, and the natural fractures; whereas in America, the tectonic activity was simple, resulting in rare faults, shale reservoirs, formed late and slightly affected by later reformation, are well preserved. (2) The sedimentary age of China marine shale is old, mostly Sinian—Permian, with relatively low TOC, and Ro of more than 2.0% in general; whereas in America, the most marine shale is Devonian and Carboniferous, with relatively high TOC, and Ro generally of 1.1%−2.0%. (3) The burial depth of China shale gas reservoirs is relatively deep, for instance, it is 5 000−8 000 m in Tarim Basin, and the surface condition is complicated, like mountain; whereas in America, the shale gas reservoirs are buried relatively shallow, and the surface mostly is plain. In view of the marine and continental superposed basins like Tarim, Ordos and Sichuan, it is necessary to have an overall consideration on the development of marine and continental, deep and shallow conventional and unconventional oil and gas at the same time. 4.2

Wellpad “factory-like” operation pattern

Wellpad “factory-like” operation pattern commonly used both at home and abroad at present mainly refers to the deployment of a batch of similar wells in areas with similar geologic conditions or areas where subsurface geological conditions are well understood, in large wellpad pattern, to conduct wellpad “factory-like” operation. In such a way, an “artificial oil and gas reservoir” that takes the horizontal well length as the volume unit and the hydraulic fracturing network as the flow channel is formed underground. For the moment, the “factory-like” operation only has been adopted in single unconventional hydrocarbon sweet spots of shale gas and tight oil in North America and China.

Fig. 6

To realize wellpad “factory-like” production, the following 4 elements are essential: (1) Overall research, batch well deployment: lay stress on overall integrated study, on the basis of finding out the basic features of hydrocarbon accumulation in the target area, uniformly deploy drilling wellpads in factory-like pattern in the area with similar geologic conditions and suitable surface conditions, perform standardized design for wellpads, and conduct batch and standard well drilling. It not only can share the surface facilities and reduce production and gathering costs, but also can enlarge the hydrocarbon drainage scope and improve reserve producing level and recovery efficiency. (2) Modular equipment, standard design: lay stress on standardization and modularization of technology and miniaturization and high power of equipment, which is not only the prerequisite for simultaneous drilling, completion, flowback and production operations in the limited wellpad space, but also the basic requirements to ensure security of the operations. (3) Cross operation, flow operation: lay stress on flow production, once a well is drilled, the rig will be slided to another borehole on the same wellpad to drill ahead, while the drilled well is completed. In this way, drilling, completion, flowback and production operations can be performed simultaneously on the same platform. The duration from spud in to initial production of the whole wellpad is shortened, and the operating cost is reduced as a result. (4) Recycling of materials and water: lay stress on recycling, it is required to recycle consumptive materials and liquid used in the course of operations as far as possible, and this is an important link to reduce the usage of consumptive materials, control waste discharge and reduce operating costs. The multilayer multi-well wellpad “factory-like” operation pattern presented in the paper refers to the deployment of a batch of wells with different well structures and completion modes in line with the different types of conventional and unconventional hydrocarbon resources in different layer series of hydrocarbon bearing units (basin, depression or sag), to to perform drilling, completion, flowback and production operations of several wells at the same time with standard and modular equipment in stream line operation mode (Fig.6). Its connotation can be extended to the whole conventional-unconventional hydrocarbon accumulation system, and

Wellpad “factory-like” operation pattern for conventional-unconventional hydrocarbon

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it is a new mode of applying thoughts and methods of system engineering, intensively collocating industrial elements like manpower, material resources, investment and organization, and using the modern hydrocarbon science and technology, information technology and management means to perform rapid operation and efficient production of oil and gas [67]. “Multi-wells” refer to drilling several or even tens of wells on one wellpad; “wellpad” refers to forming a large wellsite based on surface conditions; “factory-like” refers to performing streamline and collaborative hydrocarbon production after batch deployment of multiple wells; it has 8 characteristics: “integrity, systematicness, integration, streamline, batch, standardization, automation and benefit maximization”, and can help to realize efficient development or synchronous overall economic development of conventional and unconventional hydrocarbon. Wellpad “factory-like” operation pattern not only can reduce land occupation significantly, save consumptive material usage, control waste discharge, shorten operating cycle and reduce operational cost, but also can assemble surface facilities and reduce production cost; at the same time, in the area of hydrocarbon-bearing formation controlled by multiple wells, it can help implement synchronous fracturing or cross fracturing, create more complicated fracture network system on the whole, increase the hydrocarbon reservoir stimulation volume, and improve the initial production and ultimate recovery efficiency significantly; therefore, it provides an efficient operation mode for the realization of co-development of conventional-unconventional oil and gas. The surface conditions in the hydrocarbon resource distribution area of China is complicated, mostly mountains, loess tableland, desert and ocean; therefore, the area for rigging up is restricted, and the environment is fragile, which makes it a necessity for conventional and unconventional hydrocarbon superposed development areas like Sichuan and Ordos basins to take the wellpad “factory-like” operation pattern. 4.3

Commercial value of co-development

For the purpose of expediting development and utilization of conventional and unconventional hydrocarbon resources, “co-development” is needed at least in the following 6 aspects. (1) collaboration of exploration mode: overall research, overall deployment and overall exploration; (2) collaboration of development mode: collaborative recovery of different layer series and types of resources; (3) collaboration of surface construction: overall research, planning and construction; (4) collaboration of operation mode: wellpad type “factory-like” synchronous operation of different types of resources; (5) collaboration of policy support: work out development subsidy incentive mechanism for conventional remaining hard producing reserves and unconventional hydrocarbon; (6) collaboration of personnel training: cultivate conventional hydrocarbon undergraduate and unconventional hydrocarbon graduate in hydrocarbon institutes and universities, so as to

provide conventional and unconventional hydrocarbon industrial development with personnel guarantee and form hydrocarbon industry collaboration system.

5

Conclusions

This paper elucidates the theoretical understandings of conventional-unconventional hydrocarbon “orderly accumulation”, reveals the “orderly accumulation” pattern of different types of hydrocarbon resources inside the hydrocarbon bearing units (basin, depression or sag), points out that there is unconventional hydrocarbon distribution in the hydrocarbon supply direction of conventional hydrocarbon and there may be association of conventional hydrocarbon in the peripheral space of unconventional hydrocarbon, and the evolution from “hydrocarbon prospecting in distal trap” and “hydrocarbon prospecting near/surrounding source” to “hydrocarbon prospecting inside source” as well as the conventional and unconventional hydrocarbon resource “stereoscopic exploration, synchronous exploitation” will expedite oil and gas discovery pace and exploration and development rhythm and thus improve the economic benefit. Conventional-unconventional hydrocarbon “orderly accumulation” puts stress on adopting applicable assessment method and engineering technology to promote the co-development of conventional and unconventional hydrocarbon. Unconventional hydrocarbon focuses on studying “whether hydrocarbon is contained in the reservoir” and assessing six characteristics including “hydrocarbon source, lithology, physical property, brittleness, hydrocarbon-bearing property and stress anisotropy” and their matching relations. In addition, the paper also focuses on the exposition of assessment methods and technologies like “sweet spot area” assessment and wellpad “factory-like” operation etc. For the development and utilization of conventional-unconventional hydrocarbon resources in the future, it is required to establish a new hydrocarbon industry system of “co-development” in six aspects, namely exploration mode, development mode, surface construction, operation mode, policy support and personnel training. The reveal of “orderly accumulation, spatial paragenesis” pattern of multiple subsurface energy and mineral resources is of great importance for the search and exploitation of different kinds of solid, liquid and gaseous mineral resources.

Acknowledgement Substantial help is obtained from Zhao Wenzhi, Du Jinhu, Yang Hua, Kuang Jun, Xu Chunchun, Hu Suyun, Li Jianzhong, Yang Tao, Wu Songtao, Bai Bin, Yang Fan, Zhao Zhe, Huang Jinliang et al. in writing the paper, and we would like to take this opportunity to express our sincere thanks to all of them.

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