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RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT

Volume 40, Issue 1, February 2013

Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(1): 114.

Received date:09 Aug. 2012; Revised date:10 Nov. 2012.

*Corresponding author.E-mail: [email protected]

Foundation item:Supported by the National 973 Program (2007CB209500) and the National Carbonate Rock Major Project (2008ZX05004).

Copyright 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Large scale accumulation and distribution of medium lowabundance hydrocarbon resources in ChinaZHAO Wenzhi1,*, HU Suyun2, WANG Hongjun2, BIAN Congsheng2, WANG Zecheng2, WANG Zhaoyun2

1. PetroChina Exploration & Production Company, Beijing 100007, China;

2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China

Abstract: This paper analyzes the large-scale accumulation conditions and distribution characteristics of medium-low abundance hy-

drocarbon resources in China. Large-scale development of accumulation elements and their change in scale are the material basis of largescale oil and gas accumulation, determining the regional nature of oil and gas distribution. Liquid hydrocarbon dispersed in marine source

rocks being cracked to form a large volume of gas and coal measure source rocks expelling gas during uplift are two important factors for

the formation of large-scale hydrocarbon accumulation, which control the scale of source rocks that enter the main gas-generating stage.

Volume flow and diffusive flow are the main migration-accumulation mechanism for the large-scale hydrocarbon accumulation, which

ensures the sufficiency of hydrocarbon supply. Pancake, layer-like, and cluster are three main accumulation forms of large-scale hydro-

carbon accumulation, which ensure the scale of hydrocarbon accumulation. Middle to low abundance hydrocarbon resources are charac-

terized by near-source distribution, main-body play, late accumulation stage and single accumulation type. The periclinal area of pa-

laeo-highs in marine craton basins, the lower slopes and sags in an intra-continental depression basin, and the gentle slopes of foreland

basins are the most likely areas for the development of large-scale hydrocarbon accumulation, and they have two types of accumulation,

large area and large scope. The proposal of the large-scale accumulation of middle to low abundance hydrocarbon resources in China im-

proves the hydrocarbon discovering potential in middle to deep layers of superimposed basins and in the lower slopes and sags in depres-

sion basins, enlarges the exploration scale, and extends the hydrocarbon exploration from local second-order structure zones to the whole

basin with the main source rock as the center, and from middle layers to deep, even super-deep, layers.

Key words: middle-low abundance hydrocarbon resources; large-scale hydrocarbon accumulation; condition; distribution characteris-tics; exploration field; superimposed basin; depressed basin; onshore China

1 Overview of hydrocarbon resources in onshoresuperimposed basins in China

The mainland continent of China was formed as a result of

the collision, accretion and merge of a number of small an-

cient plates (e.g. North China Plate, Tarim Plate, Yangtze

Plate) of different scales[12]

. It has generally experience a

long period of complicated evolution and multiple periods of

geodynamic system superposition and reconstruction. The

sedimentary basins in China received Early Paleozoic marine,

Late Paleozoic marine to transitional and Mesozoic and Ce-

nozoic continental depositional architectures from the bottom

up[3]

, forming a couple of large-scale superimposed basins

with multi-cycles[46]

, e.g. the Ordos Basin, the Sichuan Basin,

the Tarim Basin, the Songliao Basin, and the Bohai Bay Basin.

Provided with abundant hydrocarbon resources, petrolifer-

ous basins with superimposed sedimentation are the current

focus of hydrocarbon exploration and reserves increase in

China. Through the exploration over the last half a century, a

number of large and medium sized oil and gas fields, e.g.

Daqing, Shengli, north Dagang, Damintun have been discov-

ered in Mesozoic and Cenozoic terrestrialformations, which

symbolize the first milestone in the founding of Chinas pe-

troleum industry[78]

. Since the late 1980s, more efforts have

been put into hydrocarbon exploration targeting Paleozoic

marine and transitional formations. Consequently, some large

and medium oil and gas fields have been discovered in suc-

cession, e.g. Jingbian Gas Field in middle Ordos Basin and

Sulige Gas Field in north Ordos Basin[9]

, Kela2, Dina and

Dabei Gas Field in Kuche foreland province in the Tarim Ba-

sin, Lunnan, Tahe and Tazhong oil and gas fields in deep ma-

rine craton carbonate measures, Puguang and Longgang gas

field in Permian and Triassic System in the Sichuan Ba-

sin[1013]

. The exploration practices verify the existence of

large oil and gas fields both in shallow to middle Mesozoic


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ZHAO Wenzhi et al. / Petroleum Exploration and Development, 2013, 40(1): 114

2

and Cenozoic terrestrial formations and in middle to deep

Paleozoic marine to transitional formations in superimposed

petroliferous basins[1416]

. According to petroleum exploration

in recent years[1724]

, there are some trends in superimposed

petroliferous basin exploration: (1) effective exploration depth

has increased continuously. The exploration has deepened by1 5002 000 m compared with previous activities. In eastern

China, continental clastic rock exploration has gone beyond

3 500 m depth, and discovered meaningful oil and gas in for-

mations deeper than 4 000 m. In western China, exploration

has been pushed down to over 5 000 m depth, and made

breakthroughs in formations deeper than 6 000 m; the deepest

exploration depth is close to 8 000 m; (2) exploration has

expanded constantly from previous second-order structure

zones to structural lows and depressions in spacious slope

areas. Large scale hydrocarbon reserves discovered to date

have made slope areas an important focus in onshore petro-

leum reserves and resulted in production increases in China;

(3) fundamental changes in prospecting targets from previous

structural reservoirs to composite stratigraphic, lithologic and

structural-lithologic reservoirs have taken place. The latter has

become a principal part in petroleum reserves increases; (4)

reservoir types have diversified greatly from mainly clastic

reservoir rocks in the past to an assemblage of clastic rocks,

carbonate rocks, volcanic rocks and metamorphic rocks. Spe-

cial reservoir types have gained an increasingly prominent

position in reserves increases; (5) most large oil and gas fields

discovered recently have medium to low abundance of hy-

drocarbons, indicating the deterioration of resource quality;but their large reserves scale indicates large-scale hydrocarbon

accumulation in the past; (6) engineering technology plays a

crucial role in not only lowering exploration cost but also

enhancing the economic value of resources.

Onshore hydrocarbon resources of medium to low abun-

dance are spread extensively across China (Figure 1). Aiming

at hydrocarbon resources with medium to low abundance in

onshore superimposed petroliferous basins, this paper probesthe geologic settings of these large-scale accumulations and

their distribution in the hope of shedding a little light on hy-

drocarbon geologic theory and to push ahead exploration and

reserves increases in the province.

2 Geologic settings for large scale accumulationof medium low abundance hydrocarbonWe have observed a special kind of hydrocarbon accumula-

tion which is low in abundance[25]

, extensive in distribution

and large in potential reserve; widely spread over large on-

shore petroliferous basins in China; we refer to them as me-

dium-low abundance hydrocarbon resources. In general thiskind of resource differs significantly from medium-high

abundance resources in terms of reservoir geometry, source

-reservoir-seal assemblage, mechanisms of hydrocarbon gen-

eration, expulsion, migration and accumulation, preservation

conditions, etc. Here we use the concept of medium-low

abundance resources in large-scale hydrocarbon accumula-

tion to indicate its accumulation and distribution features.

2.1 Concept and connotations of large scaleaccumulation of medium low abundance resources2.1.1 Definition of medium-low abundance hydrocarbon

resources

In accordance with their quality and economic value, hy-

Fig. 1 Distribution of hydrocarbon resources of medium to low abundance in China


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3

drocarbon resources occurring in major onshore petroliferous

basins in China can be classified into two categories, i.e. high

abundance resources and medium-low abundance resources.

Just as its name implies, medium-low abundance resources

refer to hydrocarbons relatively low in abundance. According

to the statistical data based on national standards for hydro-carbon geologic reserves abundance classification (for crude

oil, reserves 300104 t/km

2belong to high abundance, re-

serves 10010430010

4 t/km

2 medium abundance, and re-

serves 5010410010

4t/km

2low abundance; for natural gas,

reserves 10108 m

3/km

2 represent high abundance, re-

serves 21081010

8 m

3/km

2 medium abundance, and re-

serves


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4

O12yMiddle and Lower Ordovician Yingshan Formation, O2yjUpper Ordovician Yijianfang Formation,

O3qUpper Ordovician Qrebake Formation, O3sUpper Ordovician Sangtamu Formation

Fig. 2 Distribution of fractured vuggy hydrocarbon unit in Tahe Oilfield

Fig. 3 Reservoir petrophysical properties of different kinds of

natural gas reservoirs

drocarbon depends on the existence of accumulation elements

on a large scale and three aspects of variations in accumula-

tion conditions on a certain scale, i.e. the existence of source

kitchens and reservoir bodies on a large scale and their change

in heterogeneity, the extensive distribution of source-reser-

voir-seal assemblages, and formation uplift on a large scale.

2.2.1 Existence and variation of accumulation elements

on a large scale

2.2.1.1 Extensive development of three kinds of sourcekitchen and two kinds of reservoir bodies

There are three kinds of major source kitchens for large-

scale accumulation of medium-low abundance hydrocarbon

resources: (1) coal-measure source rocks, mainly distributed

in the Carboniferous-Permian and Triassic-Jurassic Systems;

(2) argillaceous source rocks, commonly found in the Creta-

ceous System in the Songliao Basin, the Triassic System in

the Ordos Basin, or the Carboniferous-Permian System in the

Junggar Basin, etc.; (3) cracked gas from liquid hydrocarbonsresiding in source rocks, mainly found in marine Paleozoic

measures in the Tarim and Sichuan Basin. The mass develop-

ment of source kitchens refers to the large scale of kitchens

which may provide enough hydrocarbon sources for

large-scale accumulations from medium-low abundance hy-

drocarbons as well as the large scale of kitchens during the

hydrocarbon generation and expulsion stages. For example,

the Upper Paleozoic Carboniferous-Permian coal-measure

source rocks as a whole with an area of 24104 km

2 in the

Ordos Basin have reached the peak gas generation threshold

of Roabove 1.2% at the end of the Cretaceous due to gentleformation configuration, which means over 90% of source

kitchens have entered the gas generation window. The overall

basin uplift since the Cretaceous has given rise to adsorbed

gas desorption and free gas expansion, which have then been

discharged from gas source kitchens for accumulation over an

area of 18104km

2. There is also large scale liquid hydrocar-

bons residing in marine argillaceous source rocks in the Tarim

and Sichuan Basins which have been converted to natural gas

via thermal cracking at a high-post mature stage. Cracked gas

source rocks cover an area of 7104 km

2 in the Manjar de-

pression in the Tarim Basin. The Sinian-Cambrian gas source

kitchens that have also reached the thermal cracking stage

also extend over an area greater than 8104 km

2 in the Si-

chuan Basin.


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5

The mass development of reservoir bodies refers to those

reservoir groups transformed from depositional sand bodies

which are controlled by gentle formation configuration and

formation water systems in succession and are altered by

diagenesis, or fractured vuggy unit groups in cluster distribu-

tions on a large scale which are the product of carbonate rocksaltered by epigenetic dissolution. The mass development in

the paper means the reservoir groups cover thousands of, even

tens of thousands of square kilometers (Tables 1 and 2). For

example, the proved and basically proved gas-bearing area in

the Sulige Gas Field in the Ordos Basin has exceeded 3.3104

km2with tens of thousands of relatively independent gas res-

ervoirs. Fractured vuggy carbonate reservoir groups are very

common in carbonate reservoir rocks and are distributed ex-

tensively thorughout the Tarim, Sichuan and Ordos Basins. If

each fractured vuggy unit is taken as a basic hydrocarbon

accumulation unit, this kind of reservoir group also contains

thousands of to tens of thousands of units, spreading over

thousands of or tens of thousands of square kilometers.

2.2.1.2 Mass distribution of four source-reservoir-seal

assemblages

The mass development of source-reservoir-seal assem-

blages means large-scale distribution of assemblages gener-

ated through close surface contact between source rocks, res-

ervoir rocks and caprocks or internal connection by various

channels. The large-scale distribution of assemblages is based

on the large-scale development of source rocks and reservoir

groups. According to the study, there are four kinds ofsource-reservoir-seal assemblages (Figure 4) in large-scale

hydrocarbon accumulations of medium-low abundance in

large onshore petroliferous basins in China: (1) extensive as-

semblages, in which underlying source rocks are in close

contact with overlying reservoir bodies in a sheet-like mode

on a large scale. This kind of assemblage is represented by the

Carboniferous-Permian transitional coal-measure clastic rock

assemblages in the Ordos Basin, where source kitchens are in

close contact with underlying reservoir groups, conducive to

migration and accumulation of gas discharged from source

rocks in reservoir groups; (2) sandwich or layer cake assem-

blage, in which source rocks and reservoir bodies are in alter-

nate contact with each other. This kind of assemblage is rep-resented by the Upper Triassic Xujiahe Formation in the Si-

chuan Basin, where the Xu1, Xu3 and Xu5 Members are ma-

jor gas source rocks and the Xu2, Xu4 and Xu6 Members are

major reservoir beds, which interfinger alternately over a dis-

tribution area of 11.3104 km

2. Gas discharged from source

rocks could migrate into adjacent reservoir bodies to form

large-scale gas reservoirs; (3) inter-medium assemblage, in

which source kitchens are in connection, instead of direct

contact, with reservoir groups through fault networks and the

planar distribution of unconformable surfaces which act as

passages for large-scale hydrocarbon migration and accumu-

lation. This kind of assemblage is represented by the Ordovi-

cian Yijianfang, Lianglitage and Yingshan Formations in the

slope areas of the Tazhong and Tabei uplifts in the Tarim Ba-

sin; (4) reverse flow assemblage, in which source rocks cap

the reservoir bodies and hydrocarbons are expelled from

overlying source rocks, charging downwards into underlying

reservoir bodies to form large-scale reservoirs. This kind of

assemblage is also large in scale and represented by the Ordo-

vician Majiagou Formation in the middle Ordos Basin (Figure

5), where the Carboniferous-Permian coal-measures source

kitchen directly overlies the Majisgou weathering crust reser-

voir beds. Here gas flows downwards into underlying reser-voir bodies to form gas reservoirs. At present nearly 160 gas

reservoirs have been discovered in the Jingbian Gas Field

alone, with a probable gas-bearing area of 1.0104 km

2,

proved gas reserves of 4 337108 m

3, basically proved re-

serves of 330108m

3, probable gas reserves of 2 08710

8m

3

and PPPR (proved, probable, and possible reserves) totalling

6 754108m

3.

Table 1 Overview of reservoir bodies in medium-low abundance clastic gas fields in China

HorizonSedimentary system

area/km2Sands area /km

2Reservoirs area/km

2

Single reservoir

area/km2

Reservoir-sand

area ratio/%

Xu2 Member, Hechuan 17 703 12 534 11 852 0.510.0 95

Xu2 Member, Guang'an 27 096 17 227 16 519 5.015.0 96

He8 Member, Sulige 159 386 145 591 115 043 0.31.5 79

He8 Member, Mizhi-Yulin 105 062 79 135 54 891 0.21.2 69

Table 2 Statistics of typical carbonate reservoir bodies

Class-I reservoir unit Class-II reservoir unit Class-III reservoir unit Class-IV reservoir unit

Gas field

Proved oil &

gas bearing

area/km2

Number of

reservoir

units NumberSingle reservoir

unit area/km2Number

Single reservoir

unit area/km2Number

Single reservoir

unit area/km2Number

Single reservoir

unit area/km2

Jingbian 6 000 158 28 20.060.0 53 1545 77 1030

Tahe 1 780 348 35 3.528.0 20 416 43 26 250 0.52.0


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Fig. 4 Major source-reservoir-seal assemblages in large-scale

gas reservoir groups of medium-low abundance

2.2.1.3 Large-scale variations in three aspects of accu-

mulation elements

Large-scale variations in three aspects of accumulation

elements include variation in continuity of lateral source

kitchen distribution, lateral variation in reservoir petrophysi-

cal properties and continuity, and variation in stratigraphy and

lithology. These variations would make hydrocarbon flow into

adjacent reservoir bodies, continuously or discontinuously, to

form hydrocarbon reservoirs distributed in clusters and also

ensure large scale hydrocarbon accumulation and reservoir

formation.

(1) Lateral changes in source kitchen continuity. There aretwo kinds of source kitchens for large-scale hydrocarbon ac-

cumulation: one is continuous source kitchens represented by

cracked gas from liquid hydrocarbon residing in source rocks

and coal-measures source kitchens. The other is discontinuous

source kitchens represented by the Upper Triassic Xujiahe

Formation in the Sichuan Basin. In spite of the existence of

Xujiahe coal-measure source kitchens on a large scale, the

discontinuous coal distribution leads to a wide planar varia-

tion of hydrocarbon expulsion in both intensity and quantity,

and hence the discontinuous hydrocarbon accumulation in

adjacent reservoir bodies. Xujiahe coal-measure source

kitchens with cumulative gas generation intensity higher than2010

8 m

3/km

2 account for over 80% of the total source

kitchen area. Gas source rocks mainly occur vertically in the

Xu1, Xu3 and Xu5 Members with individual gas generation

intensity of less than 15108m

3/km

2 in general. Gas genera-

tion intensity in most sections is only 81081010

8m

3/km

2.

In terms of the capacity of gas supply from source kitchens,

partial reservoir bodies do not receive enough gas due to the

discontinuous nature of the source rock distribution and it is

difficult for reservoir bodies to form continuous gas reservoirs

with high gas saturation, resulting in discontinuous hydrocar-

bon accumulation and large variation in reservoir abundance.

(2) Lateral change in reservoir petrophysical properties and

continuity. In spite of the large-scale development of reservoir

bodies on the whole, there are still some lateral changes

within the internal reservoir space and petrophysical proper-

ties owing to the lateral alteration of sedimentary environ-

ments, diagenesis and epigenetic reconstruction intensity. As a

consequence, a series of reservoir units or fractured vuggy

units with relatively good porosity, permeability and pore

throat structures are present against a background of quasi-la-

yered distribution. A single reservoir unit or fractured vuggy

unit may be small and different in scale, but the gathering of

these units would be considerably larger. For example, thesandstone reservoir beds in the Sulige Gas Field, in the Ordos

Basin basically consist of numerous tight sands and conven-

tional sands (Figure 6).

(3) Lateral change in stratum and lithology. Large-scale

reservoir bodies generated against a gentle structural setting

exhibit great spatial variations in their internal reservoir

Fig. 5 Gas reservoir section, Jingbian Gas Field, Ordos Basin


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7

Fig. 6 Reservoir section of Shihezi Formation, Ordos Basin

petrophysical properties and structures. As a result, many ge-

netic types of lithologic-stratigraphic traps come into being,

e.g. lithologic traps generated from initial sedimentation,petrophysical property traps originated from diagenesis,

stratigraphic traps composed of fractures-vugs and wall rocks

originated in epigenesis, etc. These numerous features, inde-

pendent of quasi-independent traps, often occur in clusters

and evolve into reservoir groups once charged with hydrocar-

bons. Although a single reservoir is small, tens of thousands

of units could constitute reservoir groups on a large scale with

the distribution area over thousands or even tens of thousands

of square kilometers. The large-scale hydrocarbon accumula-

tions may remedy the disadvantage of large structural trap

deficiencies in the hinterland and slope areas in large onshore

depositional basins and the disadvantage of poor caprock

conditions in vast gentle structural areas in the hinterlands in

petroliferous basins in China. Large-scale hydrocarbon accu-

mulations may still occur in those provinces with poor

caprock conditions (throat breakthrough pressure difference

between tight sands and gas sands may be only 0.30.5 MPa

in general).

2.2.2 Extensive uplift

Large-scale accumulation of medium-low abundance hy-

drocarbon features large-scale accumulation during petrolif-

erous basin uplift. Extensive uplift and denudation in a depo-sitional basin are usually regarded as a process of overlying

formation pressure drop (i.e. unloading) as well as formation

temperature drop and pressure relief. According to classic

hydrocarbon accumulation theories, formation uplift would be

taken as the cause of reservoir damage and hydrocarbon loss,

especially in those provinces with poor caprock conditions.

Here we believe that moderate uplift of large depression lake

basins in China may not lead to hydrocarbon reservoir dam-

age; instead, it becomes an important period in hydrocarbon

expulsion and accumulation. As for moderate uplift, we here

refer to an uplift process which would lift source rocks and

major target strata up to an appropriate depth interval and

meanwhile not lead to hydrocarbon losses. Based on the

analysis of uplift magnitude impact on hydrocarbon accumu-

lations in the Ordos, Sichuan, Tarim and Junggar Basins, large

depression lake basins in China have generally been uplifted

by 1 000

3 000 m and the present buried depths of majortarget strata with hydrocarbon accumulations are all deeper

than 2 000 m. Basin uplift is considered to facilitate hydro-

carbon accumulation on a large scale due to the following

three aspects: (1) in continuous burial period, hydrocarbons

may accumulate in progressively and be deposited in source

rocks at some stage, which reserves energy for hydrocarbon

expulsion at the uplift stage; (2) in the process of formation

uplift and temperature drop, hydrocarbon generation slows

down while oil and gas expand due to the pressure drop. For

example, the uplift of coal-measure source kitchens may give

rise to pressure release and gas desorption and expansion[2627]

.

Volume growth from marine liquid hydrocarbon cracking and

conversion into gaseous hydrocarbons at the mature to post

mature stages[2829]

may result in hydrocarbon discharge in a

concentrated manner from inner source rocks, i.e. large-scale

hydrocarbon expulsion; (3) large onshore depression lake

basin uplifts in China have mainly occurred after the end of the

Cretaceous, which postpones hydrocarbon accumulation and

is advantageous to the preservation of hydrocarbon reservoirs.

3 Mechanism and distribution of large scaleaccumulations of medium low abundancehydrocarbon

Large-scale hydrocarbon accumulations of medium-low

abundance need three geologic prerequisites: (1) source rocks

in close and extensive contact with reservoir rocks; (2) reser-

voir bodies with strong heterogeneity, complicated pore-throat

structures and poor petrophysical properties; (3) gently dippng

strata and lack of high-relief structural traps and good

caprocks. Accordingly, large-scale hydrocarbon accumula-

tions of medium-low abundance differ greatly from both con-

ventional hydrocarbon reservoirs and unconventional hydro-

carbon accumulations in mechanism and distribution.

3.1 Mechanism of hydrocarbon migration andaccumulationAs mentioned above, the close and extensive contact of


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8

source rocks with reservoir bodies is one of the prerequisites

for large-scale hydrocarbon accumulation at medium-low

abundance, which works in two aspects: (1) the excessive

pressure inside source kitchens would be fully converted into

effective drive to expel massive hydrocarbon generated from

source kitchens into reservoir bodies over a short distance; (2)the extensive contact guarantees extensive migration and ac-

cumulation over a short distance for large-scale hydrocarbon

accumulations.

There are two mechanisms working in the process of hy-

drocarbon accumulation in reservoir bodies with poor proper-

ties: (1) the mechanism of volume flow migration and accu-

mulation. Hydrocarbons generated intermittently in large

quantities in source kitchens, give rise to overpressure and

inner source pressure much higher than the adjacent reservoir

pressure. Thus the source-reservoir pressure difference works

as a powerful force driving hydrocarbons into tight heteroge-

neous reservoir bodies in the mode of volume flow. Mean-

while differences between source-reservoir hydrocarbon con-

centrations would also drive hydrocarbons to diffusively flow

into reservoir rocks. We found that hydrocarbon charge

through volume flow mainly occurs during the burial stage, i.e.

at a stage where high source-reservoir pressure differences

occur. Taking the Ordos Basin as an example, it is confirmed

by pressure data from inclusion tests that there is at least 7

MPa residual source-sand pressure difference in the Upper

Paleozoic Shanxi Formation and 5 MPa residual pressure dif-

ference between Shanxi Formation and adjacent Shihezi For-

mation sandstone, which would propel gases inside sourcerocks to move toward reservoir beds, i.e. gas charge with

volume flow. At a later basin uplift stage, residual source-re-

servoir pressure differences may decline gradually because

hydrocarbon generation in source kitchens comes to an end.

On the other hand, free gas volumetric expansion in micro-

pores in source rocks due to formation uplift may drive up

inner source pressures and maintain certain drainage forces in

source rocks. Meanwhile desorption of adsorbed gas in source

kitchens owing to source pressure decline during uplift would

increase free gas saturation in those micropores in source

rocks, which would also contribute towards the source-reser-

voir driving force; (2) the mechanism of diffusion flow migra-

tion and accumulation. In general, reservoir bodies in

large-scale hydrocarbon accumulations of medium-low abun-

dance often feature low porosity, low to extremely low per-

meability, high displacement pressure and high irreducible

water saturation. In addition to volume flow driven bysource-reservoir pressure differences, diffusion driven by hy-

drocarbon concentration differences would also cause hydro-

carbon migration and accumulation. Especially in those tight

reservoirs with poorer properties and pore-throat structure,

hydrocarbons may dominantly be propelled into reservoir

beds by diffusion due to concentration differences. On ac-

count of the extensive and direct contact of source rocks with

reservoir rocks, the diffusion may occur regionally on a large

scale. Therefore diffusion is another mechanism for large-scale

hydrocarbon accumulation.

3.2 Major patterns of large scale hydrocarbonaccumulationPancake, layer-like, and cluster are three major accumula-

tion patterns of large-scale hydrocarbon accumulations which

ensures the scale of hydrocarbon accumulation. Accumulation

in thin, layer-cake patterns refers to the type of discoveries

with small oil and gas column height (usually several meters

to dozens of meters) and large oil and gas bearing areas (usu-

ally thousands to tens of thousands of square kilometers); oil

and gas zones would be distributed in space like a layer-cake.

The area of this kind of reservoir could be denoted with regu-

lar squares in numerical characterization, with square heightstanding for oil and gas column height and square area oil and

gas bearing area, with the aspect ratio denoting the feature of

the thin cake like structure. Based on this notation, we

counted the medium-low abundance gas reservoir groups dis-

covered in the Ordos, Sichuan and Tarim Basins (Table 3) and

found that values of aspect ratio often exceed 1000 or ten

thousand at the most. For example, the proved gas-bearing

area in the Sulige Gas Field is about 20 800 km2and effective

gas zone thickness is 515 m; the ratio of average width of

gas-bearing area to average gas zone thickness is up to 14422

and is much higher than that in large gas fields with high

Table 3 Medium-low abundance gas reservoir and gas zone thickness, China

Gas fieldOil & gas bearing

area/km2

Reservoir

thickness/mPorosity/%

Permeability/

103m2

Reservoir-sandstone

thickness ratio /%

Reservoir

width-to-thickness ratio

Xinchang 161.20 825 3.08.0 0.104.00 9 1 311

Daniudi 1545.65 619 5.011.0 0.00110.00 28 3 574

Hechuan 1058.30 1126 7.010.0 0.00150.00 25 2 168

Guang'an 578.90 635 6.013.0 0.00110.00 20 1 322

Anyue 360.80 1036 6.014.0 0.00114.00 29 1 187

Sulige 20 800.00 515 7.011.0 0.0110.00 57 14 422

Yulin 1 715.80 330 5.011.0 0.0110.00 58 3 570

Wushenqi 872.50 512 3.514.0 0.0110.00 56 3 475

Shenmu 827.70 315 4.012.0 0.0110.00 69 3 424


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9

abundance, where the ratio is generally from less than one

hundred to several hundred. For example, the gas-bearing area

in the Kela2 Gas Field is 48 km2with a gas column height of

55 m on average, the ratio of average width to average thick-

ness is only 126; this ratio in the Puguang Gas Field is only

75.It is noted here that large-scale hydrocarbon accumulation

in thin, layer-cake patterns may come about in the provinces

with poor caprock conditions; thereby this pattern is very im-

portant for large-scale hydrocarbon accumulations of me-

dium-low abundance. For instance, the Upper Paleozoic

structure in the Sulige Gas Field is a gentle monocline, high in

the north and low in the south, with a dip angle of 13. Gas

zone thickness is usually 515 m and single gas-bearing sand

is 1 0002 500 m in length and 100250 m in width. Buoy-

ancy pressure from the gas column height is 0.15 MPa at the

most. The direct caprocks over the gas zones are tight sands

with poorer properties and their displacement pressure is

higher than 1.2 MPa according to lab data. The reservoir-seal

displacement pressure difference is larger than 0.5 MPa,

which means the buoyancy from the gas column is insufficient

for natural gas to break through caprocks and gas reservoirs

may then be preserved.

Accumulation in cluster refers to oil and gas accumulation in

a series of stratigraphic or lithologic trap groups. Terrestrial

sedimentary series, such as carbonate series in gentle platforms

in cratonic basins, transitional and continental series in onshore

depressions and continental sedimentary series in wide gentle

slopes in foreland basins, are reconstructed by positive diagene-sis and epidiagenesis to form reservoir beds with strong hetero-

geneity. Variation in lithologies and hydrodynamic energy in the

original sedimentary source area could also result in heteroge-

neous reservoirs. As a consequence, numerous independent or

quasi-independent reservoir groups may come into being; the

single reservoir scale may be small, but a gathering of these

reservoirs would become considerably large (Table 4). Oil and

gas charge into these reservoir bodies would create hydrocarbon

reservoir groups on a large scale; besides the reservoir groups

do not demand sealing conditions and some inferior sealing

conditions (usually less than several mega-pascal in pressure)are sufficient for large-scale hydrocarbon accumulations, in

which reservoirs with better abundance (i.e. sweet spots) be-

come targets for exploration. For example, the He8 Member in

the Sulige Gas Field, where thousands of gas reservoir groups,

of a small single scale, constitute the large gas field owing to

severe lateral variations in lithology and petrophysical proper-

ties of channel bar sands. Tight gas zones with low gas satura-

tion, dry zones and water zones (at the structural low in west

Sulige) are distributed continuously or discontinuously between

small conventional lithologic gas reservoirs. Unconventional

and conventional gases are mixed together on the whole. Ac-cording to studies on the Sulige Gas Field, the gas-bearing area

is close to 3.3104km

2, gas reservoirs defined by sand geome-

try are about 5104810

4, gas column height in single gas

reservoir is 26 m, the scale of OGIP is generally 3000104

10 000104 m

3, and the average reserves abundance of the

whole field is about 0.7108m

3/km

2. Tight sands on the whole

bear natural gas, and spread continuously despite their low gas

saturation.

A quasi-layered pattern is most common in hydrocarbon

accumulations in fractured-vuggy carbonate reservoir beds.

According to studies, quasi-layered fractured-vuggy karst

reservoir beds spread widely in periclinal zones at inherited

palaeohighs in cratonic basins due to the impacts of weather-

ing, karstification, bedding karst and inter-stratal karst. Oil

and gas charge into these reservoir beds will lead to a distrib-

uted configuration of quasi-layered accumulations. Take the

Ordovician Yijianfang, Yingshan and Lianglitage Formations

in the Tabei uplift slope area in the Tarim Basin as an example.

There are hundreds or even thousands of fractures and vugs

originating from multi-karstification and each fractured vuggy

unit may be regarded as a relatively independent accumulation

unit (Figure 7). Multiple quasi-layered hydrocarbon reservoir

groups can be merged together to form large oil and gas fields,e.g. the Tahe and Halahatang Oilfields, with reserve scales of

several hundred million tons to even several billion tons.

3.3 Distribution of large scale accumulations ofmedium low abundance hydrocarbon3.3.1 Proximity to source

Proximity to source implies that hydrocarbon reservoirs of

medium-low abundance are distributed in the areas within or

adjacent to effective source kitchens and reservoir distribution

is controlled by source area. There are two aspects to consider:

(1) the reservoir bodies coexist and are in close contact withsource kitchens on a large scale, which is a prerequisite of

large-scale hydrocarbon accumulation at medium-low abun-

dance; (2) there is a considerable source-reservoir pressure

difference or hydrocarbon concentration difference to guaran-

tee effective hydrocarbon accumulation on a large scale.

Table 4 Overview of typical clustered medium-low abundance gas reservoirs

Gas fieldNumber of gas

reservoirs

Gas bearing area of

single reservoir/km2

Reserves in single

reservoir/108m

3

Reserves abundance in single

reservoir/(108m

3km

2)

Gas zone thickness in

single reservoir/m

Gas saturation in

single reservoir/%

Sulige 5104810

4 0.31.5 0.31.0 0.30.7 26 4065

Jingbian 120200 20.060.0 10.060.0 0.20.7 27 6090Hechuan 150200 0.510.0 1.020.0 1.05.0 210 5065

Guang'an 3560 9.017.0 5.040.0 0.84.3 413 3560


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Fig. 7 Quasi-layered hydrocarbon distribution pattern in Tabei uplift and slope area, Tarim Basin

Based on recent studies of medium-low abundance reservoirs

discovered in the Ordos, Sichuan, Songliao, Tuha and Junggar

Basins, over 90% of reservoirs have been found to be within

the extent of source kitchens. Due to the proximity to source,

gas reservoir groups of medium-low abundance mainly occur

in synclines and wide slopes in depression basins, periclines at

palaeohighs in cratonic basins and gentle slopes in forelandbasins.

3.3.2 Large area and range plays

The reservoir assemblage (play) refers to a group of hydro-

carbon reservoirs which are created under the same or similar

forming conditions and are provided with the same or similar

genetic mechanisms and distribution. There are two assem-

blages in large-scale hydrocarbon accumulations of me-

dium-low abundance, i.e. accumulations large in area and

accumulations large in range.

Accumulations large in area denote the kind of reservoir

assemblages dominated by continuous source kitchens with

sufficient hydrocarbon supply. These mainly occur in the

Carboniferous-Permian transitional sandstone series in the

Ordos Basin, followed by some clastic series close to major

gas source kitchens, e.g. the Permian Jiamuhe, Xiazijie and

Wuerhe Formation around Manas Lake source center in the

Junggar Basin, Taiyuan Formation close to source kitchens in

the Ordos Basin, the Upper Triassic Xujiahe Formation close

to major gas source kitchens in west Sichuan Basin, and the

sandstone inside major Xu1, Xu3 and Xu5 source series are

some examples.

Accumulations that are large in range denote the kind ofextensive reservoir assemblage which contains sweet spots.

Usually seen in sandwich assemblages and reverse flow as-

semblages, Xujiahe Formation in central Sichuan Basin and

Ordovician Majiagou karstic weathering crust in central Or-

dos Basin are two typical examples of this kind of assemblage.

The single accumulation is hardly economic, but laterally

there will be dozens of or even hundreds of hydrocarbon res-

ervoirs spreading over a wide province; these reservoirs are

usually separated by water zones or tight zones into patches.Studies show there are two reasons for discontinuity of reser-

voir: one is uneven distribution of source kitchen and insuffi-

ciency of hydrocarbon supply volume, which leads to the hy-

drocarbons accumulating first in reservoirs nearest the sources;

the other is poor continuity of reservoir bodies, which results

in scattered distribution of hydrocarbon reservoirs across a

vast area. The Xujiahe gas reservoirs in the Sichuan Basin

(Figure 8), where Xu2, Xu4 and Xu6 reservoir beds vary

greatly in gas saturation and abundance in a lateral direction

due to the discontinuous distribution of Xu1, Xu3 and Xu5

coal-measure source rocks, thinning or even an absence of

source rocks in some areas. There is little chance to form gas

reservoirs in an area with thin coal-measure source rocks.

Natural gas would preferably abound in those regions with

premium source rocks superimposed with effective reservoir

beds, to form accumulations of relatively high abundance and

gas saturation.

3.3.3 Late formation

Late formation refers to hydrocarbon reservoirs of me-

dium-low abundance that are usually formed after the middle

to late stage of the Cretaceous Period in large onshore depres-

sions making up the petroliferous basins in China. The stageof reservoir formation is obviously subsequent to that of the

conventional, but the accumulation efficiency is relatively


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11

Fig. 8 Xujiahe Formation gas reservoir section, Guang'an, Sichuan Basin

high due to limited dissipation in a shorter period of hydro-

carbon loss. The late accumulation is likely due to: (1) forma-

tion uplift took place at a later stage; (2) fossil marine source

rocks resided within the liquid hydrocarbon window for a

long time (almost hundreds of millions of years) which is

enough for source rocks to be fully matured. Hydrocarbon

expulsion might be limited at an early stage and mass expul-

sion might follow the rapid burial after the Cretaceous Period;

(3) gas generation from thermal cracking might happen at a

later stage. These three factors lead to hydrocarbon discharge

on a large scale from source kitchens, especially from gas

source kitchens, at a later stage to form large-scale hydrocar-

bon reservoir groups of medium-low abundance. In addition,

natural gas adsorption in gas source kitchens due to high ad-

sorbability of coal measures and large-scale desorption and

expulsion from sources at the later uplift stage might also

cause the late accumulation. Studies show the Sulige Gas

Field in the Ordos Basin and the Xujiahe gas reservoir in the

central Sichuan Basin were all generated in the Cenozoic Era,

exactly coincident with the period of basin uplift. The explo-

ration activities in Sulige and central Sichuan demonstrate the

large scale of hydrocarbon accumulation at the uplift stage,

proving the possibility of accumulation at a later stage.

3.3.4 Simplicity in reservoir type

Simplicity in reservoir type means that there is only one

predominant reservoir type in reservoir groups of me-

dium-low abundance, namely lithologic-stratigraphic reser-

voirs, which account for over 95% of total reservoirs. This is

the inevitable result of the geologic setting, where low-relief

marine carbonate series and transitional to terrestrial coal-

measure sedimentary series, fracture-vug systems originated

from positive reconstruction and sand bodies deposited from

inherited traction currents have intense lateral heterogeneity.

These processes are very likely to form stratigraphic andlithologic traps as well as subsequent reservoir groups. Hin-

terlands in such large-scale petroliferous basins as the Ordos,

Sichuan and Tarim Basins are usually gentle in structure with

a low stratigraphic dip angle of 1-3, where reservoirs seem

unlikely to occur due to a lack of large structural traps; but the

extensive development of lithologic or stratigraphic traps

remedies the disadvantages of reservoir forming conditions.

Despite their inadequate abundance, massive oil and gas

bearing scale and less demanding requirements on sealing

conditions make it easier to create accumulations on a large

scale.

4 Discussions4.1 Boundary of clustered reservoirs

It is still in doubt whether thin layer-cake and clustered hy-

drocarbon accumulations have boundaries or not, as well as

where the boundaries are, if they exist and how the boundaries

could be defined. The authors think that the hydrocarbon ac-

cumulation process is in fact a process of hydrocarbon en-

richment in reservoir bodies; no matter how good the abun-

dance and quality are, there is always a process of minerali-

zation; nevertheless, it is more difficult and complicated to

define the boundary of medium-low abundance resources

compared with conventional resources. As for thin layer-cake

and clustered accumulations presented here, it is suggested to

differentiate the boundary of reservoir group from the border

of a single reservoir. In general the boundaries of reservoir

groups are in three forms: (1) lithologic boundary between

reservoir group and contemporaneous deposits. Taking the

Sulige Gas Field as an example, if we focus on each specific

gas sand in a continuous gas zone with superimposed

multi-layers, there should be a border of the sand because

skylight areas with no gas do exist in a large area; (2)

boundary between the reservoir group at the structural low

and the water zone; (3) regional property boundaries inside a

reservoir group due to the existence of tight lithologies. Indi-vidual reservoirs may be provided with four kinds of bounda-

ries: (1) the boundaries of clastic reservoir units are usually


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12

the property boundaries due to the existence of tight litholo-

gies; (2) gas-oil-water contact in conventional reservoirs; (3)

the boundaries of fracture-vug units in carbonate rocks, are

generally the division between dissolved pore-vug units and

tight wall rocks; (4) the gas-oil-water contact inside frac-

ture-vug units.According to the development experiences in the Sulige

Gas Field in the Ordos Basin, wells producing gas and water

or only water occur in the north and west part of the field.

This implies an evident gas-water transition feature indicating

that this part of the field may be taken as the macro-boundary

of the Gas Field. Meanwhile there are many areas with little

or no sandstone reservoir inside the gas field due to intense

heterogeneity of reservoir bodies, which are lithologic pin-

chout belts and may also be taken as the boundaries of gas

reservoirs (Figure 9).

Fig. 9 Gas reservoirs in Sulige Gas Field, Ordos Basin

4.2 Differences between the theory of large scaleaccumulation and other accumulation theories

The ideas for large-scale accumulation of medium-low

abundance hydrocarbon are very different from classic hy-

drocarbon reservoir forming theories and continuous hydro-

carbon accumulation theories [30] both in their subjects of

study and their accumulation conditions and features. Gener-

ally speaking, the differences are greater than the similarities

(Table 5). The theory of large-scale accumulation mainly fo-

cuses on the hybrid conventional and unconventional re-

sources, varies from the theory of classic reservoir formation

and continuous accumulation of conventional and unconven-

tional resources. In such provinces with good reservoir prop-

erties or favorable tectonic settings (e.g. low-relief structural

traps), the differentiation of gas, oil and water is relatively

remarkable and there are dominantly conventional hydrocar-

bon resources; while in such provinces with tight reservoirbeds or short of structural features, the differentiation of gas,

oil and water is indistinct and there are mainly unconventional

accumulations with complicated reservoir boundaries. As for

conventional reservoirs, oil and gas would combine from a

disperse state under the effect of formation water buoyancy

and discharge into reservoir bodies in volume flow; reservoirs

would be provided with high abundance and oil and gas dis-

tribution is controlled by local traps, thereby there are distinct

gas-oil-water contacts inside reservoirs. As for unconven-

tional resources, hydrocarbons usually spread inside source

kitchens and accumulate in self-source-reservoirs or coexist-ing source-reservoirs via diffusion flow under the effect of

source-reservoir pressure difference or hydrocarbon concen-

tration difference; there is no distinct differentiation of gas, oil

and water and hydrocarbons would be continuously distrib-

uted over a large area.

As for distribution, conventional reservoirs usually concen-

trate in those regions in a basin with low fluid potential and

their distribution is often dominated by large structural set-

tings. Unconventional accumulations often coexist with

source rocks and often spread inside source series or proximal

Table 5 Overview of three hydrocarbon accumulation theories

Theory Study object Static geologic element MechanismAbun-

danceAssemblage Significance

Classical

hydrocarbon

accumulation

Conventional

reservoirs

Premium

source-reservoir-seal

assemblage and traps

Separated

source-

reservoir

Limited

distribu-

tion

Darcy &

non-Darcy

flow

Volume flow

charge

Middle-

high

Structural types

followed by

composite type

Guidance to

prospect

forecast

Medium-low

abundance

Hydrocarbon

accumulation

on a large scale

Conventional-

unconventional

Transitional ac-

cumulation

Premium source and

poor reservoir-seal

assemblage

Close

source-

reservoir

contact

Extensive

distribution

Non-Darcy

flow

Volume flow

and diffusion

flow charge

Low-

middle,

Mainly

low

Extensive

distribution

(thin layer-

cake &

concentrated)

Guidance to

prospect

forecast

Continuous hy-drocarbon

accumulation

Unconventional

accumulation

Premium-source and

poor-seal assemblage,

thick and continuous

reservoir rocks

Close sour-ce-reservoir

contact

Extensive

distribution

Rate in pub-

lication

Mediumabundance

and above

Continuous

Resource

management

and evalua-

tion


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13

reservoir bodies in alternate contact with source rocks. Hybrid

reservoirs of medium-low abundance commonly occupy the

same terrains with unconventional resources, like platforms in

cratonic basins, gentle synclines in depression basins and

wide slopes in foreland basins. Large-scale accumulations

may also occur in those regions with tectonic uplift consid-ered in the past to be unfavorable for hydrocarbon prospectiv-

ity due to their poor reservoir forming conditions and those

districts with poor sealing conditions.

5 ConclusionsLarge onshore petroliferous basins in China widely contain

a kind of petroleum resource of medium-low abundance but

large-scale hydrocarbon accumulation. The key to large-scale

accumulations at medium-low abundance is the existence of

accumulation elements on a large scale and variations in ac-

cumulation conditions on a large scale. Three kinds of source

kitchens, i.e. coal-measure source rocks, marine argillaceous

source rocks and source rocks with residing liquid hydrocar-

bon, and reservoir bodies distributed over a large scale are the

physical foundation of large-scale accumulations. The exten-

sive distribution of four kinds of source-reservoir-sealing as-

semblages, i.e. extensive assemblage, sandwich assemblage,

inter-medium assemblage and reverse flow assemblage, pro-

vides conditions for large-scale accumulation. Large-scale

continuity variation in lateral source kitchen distribution,

large-scale lateral variation in reservoir petrophysical proper-

ties and their continuity, and large-scale variation in stratigra-

phy and lithologies guarantee the accumulation on a largescale. Extensive formation uplift also facilitates the overall

hydrocarbon expulsion and accumulation.

Large-scale accumulation of medium-low abundance hy-

drocarbon is realized through volume flow and diffusion flow,

which guarantees the sufficiency of hydrocarbon charge. Thin

layer-cake accumulation, quasi-layered accumulation and

clustered accumulation are three major patterns of large-scale

accumulation, which guarantees the scale of reservoir forma-

tion. Large-scale accumulations of medium-low abundance

hydrocarbon feature proximal accumulation, main body of

reservoir assemblages, late accumulation and simple reservoir

type and are mainly distributed in periclines at palaeohighs in

marine cratonic basins, wide slopes and synclines in onshore

depression basins and gentle slopes in foreland basins, which

are represented by accumulations over a large area and range.

The idea of onshore large-scale accumulation of me-

dium-low abundance hydrocarbon in China enriches and de-

velops hydrocarbon accumulation theory and promotes the

potential of discovering hydrocarbon resources in middle and

deep zones in superimposed basins. It pushes hydrocarbon

exploration ahead from local second-order structure zones to

the whole basin with major source kitchens as the focus and

from shallow and intermediate zones to deep and extremelydeep zones. It will be instructive to the extension of future

hydrocarbon exploration.

AcknowledgmentsIn addition to references, this paper also cites some research

findings from the National 973 Program (2007CB209500) and

the National Carbonate Rock Major Project (2008ZX05004).

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