1. No category

Marine and Petroleum Geology 62 (2015) 77e89
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Marine and Petroleum Geology
journal homepage: www.elsevier.com/locate/marpetgeo
Research paper
Diagenesis and reservoir quality evolution of the Eocene sandstones in
the northern Dongying Sag, Bohai Bay Basin, East China
Guanghui Yuan a, b, *, Jon Gluyas b, Yingchang Cao a, *, Norman H. Oxtoby c, Zhenzhen Jia a,
Yanzhong Wang a, Kelai Xi a, Xiaoyan Li a
a
b
c
School of Geoscience, China University of Petroleum, Qingdao, 266580, China
Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK
41 Oaken Lane, Claygate, Esher, Surrey, KT10 0RG, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 October 2014
Accepted 16 January 2015
Available online 27 January 2015
The Eocene sandstones in the northern Dongying Sag, Bohai Bay Basin, China, are reservoirs for large
accumulations of hydrocarbons. The sandstones are mainly lithic arkoses and feldspathic litharenites,
texturally and compositionally immature. These sandstones have a wide range of porosity (0.4e37%) and
permeability (0.004e6969 mD) and show an overall decrease in reservoir quality from 1500 m to 5000 m
below sea level. The reduction in reservoir quality is a product of several digenetic processes; these
include compaction, precipitation of dolomite and calcite in eodiagenetic stage; compaction, feldspar
dissolution, precipitation of quartz cements and clays (kaolin and illite) and precipitation of ferrocalcite
and ankerite in mesodiagenetic stage.
Mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones
originate from sources outside the sandstones. Carbonate cementation, together with compaction
reduced the sandstones’ porosity and permeability significantly. In a sandstone bed, marginal sandstones
with distance to sandstone/mudstone interface less than one meter always have lower porosity than
central sandstones. As burial depth exceeds 4000 m, marginal sandstones have very low porosities (<5%),
indicating that thin sandstone beds (<2 m) were totally destroyed by cementation and compaction, and
only thick sandstone beds (>2 m) can be potential effective reservoirs.
Feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute
porosity. Mineral distribution pattern and quantitative data show that leached feldspars are the internal
source of authigenic quartz and clays in sandstones, and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0.25%. However, although there is
little or no net import of matter to the sandstones, the pore architecture changes dramatically. Primary
macropores are lost as clays and quartz precipitate while the proportion of microporosity increases,
occurring mainly between clay crystals. The overall result is that permeability is significantly degraded.
© 2015 Elsevier Ltd. All rights reserved.
Keywords:
Sandstone diagenesis
Reservoir quality
Carbonate cements
Feldspar dissolution
Dongying Sag
0. Introduction
Reservoir quality is one of the key controls on prospectivity
during petroleum exploration (Taylor et al., 2010). Diagenesis,
which consists of physical effects of mechanical compaction and
chemical effects of mineral dissolution and precipitation, progressively alters porosity and permeability during burial (Bjørlykke and
* Corresponding authors. School of Geoscience, China University of Petroleum,
Qingdao, 266580, China.
E-mail addresses: yuan.guanghui86@gmail.com (G. Yuan), caoych@upc.edu.cn
(Y. Cao).
http://dx.doi.org/10.1016/j.marpetgeo.2015.01.006
0264-8172/© 2015 Elsevier Ltd. All rights reserved.
Jahren, 2012; Taylor et al., 2010; Thyne, 2001). Accurate prediction
of the porosity in shallow, little cemented sandstones can be made
based on the burial history and rock composition (Bjørlykke and
Jahren, 2012; Gluyas and Cade, 1997; Taylor et al., 2010; Thyne,
2001). However, prediction of more deeply buried sandstone is
much more difficult due to the import and export of materials that
related to chemical diagenesis (Bjørlykke and Jahren, 2012; Gluyas,
1997; Gluyas and Witton, 1997; Taylor et al., 2010; Thyne, 2001;
Tournier et al., 2010). Thus understanding quantitative diagenetic
processes, sources of cements and sinks of dissolved minerals in
sandstones are critical for quality prediction of the deeply buried
sandstones (Taylor et al., 2010).
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G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
The Dongying Sag is a prolific oil-producing province in East
China (Guo et al., 2012). Feldspar dissolution pores, carbonate cements, authigenic quartz and clays are common in the Eocene
sandstones in the northern Dongying Sag (Zhang et al., 2014).
However, diagenetic mass transfer into sediments and the impact
on reservoir quality is still the subject for intense debate. Different
opinions on diagenesis of the Eocene sandstones have been proposed: (1) CO2, Ca2þ, Mg2þ and SiO2(aq) were moved from mudstones to sandstones (Wang, 2010; Zhang et al., 2014; Zhong et al.,
2004); (2) Al3þand SiO2(aq) were removed from sandstones and
feldspar dissolution enhanced much porosity during mesodiagenesis stage (Zhu et al., 2007); (3) External carbonates minerals in
adjacent mudstones were the only source of the carbonate cements
in sandstones (Han et al., 2012).
The objectives of this article are to: (1) investigate diagenesis
and reconstruct diagenetic history of the Eocene sandstones in the
northern Dongying Sag; (2) identify sources of the cements and
sinks of the dissolved feldspars in these sandstones; (3) evaluate
controls of different diagenesis on reservoir quality.
1. Geologic background
The Dongying Sag is a sub-tectonic unit lying in the southeastern part of the Jiyang Depression of the Bohai Bay Basin, and
covers an area of 5700 km2 (Fig. 1) (Cao et al., 2014). It can be
further subdivided into five secondary tectonic zones from the
north to the south, namely the Northern Steep Slope zone, Northern Sag zone, Central Anticline zone, Southern Sag zone and the
Southern Gentle Slope zone (Fig. 1BeC) (Guo et al., 2010). The
northern Dongying Sag includes the Northern Steep Slope zone and
the Northern Sag zone (Lijin subsag and Minfeng subsag).
The tectonic evolution of the Dongying Sag is divided into a synrift stage (65.0 Ma 24.6 Ma) and a post-rift stage (24.6 Ma to the
present) (Fig. 2) (Guo et al., 2012). Sediments filled in the Dongying
Sag comprise the Paleogene Kongdian (Ek), Shahejie (Es) and
Dongying (Ed) formations, the Neogene Guantao (Ng) and Minghuazhen (Nm) formations, and the Quaternary Pingyuan (Qp)
Formation (Wang, 2010). The boundary between the Ed and Ng
formations is the main regional unconformity in the Dongying Sag
(Fig. 2) (Guo et al., 2012).
The Eocene Shehejie Formation contains the main source rocks
and reservoir rocks, and is divided into four members, Es1, Es2, Es3
and Es4 (from top to base) (Fig. 2). The main objects of this study are
the Es4 2 to Es3 2 msub-members where organic-rich source rocks
develop. The lower Es4 (Es4 2 ) consists of gray and dark-gray
mudstones, gypsum and halite, and interbedded subaqueous fan
sandstones deposited in semi-deep and deep lacustrine environment; the upper Es4 (Es4 1 ) comprises brown-gray, gray to black
mudstones, shales, dolomites and subaqueous fan sandstone interbeds that were deposited in semi-deep and deep lacustrine
environment. The lower Es3 (Es3 3 ) was deposited in semi-deep and
deep lacustrine environment and is dominated by lacustrine oil
shales, dark-gray mudstones, calcareous mudstones and subaqueous (sublacustrine) fan sandstones; the middle Es3 (Es3 2 ) includes
gray to dark-gray mudstones, calcareous mudstones, and
Figure 1. (A) Location map of the study area showing the sub-tectonic units of the Bohai Bay Basin, depressions in the Bohai Bay Basin of China are Jizhong Depression (Ⅰ), Huanghua
Depression (II), Jiyang Depression (Ⅲ), Bozhong Depression (IV) and Liaohe Depression (V) (Guo et al., 2012). (B) Structural map of the Dongying Sag. The area in the dashed green
line is the study area of this article. (C) NeS cross-section (P0 -P) of the Dongying Sag showing the various tectonic-structural zones and key stratigraphic intervals. (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
79
Figure 2. Generalized CenozoiceQuaternary stratigraphy of the Dongying Sag, showing tectonic and sedimentary evolution stages and the major petroleum system elements
(modified from Guo et al., 2012).
subaqueous (sublacustrine) fan sandstones deposited in semi-deep
and deep lacustrine environment (Guo et al., 2012).
Burial history and thermal history of the northern Dongying Sag
have been analyzed in detail using data from exploration and
production wells and the histories synthesized with the BasinMod
software by previous studies (Guo et al., 2012; Song et al., 2009).
The maximum burial depth of the Shahejie Formation in the
northern Dongying Sag occurs today at approximately 5000 m (Guo
et al., 2012; Song et al., 2009). The present-day geothermal gradient
is around 34 C/km with an average surface temperature of 14 C,
and the present-day maximum temperature is about 180 C at
5000 m (Wang, 2010). According to previous studies, the shales and
mudstones in Es4 2 eEs3 2 have total organic carbon (TOC) content of
0.5%e18.6% with an average of about 5%, and organic matter is
dominated by type-I and II kerogens (Guo et al., 2010; Zhu et al.,
2004). The average vitrinite reflectance (Ro%) varies from 0.35% to
1.5% from 2000 m to 5000 m, indicating source rocks are low
mature to mature (Guo et al., 2012). The northern Dongying Sag
becomes increasingly overpressured with the increasing depth, and
middle-strong fluid overpressure develops commonly in Es4eEs3 2
reservoirs from 2200 m to 5000 m (Cao et al., 2014; Guo et al., 2010;
Zhang et al., 2009).
2. Database and methods
Rock composition data of 831 thin section samples, 309 bulk
rock XRD data and 6335 reservoir porosity and permeability data
were collected from Geological Scientific Research Institute of
China Sinopec Shengli Oilfield Company.
With constraints of the collected data, samples were selected
from the Es4eEs3 drill cores of 25 wells in the northern Dongying
Sag for this study. 250 thin sections and 250 red epoxy resin-
impregnated thin sections were prepared for analysis of rock
mineralogy, diagenesis and visual porosity. Point counts were
performed on thin sections for the content of detrital grains and
carbonate cements with at least 300 points, which can provide a
standard deviation of 5.5% or less (Stroker et al., 2013). For the
content of feldspar dissolution pores, quartz cements and authigenic clays, 20 or 40 micrographs of 37 red epoxy resinimpregnated thin sections were taken firstly using the Zeiss microscope. Objectives of 100 for these thin sections were used, and
each micrograph has an area of 6.45 mm2. Then the target minerals
and pores in each micrograph were identified under the microscope and were drew on computer screen using CorelDRAW, and
the total area of each target mineral and pores in every micrograph
was obtained using Image-Pro Plus software. Finally, the contents
of the target minerals and pores in each thin section were obtained
by taking the average of all values in its micrographs. For mediumcoarse grained sandstones, 20 micrographs were used, while 40
micrographs were used for pebbly sandstones in order to ensure
that the coarse grain size did not produce any sampling bias. As the
authigenic illite is dispersive and it is difficult to do the quantitative
work, only two thin sections with mainly illite were analyzed. 25
SEM samples were identified using a Quanta200 SEM combined
with EDAX Energy dispersive spectroscopy.
Six core samples were prepared as thick doubly polished sections of approximately 100 mm thickness for fluid inclusion petrographic
analysis
and
microthermometric
measurement.
Microthermometry of aqueous inclusions was conducted using a
calibrated Linkam. TH-600 stage. The homogenization temperature
(Th) was obtained by cycling. Th measurements were determined
using a heating rate of 10 C/min when the temperature was lower
than 70 C and a rate of 5 C/min when the temperature exceeded
70 C. The measured temperature precision for Th is ±1 C.
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G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
Based on petrological studies, 29 organic matter-free sandstone
samples were selected for analysis of the carbon and oxygen stable
isotope composition in the carbonate cements. Tightly cemented
sandstones were disaggregated directly with a hammer and then
crushed in a mortar; porous sandstones were disaggregated using
the freezing-heating technique (Wang et al., 2005) to avoid
breaking detrital carbonate grains. Then the disaggregated samples
were sieved to pass through a 200 mesh sieve (75um) to get the
powder. The sample preparation method reduces the possibility of
sampling unwanted detrital carbonates. Isotopic data were obtained using Thermo-Finnigan MAT 253 IRMS online with Gas
BenchⅡ in Durham University. d13C and d18O values were determined on CO2 liberating from carbonate cements samples and the
LAEAC01 standard that were dissolved by 100% H3PO4 at 50 C.
Isotopic composition of CO2 is reported in units of ‰ relative to PEE
Dee belemnite (V-PDB). Replicate analysis is reproducible to ±0.1‰
for both d13C and d18O.
3.2. Sandstone petrology: detrital mineralogy
The studied Eocene sandstones are fine to coarse-grained. The
sandstones are generally texture immature. Sorting ranges from
poor to moderate sorted and roundness of the detrital grains varies
from subangular to sub-rounded.
The sandstones are mostly lithic arkoses and feldspathic litharenites (Fig. 4), compositionally immature with an average
framework composition of Q32F37L31. The detrital quartz grains are
primarily monocrystalline, ranging from 5% to 63% and detrital
feldspars ranges from 4% to 74%. Bulk rock XRD analysis data show
that, the K-feldspar content is 2%e27% with an average of 12% and
the plagioclase content is 2%e40% with an average of 25% (Fig. 4). In
general, the K-feldspar content decreases slightly as burial depth
increases but the plagioclase content shows no significant trend.
The rock fragment content ranges mainly from 3% to 88% (Fig. 4).
3.3. Sandstone petrology: diagenetic mineralogy
3. Results
3.1. Porosity and permeability versus depth
The Eocene sandstones have a wide range of porosity from 0.4 %
to 37% and the permeability ranges from 0.004mD to 6969mD
(measured over the depth interval 1500 me5000 m). In general, the
porosity and permeability of the Eocene sandstones decrease as the
burial depth increases from 1500 m to 5000 m, though anomalously high porosity and permeability exist in some depth intervals
(2800e3300 m, 3300e3700 m and 3900e4400 m respectively)
(Fig. 3A, B). Similarly, both the average and peak total visual
porosity in thin sections also decreases with the increasing depth
(Fig. 3C).
Authigenic minerals in sandstones consist mainly of carbonate
cements, quartz, kaolin, and illite. The authigenic quartz and clays
are usually associated with altered feldspars.
Four types of carbonate cements, dolomite (Fig. 5A), calcite
(Fig. 5B), ferrocalcite (Fig. 5C) and ankerite (Fig. 5D, E) were identified in sandstones. Dolomite and calcite cements occur as
microsparry or sparry interlocking mosaic of crystals with the
crystal size varies from 4 mm to 200 mm, these cements fill primary
pores and replace some detrital grains. In dolomite or calcite
cemented tight sandstones, the cement occupies almost all primary
pores and can account for 25%e30% of the sandstone volume. In
general, dolomite or calcite cemented sandstones are usually supported by detrital grains with just point contacts or have a floating
Figure 3. Core porosity (A) and core permeability (B), total porosity (C) and feldspar secondary porosity (D) in thin sections, the content of kaolin (E) and illite (F) in the Eocene
sandstones in the northern Dongying Sag.
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
81
Figure 4. Rock composition of the Eocene sandstones in the northern Dongying Sag. (Ternary plot refers to sandstone classification standard of Folk et al. (1970)).
texture, indicating little compaction when cementation occurred.
Ferrocalcite and ankerite are common cements in sandstones with
a general content of 0.5e10%. They occur as scattered euhedral
rhombs (10e100 mm), sparry crystals (50e200 mm) and clusters
with no signs of dissolution. In thin sections, ferrocalcite and
ankerite replaces stage-I quartz overgrowths, dolomite (Fig. 5C, D)
and calcite, and some ferrocalcites and ankerites were precipitated
in feldspar dissolution pores (Fig. 5E), leading to the conclusion that
ferrocalcite and ankerite formed after stage-I quartz cementation
and feldspar dissolution.
Feldspar grains in porous sandstones usually contain significant
secondary porosity (Fig. 5E, G, I) that must have developed during
burial stage. The feldspar secondary porosity in thin sections can
reach up to 4% (Fig. 3D). Significant feldspar secondary pores are
Figure 5. Thin section images of sandstones (pore space is shown in red): A, Dolomite filled all intergranular primary pores, feldspar secondary pore with no dolomite; B, Calcite
filled all primary pores; C, Dolomite was replaced by ferrocalcite; D, Dolomite was replaced by ankerite; E, Feldspar secondary pores was filled by some ankerite; F, Relationship
between ankerite and two stages of quartz cements; G- G-Feldspar secondary pores, kaolin and quartz overgrowths; H, Micropores in authigenic kaolin; I Extensive feldspar
dissolution with no dissolution of detrital carbonate grains. Do-Dolomite; Cc-Calcite; Cg-Detrital carbonate grain; Fc-ferrocalcite; An-Ankerite; FD-Feldspar dissolution pore; KKaolin; Qa-Quartz overgrowths. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
always accompanied with quartz cements and clays (Figs. 5G and
6B).
Authigenic quartz is evident in thin sections and SEM samples
(Figs.5F and 6A,B). Quartz cements represent no more than 1% of
the whole rock (Fig. 7C) and occur in forms of mainly quartz
overgrowths (Figs. 5F and 6A) and some small quartz crystals
(Fig. 6B). It is not difficult to discriminate the quartz overgrowths
from the detrital grains with the existence of some dust clay rim on
the grains in thin sections (Fig. 5F,G). Two stages of quartz overgrowths (Fig. 5F) can be identified, and thickness of the overgrowths ranges mainly from 2um to 50um. The texture relationship
between ankerite and two stages of quartz overgrowths indicates
that the stage-II quartz cementation occurred after ankerite
cementation (Fig. 5F).
Kaolin and illite are the two most important types of authigenic
clays in the Eocene sandstones. Kaolin mainly occurs as euhedral
booklets and vermicular aggregates filling primary pores within the
sandstones (Fig. 6C, D). Kaolin aggregates are rich in intercrystalline
microporosity (Figs. 5H and 6C, D). SEM observation revealed that
kaolin aggregates are composted of closely associated, thin
kaolinite platelets (Fig. 6C), and thicker, euhedral blocky crystals of
dickite (Fig. 6D). However, the transformation of kaolinite to dickite
in these sandstones still needs further research. Illite occurs as
fibrous crystal mainly in primary pores (Fig. 6E, F) and sometimes in
feldspar secondary pores. XRD data of clay minerals in the <2um
fractions of sandstones show that kaolin dominates in reservoirs
above 3100 m (T < 125 C) (Fig. 3E), and illite dominates in sandstones with depth deeper than 3100 m (T > 125 C) (Fig. 3F), which
can also be verified by the fabric in thin sections and SEM
(Fig. 6CeF).
In the section above we have considered variations in authigenic
mineral content largely in terms of regional trends associated with
increased burial depth. There are however variations on smaller
scales which correlate with lithological variations in individual
wells. For example, from marginal part to central part of sandstone
beds, the content of the carbonates cements in sandstones decreases sharply (Fig. 7A). In contrast to the carbonate cements, the
marginal sandstones usually contain few feldspar secondary pores
(0e1%), quartz cements (0e0.3%) and authigenic clays (0e1.5%),
and the central sandstones in thick beds can have more feldspar
secondary pores (1e2.5%), quartz cements (0.3e0.5%) and clays
(1e3%) (Fig. 7BeD).
3.4. Fluid inclusions
Aqueous inclusions can provide valuable information for the
precipitation temperature of authigenic minerals (Robinson and
Gluyas, 1992). The aqueous inclusions in samples occur primarily
along the healed micro-fractures in quartz grains (Fig. 8A) and
some aqueous fluid inclusions occur in quartz overgrowths
Figure 6. SEM images of sandstones. A, Quartz overgrowths and illite; B, Feldspar secondary pores and quartz crystals; C-Authigenic kaolinite in primary pores; D, Kaolinite and
dickite in sandstones; E, Transition of kaolinite to illite; F, Illite is precipitated on ankerite. Q-Quartz grain, FG-Feldspar grain, FD-Feldspar dissolution pores, An-Ankerite, K-Kaolinite,
Dick-Dickite, I-Illite; Qa-Quartz overgrowths, Qc-Quartz crystals.
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
83
Figure 7. The relationship between the content of carbonate cements, feldspar secondary pores, quartz cements, clays in clean medium sandstones-pebbly sandstones and the
distance to mudstone/sandstone interface.
(Fig. 8B). The aqueous inclusions have a diameter about 2e10 mm,
and most of them are two phase inclusions and have gas bubbles at
room temperature.
The measured homogenization temperatures (Th) of the
aqueous fluid inclusions from this study and previous studies (Han
et al., 2012; Guo et al., 2012) are shown in Table 1. Figure 9 presents
the distribution of the homogenization temperature of the aqueous
inclusions in healed microfractures in quartz grains, quartz
overgrowths and carbonate cements. The aqueous inclusions in
healed microfractures yield Th rangs mainly from 95 C to 125 C
and 150 Ce180 C. Th of the aqueous inclusions in quartz overgrowths ranging mainly from 100 C to 120 C and from 160 C to
180 C, suggests two stages of quartz cementation in mesodiagentic
stage, which is in consistent with thin section observation (Fig. 5F).
Th of the aqueous inclusions in carbonate cements by previous
studies ranges from 115 C to 140 C (Han et al., 2012; Guo et al.,
Figure 8. Photomicrographs of aqueous inclusions under transmitted light observed at room temperature in sandstones from the northern Dongying Sag. (A) Aqueous inclusions
along healed microfractures in quartz grains. (B) Aqueous inclusions in quartz overgrowths. AI-Aqueous fluid inclusions, Q-Quartz grains, Qa-Quartz overgrowths.
Table 1
Microthermometric data of the aqueous fluid inclusions in the Eocene sandstones in the northern Dongying Sag. Th: homogenization temperature; e: no available data.
Well
Strata
Depth, m
Aqueous inclusions in healed microfractures in quartz grains
Aqueous inclusions in quartz overgrowths
Data source
Size, um
Th, C (Number)
Size, um
Th, C (Number)
Tuo762
Tuo762
FS1
F8
F8
Tuo719
Es4 1
Es4 1
Es4 2
Es4 2
Es4 2
Es4 1
3438.0
3451.0
3684.9
4201
4055.35
3562.1
4e10
4e8
4e12
3e9
2e8
4e13
98e125 (12), 166e179 (7)
110e130 (8), 165e170 (1)
90e125 (10),180e185 (2)
110e125 (6), 155e170 (25)
130e180 (13)
100e110 (10)
e
7
4e10
6
3e8
5e8
e
113e117
110e125
164 (1)
140e170
100e125
Yan22-22
FS3
Es4 1
Es4 2
3403
4867
e
e
e
e
e
e
110e115 (2)
155 (2)
Wang, 2010.
Tuo731
Tuo711
FS10
FS1
Es3 2
Es3 3
Es4 2
Es4 2
2944.0
3195.6
4263.5
4323
e
e
e
e
103e118
114e130
141e169
156e179
e
e
e
e
e
e
e
160e165 (1)
Guo et al., 2012.
(14)
(16)
(29)
(14)
This study
(2),165e170 (1)
(6)
(5)
(3), 175e180 (1)
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G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
Figure 9. Histograms of homogenization temperature (Th) for aqueous inclusions in healed microfractures of quartz grains, quartz overgrowths and carbonate cements from the
Eocene sandstones in the northern Dongying Sag. AI: Aqueous inclusions.
2012), also reveals that the ferrocaltcite and ankerite formed after
the stage-I quartz cements and before stage-II quartz cements,
concluding similarly with the petrography texture relationship
(Fig. 5F).
3.5. Isotopic composition of carbonate cements
Isotopic composition of carbonate cements was measured in 29
sandstone samples, and the types and contents of carbonate cements in samples were counted with thin sections (Table 2). Most
dolomite and calcite have a relative wide range of d18O values
from 7‰ to 13‰ and d13C from þ2‰ to þ7‰. Ferrocalcite and
ankerite have a range of d18O values from 15‰ to 19‰ and d13C
from 7‰ to þ2‰. Samples contain a mixture of these cements
show intermediate values.
4. Discussion
4.1. Diagenetic sequences
The relative timing of the major diagenetic sequence of the
Eocene sandstones in the northern Dongying Sag, which has been
determined from thin sections and SEM examination, is based on
texture relationship (Figs. 5, 6, 10). In summary, the dominant
eogenetic features in the Eocene sandstones are the compaction
and the precipitation of dolomite and calcite. Subsequent mesogenetic processes experienced by these sandstones include (1)
compaction, (2) feldspar dissolution/precipitation of quartz cements and kaolin, (3) precipitation of ferrocalcite and ankerite, (4)
illitization of kaolin/feldspar dissolution/precipitation of quartz
cements and illite.
With constraints of the isotopic composition, the homogenization temperature (Th) of the aqueous fluid inclusions in cements,
and burial-thermal history of well Fengshen-1 in the northern
Dongying Sag (Song et al., 2009), the diagenetic history of the
Eocene sandstones can be summarized in Figure 10.
4.2. Sources of carbonate cements in sandstones
Detrital carbonate grains in the studied sandstones show no
signs of dissolution (Fig. 5I) (Cao et al., 2014), and the distribution
pattern of carbonate cements (Fig. 7A) suggests external sources for
these cements (Thyne, 2001).
The average d18OPDB value (0.85‰) of lacustrine sedimentary
dolomite in the Es4 Formation in the Dongying Sag (Liu, 1998)
suggests d18OSMOW value of lake water to be 4.8‰ with a water
temperature of 10 C (Matthews and Katz, 1977). With the
d18OSMOW value of 4.8‰ of pore water at eodiagenetic stage, and
different oxygen isotope fractionation factor for dolomite-water
(Matthews and Katz, 1977) and calcite-water (Friedman and
O'Neil, 1977), calculated precipitation temperatures for the dolomite and calcite cements in sandstones range mainly from 30 C to
70 C (Table 2). These dolomite and calcite have relative positive
d13C values between þ2‰ and þ7‰ (Table 2), as interbedded
mudstones contain much organic matter (TOC up to 18.6%) (Guo
et al., 2010), fermentative degradation of the organic matter must
be an important carbon source of these cements (Curtis, 1978). Bulk
rock XRD data show that about half of the mudstones in the
Es3eEs4 formations contain more than 30% carbonate minerals
(Qian et al., 2009). When some of these carbonates in adjacent
mudstones were dissolved by the CO2 generated by bacterial
fermentation (Dutton, 2008), the dissolved masses could be
transported into sandstones and provide carbon for the carbonate
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
85
Table 2
Mineralogical and isotopic composition of carbonate cements, and calculated formation temperature of cements in the Eocene sandstones in the northern Dongying Sag. Cacalcite, Do-dolomite, Fc-ferrocalcite, An-ankerite.
Well
Yong925
Yanxie21
Yan22_22
Tuo73
Yong924
Tuo717
Tuo76
Yan222
Yong928
Yan23
Tuo762
Tuo121
Tuo125
Yong92
Fengshen3
Yong920
Feng8
Yan22_22
Yan22_22
Tuo718
Yong925
Tuo767
Tuo720
Yong924
Tuo168
Tuo75
Tuo764
Tuo714
Fengshen4
Depth (m)
2545.30
3045.40
3490.60
3371.45
2866.80
2995.04
2938.12
4074.48
3756.30
3672.85
3439.13
2111.30
2462.64
2972.72
4867.00
3370.90
4197.91
3350.25
3375.30
3270.20
2496.05
3824.80
3535.00
2892.01
3110.10
2422.23
4166.10
2843.06
4476.15
Strata
Es3 3
Es4 1
Es4 1
Es4 1
Es4 1
Es3 2
Es3 2
Es4 1
Es4 1
Es4 1
Es4 1
Es3 3
Es4 1
Es4 1
Es4 2
Es4 1
Es4 2
Es4 1
Es4 1
Es3 3
Es3 3
Es4 1
Es3 3
Es4 1
Es4 1
Es4 1
Es4 1
Es3 2
Es4 2
Carbonate minerals
100%An
30%Ca þ 30%Fc þ 40%An
100%Ca
80%Ca þ 20%Do
40%Ca þ 40%Do þ 20%An
90%Ca þ 10%Do
100%Do
80% An þ 20Fc
70%An þ 30%Do
100%Do
100%Do
100%Ca
100%Do
80%Ca þ 20%An
100%An
90%An þ 10%Ca
80%Do þ 20%An
20%An þ 60%Fc þ 20%Ca
40%An þ 30%Fc þ 30%Do
100%Do
80%An þ 20%Do
95%Do þ 5%Ca
90%Do þ 10%An
20%Fc þ 80%An
70%Ca þ 30%Do
100%Do
80%Ca þ 20Do
100%Do
100%Do
Carbonate
cement
content, %
d18O-PDB
(‰)
d13C-PDB
( ‰)
d18OSMOW ¼ 4.8‰
Temp ( C)
Temp.( C)
d18OSMOW ¼ 3‰
Temp. ( C)
d18OSMOW ¼ 0‰
7
6
30
10
8
5
25
6
7
26
25
20
15
10
6
10
15
15
14
20
14
15
15
8
15
25
20
5
15
16.8
14.6
12.8
10.0
12.9
9.8
8.2
16.5
13.5
10.7
9.6
11.1
9.1
13.3
18.3
18.4
13.5
16.7
14.9
9.8
15.9
10.8
7.9
18.3
9.8
5.6
11.3
12.0
10.3
0.4
1.4
1.3
3.7
2.1
2.7
7.1
2.8
0.7
5.7
3.7
3.3
2.1
2.3
6.5
2.1
4.3
3.8
2.8
3.6
1.4
5.3
7.1
1.9
2.0
6.3
5.4
5.7
7.2
103
e
57
40
e
39
43
100
e
58.
51
46
49
61
116
118
75
e
e
52
95
59
42
117
e
30
48
66
56
120
e
70
51
e
49
54
117
e
69
62
58
59
74
136
137
89
e
e
63
111
70
52
136
e
39
60
79
67
154
e
94
71
e
69
72
150
e
91
82
80
79
99
174
175
114
e
e
84
142
92
70
175
e
55
81
102
87
Note: Only isotopic data of samples with the content of one specific type of carbonate up to 80% were used to calculate the temperature. The equations used for fractionation
between carbonates and water are: 1000lnacalcite/ferrocalcite-water ¼ 2.78*106/T2 e 2.89 (Friedman and O'Neil, 1977) and 1000lnadolomite/ankerite-water ¼ 3.06*106/T2 e 3.24
(Matthews and Katz, 1977).
cements in sandstones, and the d13C values (þ3‰ to þ10‰) (Liu,
1998) of these carbonates in mudstones also suggest that they
can be another carbon source.
Previous studies show that pore-water becomes isotopically
heavier with increasing burial as isotopic composition of the porewater was modified by reactions of feldspars alternation (Fayek
et al., 2001; Savin, 1980). Th of the aqueous inclusions in ferrocalcite and ankerite shows that these latter carbonate cements
were precipitated from 115 to 140 C (Fig. 9). With the oxygen
isotope fractionation factor for dolomite-water of Matthews and
Katz (1977) and calcite-water of Friedman and O'Neil (1977), precipitation temperatures of ankerite and ferrocalcite were calculated
with assuming d18OSMOW values of 4.8‰, 3‰ and 0‰. The results show that, when d18OSMOW value of the pore water is 3‰,
the precipitation temperature ranges mainly from 115 C to 137 C
(Table 2), which is consistent with the homogenization temperature of aqueous fluid inclusions in carbonate cements. As feldspars
in the studied sandstones were dissolved extensively, evolution of
the d18OSMOW value of the pore water from 4.8‰ to 3‰ is
acceptable. Ankerite and ferrocalcite have relative negative d13C
values from 7‰ to þ2‰ (Table 2), which indicate that decarboxylation of the organic matter in mudstones must be one
important carbon source (Curtis, 1978). Experienced relative high
temperature, large amounts of organic acids and CO2 from thermal
evolution of organic matter can dissolved some carbonates in
source rocks (Dutton, 2008), which may be another source when
the carbon was imported into the sandstones. Thus, the d13C values
(7‰ e þ2‰) probably represent a mixture of carbon from
decarboxylation of organic matter and dissolution of carbonates in
source rocks.
After deposition, porosities of the Eocene mudstones in the
northern Dongying Sag generally evolved from nearly 60% to less
than 10%e15% at present (Zhang et al., 2009). During such a period,
large amounts of advective compaction fluids were expelled from
mudstones to sandstones (Bjørlykke, 1993; Bjørlykke and Jahren,
2012). In Es4 2 -Es3 2 sub-members, the concentration of Ca2þ and
Mg2þ in pore water is up to 26,000 ppm (Cao et al., 2014), the high
concentration of Ca2þ and Mg2þ in pore water promised the
massive transfer of these masses from mudstones to adjacent
sandstones. Dissolution of some anorthite in sandstones may provide some Ca2þ, but the amount is not important as only less than
4% feldspars were dissolved in the sandstones (Milliken et al., 1994).
4.3. Sources of authigenic clays and quartz in sandstones
Precipitation of authigenic quartz, kaolin and illite occurred in
the mesodiagenetic stage (Fig. 10). Unlike the Ca2þ and Mg2þ, the
concentration of SiO2(aq) (<100 ppm) and Al3þ (<10 ppm) is
extremely low in deep buried sediments (Bjørlykke and Jahren,
2012; Ronald and Edward, 1990), with constraints of water volume and considerable heterogeneity in porosity and permeability,
none of the advective flow, thermal convection or diffusion can take
responsible for long distance and massive transfer of external
SiO2(aq) and Al3þ into sandstones (Bjørlykke et al., 1988; Bjørlykke
and Jahren, 2012), particularly in geochemical systems where
overpressure develops. Also, the distribution patterns of such cements (Fig. 7C, D) in sandstones do not support that they originate
from external sources out of sandstones.
With no external source, the most possible source for authigenic
clays should be the dissolution of feldspars within the sandstones
86
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
Figure 10. Burial, thermal and diagenetic history of the Eocene sandstones in the northern Dongying Sag (Burial and thermal history of well Fengshen-1 were modified from Song
et al., 2009).
(Giles and De Boer, 1990; Higgs et al., 2007). On the thin section
scale, we can identified that a positive relationship exists between
feldspar secondary pores and byproducts, that is little feldspar
secondary pores are commonly accompanied with little byproducts
nearby and massive feldspar secondary pores with massive
byproducts in nearby primary pores (Figs. 5G and 6B). The amount
of clay in thin sections increases linearly with the increase of
feldspar secondary porosity (Fig. 11A). The perfect positive linear
relationship between the amount of feldspar secondary pores and
clays suggest that the dissolution of feldspars is the source of the
authigenic clays in sandstones.
In sandstones with quartz grains as dominant clastic component
and with much quartz cements (1e30%), quartz dissolution at grain
contacts, quartz dissolution along stylolites, feldspar dissolution
were suggested to be internal silica sources (Tournier et al., 2010;
Walderhaug, 2000). As the studied sandstones experienced minimal pressure dissolution, the most likely source for quartz cements
should be the internal dissolution of feldspars. Indeed, there is a
positive correlation between the quantity of quartz cement and the
number of partially dissolved feldspars (Fig. 11B). At the same time,
the quartz cement content increases slightly as burial depth increases (Fig. 13), which suggests high temperature and deep burial
probably promote its development (Walderhaug, 2000).
All in all, the petrography texture relationship, the distribution
pattern and the quantitative data all suggest that feldspar dissolution is the internal source for authigenic clays and quartz in
sandstones. And the distribution pattern of authigenic minerals in
the northern Dongying Sag suggests that during the diagenetic
processes, when the temperature was below 125 C
(depth < 3100 m), feldspar dissolution was accompanied by precipitation of kaolin and quartz. When the temperature exceeded
125 C, illite became more stable than kaolin, products of feldspar
dissolution was precipitated as illite and quartz, and kaolin formed
at lower temperature was also transformed to illite (Chuhan et al.,
2001; Franks and Zwingmann, 2010; Lander and Bonnell, 2010).
4.4. Diagenetic control on reservoir quality
The porosity versus permeability diagrams of different lithology
show that with similar burial depth and cement contents, medium
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
Figure 11. Relationship between the content of feldspar secondary pores, authigenic
clays and quartz cements in the Eocene sandstones of the northern Dongying Sag.
sandstones, coarse sandstones and pebbly sandstones have the best
properties and permeability (Fig. 12C), followed by siltstones and
fine sandstones (Fig. 12B), and shaley sandstones and fine-grained
conglomerates own the worst properties and permeability
(Fig. 12A, D).
When it comes to single lithology, the reservoir properties
(porosity and permeability) of each lithology still show
87
considerable heterogeneity (Fig. 12). This heterogeneity must be
induced by the sandstone's complex diagenetic history, which
resulted in various types of diagenetic alterations that controlled
the sandstone's porosity and permeability (Salem et al., 2005).
Figure 12 show that for each lithology, the porosity and permeability of sandstones with similar cement content generally
decrease as the burial depth increases, indicating compaction can
damage the reservoir quality. Also, the porosity and permeability of
sandstones with similar depth generally decrease as the content of
carbonate cements increases, indicating cementation processes
with external mass sources can reduce porosity and permeability
significantly through occupation of pore space in sandstones
(Fig. 12).
The difference between feldspar secondary porosity and the
sum of authigenic quartz and clays in the sandstones is usually less
than 0.25% (Fig. 13), which means with the sandstones themselves
as sources and sinks of chemical diagenetic processes, these processes have little impact on absolute reservoir porosity. However,
although there is little or no net import of matter to the sandstones,
the pore architecture changes dramatically. Primary macropores
are lost as clays and quartz precipitate while the proportion of
microporosity increases, occurring between clay crystals (Nadeau
and Hurst, 1991) and within the partially dissolved remains of
feldspars. The overall result is that permeability is significantly
degraded (Yuan et al., 2013).
Compaction and carbonate cementation mainly control the
porosity evolution of the sandstones in the northern Dongying Sag.
The introduction of masses from external sources (adjacent mudstones) to sandstones resulted in extensive carbonate cementation
in marginal sandstones, and this thickness of these marginal
cemented sandstones (with cements more that 10e15%) can reach
up to one meter (Figs. 7 and 14), leading thin sandstone beds to be
tightly cemented totally.
Figure 12. Core porosity verse permeability diagrams of the Eocene sandstones with different carbonate cements, different lithology and at different depth in the northern
Dongying Sag.
88
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
Figure 13. The vertical distribution of the content of feldspar secondary pores, clays and quartz and the difference between secondary porosity and the byproducts in the Eocene
sandstones in the northern Dongying Sag. 62.5% was used in the calculation as kaolin generally own 25%e50% microporosity (Nadeau and Hurst, 1991).
From 2000 m to 5000 m, for central sandstones in thick sandstone beds (>2 m), the porosity reduced by cementation is usually
less than 10%, and compaction usually reduce more porosity than
carbonate cementation. Porosities of such central sandstones can
still be maintained to be more than 10% when the burial depth is
deeper than 4000 m (Fig. 14), which are higher than the porosity
cutoff (5%e6%) of effective hydrocarbon reservoirs in the study area
(Wang, 2010). While for marginal sandstones, their porosities are
usually much lower than that of central sandstones in thick beds.
With extensive carbonate cementation (>15% carbonate cements)
and increasing compaction, when burial depth exceeds 4000 m,
porosities of most marginal sandstones with distance to sandstone/
mudstone interface less than one meter are lower than 5% (Fig. 14),
which indicates that thin sandstone beds (<2 m) have been totally
destroyed by extensive cementation and compaction, and only the
thick beds can be potential effective hydrocarbon reservoirs.
5. Conclusion
Figure 14. Ternary plot of core porosity, volume of carbonate cements and the porosity
reduced by mechanical compaction in medium sandstones e pebbly sandstones with
initial porosity of 40%. ‘D’ represents the distance from sandstone samples to the
sandstone/mudstone interface. Note that destruction of original porosity by mechanical compaction and cementation are different for sandstones with different position in
the sandstone beds.
(1) The porosity and permeability of the Eocene sandstones in
the northern Dongying Sag are of considerable heterogeneity, with a wide range of porosity from 0.4 % to 37% and
permeability from 0.004mD to 6969mD.
(2) The Eocene sandstones in the northern Dongying Sag experienced compaction and precipitation of calcite and dolomite
in eodiagecetic stage, and in mesodiagenetic stage, main
chemical processes are feldspar dissolution, precipitation of
quartz, kaolin, illite, ferrocalcite and ankerite, and illitization
of kaolin.
(3) Different chemical diagenetic processes have different
impact on reservoir quality. With external mass sources,
carbonate cementation reduced considerable porosity and
permeability of the sandstones, particularly for sandstones at
the margin of sandstone beds. While as dissolution of feldspars provided just the internal source for precipitation of
authigenic quartz and clays in sandstones, these chemical
diagenetic processes have little impact on porosity. But
because grains are replaced by platy or fibrous clays,
permeability can be significantly reduced.
(4) Compaction and carbonate cementation mainly control the
quality evolution of the sandstones together, and their
impact is different for sandstone with different position in a
sandstone bed. Porosities of marginal sandstones are always
G. Yuan et al. / Marine and Petroleum Geology 62 (2015) 77e89
much lower than that of central sandstones in sandstone
beds. In deeply buried layers (>4000 m), thin sandstone beds
are of poor quality due to extensive cementation and
compaction, and only the thick sandstone beds (>2 m) can be
potential effective hydrocarbon reservoirs.
Acknowledgments
This study was financially supported by the Natural Science
Foundation of China Project (No. U1262203), the National Science
and Technology Special Grant (No. 2011ZX05006-003), the National Basic Research Program of China (973 Program)
(No.E1401004B), and Foundation for the Author of National Excellent Doctoral Dissertation of PR China. Thanks are also given to the
following individuals and institutions: Wen L. of China University of
Petroleum; Dr. Macpherson C.G. and Peterkin J. of Durham University; Shengli Oilfield Company of Sinopec provided all the
related core samples and some geological data of Dongying Sag.
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