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RESEARCH ARTICLE
Geochemical characteristics and hydrocarbon generation
potential of the first member of Shahejie Formation (E
2
s
1
)
source rocks in the Dongpu Depression, East China
Xueying Lyu
1
|Youlu Jiang
1
|Jingdong Liu
1
|Tianwu Xu
2
1
School of Geoscience, China University of
Petroleum (East China), Qingdao, China
2
Exploration and Development Research
Institute, Zhongyuan Oilfield Company,
SINOPEC, Puyang, China
Correspondence
Xueying Lyu and Youlu Jiang, School of
Geoscience, China University of Petroleum
(East China), Qingdao 266580, China.
Email: lv_xue_ying@163.com; jiangyl@upc.
edu.cn
Funding information
Important National Science & Technology
Specific Projects, China, Grant/Award Num-
bers: 2016ZX05006‐003 and 2016ZX05006‐
007
Handling Editor: I. Somerville
The Dongpu Depression, containing huge amounts of hydrocarbons, is a typical
petroliferous province in the Bohai Bay Basin, east China. As one set of the main
source rocks, the first member of Shahejie Formation (E
2
s
1
) shows huge exploration
potential with approximately 0.83 × 10
8
t resources remaining to be discovered. How-
ever, the geochemical characteristics and thermal evolution history of E
2
s
1
source
rocks have still not been determined. In this study, the sedimentary features were
firstly analysed based on the geological data. Then the organic matter abundance, ker-
ogen type, and thermal maturity of E
2
s
1
source rocks in the present day were evalu-
ated according to the Rock‐Eval pyrolysis, micro‐components of kerogen, total
organic carbon (TOC), and vitrinite reflectance analysis. Furthermore, the evolution
of thermal history and hydrocarbon generation was determined using the PetroMod
procedure. The results show that dark mudstones and salt rocks were well developed
in E
2
s
1
, which are propitious for organic matter to grow and preserve. E
2
s
1
source
rocks contain abundant organic matter and moderate to good generative source rock
potential. E
2
s
1
source rocks mainly consist of Type II kerogen and have entered early
to maturation stage for hydrocarbon generation. Due to the limited buried depth, E
2
s
1
source rocks only attained lower thermal maturity and generated finite hydrocarbons.
The initial rapid subsidence and the second rift after uplift erosion resulted in the
higher geo‐temperature at the end of E
3
d and the present day. Corresponding to
the geo‐temperature evolution, there are two stages of hydrocarbon generation for
E
2
s
1
source rocks: The first stage was in the late E
3
d period, and the late stage was
from the late Neogene to the present day. Moreover, the hydrocarbon generation
rate and quantity of the late stage are much higher than the first stage, indicating
the major hydrocarbon generation stage. Due to the limited hydrocarbon generation
capacity, oils generated from E
2
s
1
source rocks are low maturity; thus, it cannot
migrate for a long distance and only accumulated around the sag belts.
KEYWORDS
Bohai Bay Basin, Dongpu Depression, E
2
s
1
source rocks, geochemical characteristics, hydrocarbon
generation
Received: 30 December 2016 Revised: 17 December 2017 Accepted: 6 June 2018
DOI: 10.1002/gj.3276
Geological Journal. 2018;1–14. © 2018 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/gj 1
1|INTRODUCTION
Bohai Bay Basin is a significant petroliferous basin in China (Lyu &
Jiang, 2017). Over the years, petroleum exploration and development
in the Bohai Bay Basin has focused on the deeper strata (deeper than
3,500 m). Although the shallower strata contain a higher exploration
degree, the low mature oil shows enormous potential. The total
resource of low mature oil is 25 × 10
8
t, and the proven reserves are
only 7 × 10
8
t, accounting for only one quarter (Zhai & He, 2002). A
better understanding of its formation and distribution is vital for the
exploration in shallower strata. Similarly, the low mature oil is also dis-
tributed around the hydrocarbon generation centre. Then the issue
changes to determine the hydrocarbon generation centre for
shallower strata. In other words, the characteristics and distribution
of source rocks for low mature oil should be firstly understood. The
Dongpu Depression, located in the southeast of the Bohai Bay Basin,
east China (Figure 1), is one of the typical oil and gas‐bearing depres-
sions (Chang, Chen, Wang, Zhao, & Wang, 2007; Jiang, Chang, Lu, &
Wu, 2008; Lyu, Jiang, & Liu, 2016). Defining the geochemical features
and distribution of source rocks in shallower strata is important for the
exploration of petroleum accumulated in shallower strata in the whole
Baohai Bay Basin.
Hydrocarbon exploration in the Dongpu Depression began in
1975, when the well Pucan‐1 produced high‐yielding industrial oil
flows during drilling (Yu & Rao, 2015). And the study of the first mem-
ber of Shahejie Formation (E
2
s
1
) has started since then. Hydrocarbons
have been found in the Dongying Formation, from the first to the
fourth member of Shahejie Formation (E
2
s
1
to E
2
s
4
), but dominated
by the second and third members of Shahejie Formation, accounting
for 95.2% of the total proved reserves in the Dongpu Depression.
FIGURE 1 Tectonic units and hydrocarbon distribution in the Dongpu Depression, Bohai Bay Basin (modified after Chen, Xu, Wang, & Tan,
2013, and Y. L. Jiang, Fang, Liu, Hu, & Xu, 2016). Showing the location of the Dongpu Depression, and tectonic units, hydrocarbon
distribution, and sample wells [Colour figure can be viewed at wileyonlinelibrary.com]
2LYU ET AL.
According to the third resource assessment in China, the total
resources in E
2
s
1
in the Dongpu Depression are 1.06 × 10
8
t, and
approximately 0.83 × 10
8
t resources are remaining to be discovered
(Gao et al., 2013); thus, its proved ratio is only 21.7%, indicating its
huge exploration potential.
Previous studies have suggested that the potential source rocks in
the Dongpu Depression occur in the E
2
s
1
,E
2
s
3
, and E
2
s
4
units in the
Paleogene, and they were all deposited in an evaporitic lacustrine
environment (Jiang, Zhuo, Tan, & Lu, 2009; R. S. Lu, Zhao, & Chen,
1993; Wang et al., 2015), and the high‐mature and widely distributed
E
2
s
3
source rocks are regarded as the more prolific ones in the depres-
sion. E
2
s
1
source rocks are low‐moderate maturity due to the
shallower depth. The previous oil correlation shows that oils accumu-
lated in E
2
s
1
reservoirs may be generated from E
2
s
1
or E
2
s
3
source
rocks (Gao et al., 2013). And the E
2
s
3
sourced oils are distributed in
Pucheng, Weicheng, and Qiaokou areas, while the E
2
s
1
sourced oils
are lower maturity and have only a limited distribution in the
Liuzhuang and south Wenliu areas (Gao et al., 2013).
However, scholars have laid emphasis on the hydrocarbon gener-
ation capacity and their exploration potentials of E
2
s
3
to E
2
s
4
source
rocks (Fan, Wang, & Li, 2010; Liu & Jiang, 2013), and few studies have
focused on E
2
s
1
source rocks. Although Chang, Shen, Chen, and Wang
(2005) and Zhang, Shen, Chen, and Wang (2007) have evaluated the
hydrocarbon‐generating ability of E
2
s
1
source rocks and suggested
their better organic matter type, low to moderate maturation and
about 2,500 m depth of hydrocarbon generation threshold, and lower
hydrocarbon‐generating intensity, the geochemical features of E
2
s
1
oils and source rocks are still completely unclear. With a growing
degree of exploration, an increasing number of wells were drilled
through E
2
s
1
, but the geochemical features and thermal evolution his-
tory of the source rocks have been seldom investigated, and thus, the
timings of hydrocarbon generation and charging are poorly under-
stood. In addition, multiple sub‐sags are present in the Dongpu
Depression, but heterogeneity studies in hydrocarbon generation
and thermal evolution of E
2
s
1
source rocks among them have not been
carried out. Moreover, hydrocarbons generated from E
2
s
1
source
rocks are low mature (Gao et al., 2013) and mainly accumulated in
E
2
s
1
reservoirs as mentioned above. Nevertheless, few researches
have discussed the developmental features and physical property of
E
2
s
1
reservoirs in the Dongpu Depression, and thus, the favourable
exploration targets are ambiguous. In addition, these previous studies
focused on Pucheng or Liuzhuang area in the Dongpu Depression
(Chang et al., 2007; Gao et al., 2013), meaning that no regional or
comprehensive researches have been performed, and therefore lim-
ited the hydrocarbon exploration.
The quality of source rocks directly influences the hydrocarbon
exploration potential (F. J. Jiang, Pang, Bai, Zhou, Li & Guo 2016;
Prinzhofer, Mello, & Takaki, 2000). Therefore, the evaluation of source
rocks is of great importance and necessary. In practical application,
source rocks are evaluated based on the analyses of their geological con-
ditions and geochemical characteristics (Jiang, Pang, et al., 2016; Peng
et al., 2016; Shalaby, Hakimi, & Wan, 2011). The geological conditions,
including the basin type, tectonic settings, and sedimentary environment,
are of critical importance for the formation of source rocks and further
control their spatial and vertical distribution and organic matter types
(Peng et al., 2016; Tissot & Welte, 1984; Welte, 1965). The thermal evo-
lution or burial history of a depression directly determines the thermal
evolutionof source rocks and the hydrocarbon‐generating process (Peng
et al., 2016; Ross & Bustin, 2008; Shalaby et al., 2011). Geochemical
characteristics of source rocks, including their organic matter abundance,
types, and maturity, fundamentally determine the hydrocarbon genera-
tion ability (Hanson, Ritts, & Moldowan, 2007; Pan, 1941; Peng et al.,
2016). The organic matter abundance is usually assessed by the total
organic carbon (TOC), chloroform bitumen “A”, and hydrogen index (HI).
And the maturity can be represented by the temperature of peak
pyrolysis (T
max
) and vitrinite reflectance (R
o
).
To deeply and systematically understand the hydrocarbon gener-
ation potential of E
2
s
1
source rocks, this paper evaluated their organic
matter abundance, types, and maturity and combined the estimate
results with basin modelling of typical wells to ascertain the timing
of hydrocarbon generation. Furthermore, differences in hydrocarbon‐
generating features among different sags are discussed.
2|GEOLOGICAL SETTING
2.1 |Tectonics and stratigraphy
The Dognpu Depression, which covers an area of about 5,300 km
2
,is
located in the south‐western Bohai Bay Basin, east China (Chang,
2007; Chen et al., 2013; Lyu et al., 2016). The Dongpu Depression is
an NNE‐trending depression, narrow in the north and broad in the
south in plane view (Figure 1). The depression is bounded to the east
by the West Shandong Uplift and Lanliao Fault, by the Neihuang Uplift
in the west, by the Lankao Uplift in the south, and by the Shenxian
Depression in the north. Within the depression, four tectonic units
are identified, namely, Eastern Sag Belt, Central Uplift Belt, Western
Sag Belt, and Western Slope Belt from east to west (Jiang et al., 2008;
R. S. Lu et al., 1993). Moreover, several secondary hydrocarbon gener-
ation units exist in the depression, including Pucheng‐Qianliyuan and
Gegangji sags in the Eastern Sag Belt, and Liutun‐Haitongji and
Nanhejia‐Menggangji sags in the Western Sag Belt. These sags all con-
tain great hydrocarbon generation capacity, especially the Qianliyuan
Sag, generating and providing hydrocarbons for the majority oil‐and
gas‐bearing units.
As a Cenozoic extensional rift basin developed on the Paleogene‐
Mesozoic sedimentary basement, the Dongpu Depression underwent
two evolutionary stages: rifting during the Eocene–Oligocene under
NW–SE extension and followed by subsidence and deposition during
the Neogene and Quaternary (Chen et al., 2013; Wang et al., 2015).
These evolution cycles can be divided into five stages, including the initial
rifting during E
2
s
4
period, followed by an advanced rifting during E
2
s
3
deposition, post‐rifting from E
2
s
2
to E
2
s
1
periods, rifting termination dur-
ing E
3
d period, and subsidence from Ng to Quaternary period (R. S. Lu
et al., 1993; Wang et al., 2015). In addition, a right‐handed wrench was
superimposed over the extension in the Oligocene. However, this
wrench mainly affected some areas of the northern Dongpu Depression
and became weak southward, resulting in mainly affecting the basement.
The synrift sediments are composed of the Kongdian (E
1
k),
Shahejie (E
2
s), and Dongying (E
3
d) formations, from the bottom to top
LYU ET AL.3
(Figure 2). And the Shahejie Formation can be further subdivided into
four members, that is, the first to fourth members (top to bottom).
During the deposition period of E
2
s
1
, the depression was
experiencing the second rifting and subsidence cycle. The Dongpu
Depression subsided in the relatively equable tectonic background
and deposited dark lacustrine mudstones and siltstones and evapo-
rates in the northern depression (Figure 2; Chen et al., 2013).
2.2 |Sedimentary characteristics
During the deposition period of E
2
s
1
, the depression was experienc-
ing the second rifting and subsidence cycle (Chen et al., 2013; R. S.
Lu et al., 1993). But the subsidence intensity is much weaker than
the first cycle during the E
2
s
3
deposition (R. S. Lu et al., 1993).
Due to the strong activities of the southern sections of the Huanghe
and Changyuan faults, the Menggangji Sag, located in the south of
the Western Sag Belt (Figure 1), became the subsiding centre of
E
2
s
1
with a thickness up to 600 m. However, the sedimentary centre
was located around Wenliu area, 60 km to the northeast. The delta
depositional system was well developed in the depression, with a
provenance from the northeast and southwest uplifts. Dark mud-
stones of pre‐delta and shallow‐semideep lacustrine facies were well
developed (Figure 3).
The sedimentary system of E
2
s
1
can be further subdivided into two
stages resulting from the different sedimentary environments. During
the early E
2
s
1
period or the deposition of lower E
2
s
1
, the paleoclimate
was arid and the depositional system was closed, suitable for the depo-
sition of salt rocks (Figure 3a). Thereafter, the paleoclimate became
damp, and a large amount of water was supplied, resulting in the devel-
opment of freshwater lacustrine facies (Figure 3b). Besides, the condi-
tions are also propitious for organic matter to grow and preserve.
Figure 4 shows the thickness distribution of E
2
s
1
source rocks in
the Dongpu Depression. The E
2
s
1
source rocks are distributed over
the whole study area, but the total thickness is relatively thin. In addi-
tion, several sedimentary centres existed in the depression, including
Pucheng, Qianliyuan, Menggangji, and Haitongji sags (Figure 2), with
the maximum total thickness of E
2
s
1
source rocks of 240, 350, 250,
and 250 m, respectively. In general, E
2
s
1
source rocks gradually
thicken westward and northward.
3|METHODOLOGY
3.1 |Samples and analytical methods
According to the Zhongyuan Oilfield, a total of 182 wells have been
drilled through E
2
s
1
in the Dongpu Depression, and they are widely
FIGURE 2 Generalized stratigraphy of Paleogene in the Dongpu Depression (modified after Wang et al., 2015), showing the lithology, thickness,
and sedimentary environment of formations
4LYU ET AL.
distributed throughout the depression. The E
2
s
1
source rock thick-
nesses were obtained from the well logging and drilling data.
To evaluate the abundance, types, and thermal maturity of
source rocks, more than 300 E
2
s
1
rock samples from 33 wells were
collected in the Dongpu Depression. Among them, pyrolysis analyses
were conducted on 218 samples from 13 wells, the total organic
carbon (TOC) was determined for 101 samples from 21 wells,
vitrinite reflectance (R
o
) was measured for 36 samples from 13 wells,
and kerogen types were determined for 18 samples from five wells.
Rock‐Eval pyrolysis was performed using the Rock‐Eval II appara-
tus in line accordance with the petroleum industry standard of China
(CNPC, 1996). Parameters obtained compose of TOC, S
1
,S
2
,S
3
, and
the temperature at peak S
2
(T
max
). Then oxygen index (OI) and hydro-
gen index (HI) were calculated using the formulas proposed by
Espitalié et al. (1977) and Peters and Cassa (1994). TOC was measured
after removing carbonate by an LECO CS‐230 analyser. Vitrinite
reflectance (Ro) was measured using an oil immersion lens and a
reflected light microscope. The kerogen type can be directly reflected
by its micro components. They were measured as described by Cao
FIGURE 3 Sedimentary facies of E
2
s
1
in the
Dongpu Depression (according to Lu et al.,
1993). (a) Lower E
2
s
1
; (b) upper E
2
s
1
. Salt
rocks deposited during the early E
2
s
1
period,
while freshwater lacustrine facies were
developed in late E
2
s
1
period, both propitious
for organic matter to grow and preserve
[Colour figure can be viewed at
wileyonlinelibrary.com]
FIGURE 4 Thickness distribution of E
2
s
1
source rocks in the Dongpu Depression. E
2
s
1
source rocks are widely distributed in the
depression with thickness ranging from 0 to
400 m and developed well in the north and
southwest sags [Colour figure can be viewed
at wileyonlinelibrary.com]
LYU ET AL.5
(1985) under transmitted light. Table 1 shows the Rock‐Eval analysis
results with calculated parameters for several samples.
3.2 |Modelling procedures
Basin modelling has been extensively used as a tool to restore the his-
tory of burial and thermal evolution of a basin for a long time (Makker
et al., 2016; Shalaby et al., 2011; Welte & Yukler, 1981). Then the
paleo‐depth and the hydrocarbon generation and expulsion timing
can be determined. Some simulation parameters are needed for the
modelling, including the geological ages and present‐day depth or
thickness of formations, tectonic movement, lithology, paleo tempera-
ture (SWIT), water depth (PWD) and heat flow (HF), and the proper-
ties of source rocks (Jiang, Fang, et al., 2016; Shalaby et al., 2011).
As described in the geological setting section, the major tectonic
movement in the Dongpu Depression is erosion during the late
Dongying period, and the erosion thickness was obtained according
to X. S. Lu, Jiang, Chang, and Wu (2007). Furthermore, the modelling
results were calibrated on the basis of the measured data of tempera-
ture, pressure, and R
o
. The main input data are shown in Table 2.
4|GEOCHEMICAL CHARACTERISTICS
4.1 |Organic matter abundance
Rock‐Eval pyrolysis indicates that the samples from E
2
s
1
source rocks
have a relatively intense variation in total organic carbon (TOC) values,
ranging from 0.21% to 10.1% with a mean value of 1.27%. Similarly,
hydrocarbon potential yield (S
1
+S
2
) values range from 0.16 to
72.93 mg HC/g rock with a mean value of 5.26 mg HC/g rock (Table 3).
The chloroform bitumen “A”content values are in the range of 0.01–
0.4%, with a mean value of 0.15%. However, the total hydrocarbons
(HC) values are relatively higher, in the range of 67 to 7,175 ppm with a
mean value of 928 ppm. Therefore, the E
2
s
1
source rocks in the Dongpu
Depression are moderate to good source rocks (Table 3; Figure 5).
Since the samples analysed were mainly from wells located on
structural highs, in other words, few samples were from the centre of
sags, where higher TOC contents and thicker source rocks may occur
(Peng et al., 2016). Thus, parameters to evaluate the organic matter
abundance listed in Table 3 are underestimated and cannot reflect the
real abundance of E
2
s
1
source rocks in the Dongpu Depression. Hence,
to further understand the organic matter abundance in sags, it is neces-
sary to know the TOC distribution in plan view. Figure 6 shows the dis-
tribution of present‐day or residual TOC contents for E
2
s
1
source rocks
in the Dongpu Depression. It is obvious that TOC contents are relatively
unevenly distributed in the whole depression, higher in the north and
lower in the south. In addition, several centres occur in the two sag belts
with higher TOC contents, including the Qianliyuan, Gegangji, Liutun,
and Nanhejia sub‐sags. The maximum value can be up to 1.4%, 1.2%,
1.5%, and 1.5%, respectively.
However, hydrocarbon generation and expulsion will deplete the
organic matter and therefore decrease the TOC contents (Daly &
Edman, 1987; Peng et al., 2016). Thus, the original TOC contents must
be larger than the present‐day values. It can be inferred that the E
2
s
1
source rocks in the Dongpu Depression have much larger organic mat-
ter abundance during the timing of hydrocarbon generation.
4.2 |Type of organic matter
The type of organic matter can be determined by T
max
, the hydrogen
index (HI), oxygen index (OI), and the micro components of kerogen.
The values of HI (S
2
/TOC) and OI (S
3
/TOC) can be calculated using
the Rock‐Eval pyrolysis data. The type index T can be calculated
according to the contents of micro components of kerogen,
TABLE 1 Geochemical results of Rock‐Eval analysis with calculated parameters of samples E
2
s
1
source rocks in the Dongpu Depression (col-
lected from Zhongyuan Oilfield)
Wells Depth (m) TOC (wet %) S
1
(mg/g) S
2
(mg/g) S
3
(mg/g) S
1
+S
2
(mg/g) Tmax (°C) PI HI OI
H81 2,150.50 1.13 0.04 0.96 1.51 1.00 420 0.04 85 134
H81 2,174.66 1.74 0.19 6.14 1.62 6.33 432 0.03 353 93
M1 3,195.00 0.84 0.08 0.77 0.86 0.85 428 0.1 92 102
P1–154 2,364.32 1.85 0.3 6.56 0.62 6.86 427 0.04 355 34
P60 2,205.82 3.85 0.31 21.68 1.74 21.99 424 0.01 563 45
PS10 2,666.00 1.55 0.18 6.47 6.65 427 0.03 417
PS12 2,725.00 0.91 0.24 2.48 2.72 431 0.09 273
PS13 3,306.25 0.12 0.66 0.16 0.82 433 0.80 133
PS6 3,195.00 0.43 0.08 0.42 0.50 432 0.16 98
PS7 2,449.00 1.15 0.47 3.71 4.18 429 0.11 322
PS8 3,441.50 2.35 0.25 7.33 7.58 425 0.03 312
Q11 2,400.02 0.63 0 0.1 0.25 0.10 434 0.01 16 40
Q11 2,467.81 0.12 0.02 0.15 0.47 0.17 433 0.12 125 392
Q11 2,477.22 0.55 0.04 2.27 0.83 2.31 438 0.02 413 151
Q11–6 2,429.68 1.24 0.02 1.48 0.44 1.50 438 0.01 119 35
Wen164 1,873.57 9.90 7.38 43.28 50.66 418 437
Note. TOC: total organic carbon, wt.%; S
1
: volatile hydrocarbon (HC) content, mg HC/g rock; S
2
: remaining HC generative potential, mg HC/g rock; S
3
:
carbon dioxide content, mg CO
2
/g rock; HI (hydrogen index) = S
2
× 100/TOC, mg HC/g TOC; OI (oxygen index) = S
3
× 100/TOC, mg HC/g TOC; PI
(production index) = S
1
/(S
1
+S
2
).
6LYU ET AL.
comprising sapropelite, exinite, inertinite, and vitrinite. And T> 80%
indicates Type I kerogen, while T ranging from 40% to 80% represents
Type II
1
kerogen, in the range of 0% to 40% indicates Type II
2
kerogen,
and T< 0 represents Type III kerogen (Jiang & Zha, 2006).
E
2
s
1
source rocks contain higher contents of sapropelite and
exinite, which are rich in hydrogen and propitious to generate hydro-
carbons. The type index (T) ranges from 46% to 96.5% for E
2
s
1
source
rocks in the Dongpu Depression, indicating the kerogen of Type I and
Type II
1
, taking an equal share among the tested samples (Figure 7).
However, the plots collected are less and may not represent the real
percentage of each type. The HI‐T
max
diagram (Mukhopadhyay, Wade,
& Kruge, 1995) can also be used to classify the type of organic mat-
ters. As shown in Figure 8a, the HI values are distributed between
16.3 and 1259 mg HC/g TOC, and the majority of samples plot in
the mature area of Type II kerogen mixed with some Type I and Type
III kerogen. Besides, Zhang et al. (2007) suggested that kerogen from
E
2
s
1
source rocks are mainly of Type II
1
and Type II
2
according to
the modified van Krevelen plots. Consequently, the E
2
s
1
source rocks
in the Dongpu Depression are dominated by Type II kerogen, espe-
cially the Type II
1
.
4.3 |Thermal maturity of organic matter
Vitrinite reflectance (R
o
) and T
ma
x can both be used to determine the
maturity of organic matter. In general, the T
max
of 430°C and R
o
of
0.7% indicates the top of the oil window for type II kerogen
TABLE 2 Main input data for Basin modelling of the thermal and hydrocarbon generation history in the selected wells
Formation
Deposition age (Ma)
SWIT
(°C)
HF
(mW/m
2
)
PWD
(m) Lithology
Well PS6 Well PS12 Well PS13
From To
Top
(m)
Bottom
(m)
Thick
(m)
Top
(m)
Bottom
(m)
Thick
(m)
Top
(m)
Bottom
(m)
Thick
(m)
Ng‐Q 17 0 12 1.55 10 SHALE 9 2,388 2,379 5 1,509 1,504 0 1,804 1,804
E
3
d 33 27 14 1.6 15 SHALE & SANDS 2,388 3,121 733 1,509 2,607 1,098 1,804 2,864 1,060
E
2
s
1
35 33 15 1.7 50 SHALE & SANDS 3,121 3,438 317 2,607 2,972 365 2,864 3,321 457
E
2
s
2
38 35 15 1.7 20 SHALE & SANDS 3,438 4,105 667 2,972 3,653 681 3,321 4,421 1,100
E
2
s
3
45 38 15 1.75 50 SHALE & SANDS 4,105 4,805 700 3,653 5,000 1,347 4,421 5,150 729
E
2
s
4
50.5 45 15 1.75 50 SHALE & SANDS 4,805 4,834 29
Note. SWIT: paleo‐temperature, °C; HF: heat flow, mW/m2; PWD: paleo water depth, m. The erosion during the Dongying Period occurred from 27 to 17 Ma.
TABLE 3 Organic matter abundance of E
2
s
1
source rocks in the
Dongpu Depression
TOC(%) “A”(%)
S
1
+S
2
(mg/g)
HC
(ppm) Evaluation
Minimum 0.21 0.01 0.16 67 Moderate to
goodMaximum 10.1 0.4 72.93 7175
Mean 1.27 (219) 0.15 (57) 5.26 (73) 928 (40)
Note. HC: total hydrocarbon, ppm; TOC: total organic carbon, wt.%; S
1
:
volatile hydrocarbon (HC) content, mg HC/g rock; S
2
: remaining HC gener-
ative potential, mg HC/g rock; “A”: chloroform bitumen “A”,%.
FIGURE 5 S
2
versus total organic carbon (TOC) for samples of E
2
s
1
source rocks in the Dongpu Depression (modified after Shalaby et al.,
2011). Values of TOC and S
2
in E
2
s
1
source rocks are relatively lower,
indicating moderate to good generative potential [Colour figure can be
viewed at wileyonlinelibrary.com]
LYU ET AL.7
(Bordenave, Esoitalie, Leolat, Ouidin, & Vandenbroucke, 1993; Tissot
& Welte, 1984). The R
o
values of E
2
s
1
source rocks in the Dongpu
Depression range from 0.38% to 1.33%, most of which are greater
than 0.5%, indicating that the E
2
s
1
source rocks have been mature
and mainly generated mature oil (Figure 8b), which is in agreement
with the T
max
values ranging from 416 to 461°C with more than
76% greater than 430°C. Furthermore, R
o
increases logarithmically
with the increasing depth and began to be greater than 0.5% when
the depth is deeper than 2,300 m and more than 0.7% when depth
reaches 2,700 m (Figure 8b).
It is noteworthy that samples are located on the structural highs
and cannot reflect the real thermal maturity in sags. In this study, we
use the contour map (Figure 9) of Ro values for E
2
s
1
source rocks to
infer their thermal maturity in deeper structural sags in the Dongpu
Depression.
As shown in Figure 9, the thermal maturity of E
2
s
1
source rocks is
relatively lower with R
o
value less than 1.3% throughout the whole
depression. The R
o
value ranges from 0.2% to 1.1% for E
2
s
1
source
rocks in the Dongpu Depression and is higher in the western and east-
ern sag belts, especially in the Menggangji Sag with R
o
as high as 1.1%.
Moreover, E
2
s
1
source rocks are low mature (0.5% < R
o
< 0.7%) in
most areas in the depression, mature (0.7% < R
o
< 1.0%) in the
Qianliyuan, Menggangji, Haitongji, and Nanhejia sags, and high mature
(R
o
> 1.0%) only in the centre of the Menggangji Sag. Therefore, E
2
s
1
source rocks have generated low mature oil almost throughout the
whole depression, entered the oil window in these two sag belts and
reached peak oil generation stage only in the Menggangji Sag. Overall,
these two sag belts are the hydrocarbon generation centres for E
2
s
1
source rocks in the Dongpu Depression.
In general, the area with thicker mudstone and higher TOC con-
tents should have relatively higher maturation, especially in the deeper
sags. However, the Wenliu area as the sedimentary centre of E
2
s
1
con-
tains the relatively thickest mudstone and higher TOC contents but
lower R
o
value (maturity). Considering the depositional environment
FIGURE 6 Residual TOC distribution of E
2
s
1
source rocks in the Dongpu Depression
(according to the data provided by the
Zhongyuan Oilfield). TOC contents are higher
in the north and lower in the south. And
several centres occur in the sag belt [Colour
figure can be viewed at wileyonlinelibrary.com]
FIGURE 7 Frequency distribution of T index for E
2
s
1
source rocks in
the Dongpu Depression. E
2
s
1
source rocks are of Type I and II
1
kerogen,
which is prior to oil generation, and the percentage of the two types is
equivalent [Colour figure can be viewed at wileyonlinelibrary.com]
8LYU ET AL.
and tectonic settings of E
2
s
1
, during the early E
2
s
1
period, a set of evap-
oritic rocks developed around the Wenliu area in the northern part of
the depression (Figure 3) that resulted from the arid and closed climate
and converted to the lake facies sediments due to the humid climate
during the late E
2
s
1
. The salt and anhydrite rocks of lower E
2
s
1
are
mainly distributed in the Wenliu and Pucheng areas, with thickness
ranging from 30 to 240 m (Qi, Huang, Shou, & Li, 1992) and covered
by dark mudstones depositing in deep lake facies (Figure 3). Because
of the lower thermal conductivity, the salt and anhydrite rocks of lower
E
2
s
1
can hinder the transmission of geothermal heat and thus result in
the relatively lower thermal maturity (or R
o
) of dark mudstones in upper
E
2
s
1
, as indicated in Figure 9.
4.4 |Hydrocarbon generation potential
In this study, the organic matter abundance analysis suggested that
moderate to good generative potential is found in the E
2
s
1
source
rocks. The E
2
s
1
source rocks have relatively higher TOC contents
above 1.0% but lower S
2
yields mainly lower than 10 mg/g rock
(Figure 5). However, the other parameters, including “A,”(S
1
+S
2
)
FIGURE 8 Hydrogen index versus T
max
(a; modified from Peng et al., 2016) and R
o
profile (b) of E
2
s
1
source rocks in the Dongpu
Depression. E
2
s
1
source rocks are dominant
by the Type II kerogen and have been mature
with R
o
between 0.5% and 1.3% [Colour
figure can be viewed at wileyonlinelibrary.
com]
FIGURE 9 Present‐day R
o
distribution of
E
2
s
1
source rocks in the Dongpu Depression
(according to the data provided by the
Zhongyuan Oilfield). R
o
value of E
2
s
1
source
rocks is less than 1.3% throughout the whole
depression and relatively higher in the
western sag belt [Colour figure can be viewed
at wileyonlinelibrary.com]
LYU ET AL.9
and HC, are relatively great (Table 3) and indicate a better potential.
Therefore, the dark mudstones of E
2
s
1
are moderate to good poten-
tial source rocks in the Dongpu Depression. E
2
s
1
source rocks of
Type II
1
kerogen dominantly generate oil, as supported by the higher
contents of sapropelite and exinite beneficial for hydrocarbon
generation.
In addition, T
max
versus production index can tell the origin of
hydrocarbons generated (Shalaby et al., 2011) and shows that sam-
ples from E
2
s
1
source rocks in the Dongpu Depression have entered
the mature threshold and generated majority hydrocarbons non‐
indigenously with a few generated indigenously (Figure 10). More-
over, samples plotted in the hydrocarbon generation indigenous area
are from well PS10 and well PS12 located in the Qianliyuan Sag and
well PS13 in Haitongji Sag; the areas have much higher TOC con-
tents and thermal maturity and thus have higher hydrocarbon gener-
ation capacity. This point agrees with the opinion proposed by Gao
et al. (2013), suggesting that majority of hydrocarbons stored in
E
2
s
1
reservoirs are sourced by E
2
s
3
source rocks and only generated
from E
2
s
1
source rocks in the Liuzhuang and south Wenliu areas. In
other words, E
2
s
1
source rocks contain limited hydrocarbon
generation potential.
5|EVOLUTION OF HYDROCARBON
GENERATION FOR E
2
s
1
SOURCE ROCKS
To better understand the evolution history of thermal and hydrocar-
bon generation for E
2
s
1
source rocks in deeper sags in the Dongpu
Depression, three wells were selected for the numerical modelling
using 1D modelling in PetroMod 2012 procedure, including Well
PS12 located in Qianliyuan Sag, Well PS13 in Haitongji Sag, and Well
PS6 in Menggangji Sag (Figure 1), where the mature E
2
s
1
source rocks
are distributed.
5.1 |Burial history
The tectonic evolution prominently affected burial history and ther-
mal evolution of a depression. Aiming to determine the timing of
hydrocarbon generation, the burial and thermal history of the depres-
sion needs to be understood initially. In the Dongpu Depression, the
sedimentary rate can be calculated using the sedimentation time and
current thickness for E
2
s
1
. Three sags experienced the same burial
history in the depression, and well PS12 was taken as a typical exam-
ple, as shown in Figure 11. The rapid burial resulted in the rapid
deposition of E
2
s
1
during the early Oligocene (35–27 Ma), subsiding
about 300 m per million years. Following this subsidence, a tectonic
uplift occurred in the late Dongyin period (27–17 Ma) due to the
Dongying movement caused by the Himalayan Orogeny, resulting in
great erosion of Dongying Formation of about 800 m. From the Neo-
gene Period, the strata began to subside again for 70 myr. And the
re‐burial depth is much greater than the erosion thickness, leading
to the overcompensation and re‐evolution of E
2
s
1
source rocks. A
total of 365 m sediment were deposited during E
2
s
1
in the Dongpu
Depression.
FIGURE 10 PI versus T
max
for samples from E
2
s
1
source rocks in the
Dongpu Depression (modified after Shalaby et al., 2011). E
2
s
1
source
rocks have entered mature threshold and generated majority
hydrocarbons non‐indigenously with only a few indigenously [Colour
figure can be viewed at wileyonlinelibrary.com]
FIGURE 11 Burial history with paleo‐temperature distribution in well PS12, Dongpu Depression; the right diagrams show the calculated (lines)
and measured (solid circles) values of R
o
and temperature. The calculated curves of temperature and R
o
are well matched with the measured
samples [Colour figure can be viewed at wileyonlinelibrary.com]
10 LYU ET AL.
5.2 |Thermal and maturation evolution history
The thermal history of a basin can be simulated using 1D modelling,
and several parameters need to be determined. The paleo‐tempera-
ture and paleo‐water depth were referred to Chang (2007). In addition
to these two parameters, the heat flow and tectonic evolution are nec-
essary to qualitatively reconstruct the thermal history of the Dongpu
Depression. The thermal history of a basin is significantly controlled
by heat flow (Shalaby et al., 2011). During rift stage, the thinning of
the lithosphere could lead to an increase in heat flow (Shalaby et al.,
2011). Nevertheless, its value is difficult to define in the geological
past. Hence, measured vitrinite reflectance (R
o
) and temperature data
were used for the estimation and calibration of the heat flow values in
the 1D modelling. In the modelled wells, the value of heat flow ranges
from 57.5 to 61 mW/m
2
, used for numerical modelling in the depres-
sion. As shown in Figure 11, the applied heat flow values contribute to
a good match between calculated and measured R
o
values.
The temperature increases with depth from a surface value. Due
to the initial rapid subsidence and the second rift after uplift erosion,
the temperature reached a higher value at the end of E
3
d and the
present day, and up to 200°C at the present day because of the
deeper burial than during the initial subsidence. For E
2
s
1
, temperature
values have a similar evolution trend, initially increased from 15 to
104°C from 35 to 27 Ma, followed by a decrease to 62°C from 27
to 17 Ma, thereafter, rising again to 117°C since 17 Ma to the present
day (Figure 11).
5.3 |Timing of hydrocarbon generation
On the basis of thermal history analysis, the timing of hydrocarbon
generation from E
2
s
1
source rocks was discussed and determined.
Hydrocarbon generation features were estimated assuming mainly
Type II kerogen and using the kinetic model proposed by Burnham
(1989). The amount of hydrocarbon generated was evaluated on the
basis of the transformation ratio and kerogen type. The transforma-
tion ratio refers to the ratio of the amount of hydrocarbons generated
to the total amount of hydrocarbons that the kerogen is capable of
generating (Shalaby et al., 2011). Figure 12 shows the maturation evo-
lution of studied wells. The simulation indicates that thermal evolution
history of E
2
s
1
source rocks among the studied three wells were sim-
ilar, but some differences still occur.
In the Qianliyuan Sag as sampled by well PS12, E
2
s
1
source rocks
were immature with R
o
less than 0.5% during early E
2
s
1
to late E
3
d
period (35–28 Ma). Since the late E
3
d period (28 Ma), E
2
s
1
source
rocks began to become mature with R
o
above 0.5%. Due to the uplift
from 27 to 17 Ma, the geotemperature and pressure decreased and
FIGURE 12 Burial history with R
o
for E
2
s
1
source rocks in the studied wells, Dongpu Depression. Thermal evolution history of E
2
s
1
source rocks
among the studied three wells was similar, but the timing of mature stage in the north sags (a and b) is earlier than in the south sags (c) [Colour
figure can be viewed at wileyonlinelibrary.com]
LYU ET AL.11
resulted in the cessation of hydrocarbon generation, whereas some
petroleum was expelled during the uplift stage. From the late Neogene
until the present (12–0 Ma), the temperature rose again and exceeded
the threshold temperature due to the secondary subsidence. E
2
s
1
source rocks developed again and generated hydrocarbons secondar-
ily. During the late hydrocarbon generation stage, the E
2
s
1
source
rocks were at a higher maturation stage than the first stage with R
o
even greater than 0.7%, corresponding to the early oil window at a
depth greater than 2,800 m (Figure 12a). The hydrocarbon generation
curves (Figure 13a) show that hydrocarbon generation rate was rela-
tively lower, and first generation from E
2
s
1
source rocks started at
the late E
3
d period (33 Ma) and peaked at 27 Ma and continued for
11 myr until the early uplift stage due to the Dongying movement.
Then hydrocarbon generation ceased during the late uplift stage.
Thereafter, hydrocarbon generation started again in a relatively higher
rate since the early Minghuazhen period (12 Ma). The transformation
ratio reaches 8.84% at the present day, with oil as the major product.
In the Haitongji Sag as sampled by well PS13, E
2
s
1
source rocks
have a similar evolution history as the Qianliyuan Sag mentioned above.
However, well PS13 contains higher maturity oil than well PS12 at the
present day, mainly resulting from the thicker sediments after uplift.
The E
2
s
1
source rocks were at higher maturation stage with R
o
even
greater than 0.7% since the late Neogene, corresponding to the early
oil window at a depth greater than 2,700 m (Figure 12b). The hydrocar-
bon generation curves (Figure 13b) show that hydrocarbon generation
rate is consistent with Qianliyuan Sag, but with much higher values. The
transformation ratio is larger than Qianliyuan Sag as well and reaches
44.66% at the present day, with oil as the major product.
In the Menggangji Sag as sampled by well PS6, E
2
s
1
source rocks
have a very different evolution history to the Qianliyuan and Haitongji
sags and only generated hydrocarbons for the first time during the late
Neogene to the present day. E
2
s
1
source rocks were immature with R
o
less than 0.5% since the early E
2
s
1
period until the early Neogene (35–
11 Ma). Since the early Neogene (11 Ma), E
2
s
1
source rocks began to
be mature with R
o
above 0.5%. From the late Neogene (3 Ma) to the
present day, the E
2
s
1
source rocks were at higher maturation stage
with R
o
even greater than 0.7%, corresponding to the early oil window
at depth greater than 3,100 m (Figure 12c). The hydrocarbon genera-
tion curves (Figure 13c) show that hydrocarbon generation rate was
situated between the Qianliyuan and Haitongji sags. And first genera-
tion from E
2
s
1
source rocks started at the late E
3
d period (33 Ma) and
peaked at 27 Ma and continued at a very low rate for 11 myr until the
early uplift stage due to the Dongying movement. Then hydrocarbon
generation ceased during the late uplift stage. Thereafter, hydrocarbon
generation started again at a relatively higher rate since the early
Minghuazhen period (12 Ma). The transformation ratio reaches
27.8% at the present day, with oil as the major product.
In the depression, the thickness of the E
2
s
1
ranges from 50 to
600 m. E
2
s
1
source rocks have a moderate to good hydrocarbon
generation potential. Two stages of hydrocarbon generation for
E
2
s
1
source rocks existed in the Dongpu Depression. The first stage
was from the middle to late E
3
d period (33–22 Ma), and the second
stage was from the late Neogene to the present day (12–0 Ma).
However, not all the E
2
s
1
source rocks in the depression contain
two hydrocarbon generation stages. Moreover, the late stage was
the major hydrocarbon generation stage, with higher rate and quan-
tity. However, relatively lower maturity leads to limited amounts of
generated hydrocarbons. Oils produced from E
2
s
1
source rocks are
low mature and cannot migrate for a long distance and accumulate
in situ therefore. Areas around Qianliyuan, Haitongji, and Gegangji
sags might be the effective accumulation units for oils sourced from
E
2
s
1
source rocks.
FIGURE 13 Calculation of transformation ratios and hydrocarbon generation from E
2
s
1
source rocks in the studied wells, Dongpu Depression.
The hydrocarbon generation curves are similar in the north and south sags with two stages of hydrocarbon generation and dominant by the later
stage, but the hydrocarbon generation in the first stage of well PS6 (c) is obviously weaker than well PS12 (a) and PS13 (b) [Colour figure can be
viewed at wileyonlinelibrary.com]
12 LYU ET AL.
6|CONCLUSIONS
The dark mudstones of E
2
s
1
are significant source rocks in the Dongpu
Depression. Samples have been collected to evaluate their geochemi-
cal features. E
2
s
1
source rocks contain abundant organic matter and
moderate to good generative source rock potential. They consist of
Type II kerogen and mixed with several Type I and Type III kerogen
with the HI value ranging from 16.3 to 1259 mg HC/g TOC. R
o
and
T
max
values suggest that E
2
s
1
source rocks have entered early to
mature stage for hydrocarbon generation. In view of these data, the
E
2
s
1
source rocks have only been buried a limited depth and attained
lower thermal maturity, resulting in finite generated hydrocarbons.
Only the Qianliyuan, Haitongji, and Menggagnji sags reached maturity
with R
o
greater than 0.5%. Numerical modelling of three typical wells
illustrates that there are two stages of hydrocarbon generation for
E
2
s
1
source rocks in the Dongpu Depression and predominated in
the late stage. Besides, the Menggangji Sag, which contains the
highest maturity, only performed the late stage of hydrocarbon gener-
ation. The depth of the oil window varies from 2,700 to 3,100 m. Due
to the limited hydrocarbon generation capacity, oils generated from
E
2
s
1
source rocks only accumulated around the three sags, that is,
Qianliyuan, Haitongji, and Menggangji sags.
ACKNOWLEDGEMENTS
This work is granted by the Important National Science & Technology
Specific Projects (Grants 2016ZX05006‐007 and 2016ZX05006‐003).
We are also grateful for the materials provided by the Zhongyuan
Oilfield Company, SINOPEC Company Ltd.
ORCID
Xueying Lyu http://orcid.org/0000-0002-0688-5503
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How to cite this article: Lyu X, Jiang Y, Liu J, Xu T. Geochem-
ical characteristics and hydrocarbon generation potential of
the first member of Shahejie Formation (E
2
s
1
) source rocks in
the Dongpu Depression, East China. Geological Journal.
2018;1–14. https://doi.org/10.1002/gj.3276
14 LYU ET AL.
