Hydrocarbon charge history of the Paleogene reservoir in the northern Dongpu Depression, Bohai Bay Basin, China
Hydrocarbon charge history of the Paleogene reservoir in the northern Dongpu Depression, Bohai Bay Basin, China
You-Lu Jiang 0 1 2
Lei Fang 0 1 2
Jing-Dong Liu 0 1 2
Hong-Jin Hu 0 1 2
Tian-Wu Xu 0 1 2
0 Edited by Jie Hao
1 SINOPEC Zhongyuan Oil Company , Puyang 457001, Henan , China
2 CNOOC Research Institute , Beijing 100028 , China
The hydrocarbon charge history of the Paleogene in the northern Dongpu Depression was analyzed in detail based on a comprehensive analysis of the generation and expulsion history of the major hydrocarbon source rocks, fluorescence microscopic features and fluid inclusion petrography. There were two main stages of hydrocarbon generation and expulsion of oil from the major hydrocarbon source rocks. The first stage was the main hydrocarbon expulsion stage. The fluorescence microscopic features also indicated two stages of hydrocarbon accumulation. Carbonaceous bitumen, asphaltene bitumen and colloidal bitumen reflected an early hydrocarbon charge, whereas the oil bitumen reflected a second hydrocarbon charge. Hydrocarbon inclusions also indicate two distinct charges according to the diagenetic evolution sequence, inclusion petrography features combined with the homogenization temperature and reservoir burial history analysis. According to these comprehensive analysis results, the hydrocarbon charge history of the Paleogene reservoir in the northern Dongpu Depression was divided into two phases. The first phase was from the late Dongying depositional period of the Oligocene to the early uplift stages of the late Paleogene. The second phase was from the late Minghuazhen period of the Pliocene to the Quaternary. Reservoirs formed during the first period were widely distributed covering the entire area. In contrast, reservoirs formed during the second period were mainly distributed near the hydrocarbon generation sags. Vertically, it was characterized by a single phase in the upper layers and two phases in the lower layers of the Paleogene.
Dongpu Depression; Hydrocarbon charge history; Hydrocarbon generation and expulsion history; Fluid inclusion; Petrography; Fluorescence microscopy
School of Geosciences, China University of Petroleum,
Qingdao 266580, Shandong, China CNOOC Research Institute, Beijing 100028, China SINOPEC Zhongyuan Oil Company, Puyang 457001, Henan, China
Hydrocarbon charge history is an important issue in the study
of pool-forming. Determination of the hydrocarbon charge
history is helpful to correctly understand the oil and gas
reservoir formation and distribution and has important
practical value for guiding petroleum exploration.
Traditional hydrocarbon charge history analytical methods
include the hydrocarbon generation and expulsion history
analytical method, the trap development history method and
the reservoir saturation pressure method. Since 1990s, many
new methods such as fluid inclusion studies, reservoir
bitumen analysis and diagenetic mineral dating have been widely
used. In recent years, the fluid inclusion method has been
widely used and has achieved good results
(Zhao et al. 2013;
Guo et al. 2012; Yang et al. 2014; Xiao et al. 2012, 2016;
Jiang et al. 2015a, b, c; Liu et al. 2007a, b, 2013; Gui et al.
2015; Wu et al. 2013; Wang et al. 2015a, b)
. Fluid inclusions
can provide valuable information on the reservoir pressure
and temperature at the time of the fluid migration and
entrapment as well as on the compositions of the fluids
involved in diagenesis and may thus provide important
insight into the mineral diagenesis and fluid dynamics within
(Doln´ıcˇek et al. 2012; Shan et al. 2015;
Wang et al. 2015a, b; Guo et al. 2014; Lu¨ et al. 2015; Li
. Fluid inclusion entrapment temperature in
conjunction with burial and thermal history plots can be used to
determine the petroleum charge history
(Parnell 2010; Liu
et al. 2011; Pang et al. 2015)
. The key of fluid inclusion
study—one of the most important methods—is the accurate
division of hydrocarbon fluid inclusion formation stages. In
hydrocarbon-rich depressions with multiple sets of
hydrocarbon source rocks, oil and gas that generated in the same
period but different structural positions could have different
temperatures, different maturity and different fluorescent
colors. Therefore, the fluorescent color and homogenization
temperature cannot be used as the absolutely effective basis
for dividing the phases of hydrocarbon inclusions
. Due to the irreversibility of diagenesis, and the
simultaneity of the inclusions and their host minerals
formation, it is more reliable to determine the hydrocarbon
charge history according to the order of formation of the host
(Liu et al. 2007a, b; Tao 2006)
. But reservoirs of the
same period but at different depths may be in different
diagenetic evolution stages. So, fluid inclusions of different host
minerals may be formed in the same period. Therefore, the
fluid inclusion forming periods cannot be simply divided
according to the sequence of host minerals in the diagenetic
evolution stage. Because of the complexity of the formation
and evolution of oil and gas reservoirs, a single method is
often limited in the hydrocarbon charge history analysis, and
it should be combined with a variety of methods, to ensure
the reliability of the results. In this study, we integrated a
variety of research methods to determine the hydrocarbon
charge history of the Paleogene reservoir in the north of the
Dongpu Depression. The hydrocarbon charge stages were
divided qualitatively based on the analysis of generation and
expulsion stages of major hydrocarbon source rocks and
fluorescence microscopic features
(Jarmołowicz-Szulc et al.
; the hydrocarbon fluid inclusion forming periods were
divided according to the diagenetic evolution sequence of
host minerals combined with reservoir burial evolution
history. Then, the hydrocarbon charge time was determined
according to the inclusions’ homogenization temperature in
conjunction with burial and thermal history plots.
2 Geological setting
The Dongpu Depression is located to the southwest of the
Bohai Bay Basin with an area of about 5300 km2. Faults
are developed in the depression
(Chen et al. 2007; Jiang
et al. 2015a)
, and controlled by this, a tectonic framework
of ‘‘two sags, one uplift and one slope’’ was formed
(Fig. 1). The depression underwent the rift stage in the
Paleogene and the depression stage in the Neogene and the
Quaternary. From the Kongdian period to the Es4 period
which was the early rifting stage, the former cratonic basin
disintegrated and formed a half-graben basin prototype.
During the Es3 period, namely the strong rifting stage, the
fault activity was strong, and the basic tectonic framework
was established. The advanced fault depression stage was
from the Es2 period to the Dongying period, during which
the fault activity weakened, and uplift occurred in the late
Dongying period, and there was great erosion
et al. 2007)
. During the Neogene and Quaternary which
was the depression stage, the tectonic activity was weak
and most of the faults stopped being active
(Jiang et al.
. There is a series of hydrocarbon generation
subdepressions in the depression, two of which are in the study
area, the Liutun-Haitongji sub-depression and the
Qianliyuan sub-depression. Source rocks in the depression
include the coal measures of the Carboniferous-Permian
and the mud shale of the third and first member of the
Shahejie Formation. The main hydrocarbon source rocks
are the shale of the middle-lower sections of the third
member of the Shahejie Formation. The reservoir strata are
the sand layers of the Shahejie Formation. The several sets
of mudstones and gypsum in the Shahejie Formation form
high-quality caprocks. They were superposed and formed
many sets of source rock–reservoir–caprock assemblage.
More than 80% of the oil and gas reserves found in the
depression are distributed in the north area (Fig. 1). Thus,
the reservoir formation process of the northern part is also
largely representative of the whole depression.
3 Methodologies and samples
3.1 Hydrocarbon generation and expulsion history modeling
The hydrocarbon generation and expulsion history of the
source rocks in the Dongpu Depression were simulated
using the basin modeling method
(Makeen et al. 2016)
simulation parameters include stratigraphic ages, formation
depth/thickness, erosion thickness, lithology, boundary
conditions and source rock properties. The stratigraphic
ages, formation depth/thickness, boundary conditions (heat
flow, paleowater depth and sediment water interface
temperature) and source rock properties (thickness,
distribution, TOC and HI value) used the third resource evaluation
results of Dongpu Depression. The erosion thickness was
obtained using the vitrinite reflectance and sonic log
(Lu et al. 2007)
. The porosity–depth curves of
sandstones and mudstones are fitted according to the
measured porosity and the porosity calculated from sonic
logs, which have been used in the compaction correction of
the simulation. The simulation results were calibrated and
Bohai Bay Basin
corrected using the measured temperature, pressure and
vitrinite reflectance data.
3.2 Fluorescence microscopy
Fluorescence microscopy has been widely used in the field
of biological sciences (Lin et al. 2015; Muhammad and
Asifullah 2016). Since as early as the nineteenth century, it
has been applied in petroleum geology research and has
played an important role in the theory of petroleum origin
and the search for oil and gas reservoirs
(Lang et al. 2008)
Fluorescence microscopy uses ultraviolet or blue light as a
light source to stimulate the asphalt material in the rocks to
produce visible fluorescence and can be used to observe the
distribution of the petroleum asphalt directly. Different
components in the petroleum such as saturated
hydrocarbons, aromatic hydrocarbons, non-hydrocarbon and
asphaltene will show different fluorescence under the
ultraviolet or blue light excitation. Different luminous pitch
contains different petroleum components and will display
Wei326 Wei42 Pu120
(Lang et al. 2008)
. Oil and gas
reservoirs formed in different periods which have
experienced different evolution processes have different
components. Therefore, the hydrocarbon charging stages can be
analyzed by observing the fluorescent pitch in the reservoir.
3.3 The fluid inclusion petrography analytical method
Fluid inclusion petrography studies the relationship
between hydrocarbon inclusions and the host minerals to
establish the relationship between the hydrocarbon
migration, accumulation and the diagenetic evolution time. The
qualitative research of the fluid inclusion petrography is
mainly based on the occurrence, fluorescence
characteristics and the phase state of hydrocarbon inclusions.
The key of the fluid inclusion study is the accurate
division of hydrocarbon fluid inclusion formation stages.
The determination of the diagenetic evolution sequence is
the premise of the division of the inclusion formation
stages, and the relative position of the host minerals in the
diagenetic evolution sequence is the fundamental basis for
the correct classification of the inclusions.
The homogenization temperature of the fluid inclusions
represents the temperature when the inclusion is formed,
which can determine the time of oil and gas filling
combined with the buried heating history. Oil inclusions are
generally not captured at natural gas saturation, but when
restored to a single-phase state it is in saturation condition,
so the homogenization temperature of the oil inclusion is
generally lower than the trapping temperature, while the
homogenization temperature of the associated water
inclusions is generally close to the capture temperature.
Therefore, when analyzing the oil and gas reservoir
forming period, we usually use the brine inclusions.
In this study, the diagenetic evolution sequence was
determined by thin-section identification; then, inclusion
formation stages were divided according to the host
minerals’ formation sequence combined with the inclusion
petrography analysis. Homogenization temperature tests
were carried out for the observed hydrocarbon inclusions
and their associated water inclusions. The time of
formation of hydrocarbon inclusions of each phase was
determined by the analysis of the homogenization temperature
in conjunction with burial and thermal history, which is
namely the hydrocarbon charging time.
3.4 Instruments and samples
In this study, a Zeiss AXIO Imager D1m digital polarized fluorescence microscope was used in the fluorescence microscopy, cast thin-section observations and inclusion thin-section observations, and a Linkam THMS600 gas
flow heating/freezing system was used in the
homogenization temperature test.
The core sampling was carried out in the oil and gas
intervals of the Shahejie Group of 49 wells (Fig. 1). A total
of 136 inclusion thin sections were made for the
fluorescence microscopic observation and the fluid inclusion
studies. A total of 23 cast thin sections were made for the
4 Results and discussion
4.1 Hydrocarbon generation and expulsion histories
The results of simulation of the evolution of major source
rocks in study area showed that in the early sedimentary
period of the second section of the Shahejie group (Es2), all
source rocks were at an immature stage and no hydrocarbon
was expelled. From the Es1 period to the early Dongying
period, source rocks were in the low mature stage and there
was some hydrocarbon generated but only a little expelled. In
the middle and late period of Dongying group (about
31–27 Ma), most of the source rocks were at the mature to
highly mature stage, at the peak hydrocarbon generating and
expulsion period. Some hydrocarbon was expelled at the
early uplift stage (27–23 Ma). Then, the evolution of source
rocks stagnated until the early Minghuazhen period in the
Neogene. From the late Minghuazhen period until now
(about 5–0 Ma), the burial depth and maturity of source
rocks reached and exceeded that before the uplift, and the
evolution of source rocks continued. This was the second
stage of hydrocarbon generation and expulsion, in which the
source rocks were at the high maturity stage in the upper
layers and the over mature stage in the lower layers.
However, the source rocks which entered into the second stage
were relatively limited, mainly distributed in the deep
hydrocarbon generation sags.
In conclusion, there were two hydrocarbon generation and
expulsion stages in the study area. The first stage was from
the middle and the late Dongying deposition period to the
early uplift stage in the late Paleogene (about 31–23 Ma),
and the second was from the late Minghuazhen period of
Neogene to Quaternary (about 7–0 Ma). According to the hydrocarbon expulsion quantity in each stage, the first stage was the main hydrocarbon expulsion stage (Fig. 2).
4.2 Fluorescence microscopy characteristics
4.2.1 Fluorescence characteristics of different types
A lot of carbonaceous asphalts (without fluorescent), bituminous asphalts, colloidal asphalts and oleaginous
asphalts were seen in the samples of the reservoirs in the
study area (Fig. 3; Table 1).
Carbonaceous asphalts are the production of petroleum
(Shokrlu and Babadagli 2013)
. These are black
under both the fluorescence and transmitted light. The
occurrences can be divided into three types: (1) distributed
in the secondary pore spaces between the quartz particles,
blocky with straight clear edges (Fig. 3a, b). (2) Distributed
around the particles, ring banded or irregular shape,
equivalent to the cements (Fig. 3c, d). (3) Distributed in
fractures, banding (Fig. 3e, f).
The bituminous asphalts were formed due to the
condensation of the resin and other heavy components of
petroleum and were the residues of previous reservoirs
(Qin and Guo 2002; Shalaby et al. 2012)
. Their main
ingredients are resin, non-hydrocarbon and asphaltene,
which are not soluble in petroleum ether. Bituminous
asphalts are common in the samples of study area, and their
b Fig. 3 Photographs of asphalt from the northern Dongpu Depression
under fluorescence and transmitted light. a–f Carbonaceous asphalt,
g–j bituminous asphalt, k–l colloidal asphalt; m–r oleaginous asphalt;
a, b Qiao21, 2525.4 m; c, d Wei145, 2785.32 m; e, f Wei47,
2863.78 m; g, h Hu40, 2660.3 m; i, j Qiao21, 2525.4 m; k, l Qiao59,
4556.8 m; m, n Liu6, 3882.8 m; o, p Pu120, 3264 m; q, r Pu80,
main features and occurrence can be divided into two
categories: (1) distributed in the intergranular pores, with
weak intensity yellow fluorescence, layered, and
disseminated along both sides of the pores, gray or gray brown
under transmitted light (Fig. 3g, h). (2) Clustered or blocky
distributed in the intergranular pores, yellow fluorescence,
dark brown under transmitted light, disseminated to the
surrounding cements (Fig. 3i, j).
The colloidal asphalts were distributed on the surface of the quartz grains like films, yellow brown fluorescent, disseminated to the interior of the particles in microfractures (Fig. 3k, l).
The oleaginous asphalts were mainly composed of light
petroleum components, which were widely distributed in
the reservoirs. Their occurrence can be divided into three
types: (1) distributed along the calcite surface and in the
cleavage cracks, yellow to yellow green fluorescence and
colorless under transmitted light (Fig. 3m, n). (2)
Distributed along the surface of mineral particles like thin
films, yellow to yellow green fluorescence, disseminated to
the interior of mineral particles (Fig. 3o, p). (3) Adsorbed
in the intergranular spaces, mostly distributed in the parts
where argillaceous matrix or clay mineral content is high,
filling and disseminated to the matrix and matrix
contraction joints, yellow green fluorescence (Fig. 3q, r).
4.2.2 Cause analysis and phase partition of asphalts
There are many reasons for the formation of asphalts in
reservoirs. They are generally considered to be from liquid
petroleum. There are 3 common reasons, which are thermal
evolution, cold metamorphism and gas-washing
(Luo et al.
. In addition to the nature of the oil, the
environmental factors such as reservoir temperature, volume and
pressure are very important to promote the occurrence of
asphaltene deposition, and among which, pressure is the
most important factor. A decrease in pressure can decrease
the solubility of asphaltene in crude oil and thus cause
deposition of asphalt
(Qin and Guo 2002)
When the temperature of crude oil is over 150 C, it will
become unstable, macromolecular hydrocarbons and other
heterocyclic compounds will be gradually transformed into
low molecular compounds (condensate and gas
hydrocarbon) and asphalts
(Guo et al. 2008)
. The representative
geothermal gradient in the Dongpu Depression is 3.4 C/
(Liu et al. 2007a, b)
. The corresponding depth to
reach the temperature of crude oil cracking is about
4400 m. But most of the carbonaceous asphalts observed in
the study area did not reach this depth and were not of
pyrolytic origin. However, asphalts in the deep reservoir
could be formed by oil cracking.
The Dongpu Depression experienced a substantial rise in
the Paleogene period (27–17 Ma), the formation suffered an
intense erosion, and denudation thickness was up to more
than 2000 m, with an average of about 1000 m
(Lu et al.
. Strong tectonic uplift decreased the oil’s burial depth
and temperature and pressure became lower. Decrease in
pressure caused the decrease in the solubility of asphaltene
in crude oil and caused an increase in resins and bitumen
and decrease in light components in the petroleum, which
formed the carbonaceous asphalts, bituminous asphalts and
colloidal asphalts (Fig. 2). Because of the stably distributed
salt rocks in the first section of the Shahejie group and as the
oil-bearing strata are mainly located under the salt rocks,
the sealing and preservation condition is good, so the
asphalt should not be formed by oxidation.
The above analysis indicates that carbonaceous asphalts,
bituminous asphalts and colloidal asphalts were formed
due to the strong uplift and erosion, which reflected the
early oil and gas injection phase, while the oil and gas
injected in the late period were not subjected to alteration
and showed mostly oleaginous asphalts.
4.3 Diagenetic evolution sequence
From the detailed microscopic identification of thin sec
tions, the diagenesis of reservoir sandstones of the Shahejie
Formation in the study area mainly includes compaction,
cementation, metasomatism and dissolution (Fig. 4).
According to the ‘‘Standard for the division of diage
netic stages of clastic rocks’’
(Ying et al. 2004)
paleotemperature, vitrinite reflectance (Ro) and authigenic
clay mineral assemblages, the diagenetic stages of
sandstones of the Shahejie Formation in the northern Dongpu
Depression were divided into the early diagenetic stage B,
middle diagenetic phase A and middle diagenetic phase B.
The schematic diagram of diagenetic stages division of the
reservoirs of the Shahejie Formation in the northern
Dongpu Depression was compiled (Fig. 5). The charac
teristics of each diagenetic stages are as follows.
The early diagenetic stage B: depth from 2000 to 2500 m,
temperature \95 C, Ro \ 0.5%, equivalent to the
semimature period of the organic matter evolution, and the
smectite layer of I/S mix-layer was in the second quick
transformation zone (from 50% to 20%). In this stage, the diagenesis
was dominated by compaction and early carbonate
cementation. The cementation included calcite cementation,
secondary outgrowth of quartz and the precipitation of dolomite.
The middle diagenetic phase A1: depth from 2500 to
3300 m, temperature from 95 to 135 C, Ro from 0.5% to
0.7%, in the low mature stage of organic matter evolution.
In this stage, the main diagenesis included dolomite
cementation, ferroan dolomite cementation and clastic
particles replacement by calcite, calcite replacement by
ferroan calcite and the dissolution of quartz and feldspar
particles in the later stage.
The middle diagenetic phase A2: depth from 3300 to
4200 m, temperature from 135 to 160 C, vitrinite
reflectance Ro 0.7% to 1.3%, in the mature stage of organic
matter. This stage was characterized by a large amount of
dolomite and ferroan dolomite cementation, and the
dissolution of quartz and feldspar.
The middle diagenetic stage B: burial depth of 4200 to
5000 m, temperature of [160 C, vitrinite reflectance
Ro [ 1.3%, the main features of this stage are the
secondary outgrowth of quartz and the metasomatism of
Microfractures are important places to capture fluid
inclusions. Understanding the cause of the microfractures
and their formation sequence in the diagenetic stage is the
basis for determining the phase of hydrocarbon inclusions
in the microfractures. Microfractures containing
hydrocarbon inclusions in the study area include the internal
cracks of quartz particles (Fig. 6e, f), and the cracks
through quartz grains (Fig. 6i–l). The former did not cut
through the quartz grains and the latter cut through the
whole quartz grains. Research suggests that the internal
cracks of quartz particles were formed due to the pressure
of the overlying strata exceeding the critical fracture
pressure of the debris particles
(Li et al. 2014)
fractures had been gradually healed due to late diagenesis.
In the samples, the internal cracks of quartz appeared at
depths below 2500 m, which is equivalent to the middle
diagenetic phase A1 and the later diagenetic stages. The
cracks through quartz grains appeared in the whole section,
not controlled by the depth. This indicates that the cracks
through quartz grains should not be the result of diagenesis
and presumably be formed due to the stress field changes
caused by the tectonic uplift during late Paleogene, which
captured the fluid inclusions during the subsidence in the
4.4 Characteristics of fluid inclusion petrography
4.4.1 Fluid inclusion petrography
Generally speaking, petroleum inclusions formed in the
low mature phase of hydrocarbon source rock evolution
had a high density, with a high content of heavy
hydrocarbon and asphaltene. And their fluorescence is mainly
brown. As the maturity increases, or the migration and
cracks of quartz grains were formed in the middle
diagenetic phase A1, when buried deep below 2500 m. The
feldspar dissolution pores and fractures were the result of
feldspar dissolution, which happened in the late period of
middle diagenetic phase A1, when buried below 3000 m.
The cracks through quartz grains were formed after the
uplift in the late Paleogene.
The hydrocarbon inclusions in different minerals or
fractures distributed in different depths: The inclusion in
the carbonate cements distributed at depths of
2000–2700 m, among which the early carbonate cements
distributed at 2000–2500 m and the late carbonate cements
in 2500–2700 m. The inclusions in the internal cracks of
quartz grains distributed at depths of 2500–4000 m, and
inclusions in the feldspar dissolution pores and fractures of
the quartz grains are distributed at 3100–4000 m.
Inclusions in the cracks through quartz grains are distributed in
Samples of different depths have different burial histo
ries. The geological periods corresponding to the
diagenetic stages of the host minerals are estimated in the
reservoir burial history, from the time of formation of
different host minerals of hydrocarbon inclusions (Fig. 7).
The results show that the inclusions in the carbonate
differentiation increased, crude oil density became low,
and fluorescence colors of the petroleum inclusions
captured in this stage are mainly white, yellow or yellow
green. Petroleum inclusions formed in the highly mature
phase have fluorescence colors mainly milky blue or blue
(Liu et al. 2007a, b)
. Fluorescent colors of hydrocarbon
inclusions observed in the study area include yellow,
yellowish white, yellow green and blue white, which indicates
that the oil and gas in the study area were mainly charged
at mature and highly mature stages (Fig. 6).
Hydrocarbon inclusions in the study area are mainly
gas–liquid two-phase inclusions, mainly distributed in the
carbonate cements (including calcite dolomite and ferroan
dolomite), feldspar dissolution pores, internal cracks in
quartz grains and late cracks through quartz grains (Fig. 6).
4.4.2 Phase division of fluid inclusions
According to the analysis of diagenetic evolution sequence,
the calcite and dolomite were early carbonate cements,
formed in the early diagenetic stage B, when the reservoir
depth was up to 2000 m. The ferroan dolomite was late
carbonate cement formed in the middle diagenetic A phase,
when the reservoir was buried below 2500 m. The internal
Depth Microscopic sketches Main diagenesis
quartz & feldspar
resulting in late
Host minerals distribution
ttrrcnbaaoaLee ilttrrzenanauQ rcksca
irsdap rsope r
s n cks trz
ledF lituo raC uaq
Reservoir burial history
Es4 Es3 Es2-1 Ed Uplift N-Q
Period 1 Period 2
Sample depth 2000-2500m
Sample depth 2500-2750m
Sample depth >2500m
Sample depth >3100m
Formation temperature, °C
Eastern sag belt
Western sag belt
Wenxi fault zone
Wendong graben zone
Wenzhong Horst zone
Wendong rolling anticlinal zone
Wendong reverse roof ridge zone
Pucheng reverse roof ridge zone
Western slope belt
Weicheng Horst zone
cements, internal cracks of quartz grains and feldspar
dissolution pores were formed in the late Dongying deposition
period, and they were formed in the same period. The
hydrocarbon inclusions occurred in the cracks through
quartz grains were formed in the reburial period after the
tectonic uplift, the second phase of hydrocarbon inclusions.
Therefore, the hydrocarbon inclusions can be divided into
two phases, which reflects the two hydrocarbon
accumulation processes, the main characteristics of each stage of
the hydrocarbon inclusions are as follows.
The first phase of hydrocarbon inclusions is distributed in
carbonate cements, internal cracks of quartz grains and
feldspar dissolution pores which are gas–liquid two-phase
hydrocarbon inclusions with sliver shape or irregular shape,
isolated or sporadic distribution, variable size, yellow,
yellowish white or yellowish green fluorescence, and brown,
light brown or colorless under transmitted light. They
correspond with the early oil and gas filling (Fig. 6a–h).
The second phase of hydrocarbon inclusions occurs in
cracks through quartz grains in a beaded distribution, with
ellipse shapes, usually small, yellow green to bluish green
in fluorescence, colorless under transmitted light. They
correspond to the late highly mature stage of oil and gas
filling (Fig. 6i–l).
4.5 Inclusion homogenization temperature characteristics and determination of hydrocarbon accumulation timing
The time of formation of hydrocarbon inclusions of
different phases was determined by the combined analysis of
homogenization temperature and reservoir burial heating
history (Fig. 8). The homogenization temperature of fluid
inclusions was tested, and the hydrocarbon charge period
was determined in different tectonic units and different
layers in this study (Table 2).
Based on the comprehensive analysis of hydrocarbon
generation and expulsion histories of major source rocks,
fluorescence microscopic characteristics and reservoir fluid
inclusions, the pool-forming period was divided into two
phases: The first phase is from late Dongying deposition
period to the early uplift stage in late Oligocene and the
second is from late Minghuazhen deposition period in
Pliocene to Quaternary. The first period is the main accu
By comparing the pool-forming time in different
tectonic belts and different oil and gas bearing series, it can be
seen that the first stage of hydrocarbon accumulation
occurs widely, distributed in the whole region, and the
second stage of the hydrocarbon reservoirs was mainly
distributed near the sub-depressions, including the Puwei
sag belt, Huqing area, Qiaokou and Baimiao area,
Wendong area, the south of Wenxi area, the west of Liuzhuang
area, Qianliyuan sub-sag and Haitongji sub-depression.
The central uplift belt and the western slope zone of
northern Xinzhuang, Mazhai, and the distant-free
hydrocarbon accumulation area only have the first stage of
poolforming. In the vertical direction, the first stage is widely
distributed in a large depth span, mainly distributed in the
depth range of 2000–3500 m. From the view of layers,
there was a single stage in the upper layers and two stages
in lower layers. Specifically, the second section of the
Shahejie group and the upper and middle layers of the third section of the Shahejie group were mainly in the first stage, while the lower part of the third section of the Shahejie group was in two stages (Fig. 9).
The difference of the hydrocarbon charge history in
different parts and different layers is closely related to fault
activity. The fault system is highly developed in the
Dongpu Depression and is the main channel for oil and gas
migration. The activity rate of the main secondary oil
source faults was calculated using the fault activity rate
method (Table 3). The main oil source faults in the study
area became active in the early stage of Es3. The activity
rate was the highest in the late Es3 and Es2 and decreased
gradually in the Es1 and Dongying period. The rate of fault
activity could not be calculated during the uplift period
because there is no sediment, but the tectonic movement in
this period is very intense, and it is inferred that most faults
are active. Most of the faults have not cut through the
Guantao Formation, so that most of the fault activity stopped in the Early Neogene.
In the first stage of oil and gas accumulation, the source of oil and gas was abundant, and the fault activity was
Samples with the 1st type
of hydrocarbon inclusions
2 types of
strong, so the oil and gas migrated to different layers and
depths to accumulate. During the period of Dongying
movement, an uplift occurred throughout the depression,
the fault activity increased, and had great influence on the
distribution and reorganization of oil and gas reservoirs,
which resulted in a wide range of oil and gas distribution in
the plane and vertical direction. In the second phase of
hydrocarbon charging period, most of the faults were not
active, and due to the plugging of salt rocks, oil and gas
could not migrate along faults at a large-scale, so most
accumulated near the sources. This results in the oil and
gas formed in the second phase being mainly distributed in
the vicinity of the sags in the horizontal plane and mainly
concentrated in the main hydrocarbon generation layers
1. The main hydrocarbon source rocks of the Dongpu
Depression mainly had two expulsion periods. The
fluorescence microscopic features also indicated two
stages of hydrocarbon accumulation. The
carbonaceous asphalts, bituminous asphalts and colloidal
asphalts reflect an early oil and gas injection phase,
whereas the oleaginous asphalt reflects a second oil
and gas injection phase.
2. According to the diagenetic evolution sequence, fluid
inclusion petrography and the reservoir burial history
analysis, hydrocarbon inclusions were divided into two
formation phases. Phase I occurs in carbonate cements,
internal cracks of quartz grains and dissolution pores of
feldspars, corresponding to the early hydrocarbon
filling. Phase II occurs in late cracks through quartz
grains, corresponding to the late oil and gas filling.
3. Based on the comprehensive analysis of the hydrocarbon generation and expulsion histories of the major source rocks, fluorescence microscopic characteristics
and reservoir fluid inclusions, the pool-forming in
northern Dongpu Depression is found to be in two
phases. The first phase was from the late Dongying
depositional period to the early uplift period in the late
Oligocene, while the second was from the late
Minghuazhen period of the Pliocene to the Quaternary,
with the first phase being the main hydrocarbon charge
phase. The first phase of hydrocarbon reservoir
charging was distributed across the whole region, whereas
the second phase of the hydrocarbon reservoir
accumulation was mainly distributed near the sub-sags and
mainly in the hydrocarbon generation layers. In the
vertical direction, it was characterized by a single
phase in the upper layers and two phases in the lower
layers within the Shahejie group.
Acknowledgements This work was supported by the Important
National Science & Technology Specific Projects (Grant No. 2011ZX05006-003/004).
Open Access This article is distributed under the terms of the
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