Science in China Series D: Earth Sciences
© 2007
Science in China Press
Springer-Verlag
Characteristics of petroleum accumulation in syncline
of the Songliao basin and discussion on its accumulation
mechanism
WU HeYong1,2†, LIANG XiaoDong1, XIANG CaiFu3 & WANG YueWen1
1
Daqing Oil Field Exploration and Development Research Institute, Daqing 163712, China;
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
3
Basin & Reservoir Research Center of China University of Petroleum, Beijing 102249, China
2
The relation between oil and water in reservoirs with low and ultra-low permeability is very complicated.
Gravitational separation of oil and water is not obvious. Normal reservoirs are located in depression
and structural high spot, oil and water transitions are located in their middle. Stagnation is the key fact
of oil-forming reservoir in the axis of a syncline based on the research of oil, gas and water migration
manner, dynamics and non-Darcy flow in the Songliao basin. In low and ultra-low permeable reservoir,
gas and water migrate easily through pore throats because their molecules are generally smaller than
the pore throats; but the minimum diameter of oil droplets is larger than pore throats and they must be
deformed to go through. Thus, gas and water migrate in advance of oil, and oil droplets remain behind.
Pressure differential and the buoyancy force in a syncline reservoir are a main fluid driving force; and
capillary force is the main resistance to flow. When the dynamics force is less than resistance, oil is
immobile. When the buoyancy force is less than the capillary force, a gravitational separation of oil and
water does not occur. The reservoir in the mature source rock of a syncline area with the low and ultra-low permeability belongs to an unconventional petroleum reservoir.
oil droplet, pore throat, syncline oil reservoir, low to ultra-low permeable reservoir, stagnation, non-Darcy flow
The earliest anticlinal reservoir was proposed by T. S.
Hunt in 1861[1]. He pointed out that petroleum accumulates in an anticline or a structure high, where gravitational separation occurs, so the hypothesis was called the
anticlinal theory. The anticlinal theory is followed by
geologists and it directs the decision-making of explora―
tion[2 4]. Since then, petroleum geology followed the
traditional static analysis of source rock, reservoir, caprock, trap, preservation condition. Furthermore, it emphasizes interrelations between each static factor[5] and
dynamic factors and system analysis methods of forming
reservoir dynamics[6]. The above petroleum migration
and accumulation theories are based on the buoyancy
effect and petroleum accumulation in a trap or stratigraphic trap or lithologic trap in the form of discrete
accumulation. Law et al. [7] proposed a conceptbasin
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― centered accumulation of unconventional petroleum system[7]. They thought that the unconventional
petroleum system has no bearing upon a structure, stratigraphic and lithologic traps, owing to having no
influence by gravitational separation. There is an oil
field belt in a large scale. They took some sorts of the
natural gas resources as example, such as coal bed gas[8],
deep basin gas[9] and natural gas hydrate[10] and so on,
which are unconventional gas resources. Domestic
scholars at the beginning of this century took notice of
one situation in which an oil seep occurred in the axis of
syncline, and called it sag wide oil-bearing theory[11].
The senior author of this paper detected such a situaReceived June 26, 2006; accepted December 6, 2006
doi: 10.1007/s11430-007-0031-y
†
Corresponding author (email: [email protected])
Sci China Ser D-Earth Sci | May 2007 | vol. 50 | no. 5 | 702-709
tion in Fuyang oil layers (Fuyu and Yangdachengzi oil
layers, Figure 1) in the Sanzhao sag of the Songliao basin in 1986. This is a large petroleum province with
highly mature source rocks and high pressure hydrocarbon expulsion. More than 90% of compact layer did not
contain water. It is different from structure trap, conventional stratigraphic trap and lithologic trap. Subsequently,
similar situations were reported in the Ordos Basin[12]
and the Bohai Gulf Basin[13]. We consider that the oil
migrates in the low to ultra-low permeable reservoir
since the diameter of an oil droplet is larger than that of
a pore throat, the oil droplet must be deformed to go
through the pore throats (the mean permeability of low
permeable reservoir: 50×10−3―10.1×10−3 μm2; the one
−3
of especially low permeable reservoir: 10×10 ―
1.1×10−3 μm2; the one of ultra-low permeable reservoir:
1.0×10−3― 0.1×10−3 μm2)[14]. By contrast, gas and water pass easily through pore throats, owing to their
smaller diameter; oil stagnation becomes the key of petroleum accumulation in synclines. The study and application to the situation and its accumulation mechanism
breaks a conventional exploration idea. The idea that
petroleum stagnates in syncline in a large scale will increase greatly the domain of oil exploration.
1 Geological characteristics in the Songliao basin
The Songliao basin is located west of the Tanlu fault
zone, and occupies three provinces of northeast China
and part of Inner Mongolia. Its area is about 26×104 km2.
The Daqing oil field, which is one of the biggest nonmarine oil field in the world, is contained within the basin. The basin contains mainly Mesozoic and Cenozoic
strata over 10 km thick. The basin is a double-layer
model of downside fracture and upside sag. Its fracture
stage is pre-Cretaceous involving flysch and pyroclastic
strata. Its sag stage is between the Quantou stage and the
Nenjiang stage involving three sets of cycles of lake and
fluvial facies. Lacustrine mudstone and glutenite are the
main source rock and reservoir. The basin began to uplift
and be denuded after the Nenjiang stage. It turned into
wilt phases and its depocenter migrates unceasingly to
the west, with fluvial facies deposits being primary.
There are five sets of oil gas combination. Middle and
under part combination are primary (Figure 1).
2 The characteristics of syncline oil
reservoir in the Songliao basin
The Songliao basin is a large-scale nonmarine petroliferous basin. We have discovered more and more geological phenomena which are not explained by conventional petroleum migration and accumulation theory. In
the center of syncline in this basin, oil and water in the
reservoir are not gravitationally separated. The phenomena cannot be explained with the differential accumulation principle.
2.1 Oil reservoirs exist generally in the center of
syncline
We discovered large-scale and continue petroleum reserves not only in Daqing placanticline but also in the
syncline area. The demonstrated reserves in syncline
area surpassed 1200000000 tons, which is important to
the stable production and high production of Daqing oil
field. Statistics have indicated that syncline oil reservoirs
as a result of stagnation accounting for about 87% outside Daqing placanticline. They have no relation with
structure and stratigraphic lithologic trap, and the gravitational separation is not obvious (Figure 1), so they belong to unconventional petroleum system. Take the Putaohua oil bed of Talaha-Changjiaweizi syncline in the
west of the basin as example, we have already discovered many highly commercial wells, in which the oil
production of Gu88 well surpasses 100 m3/day. In recent
years, exploration has expanded to the syncline center.
The properties of reservoir are different obviously
between the conventional oil reservoir and the syncline
oil reservoir. The permeability of the former (for example, Daqing placanticline) is bigger than 10×10−3 μm2,
but that of the latter is smaller than 10×10−3 μm2. But
their sedimentary environment and sedimentary facies
are similar, and their percentage of the sand is between
20%―40%. Gas will be on the top and oil on the middle
owing to gravitational separation to an identical set of
reservoirs, demonstrating that there is a unification
oil-water boundary in the reservoir. In the Songliao basin, the syncline oil reservoirs owing to stagnation are in
the central part, the oil-water transition zones owing to
half stagnation are in both sides, and conventional oil
reservoirs are in the anticline (Figure 2).
2.2 Inverted relation between oil and water
We have found that 53% oil reservoirs belong to syn-
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703
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WU HeYong et al. Sci China Ser D-Earth Sci | May 2007 | vol. 50 | no. 7 | 702-709
Figure 1 Geologic characteristics and syncline oil reservoir of the Songliao basin, northeast China. 1, Primary structural line; 2, secondary structural line; 3, volcanic rock and pyroclastic
rocks; 4, conglomerate; 5, glutenite; 6, sandstone; 7, sandy mudstone; 8, mudstone; 9, vitrinite reflectance of Qingshankou Formation source rock; 10, locality of Figure 2; 11, syncline oil
reservoir area of Putaohua oil bed; 12, structure and stratigraphic lithologic oil reservoir area of Putaohua oil bed; 13, syncline oil reservoir area of Fuyang oil layers; 14, structure and stratigraphic lithologic oil reservoir area of Fuyang oil layers; 15, permeability of Putaohua oil bed (×10−3 μm2).
Figure 2 Oil reservoir profile of Talaha-Changjiaweizi syncline in the Songliao basin, northeast China. 1, Oil layer; 2, oil and water layer; 3, water layer;
4, empty layer.
cline oil reservoirs in the Putaohua oil bed of Gulong
sag in the Songliao basin. Water layers are above the oil
layer, and there is no edge water and bottom water in the
oil reservoir. But the inverted phenomenon does not exist in the anticline and the sandstone lens or the
lithological pinchout oil reservoir where the permeability is high.
3 Discussion on mechanism of petroleum accumulation in syncline
The phenomenon of an inverted relation between oil and
water indicates that the Gulong sag is unconventional oil
reservoir. We consider that stagnation is the key to this
kind of unconventional oil reservoir forming in a syncline. We discuss the stagnant mechanism in three aspects: difference of migration path ways and rates
among oil, gas and water; dynamical condition; and the
status of fluid flow through porous medium.
3.1 Difference of migration path ways among oil, gas
and water
Migration path ways of gas, water and oil through porous media are different. Gas and water flow generally
as molecules, and their diameters are generally smaller
than the pore throats diameter of low permeable reser-
voir (in low to ultra-low permeable reservoir of Putaohua oil bed in the Songliao basin, the average pore
throats diameter is: n×10―n×100 nm). But petroleum is
some organic macro-molecules and they migrate in the
form of oil droplets. The oil droplets diameters are associated with properties of oil (n×100―n×10000 nm)[15].
Generally, they are bigger than the pore throats diameter
of this kind of reservoir, so oil droplets are blocked and
deformed when they pass through the pore throats. By
contrast, gas and water can pass easily. Therefore, the
difference of size between oil droplets and water and gas
molecules is the main reason that oil droplets are delayed passing through a reservoir with low porosity and
permeability.
Physical resistance to oil droplet movement is associated with anisotropy of pore structure except properties
of oil in the process of migration. The oil droplets are
subject to three forces. The first is the result from the
change of capillary pore size, and the directions of the
force are in agreement with capillary pore size increasing. The second is because of wetting hysteresis of the
non-wetting phase (oil in a general way) in the process
of movement, and its contact angle between advance and
backlash is different which block the non-wetting phase.
The direction of third force is vertical to the capillary
WU HeYong et al. Sci China Ser D-Earth Sci | May 2007 | vol. 50 | no. 5 | 702-709
705
pore wall and points to non-wetting phase. It is a kind of
pressure to non-wetting phase, and the force is associated with a kinetic friction and deformation of non-wetting phase[16]. The resistance of non-wetting phase
would increase when aeolotropism rise of pore throats or
the properties of oil become thickness (Figure 3). The
third force is zero in Figure 3(a), because the pore size is
invariable. The third force in Figure 3(b) is not zero as a
result of the pore size changing. The variance of diameter is the same as in Figure 3(c) and (d), but the third
force in Figure 3(c) is smaller than in Figure 3(d), because the diameter of Figure 3(c) changes continuously.
3.2 Dynamic conditions of petroleum reservoir formation
The buoyancy (F), the internal fluid pressure difference
in basin (δp) and the capillary force (PC) control the dynamic process of oil driving water. F and δp are the
Figure 3
power of migration, but PC always blocks the movement
of the non-wetting phase. The magnitude between drive
and resistance determines whether the petroleum is detained. The interrelation among them determines
whether gravitational separation of oil and water occurs.
The complex dynamics process can be explained by the
dynamics model of Berkenpas which is an inversion
model of gas and water[17].
It is a process where oil displaces water when oil migrates in a reservoir. Stagnation occurs when the power
(interior fluid pressure difference in basin is primary) is
insufficient to overcome the resistance (the force makes
oil droplets deform). The dynamic condition changes
continuously in the process of oil migration. There is
hardly any mobile water in the reservoir in the axis of a
syncline, and buoyancy does not have an effect. It produces generally pure oil or few emulsified water in the
reservoir (Figure 4(a)). Figure 4(a) corresponds to the
Oil droplets deform in the process of migration in different pore structures.
Figure 4 Theoretical model of the oil reservoir accumulation in syncline by stagnation (modified from Berkenpas[17]). 1, Oil; 2, gas; 3, mobile formation
water; 4, boundary layers (immovable irreducible water).
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WU HeYong et al. Sci China Ser D-Earth Sci | May 2007 | vol. 50 | no. 7 | 702-709
stagnant area in Figure 2. Oil could migrate up-dip when
the fluid pressure difference is larger than the resistance.
The fluid pressure difference could diminish gradually
along the migration path, simultaneously, the porosity
and permeability of a reservoir are improved, and the
resistance is reduced gradually. It is the transition zone
between oil and water when the power and the resistance
balance (Figure 4(b), (c), half stagnant area of Figure 2).
Oil is stagnant under the transition zone, migrates up-dip,
and conventional petroleum reservoirs are above the
transition zone (Figure 4(d), unrestricted flow area in
Figure 2).
Gravitational separation is the result of interaction of
buoyancy and capillary force. The buoyancy which oil
droplets receives is smaller by a long way than the capillary resistance in the axis of a syncline, thus gravitational separation does not exist. The mobile water appears gently and begins to surround oil droplets along
the up-dip direction where the pores become larger. The
capillary resistance which oil droplets receives diminishes gradually, gravitational separation occurs when the
buoyancy is bigger than the capillary resistance.
3.3 The state of fluid flowing through porous medium
Filtration theory of underground fluid considers that the
flow velocity through porous media is proportional to
pressure gradient in Darcy flow. But there are nonlinear
relations in non-Darcy flow with a low speed, in which
there is a starting pressure gradient[18]. Oil droplets migrate only when they overcome resistance in the low to
ultra-low permeability reservoir, in which starting pressure gradient exists, so it is a kind of non-Darcy flow.
The non-Darcy flow with low speed intensifies stagnation in the low to ultra-low permeability reservoir. The
traditional petroleum migration and accumulation theory
is based on Darcy flow theory. Petroleum exploration
should pay more attention to low porosity and permeable reservoirs, so we should understand in depth the
stagnation of fluid flowing at a low speed going through
porous medium. Wu Jingchun et al.[19] did a set of experiments about characteristics of single phase low velocity of non-Darcy flow in a low permeable reservoir in
the eastern Daqing, with the permeability coefficient
being 0―50×10−3 μm2 in the experiments. The experimental results indicate that the reservoirs have the characteristic of low speed non-Darcy flow.
4 Discussions
The sphere of application is different between the
mechanism of petroleum accumulation in syncline and
the traditional petroleum migration and accumulation
theory. The former is suitable to the low to ultra-low
permeable reservoir. The permeability of a reservoir in
syncline areas of the Songliao basin is generally smaller
than 50×10−3 μm2. The latter is suitable to medium to
high permeable reservoir. Therefore, the former is a
supplement to the latter.
4.1 The contrast between the mechanism of petroleum migration and accumulation in syncline and the
traditional mechanism of petroleum migration and
accumulation
The syncline oil reservoir is the unconventional oil reservoir which forms in low to ultra-low permeable reservoir, and the contrast between it and the traditional
mechanism of petroleum migration and accumulation is
summarized in Table 1. Its mechanism is non-Darcy
flow with low to ultra-low speed, and oil accumulates in
reservoir by stagnation driven by the fluid pressure difference. The mechanism of traditional petroleum migration and accumulation is Darcy flow with medium to
high speed and differential accumulation. Their accumulation spots are different, too. Traditional petroleum
accumulation is in the structural high stratigraphic trap
where is corresponding to the area with low potential of
liquid. But the syncline reservoir accumulates in the axis
of syncline. It lies in correspondingly high potential area
of liquid, generally, with overpressure phenomenon existing there. Their flow mechanisms are different, so are
the development ways. The related mechanism needs to
be discussed further.
The main difference between a syncline reservoir and
stratigraphic lithologic reservoir is the difference between conventional and unconventional oil reservoir.
Based on the discussion of reservoir forming mechanisms, conventional stratigraphic lithologic reservoir
belongs to Darcy flow. The traditional mechanism emphasizes the cap rock, source rock, reservoir lens
wrapped by non-permeable rock layers, and up-dip pinchouts. It also emphasizes the seals in the vertical and
lateral directions. But syncline reservoir emphasizes
only the stagnation in low to ultra-low permeable reservoir. Only when the fluid pressure difference surpasses
the pressure which the low speed non-Darcy flow re-
WU HeYong et al. Sci China Ser D-Earth Sci | May 2007 | vol. 50 | no. 5 | 702-709
707
Comparison of the mechanisms between conventional and unconventional oil reservoir
Conventional oil gas
reservoir
Unconventional oil gas reservoir
(Half or non-oil and water gravitational separation)
(Oil and water gravitational
Classification
separation)
Traditional petroleum migraMechanism of petroleum accumulation in
Deep basin gas
tion and accumulation
syncline
High to medium permeability
Applicable scope
Low to ultra-low permeable reservoir
Low to ultra-low permeable reservoir
the reservoir
Abnormal pressure differential of natural
Power of reservoir
Buoyancy, accumulations in
Fluid pressure difference, accumulations in
gas, concentration difference, accumulaaccumulation
relatively low potential area
relatively high potential area
tions in relatively low potential area
Flowing way in which
fluid flows through
Darcy flow
Low speed non-Darcy flow
Concentration diffusion
porous medium
Compact reservoir in the scope of source
Low to ultra-low permeable reservoir in the
Relations of
Source-reservoir-oil reservoir
rock discharge. Reservoir and oil reservoir
scope of source rock discharge hydrocarbon.
source-reservoir
connect indirectly
connect directly
Reservoir and oil reservoir connect directly
Displacement ways to
From the top downwards
From low to high spot
From low to high spot
primitive water
in trap
Regional structure low spot or slope, synReservoir structure spots Regional structure high spot
Regional structure low spot, syncline area
cline area
Hydrodynamic trap, stratigraphic lithologic
Structure, stratigraphic
Low porosity reservoir, oil droplets are stagTrap way
trap, structural trap. Dynamic balance of
lithologic trap
nant or partly stagnant
accumulation and diffusion
Insufficient gravitational separation of oil
Sufficient gravitational sepaand water, edge water and bottom water does
Insufficient gravitational separation of gas
Distribution of oil and
ration of oil and water, edge
not exist, emulsified water exists, sometimes and water, there is no obvious bottom water
water
water and bottom water exist
free water exists
Bottom water drives, elastic
Development way
Fluid pressure difference drives, elastic drive
Natural gas concentration difference drive
drive
Injection-production
Development by natural gas pressure drop
Balanceable water flooding
Over balanceable water flooding
system
(does water flooding)
Table 1
quires, would oil flow through porous medium, that is to
say, it flows from relatively high potential area to relatively low potential area.
4.2 The contrast between syncline oil reservoirs and
deep basin gas
Stagnation is the key factor that results in oil accumulation in a syncline. Since the molecular diameter of natural gas is small, gas migrates primarily by concentration
diffusion in deep basin gas, and its mechanism is a kind
of dynamical equilibrium[20,21]. Two dynamical equilibrium processes exist in deep basin gas. They are between
fluid pressure and capillary pressure as well as supply
and diffusion of natural gas. There is no syncline reservoir discovered in research to deep basin gas in a syncline of a basin center. It should be the reason that oil
had become cracked gas when it is buried to a certain
depth.
5 Conclusions
First, syncline oil reservoir is the unconventional oil
reservoir which forms in a low to ultra-low permeable
reservoir by stagnation.
708
Second, the mode of gas, water and oil droplets flowing through porous medium is different. Gas and water
flow at the molecular level, and they could pass through
the pore smoothly. But the minimum diameter of oil
droplets is generally bigger than the pore throats diameter of a low to ultra-low permeable reservoir; thus, they
must be deformed to pass through the pore throats.
Therefore, water and gas pass through in advance, oil
droplets are delayed or stagnate.
Third, the main force is the internal fluid pressure
difference, the buoyancy and the capillary force in a basin. The former two are the driving force of migration;
the latter is the resistance of migration to oil. Oil would
be detained when driving force is smaller than resistance.
The buoyancy is smaller than the capillary resistance in
a syncline reservoir, so normal gravitational separation
would not exist.
The authors would like to thank academician Jia Chengzao, Professor Li
Sitian and Professor Li Mingcheng et al. for their help in researching, and
academician Han Dakuang, academician Qiu Zhongjian, Professor Zha
Quanheng, Professor Jin Zhijun, Professor Zhao Wenzhi, Professors Pang
Xiongqi and Yang Changchun et al. for their amending the paper.
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