Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments, central Bohai Sea, China
Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic-carbonate sediments, central Bohai Sea, China
Zheng-Xiang Lu¨ 0 1
Shun-Li Zhang 0 1
Chao Yin 0 1
Hai-Long Meng 0 1
Xiu-Zhang Song 0 1
Jian Zhang 0 1
0 State Key Laboratory of Oil-Gas Reservoirs Geology and Exploitation, Chengdu University of Technology , Chengdu 610059, Sichuan , China
1 College of Energy Resources, Chengdu University of Technology , Chengdu 610059, Sichuan , China
The characteristics and formation mechanisms of the mixed siliciclastic-carbonate reservoirs of the Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and scanning electron microscopy observations, X-ray diffractometry, carbon and oxygen stable isotope geochemistry, and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type I and Type II mixed siliciclastic-carbonate sediments, and the dominant pore types include residual primary intergranular pores and intrafossil pores, feldspar dissolution pores mainly developed in Type II sediments. Type I mixed sediments are characterized by precipitation of early pore-lining dolomite, relatively weak mechanical compaction during deep burial, and the occurrence of abundant oil inclusions in high-quality reservoirs. Microfacies played a critical role in the formation of the mixed reservoirs, and high-quality reservoirs are commonly found in high-energy environments, such as fan delta underwater distributary channels, mouth bars, and submarine uplift beach bars. Abundant intrafossil pores were formed by bioclastic decay, and secondary pores due to feldspar dissolution further enhance reservoir porosity. Mechanical compaction was inhibited by the precipitation of pore-lining dolomite formed during early stage, and oil emplacement has further led to the preservation of good reservoir quality.
High-quality reservoirs; Paleogene Bohai Sea; Mixed sediments
In addition to carbonate and clastic reservoir rock types,
magmatic, metamorphic, shale and mixed siliciclastic–
carbonate sedimentary reservoirs can also be considered as
important targets for oil and gas exploration and
development (Ge et al. 2011; Tong et al. 2012; Xiao et al. 2015;
Palermo et al. 2008). The concept of ‘‘mixed sediments’’
was firstly proposed by Mount (1984) and is commonly
referred to as sediments that are composed of mixtures of
siliciclastic and carbonate material (including allochemical
particles) (Lubeseder et al. 2009; Brandano et al. 2010; Xu
et al. 2014). Many Chinese and foreign scholars have made
in-depth studies of the formation mechanisms of this type
of sediment and suggested that it can be developed in both
marine and lacustrine environments. Influenced by sea
(lake)-level fluctuations, structural changes, storm, current
and tidal actions, mixed siliciclastic–carbonate sediments
are widely distributed in transitional marine-terrestrial,
continental shelf, and slope environments (Garc´ıa-Hidalgo
et al. 2007; Zonneveld et al. 2012). Under certain
conditions, mixed siliciclastic–carbonate sediments may be rich
in oil and gas. For example, hydrocarbon accumulations
have been discovered in the high-quality mixed
siliciclastic–carbonate reservoirs in China, such as the Bohai Bay
Basin, the Qaidam Basin and the Sichuan Basin (Feng et al.
2011a, b, 2013; Zhang et al. 2006; Liu et al. 2011;
Garc´ıaHidalgo et al. 2007). Although carbonate and clastic
reservoirs have been the subject of intensive study by a
large number of researchers, mixed siliciclastic–carbonate
reservoirs have received less attention. Previously, studies
of mixed siliciclastic–carbonate reservoirs have mainly
focused on petrography, structure, classification, the
establishment of depositional models (Caracciolo et al.
2012; Sha 2001; Zand-Moghadam et al. 2013; Zonneveld
et al. 2012; Ma and Liu 2003), and the reconstruction of the
sedimentary environment on the basis of sequence
stratigraphy, sea level change and paleoclimate (Anan
2014; Campbell 2005; Moissette et al. 2010). However, the
microscopic features and the formation mechanisms of
high-quality reservoirs have not been well investigated,
which has restricted the exploration and development of
mixed siliciclastic–carbonate reservoirs.
The Bohai Bay Basin is an important petroliferous basin
in North China. In the Paleogene, steep slope zones were
well developed and are represented by a series of high
steep fault noses (Lu 2005; Guan et al. 2012). The
tectonically induced physiographic changes controlled the
distribution and areal extension of mixed
siliciclastic–carbonate sediments. For example, typical mixed sediments
composed of lacustrine carbonate and siliciclastic material
are widely distributed in the Shijiutuo Uplift in the central
basin, and a large number of high-quality reservoirs are
developed in them (Liu et al. 2011; Song et al. 2013).
Statistics suggest that reservoir quality is one of the key
controls on prospectivity during petroleum exploration and
production. The study of the characteristics and formation
mechanisms of mixed siliciclastic–carbonate reservoirs is
therefore of significant importance for guiding the oil and
gas exploration and production in the Bohai Sea. The
purpose of this paper is to compare different types of mixed
siliciclastic–carbonate reservoirs, to describe the main
features of high-quality reservoirs, and to determine the
formation mechanisms of high-quality reservoirs by
integrating geological and geochemical data.
2 Geological setting
The Bohai Bay Basin is a Cenozoic rift basin superimposed
on the Paleozoic basement of the North China platform (Lu
2005). The study area is located in the Shijiutuo Uplift in
the central Bohai Bay Basin and bounded by two large
hydrocarbon generation sags—Bozhong Sag and Qin’nan
Sag (Fig. 1). The hydrocarbon accumulation condition is
excellent with high-quality source rocks and a series of
Paleogene high steep fault nose traps developed (Guan
et al. 2012). The mixed sedimentary reservoirs in the study
area are mainly developed in the Paleogene Shahejie
Formation (E2s). The Shahejie Formation is 300–400 m thick
with burial depths [3000 m and conformably overlies the
Kongdian Formation (E2k) and underlies the Dongying
Formation (E2d). Because economically significant
hydrocarbon accumulations have been found in the mixed
reservoirs, the mixed siliciclastic–carbonate reservoirs
have been the focus of study in recent years (Wang et al.
3 Samples and experimental methodology
In this study, 240 mixed siliciclastic–carbonate sediment
samples from 12 wells in the Shijiutuo Uplift in the central
Bohai Bay Basin, such as Well HD2 and Well HD5, were
selected for porosity and permeability measurements. The
locations of sample wells are shown in Fig. 1. The
microscopic features, such as petrology, pore space types,
and diagenesis, were obtained from 122 thin sections with
different physical properties. Multi-purpose thin sections
were prepared with blue-dyed epoxy impregnation and
double-sided polishing. The mineral composition was
identified by polarized light microscopy, and X-ray
diffraction (XRD) analyses were carried out on twenty-two
bulk samples and \2 lm size fractions using a Rigaku
DMAX-3C diffractometer. The chemical composition of
grain-coating and dissolved minerals was determined
quantitatively by electron microprobe analysis (EMPA)
using a Shimadzu EPMA-1720 and a JEOL JXA-8100
electron microprobes (operating conditions: 15 kV
accelerating voltage, 10 mA current, 1 lm beam diameter).
Fifteen double-thickness polished thin sections were
selected for microthermometric measurements.
Homogenization temperatures were measured using a Linkam
THMS-600 heating/cooling stage. Only primary fluid
inclusions with both aqueous and hydrocarbon phases were
selected from authigenic minerals to determine their
minimum precipitation temperatures (Liu et al. 2005; Lu¨ et al.
2015; Guo et al. 2012; Tian et al. 2016). In-situ carbon and
oxygen isotope analysis was performed using an Nd:YAG
laser microprobe. Laser probe microsampling of C and O
from carbonate cements for isotopic analysis was achieved
by focusing a laser beam with a wavelength of 1064 nm
and a diameter of 20 lm onto a sample situated in a
vacuum chamber to ablate a small area on the sample and
liberate CO2 gas. After purification, the CO2 gas was led
directly into a Finnigan MAT 252 mass spectrometer for
isotopic analysis. After obtaining the isotopic values, the
dolomite formation temperature (T) was calculated using
the empirical formula proposed by Hu et al. (2012):
dWÞ þ 0:14ðdC
where dC is the d18O of a dolomite precipitate, and dW is
the d18O of parent water.
Fig. 1 Location map and tectonic elements of the central Bohai Sea
The timing of feldspar dissolution was mainly
determined based on the fluid inclusion temperatures of the
dissolution products (authigenic quartz). Sixty-two samples
were observed under a DM4500P fluorescence microscope
in order to identify possible petroleum inclusions. Fifteen
inclusions were also examined using a Renishaw inVia
laser Raman microprobe with a wavelength of 514.5 nm to
document the existence of hydrocarbons.
The mixed siliciclastic–carbonate sediments were
deposited in a fan delta environment (Guan et al. 2012;
Zhang et al. 2015; Ni et al. 2013). In order to illustrate the
relationship between petrophysical properties and
sedimentary microfacies, the sedimentary facies were identified
by analyzing rock textures and well log data for Well HD2
and Well HD5, in which core porosity and permeability
4.1 Rock types
The E2s mixed sediments are composed of siliciclastic and
lacustrine carbonate rocks. For the siliciclastic grains,
carbonate grains, matrix, and micrite that constituted the
Fig. 2 Rock types of E2s mixed siliciclastic–carbonate sediments. 1:
sand (gravel) rock, 2: carbonate siliciclastic mixed sedimentary rocks,
3: carbonate-bearing siliciclastic mixed sedimentary rocks, 4:
siliciclastic carbonate mixed sedimentary rocks, 5: carbonate/siliciclastic
mixed sedimentary rocks, 6: carbonate-bearing argillaceous
siliciclastic mixed sedimentary rocks, 7: siliciclastic-bearing carbonate
mixed sedimentary rocks, 8: siliciclastic-bearing micrite carbonate
mixed sedimentary rocks, 9: carbonate, 10: mudstone (micritic
mixed sediments, the content of the former two was,
respectively, not less than 10%, while the latter two
accounted for less than 50%. The identification results of
122 thin sections show that (Fig. 2) E2s mixed siliciclastic–
carbonate sediments were divided into three classes. Class I
was mainly composed of siliciclastic carbonate mixed
sedimentary rocks and siliciclastic-bearing carbonate
mixed sedimentary rocks. It represented up to 55% with
carbonate particles content of more than 50% (4, 7 area in
Fig. 2). Carbonate grains were mainly bioclasts,
accounting for 65% (103 sampling points), followed by oolites and
arenes; Class II was mainly composed of carbonate
siliciclastic mixed sedimentary rocks and carbonate-bearing
siliciclastic mixed sedimentary rocks, accounting for 30%,
with siliciclastic particles content of more than 50% (2, 3
area in Fig. 2); Class III was uniformly with less than 50%
of siliciclastic grains and of carbonate grains (5, 6 and 8
area in Fig. 2). It was in the lowest content, only
accounting for 16%. The interstitial material was mainly
dolomite, followed by calcite and small amounts of
argillaceous matrix, which was well-sorted and
sub-rounded to rounded.
4.2 Diagenetic features
From the contact relationship of grains in E2s mixed
siliciclastic–carbonate sediments, it showed that the
compaction was not strong, mainly composed of point-line
contact (Fig. 3a, b).
4.2.2 Precipitation of authigenic minerals
There were numerous types of authigenic minerals formed
in E2s mixed siliciclastic–carbonate sediments. As with the
different proportions of siliciclastics and carbonate, it led
to the differences of authigenic mineral content in the
mixed siliciclastic–carbonate sediments. In the mixed
siliciclastic–carbonate sediments with a high proportion of
carbonate, authigenic dolomite, calcite and other carbonate
minerals were in high proportions and authigenic clay was
in small proportions. However, in the mixed siliciclastic–
carbonate sediments with a high proportion of siliciclastic
rocks, the authigenic minerals were dominated by
kaolinite, illite, and quartz, and the authigenic carbonate minerals
were in minor amounts.
Among authigenic carbonate minerals, dolomite made
up the largest share, followed by calcite; in addition, there
were minor amounts of ankerite and ferroan calcite. The
occurrence states of dolomites were a pore liner (Fig. 3a),
pore fillings (Fig. 3a, c) and replacement particles (Fig. 3d,
e). Calcite mainly occurred as local replacement particles.
Authigenic clay minerals included kaolinite (Fig. 3f) and a
small amount of illite (Fig. 3f). Authigenic quartz was
distributed in the pores in the form of small crystals
(Fig. 3f), and pyrite can be occasionally seen.
Dissolution was well developed in the E2s mixed
siliciclastic–carbonate sediments, and it effectively improved
the quality of reservoirs with high proportion of siliciclastic
rocks. The dissolved minerals were mainly feldspar,
especially albite and K-feldspar (Fig. 3d, e). A small
amount of carbonate minerals, such as dolomite and
ankerite, were dissolved but this made little contribution to
4.3 Reservoir space features
The reservoir space of E2s mixed siliciclastic–carbonate
sediments was dominated by residual primary intergranular
pores and dissolved pores, with minor intercrystalline
porosity. Primary pores mainly included residual primary
intergranular pores and intrafossil pores (Fig. 3a).
Dissolved pores mainly included intergranular dissolved pores
in feldspars and rock fragments (Fig. 3d) and
intercrystalline pores mainly included intercrystalline pores in
kaolinite (Fig. 3f).
4.4 Petrophysical features
The porosity of E2s mixed siliciclastic–carbonate
sediments ranged between 0.45% and 36%. In the 240 samples,
76% samples had a porosity of over 15% (Fig. 4).
Permeability mainly ranged between 0.014 and 11259 mD.
Most samples had a permeability of over 10 mD,
accounting for 53% of the total samples (Fig. 5).
4.5 Features of sedimentary microfacies
The E2s in the study area was deposited in a continental
offshore lacustrine and near-source fan delta depositional
environment (Guan et al. 2012; Zhang et al. 2015; Ni et al.
2013). The mixed siliciclastic–carbonate sediments were
mainly developed in delta front sandbars and shallow
lacustrine underwater uplift beach bars, followed by delta
front underwater distributary channels. Front sandbars
were divided into mouth bar and distal bar microfacies with
reverse grain size grading and funnel-shaped gamma-ray
(GR) curves, but the former showed lower GR curves.
Underwater uplift beach bars were characterized by fine
grain size, good sorting, low content of matrix and micrite
and a box-shaped GR curve. Underwater distributary
channels abruptly contacted with underlying strata, with
coarse grain size at the bottom and minor gravel (Fig. 6).
Dol-Pore filling dolomite
P1-Primary intergranular pore P2-Intrafossil pores
Dol1-Pore-lining dolomite Dol2-Pore filling dolomite
F1-Yellow fluorescence F2-Green fluorescence
Dol-Pore filling dolomite
f-Dissolution feldspar residual
Fig. 3 Photomicrographs of a residual intergranular primary pore
and intrafossil pores, pore-lining dolomite and filling dolomite,
pointline contact, Well HD2, 3762.6 m, polarized light. b Two phases of
hydrocarbon charging in intergranular dissolved pores and residual
intergranular primary pore, Well HD2, 3774.33 m, fluorescence
microscope. c Pore filling dolomite, pore is poorly developed, Well
HD2, 3774.33 m, polarized light. d Multiphased authigenic dolomite,
dolomite replaces feldspar, feldspar (EPMA: Na2O: 0.2%, K2O:
16.5%, Al2O3: 18.3%, SiO2: 64.6%) dissolution, Well HD5,
3382.1 m, polarized light. e multiphased authigenic dolomite,
dolomite replaces feldspar, feldspar dissolution, Well HD5,
3382.1 m, Cathodoluminescence. f Kaolinite, authigenic quartz, illite,
Well HD5, 3486.5 m, Scanning electron microscope
According to the sedimentary microfacies and the statistics
of components in the 156 samples, the content of
siliciclastic particles decreased from 83% to 18% from delta
front mouth bar—distal bar—shallow lake underwater
uplift beach bar facies.
4.6 Features of high-quality reservoirs
The reservoirs with porosity of [15% and permeability of
[10 mD were generally referred to as high-quality
reservoirs in this paper.
Dol-Pore filling dolomite
f-Dissolution feldspar residual
Fig. 4 Porosity distribution histogram of E2s mixed siliciclastic–
Fig. 5 Permeability distribution histogram of E2s mixed siliciclastic–
The sedimentary microfacies and the corresponding 106
groups of petrophysical data of the coring interval in Well
HD2 and HD5, as well as the petrophysical data of the 50
sidewall cores of the other wells showed that the mixed
siliciclastic–carbonate sediments developed in mouth bar,
distributary river channel, and underwater beach bar
microfacies had good physical properties, but high-quality
reservoirs were basically not developed in the other
microfacies (Fig. 7). Seventy-six percentage of the
highquality reservoirs were developed in Class I mixed
siliciclastic–carbonate sediments, and their porosity and
bioclastic content had good positive correlation (Fig. 8).
Seventeen percentage of the high-quality reservoirs
occurred in Class II mixed siliciclastic–carbonate
sediments, and only 7% high-quality reservoirs occurred in
Class III mixed siliciclastic–carbonate sediments. For
diagenetic features, the vast majority of high-quality
reservoirs were composed of pore-lining dolomite (Fig. 3a),
with minor authigenic calcite. The mixed siliciclastic–
carbonate sediments with pore-filling dolomite (Fig. 3c)
and calcite were poor in physical properties. Dissolution
was common in Class II mixed siliciclastic–carbonate
sediments. Through comparing the reservoir space of
highquality reservoirs and poor-quality reservoirs, it can be
seen that Class I mixed siliciclastic–carbonate sediments
were dominated by primary porosity, such as intrafossil
pores, followed by residual intergranular primary pores
(Fig. 3a), while Class II mixed siliciclastic–carbonate
sediments were dominated by residual intergranular
primary and dissolved porosity.
5.1 Genesis of primary pore development
Primary pores were pervasive in high-quality mixed
siliciclastic–carbonate sedimentary reservoirs, especially in
the reservoirs of Class I mixed siliciclastic–carbonate
sediments. According to statistics of the microscopic pore
type and plane porosity of the 87 cast thin sections of Class
I mixed siliciclastic–carbonate sediments and 20 cast thin
sections of Class II mixed siliciclastic–carbonate
sediments, the primary plane porosity of Class I accounted for
90% of the total, while the primary plane porosity of Class
II accounted for 42% of the total. The sedimentary
microfacies of different types of mixed
siliciclastic–carbonate sediments indicate that rocks were formed due to
the mixed deposition of the siliciclastic grains and matrix
in fan delta facies, and the carbonate particles and micrite
deposited in the lacustrine facies. Carbonate particles
mainly occurred in lacustrine high-energy underwater
beach bars far away from terrigenous provenance, so that
Class I mixed sediments with low micrite content and high
primary intergranular porosity were well developed,
whereas terrigenous clastics were common in the mouth
bars near terrigenous provenance, so that Class II mixed
sediments with low matrix content were developed.
Sixty-four percentage of carbonate particles were
bioclasts. The bioclastic content and reservoir porosity showed
a positive correlation in the study area (Fig. 8), since a
large amount of intrafossil pores were formed due to
biological decay. Most intrafossil pores were well preserved
during burial process, so intrafossil pores are well
developed in rocks (Fig. 3a). Thus, the bioclasts in the
highquality reservoirs in Class I mixed sediments contributed
largely to the primary porosity.
Pore-lining dolomites were common in high-quality
reservoirs, and they represented the formation features of
vadose zone–phreatic zone as indicated by blade- and
overhang-shaped distribution features. The analytical
Fig. 6 Sedimentary microfacies of HD5 mixed siliciclastic–carbonate sedimentary interval (3360–3430 m)
results of isotopic temperatures (Table 1) showed that
pore-lining dolomite was formed at a temperature of
29–83 C; together with the geothermal gradient of this
region (Liu et al. 2012), it is inferred that the pore-lining
dolomite in stage 1 (the earliest stage) was formed at a
paleoburial depth of less than 150 m and the liner dolomite
in stage 3 (the latest stage) was formed at a paleoburial
depth of less than 1700 m and the pore-lining dolomites
were formed early. It is indicated by the microscopic
observation of early pore-lining dolomite development in
reservoirs that the compaction was not strong. Under the
burial condition of 4000 m, the point-line contact in grains
was ubiquitously seen and the primary pores were well
developed (Fig. 3a). By comparing the mixed siliciclastic–
Mouth bar Distributary Underwater
channel beach bar
Fig. 7 Physical properties of different sedimentary microfacies of
E2s mixed siliciclastic–carbonate sediments
5.2 Genesis of dissolved pore development
The dissolved pores were well developed in E2s Class II
mixed siliciclastic–carbonate sediments. The statistics of
the pore types and content of 20 cast thin sections showed
that the plane porosity of dissolved pores accounted for
58%. The crystal optical features of dissolved minerals
showed that the dissolved minerals were mainly feldspar.
Furthermore, the microprobe component analysis results of
erosion remnants confirmed that the dissolved minerals
were mainly albite and K-feldspar (Table 2). In addition,
the authigenic clay minerals were kaolinite and illite,
indicating that dissolution took place under a K-rich
condition, resulting in the further transformation from kaolinite
to illite (Zhang et al. 2007). The inclusion temperature of
the authigenic quartz in mixed sedimentary reservoirs
ranged between 122–143 C, and the authigenic quartz was
formed due to the dissolution of feldspar. Thus, it is
inferred that the dissolution of feldspar took place from late
middle diagenetic stage to early epidiagenetic stage. The
temperature ranges coincided with the temperature ranges
of organic matter maturity stage, indicating that the
abundant acidic fluids were discharged during organic matter
evolution which had created conditions for the formation of
dissolved pores in feldspar (Meng et al. 2010; Cao et al.
5.3 Early phase and multiphased hydrocarbon charging on pores
The microscopic fluorescence features of E2s mixed
sediments reflected multistage hydrocarbon charging features.
For example, residual primary pores and intragranular
dissolved pores had two types of completely different
fluorescence, indicating at least two stages of hydrocarbon
charging. The early stage residual intergranular primary
pore showed yellow fluorescence, and the late stage
showed green fluorescence, which was mainly from the
dissolved pores in oolite (Fig. 3b). Hydrocarbon
components were detected in the inclusions in temperature range
of 73–87 C and 119–129 C with laser Raman (Table 3),
indicating at least two stages of hydrocarbon charging.
Pore-lining dolomite in Stage 1
Pore-lining dolomite in Stage 2
Pore-lining dolomite in Stage 2
Pore-lining dolomite in Stage 2
Pore-lining dolomite in Stage 3
Fig. 8 Relation between the bioclastic content and porosity of
E2s mixed siliciclastic–carbonate sediments
carbonate sediments with and without pore-lining in early
stage, it can be seen that the rocks without development of
pore-lining in early stage mostly represented line contact,
with low primary porosity (Fig. 3c). Therefore, the
formation of the pore-lining dolomites in early stage
effectively weakened the destruction of compaction on pores
and was favorable to the preservation of intergranular
Table 1 C and O isotope distribution of the pore-lining dolomite in E2s mixed siliciclastic–carbonate sediments
Well depth, m
d13C PDB, %
d18O PDB, %
Well depth, m
Table 3 Gas phase components of the inclusions in E2s mixed siliciclastic–carbonate sedimentary reservoirs
Well depth, m
Gas phase, %
Combined with the paleogeothermal gradient in the study
area, it is inferred that the reservoirs were buried at less
than 1500 m when there was hydrocarbon charging at the
earliest time. Generally, the early hydrocarbon charging
can inhibit cementation and also reduced further
compaction. Thus, the pores in reservoirs were effectively
preserved (Meng et al. 2010; Cao et al. 2014).
The E2s high-quality mixed siliciclastic–carbonate
sedimentary reservoirs in the central Bohai Sea were deposited
in a fan delta-lacustrine environment. The rocks were
formed due to the mixed deposition of the siliciclastic
material in fan deltas and carbonate particles deposited in
lacustrine environments. The mixed sediment content of
the carbonates gradually increased from a near provenance
region to lacustrine underwater high-energy beach bars.
The E2s high-quality mixed siliciclastic–carbonate
sedimentary reservoir rocks are mainly developed in Class
I, followed by Class II. The development of the
highquality reservoirs of Class I siliciclastic–carbonate
sediments was mainly controlled by a high-energy depositional
environment, high bioclastic content and pore-lining
dolomite and hydrocarbon charging in the early stage.
Primary pores were developed in the underwater uplift
beach bars with strong hydrodynamic conditions and low
micrite content. Intrafossil pores were common due to soft
biological decay, forming the main reservoir space of the
high-quality reservoir rocks of Class I. The development of
early stage pore-lining dolomite effectively weakened the
destruction of mechanical compaction on pores. The
hydrocarbon charging in the early stage effectively
preserved reservoir pores. The development of the
highquality reservoirs of Class II mixed siliciclastic–carbonate
sediments was mainly controlled by high-energy
depositional environments, feldspar dissolution, pore-lining
dolomite and hydrocarbon charging in the early stage. The
intergranular primary pores were formed in a high-energy
environment, such as fan delta front mouth bars and
underwater distributary channels. Feldspar dissolution
further improved reservoir properties. The hydrocarbon
charging in the early stage and the formation of pore-lining
dolomites effectively reduced the destruction of
mechanical compaction on pores. Therefore, the E2s mixed
siliciclastic–carbonate sediments in the central Bohai Sea had
good geological conditions for high-quality reservoir
accumulation, and it is prospective for exploration and
Acknowledgements This work was financially supported by the
National Science & Technology Specific Project (Grant No.
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