fluid mixing induced by hydrothermal activity in the ordovician carbonates in tarim basin, china
TRANSCRIPT
Fluid mixing induced by hydrothermal activity in theordovician carbonates in Tarim Basin, China
L. J IANG1 , 2 , W. PAN3 , C. CAI1 , L . J IA1 , L . PAN4 , T . WANG1, H. LI1 , S . CHEN1 , 5 AND Y.
CHEN1
1Key Lab of Petroleum Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China;2Department of Geology and Geophysics, School of Environmental Sciences, University of Liverpool, Liverpool, UK; 3Tarim
Oilfield Company, PetroChina, Korla, Xinjiang, China; 4PetroChina Hangzhou Research Institute of Geology, Hangzhou,
China; 5Energy Resource Department, China University of Geosciences, Beijing, China
ABSTRACT
Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into
Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteris-
tics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We pres-
ent a genetic model resulting from the study of d13C, d18O, d34S, and 87Sr/86Sr isotope values and fluid
inclusions (FIs) from fracture- and vug-filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from
Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of
hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10°C
higher than the paleo-formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The
mixing of hot (>200°C), high-salinity (>24 wt% NaCl), 87Sr-rich (up to 0.7104) hydrothermal fluid with cool (60–
100°C), low-salinity (0 to 3.5 wt% NaCl), also 87Sr-rich (up to 0.7010) meteoric water in the Ordovician unit
was supported by the salinity of fluid inclusions, and d13C, d18O, and 87Sr/86Sr isotopic values of the diagenetic
minerals. Up-migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot
brine with high sulfate concentrations which promoted thermochemical sulfate reduction (TSR) in the Ordovician,
resulting in the formation of 12C-rich (d13C as low as �13.8&) calcite and 34S-rich (d34S values from 21.4& to
29.7&) H2S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Ta-
rim Basin was facilitated by deep-seated faults and up-reaching faults due to the pervasive Permian magmatic
activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR, and faulting may have locally dissolved the
host carbonates and increased the reservoir porosity and permeability, which has significant implications for
hydrocarbon exploration.
Key words: carbonates, dolomitization, hydrothermal fluids, meteoric water, Tarim Basin, thermochemical sulfate
reduction
Received 30 July 2014; accepted 20 November 2014
Corresponding author: Prof Dr Chunfang Cai, Key Lab of Petroleum Resources, Institute of Geology and Geo-
physics, Chinese Academy of Sciences, Beijing 100029, China.
Email: [email protected]. Tel: +86 108 299 8127. Fax: +86 106 201 0846
Geofluids (2014)
INTRODUCTION
In the past few decades, the question of hydrothermal
activity due to tectonic movements has gained increasing
attention and interest by petroleum geologists in carbonate
reservoir exploration (Qing & Mountjoye 1994; Lavoie
et al. 2005; Davies & Smith 2006; Saller & Dickson
2011). This is because not only the hydrothermal activity
itself can cause faults and fractures that improve the
reservoir quality, but also the hydrothermal fluid mixing
and hydrothermal dolomitization could either dissolve cal-
cite or led to replacement of calcite by dolomite that both
created secondary porosity (Corbella et al. 2004; Lavoie
et al. 2005; Smith 2005; Davies & Smith 2006; Wendte
2006; Wendte et al. 2009; Lapponi et al. 2014). Lower
Ordovician carbonates in the Tarim Basin represent an
important hydrocarbon reservoirs associated with karstifica-
tion (most commonly in the Yinshan Formation-O1-2y),
© 2014 John Wiley & Sons Ltd
Geofluids (2014) doi: 10.1111/gfl.12125
thermochemical sulfate reduction (TSR), and dolomitiza-
tion (most commonly in the Penglaiba Formation-O1p)
(Cai et al. 2001a,b, 2009b; L€u et al. 2008; Yang et al.
2012). Significantly, hydrothermal minerals such as calcite,
saddle dolomite, fluorite, barite, anhydrite, and quartz have
been found to occur both in the subsurface cores of Taz-
hong and Tahe areas (Cai et al. 2008; Li et al. 2011; Yang
et al. 2012) and in outcrops of NW Tarim region in the
Ordovician strata (Xing & Li 2012; Dong et al. 2013;
Zhang et al. 2014), as supported by their high homogeni-
zation temperatures in fluid inclusions, positive Eu anoma-
lies as well as characteristic d18O, d13C, and 87Sr/86Sr
isotopic values of the deep burial diagenetic minerals. Pre-
vious studies have argued that hydrothermal fluids during
this period may have been related to Permian magmatic
activity based on the detailed petrological and geochemical
evidence (Cai et al. 2008; L€u et al. 2008; Xing & Li
2012; Dong et al. 2013; Zhang et al. 2014).
However, influx of meteoric water also occurred locally
in the Ordovician strata during hydrothermal fluid move-
ment, leading to a significant decrease in the paleo-water
salinity and Mn abundance (Cai et al. 2008; Li et al.
2011; Zhang et al. 2014), as well as an increase in the
depth of karstification (L€u et al. 2008). Thus, fluid mixing
during the Permian hydrothermal activity in Tarim Basin
was a complicated chemical and physical process, which
not only altered the hydrocarbon properties (Xing & Li
2012) but also significantly affected the reservoir porosity
(L€u et al. 2008; Yang et al. 2012). Although hydrothermal
diagenetic minerals in Ordovician strata in Tarim Basin
have been reported in previous studies, there is no consen-
sus on the origin and geochemical composition of these
hydrothermal fluids, and there is no comprehensive model
to explain all the observations.
In this study, detailed geochemical and petrological
analyses have been performed on the deep burial diagenetic
minerals in Ordovician strata both from subsurface cores in
Tazhong area and from outcrop samples in Taxibei area, in
an attempt to determine the origin of fluids and fluid flow
and mixing during this hydrothermal event. Specifically, we
have addressed the following questions:
(1) What are the petrological and geochemical characteris-
tics of diagenetic minerals during the hydrothermal
event in the Ordovician strata in the Tarim Basin,
China?
(2) What is the model for fluid mixing during the hydro-
thermal event?
(3) What are the implications for the hydrothermal event
on the reservoir quality of carbonate rocks from Ordo-
vician strata?
GEOLOGICAL SETTING
The Tarim Basin is located in the northwest of China
(Fig. 1A). The detailed geology of the Tarim Basin has
been reported in Cai et al. (2009a). Briefly, during the
Cambrian and Ordovician, the study area was located on a
stable shallow marine carbonate platform to deep basin
environment and resulted in the deposition of up to
2000 m carbonates and mudstone, including platform
(A) (B)
Fig. 1. Map of the Tarim Basin showing tectonic units, locations of sampled wells and outcrops (A); lithology of Cambrian to Ordovician strata in the Tarim
Basin (B).
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2 L. JIANG et al.
facies limestone and dolomite, slope facies limestone and
marlstone, and basinal facies mudstone, shale, and marl-
stone (Fig. 1B). During the Silurian to Carboniferous, the
sea level decreased and the Tarim Basin evolved to
undergo deposition of a marine sandstone and mudstone
sequence. The study area had further evolved to result in
the deposition of lacustrine sediment and volcanic rock
during the Permian, and fluvial sandstone and mudstone
during the Mesozoic and Cenozoic.
Several orogenies have led to formation of multiple un-
conformities and resulted in much missing section in the
Tarim Basin. For example, the Middle and Upper Ordovi-
cian in the eastern Central Tarim and Taxibei areas were
completely removed by erosion during the late Caledonian
orogeny at the end of Ordovician. There are no Devonian
or Silurian strata remaining in the majority of the Taxibei
area due to the early Hercynian orogeny at the end of the
Devonian. Moreover, the late Yanshanian orogeny, during
the Neogene, resulted in either no sedimentation or com-
plete removal of the Cretaceous in the Southwest Depres-
sion, eastern Central Tarim. These orogenies, resulting in
fracturing and tilting, have been linked to large-scale fluid
cross-formational flow in the Tarim Basin.
Notably, intense volcanic activities have extensively taken
place in Tarim Basin as a result of convergent subduction
of Middle Tianshan arc to the north in Early Permian
(Chen et al. 1997; Tang et al. 2004). At the end of Perm-
ian, the further amalgamation led to an extensive uplift
and subsequent long-term subaerial exposure and erosion
of the Tarim Basin. The presence of alkaline basalt, dia-
base, and granite intrusions led to invasion of the hydro-
thermal fluids and the growth of hydrothermal minerals
(e.g., fluorite, quartz, barite, saddle dolomite, and calcite)
in the Upper Cambrian to Ordovician strata in study area
(Pan et al. 2009; Yang et al. 2012; Dong et al. 2013).
The burial and geothermal histories of different tectonic
units of the Tarim Basin have been reported previously by Ye
(1994) and Cai et al. (2009a) and references therein. Briefly,
the Ordovician strata in Tazhong area have been buried to
>4000 m, with paleo-temperatures up to about 140°C(Fig. 2A,). In contrast, the burial history of Taxibei outcrops
area shows that the Ordovician strata were buried to about
4000 m during middle Permian before the whole section
was uplifted to the surface in early Triassic (Fig. 2C), and
the maximum paleo-temperature was <140°C (Fig. 2B).
METHODS
A total of 120 carbonate rock samples, with some associ-
ated with anhydrite, barite, fluorite, pyrite, and elemental
sulfur samples from the Ordovician unit, were collected
from Keping, Qingsong, Dawangou, Qieditage, Tuopu-
lang, and Xike’er outcrops in NW Tarim Basin and from
subsurface cores in TZ3, TZ12, TZ161, TZ162, TZ243,
TZ45, TZ62, TZ826, and ZG12,ZG171, ZG 431,
ZG442H, ZG45, ZG51, and ZG9 wells in the Central Ta-
rim Basin (Fig. 3). Selected samples were used for petrol-
ogy and geochemical analyses, including transmitted
microscope and fluid inclusion analyses, carbon, oxygen,
and strontium isotopic analyses. Finely polished and etched
slabs and thin sections were stained with Alizarin Red S
and potassium ferricyanide to distinguish calcite from dolo-
mite and their ferroan equivalents.
Fluid inclusions in double-polished wafers were studied
using a Linkam THMSG 600 heating–cooling stage con-
nected to Zeiss petrographic microscope which was used
(A) (B)
Fig. 2. Burial histories of the Ordovician sediments in different tectonic units: (A) from well TZ29 in Tazhong area and (B) from outcrops in Taxibei area.
© 2014 John Wiley & Sons Ltd
Hydrothermal fluid in ordovician carbonates in Tarim Basin 3
to obtain temperature data. UV fluorescence was per-
formed on these doubly polished wafers to determine
whether they were oil or aqueous inclusions, to identify
inclusion types for further analysis, and to determine their
relationship to the host minerals. Instrumental precision is
�0.1°C. Ice melting temperatures were converted to salin-
ity using standard equations (Oakes et al. 1990; Bodnar
2003).
Fine powder samples of calcite and dolomite were
extracted using a dentist’s drill and subject to carbon and
oxygen isotopes analyses. About 30–50 mg of drilled out
sample was reacted overnight with 100% phosphoric acid
at 25°C under vacuum to release CO2 from the carbonate
minerals. The CO2 was then analyzed for carbon and oxy-
gen isotopes on a Finnigan MAT251 mass spectrometer
standardized with NBS-18. All carbon and oxygen data are
reported in units per mil relative to the Vienna Peedee Bel-
emnite (VPDB) standard, respectively. The precision for
both d13C and d18O measurements is �0.1&.
About 60 mg drilled samples were used for strontium
isotope analyses. Calcite was leached in 0.5 M sub-boiling
distilled acetic acid at room temperature for 4 h, and dolo-
mite was dissolved in 3.4 M acetic acid at 60°C for 24 h,
respectively (Jiang et al. 2013). Anhydrite samples and bar-
ite samples were digested with 2.5 M HCl at 90°C for
24 h and with HF+HNO3+HClO4 at 190°C for 7 days in
Teflon capsules, respectively. Strontium in each component
was further separated by the conventional cation exchange
techniques using ion exchange resin (packed with Bio-Rad
AG50Wx8). Strontium isotope analyses were performed on
a Finnigan MAT-262 multicollector thermal ionization
mass spectrometer (TIMS) at Institute of Geology and
Geophysics, Chinese Academy of Sciences (IGGCAS), Beij-
ing. All 87Sr/86Sr ratios were adjusted to 0.710253 for
NBS987.
Sulfur isotopic determinations were carried out on a
Thermo Finnigan Delta S mass spectrometer, calibrated by
a series of IAEA standards. The pyrite and elemental sulfur
was liberated as H2S with chromic chloride solution (Can-
field et al. 1986). 0.1 M AgNO3 was subsequently added
to precipitate Ag2S for stable S isotope composition
(34S/32S) analysis. The d34S values are reported relative to
the Vienna Canyon Diablo troilite (VCDT) standard with
reproducibility better than �0.3&.
RESULTS
Petrology
Fracture- and vug-filling calcite, saddle dolomite, fluorite,
barite, pyrite, sphalerite, galena, and elemental sulfur were
commonly observed in the Ordovician unit both in NW Ta-
rim Basin and in Central Tarim Basin, and many of these
minerals were hydrothermal in origin based on their fluid
inclusion-derived precipitation temperatures being 10–50°Chigher than the highest paleo-temperature inferred from the
burial–thermal history (Fig. 2; Machel & Lonnee 2002; Cai
et al. 2008). This temperature anomaly is too great to be
explained by uncertainty or error in the modeled thermal
history of the area. In this study, we mainly focused on the
Fig. 3. Distribution of hydrothermal minerals
in the Ordovician strata in Central Tarim.
Data from a* (Cai et al. 2008) and b* (Yang
et al. 2012) are incorporated; HTD stands for
hydrothermal dolomite.
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4 L. JIANG et al.
late diagenetic minerals that were more likely to be of hydro-
thermal origin, whereas primary carbonate host rock compo-
nents and early diagenetic minerals were not the main
concern in this contribution. For clarification, we have listed
the paragenetic sequence in Fig. 4, in which the relatively
early diagenetic events were adapted from Cai et al. (2008),
Li et al. (2011), and Dong et al. (2013).
Calcite occurs commonly as cements in enlarged dissolu-
tion vugs and fractures and exhibits a milky white appear-
ance and coarse crystal sizes, commonly associated with
abundant solid bitumen (pyrobitumen) (Fig. 5A,D). Less
commonly, calcite has replaced limestone host rock or bar-
ite and anhydrite (Fig. 5B,G). Barite occurs as tabular,
vug-fillings, and crosscuts giant calcite crystals (Fig. 5B).
Abundant fluorite crystals are present in the caves (Fig 5C)
close to the Ordovician Lianglitage Formation/Yinshan
Formation (O3l/O1-2y) unconformity and are closely asso-
ciated with calcite and anhydrite in some outcrops
(Fig. 5D). Saddle dolomite can also be frequently observed
in the Ordovician unit. The amount of dolomite seems to
increase with increasing age (Fig. 5E), possibly suggesting
that the dolomitizing fluid was likely from the deeper
strata. The saddle dolomites occur close to fractures and
show coarse crystal up to 3 mm (Fig. 5F). The occurrence
of anhydrite dissolution and replacement of anhydrite by
calcite was visible in several thin sections (Fig. 5G). Volu-
metrically minor yellow elemental sulfur was also locally
observed in secondary dissolution pores (Fig. 5H). Euhe-
dral and coarsely crystalline pyrite frequently occurs in
micro-fractures, dissolution vugs, and intergranular cracks
(Fig. 5I), commonly representing less than 1%, but locally
up to 3%, by volume.
Good carbonate reservoirs in the Ordovician strata in
Tarim Basin are commonly associated with faults and
fractures although some of them are filled with the hydro-
thermal minerals mentioned previously (Fig. 5J). Hydro-
thermal dolomitization (HTD) is another common
phenomenon present in both the NW Tarim Basin and
Central Tarim Basin (Fig. 5K); point counting indicates
that these HTDs have relatively high porosity (Fig. 5L)
(locally up to >10%; Fig. 4). Less commonly, in some out-
crops in NW Tarim, large numbers of deep-seated fractures
have been found connecting with giant caves (Fig. 5M),
and these fractures and caves were partially filled by hydro-
thermal minerals (e.g., fluorite, calcite, anhydrite, sulfur).
Therefore, the above findings probably suggest that deep-
seated fractures may have provided pathways for hydrother-
mal fluids to flow and may have been responsible for the
formation or preservation of part of these caves in deep
burial environments.
Fig. 4. Paragenetic sequence of the Ordovician unit in Tarim Basin summarizing major products of seawater, near-surface, shallow-burial, mesogenetic to
deep burial, uplifting diagenesis, and the temperature and porosity in each diagenetic realm. Temperature and porosity data have been revealed by fluid
inclusion analysis and point counting for the thin sections, respectively. Relatively early diagenetic events (seawater diagenesis, near-surface realm, and shal-
low-burial realm) were adapted from Cai et al. (2008), Li et al. (2011), and Dong et al. (2013).
© 2014 John Wiley & Sons Ltd
Hydrothermal fluid in ordovician carbonates in Tarim Basin 5
(A) (B) (C)
(D) (E) (F)
(G) (H) (I)
(J)
(M)
(K) (L)
Colouronline,B&W
inprint
© 2014 John Wiley & Sons Ltd
6 L. JIANG et al.
Fluid inclusion (FI) homogenization temperature and
salinity
Doubly polished detachable wafers (45 in total) of fluorite
and dolomite samples were prepared from subsurface core
samples. Petrography, using transmitted light and UV fluo-
rescence, was performed on doubly polished wafers to
identify inclusion types for further analysis and to deter-
mine their relationship to the host minerals. All minerals
contain primary, two-phase aqueous inclusions. These fluid
inclusions showed no signs of deformation or leakage,
suggesting that the homogenization temperature data are
not an artifact of postformational processes.
Calcite and barite have homogenization temperatures
ranging from 60°C to 180°C (Fig. 6A) and 100°C to
160°C (Fig. 6E), respectively. Note that calcite and barite
with relatively high temperatures have relatively high
salinity values, lying between 14 wt% and 24 wt%
(Fig. 6B) and between 14 wt% and 22 wt% (Fig. 6F),
respectively.
Dolomite samples have a relatively narrow range of
homogenization temperatures, varying from 70°C to
(A) (B)
(C) (D)
(E) (F)
(G) (H)
Fig. 6. Histograms showing homogenization
temperatures and salinities measured from
fluid inclusions of hydrothermal minerals from
the Upper Ordovician strata in Tarim Basin.
Fig. 5. (A) late-stage fracture-filling calcite (yellow arrow) in O3l, from well TZ42, depth 5380 m; (B) late-stage calcite (in red color) and barite (yellow
arrow) replace the limestone host rock in O3p, from well TZ162, depth 5981.4 m; (C) giant cave presented in the Ordovician O3t/O1-2y unconformity, with
fluorite, calcite, and anhydrite locally filling in, from Xike’er outcrop; (D) calcite (write color) and fluorite (purple color; yellow arrow) filling in the fracture in
the Ordovician Yijianfang formation, from Xike’er outcrop; (E) dolomitization along the fractures showed in the yellow framed area, dolomite amount increas-
ing with increased depth, from Qingsong outcrop; (F) saddle dolomite shows undulose extinction under crossed nicols, from Qingsong outcrop; (G) the
photograph shows the dissolution of anhydrite and replacement by calcite; blue dash represents the corrosion interface between anhydrite and calcite, from
well TZ826, Upper Ordovician; (H) the photograph shows yellow elemental sulfur (yellow arrow) filling the isolated pores associated with bitumen, O1 from
well TZ243, depth 5750 m; (I) the photograph shows coarse-crystallized pyrite distributed along the micro-fractures, O1-2y from well TZ3; (J) fracture-related
porosity and late diagenetic infillings (calcite, pyrite, sulfur, etc.) in limestone, from well TZ3, depth 3708.1 m; (K) the photograph shows fracture-related
hydrothermal dolomite (HTD), suggesting fracture provided pathways for hydrothermal fluids for dolomitization, from well TZ12, depth 5237.0 m; (L) thin
section made from (K), showing relatively high porosity (in red color); (M) photograph shows fracture-controlled caves in the Ordovician strata, hydrothermal
minerals (e.g., fluorite, calcite, anhydrite, sulfur) commonly occurred in the caves, outcrop from Xike’er outcrop.
© 2014 John Wiley & Sons Ltd
Hydrothermal fluid in ordovician carbonates in Tarim Basin 7
140°C, with an average value of 97°C (Fig. 6C). Paleo-
water present during dolomite growth has salinities from
16 wt% to 23.3 wt%, with an average value of 21 wt%
(Fig. 6D). Salinities slightly increase with the increasing
temperatures of trapping both for the entire dataset and
for individual samples.
Fluorite samples show a wide range of homogenization
temperatures varying from 142°C to 283°C, with an aver-
age value of 179°C (Fig. 6G). Water salinities at the time
of fluorite growth ranged from 0.5 wt% to 18.3 wt%, with
an average value of 6 wt% (Fig. 6H). The relationship
between salinity and temperature is poor; however, when
the salinities are higher than 10 wt%, the trapping tempera-
ture is lower than 150°C.
C/O/S/Sr isotope data
Carbon and oxygen isotopic composition of the Ordovi-
cian carbonates are presented in Fig. 7 and Table 1. Frac-
ture- and vug-filling calcites show broadly similar d13Cvalues falling within a range between �7.11& and 2.49&,
with d18O values between �17.39& and �5.41&(Fig. 7). Saddle dolomite has a narrow range of both d13Cvalues (between �1.37& and �0.76&) and d18O values
(between �8.33& and �7.98&; Fig. 7).
Sulfur isotopic composition of pyrite from four subsur-
face core samples and of one elemental sulfur core sample
are presented in Table 1. The pyrites show a narrow range
from 22.3& to 29.7&, and the elemental sulfur sample
has d34S ratio of 21.4&.
Strontium isotopic compositions of the dolomite, calcite,
anhydrite, fluorite, and barite are presented in Fig. 8 and
Table 1. Calcite and saddle dolomite samples in the Ordovi-
cian show 87Sr/86Sr ratios from 0.708823 to 0.710445 and
from 0.708824 to 0.709840, respectively. Anhydrite samples
have 87Sr/86Sr ratios from 0.709110 to 0.710136. One bar-
ite sample has a high 87Sr/86Sr ratio of 0.710009 (Table 1).
DISCUSSION
Hydrothermal origin of deep burial diagenetic minerals
Hydrothermal minerals (e.g., saddle dolomite, fluorite,
barite, calcite, pyrite, galena, sphalerite) in the Ordovician
strata in Tarim Basin occur as fracture- and vug-fillings or
cements in fault zones, or as cements or infillings in caves
either close to or far away from the unconformity, which
connected with deep-seated fractures (Figs 5 and 10; Cai
et al. 2008; Pan et al. 2009; Yang et al. 2012). Thus, the
hydrothermal minerals may have been controlled by local
structures, and fracture networks may have provided favor-
able conduits and pathways for the deep basinal hydrother-
mal fluids (Cai et al. 2001a; L€u et al. 2008; Li et al. 2011;
Yang et al. 2012; Dong et al. 2013). Apart from the
hydrothermal mineral assemblage, it has been commonly
reported that granite intrusions and diabase dikes, as well
as the metamorphic limestone, outcrop in the NW Tarim
Basin (Pan et al. 2009; Zhang et al. 2014). Hence, it is
possible that the late-stage diagenetic minerals in this study
are hydrothermal in origin or are closely related to the
hydrothermal activity.
Two-phase aqueous inclusions with relatively high
homogenization temperatures were trapped within saddle
dolomite, fluorite, calcite, and barite. The two-phase aque-
ous inclusions in saddle dolomite in the Keping, Penglaiba,
and Qingsong outcrops show that dolomite growth
occurred during burial diagenesis when the temperatures
were between 70°C and 138°C (Fig. 6C), being similar to
the reported Upper Cambrian hydrothermal dolomite
(HTD) temperature in the Keping outcrop (Dong et al.
2013), and within the HTD precipitation temperature
ranges worldwide as summarized by Davies & Smith
(2006). The two-phase aqueous inclusions in fluorite in
the Sanjianfang, Xikeer, and Tumuxiuke outcrops show
that fluorite growth occurred during hydrothermally influ-
Fig. 7. Cross-plot of d18O and d13C values in
the fracture-, vein-, and vug-filling calcite and
saddle dolomite in the Ordovician unit in
Tarim Basin.
© 2014 John Wiley & Sons Ltd
8 L. JIANG et al.
enced burial diagenesis when the temperature was between
142°C and 283°C (Fig. 6G), which is significantly higher
than the reported fluorite temperatures in the Tazhong
area (Cai et al. 2008; Li et al. 2011). Burial diagenetic
vug- or fracture-filling calcite, both in this study and in
previous studies, has a relatively wide precipitation temper-
ature range, from 80°C to 160°C (Fig. 6A; Li et al.
2011). Barite has also precipitated at temperatures up to
160°C, which is significantly higher than the highest burial
temperature (140°C; Fig. 2) in the Ordovician strata
(Fig. 6G), whereas Upper Cambrian to Ordovician vug-
filling quartz cement has been reported to have fluid inclu-
sion homogenization temperatures even up to 300°C(Xing & Li 2012). The formation temperatures of all
above-mentioned minerals (dolomite, calcite, fluorite, bar-
ite, quartz) have their highest homogenization tempera-
tures significantly higher (>10°C; Fig. 6) than the hottest
burial temperatures (Fig. 2), suggesting there was an inva-
sion of hydrothermal fluid into the Ordovician strata in the
Tarim Basin (Smith 2005; Davies & Smith 2006; Cai et al.
2008; Wendte et al. 2009; Lapponi et al. 2014).
Two-phase aqueous inclusions in saddle dolomites, vug-
and fracture-filling calcites, and barites all show relatively
high salinities (up to 24 wt% NaCl; Fig. 6D), and their
salinities systematically increase with increasing fluid
inclusion homogenization temperatures. There are only a
few ways for formation waters to achieve salinities >24 wt%
NaCl: (i) extreme evaporation of seawater or connate water
Table 1 d13C, d18O, and 87Sr/86Sr values of the dolomites in the Ordovician Strata.
sample formation description d13C& VPDB d18O& VPDB d34S& VCDT 87Sr/86Sr
KP10-1 O1p Fracture-filling calcite �0.84 �14.86 – 0.709286QD-1 O Fracture-filling calcite �1.61 �7.49 – 0.708823QD-2 O Fracture-filling calcite �1.49 �7.65 – 0.708935TZ12-1 O3l Vug-filling calcite – – 0.709774
TZ12-2 O3l Vug-filling calcite – – 0.709211TZ45-1 O3l Vug-filling calcite – – 0.707970TZ45-2 O3l Vug-filling calcite – – 0.709408QS-9-1 O1-2y Vug-filling calcite 2.49 �16.57 – 0.709249QS-9-2 O1-2y Vug-filling calcite 0.82 �13.79 – 0.709462QS-11 O1-2y Vug-filling calcite �1.86 �10.50 – 0.708985
DWG-1 O Vug-filling calcite �7.11 �14.14 – 0.709226QS-16 O1-2y Vug-filling calcite �1.75 �10.32 – 0.709260KP10-2 O1p Vug-filling calcite �0.80 �8.28 – 0.709171KP-14 O1p Vug-filling calcite �2.29 �11.37 – 0.709083TPL-1 O Vug-filling calcite �1.66 �16.13 – 0.710313TPL -2 O Vug-filling calcite �2.32 �17.39 – 0.710445TPL -3 O Vug-filling calcite �2.72 �14.99 – 0.709537
TPL -4 O Vug-filling calcite �2.66 �15.50 – 0.709625TPL -5 O Vug-filling calcite �4.98 �13.30 – 0.709905XKE-1 O Vug-filling calcite 0.63 �5.41 – –XKE-2 O Vug-filling calcite 0.66 �7.65 – –KP-13 O1p Saddle dolomite �0.76 �7.98 – 0.708824QS-13-1 O1-2y Saddle dolomite �1.37 �8.33 – 0.709231
QS-13-2 O1-2y Saddle dolomite �0.88 �8.30 – 0.709840TZ161 O3l Pyrite – – 24.6 –Z12 O1-2y Pyrite – – 29.0 –TZ12-1 O3l Pyrite – – 29.7 –TZ12-2 O3l Pyrite – – 22.3 –TZ826 O Elemental sulfur – – 21.4 –TPL -6 O Anhydrite vein – – – 0.710015TPL -7 O Anhydrite vein – – – 0.710136TPL -8 O Anhydrite vein – – – 0.709910XKE-3 O Anhydrite vein – – – 0.709124TZ12-16-a a* O3 Anhydrite vein – – 24.5 –XKE-2 O Barite vein – – – 0.710009TZ12-17-c a* O3 Barite vein – – 46.6 0.708969
TZ12-9 a* O3 Barite vein – – 45.3 0.709023TZ12-16-d a* O3 Barite vein – – 42.1 0.709568TZ12-17-e a* O3 Barite vein – – – 0.709577TZ45-7-b a* O3 Fluorite vein – – – 0.710364TZ45-12 a* O3 Fluorite vein – – – 0.709514TZ45-6094.6 a* O3 Fluorite vein – – – 0.709124
TZ45-6100.2 a* O3 Fluorite vein – – – –TZ45-6100.9 a* O3 Fluorite vein – – – 0.709305TZ45-6061.8 a* O3 Fluorite vein – – – 0.708891
–: data not measured or unavailable; a* data from Cai et al. (2008).
© 2014 John Wiley & Sons Ltd
Hydrothermal fluid in ordovician carbonates in Tarim Basin 9
(Cai et al. 2001a; Davies & Smith, 2006) which then pen-
etrates underlying porous formation and (ii) dissolution of
buried halite-bearing evaporites (Worden 1996; Stueber
et al. 1998; Jiang et al. 2014a,b). In the Ordovician strata
(and the water had a salinity of >24 wt% NaCl) in Tarim
Basin, dissolution of evaporites seems to be the best sce-
nario to explain the elevated salinity in formation salinity,
which was supported by the geographic and stratigraphic
trends of water chemistry in the Ordovician strata (Cai
et al. 2001a), as well as supported by the presence of evap-
orites in Middle Cambrian strata (Fig. 1). These Cambrian
evaporites may have dissolved and migrated into the Ordo-
vician strata conducted by faults during the Permian
hydrothermal activity mentioned previously (Cai et al.
2001a; Dong et al. 2013).
Meteoric water origin of diagenetic minerals associated
with hydrothermal minerals
Both deep-seated faults and up-reaching faults have been
formed during the hydrothermal event (L€u et al. 2008),
which may have brought deeper high saline water and
meteoric water into the Ordovician strata in the Tarim
Basin, resulted in fluid mixing and hydrothermal mineral
precipitation. Fluorite and calcite samples, associated with
the hydrothermal calcite, dolomite, anhydrite, and barite,
show relatively lower homogenization temperatures
(<120°C) with salinities much lower than the coexisting
hydrothermal minerals in the NW Tarim area (Fig. 6).
Late-stage diagenetic fracture-filling calcite, barite, and
fluorite with low temperatures and salinities have also been
reported in the Ordovician strata in Central Tarim area
(Cai et al. 2008; Li et al. 2011). Importantly, low-temper-
ature (as low as 60°C) calcite and fluorite have similar REE
patterns with significant high Eu anomalies to their high-
temperature (up to 180°C) counterparts, also indicating
the invasion of hydrothermal fluid (Cai et al. 2008). In
addition, the salinity of hydrothermal dolomite broadly
decreased with the decreasing temperature, which probably
suggests that there is an episode of influx of fluid with low
temperature and low salinity (most likely meteoric water)
during the formation of the saddle dolomite. Therefore,
the above findings all suggest that there must have been an
invasion of fresh water during the hydrothermal period,
and the mixing of fresh water with saline hydrothermal flu-
ids resulted in the diagenetic minerals having a wide range
of homogenization temperatures (from 60°C to 240°C)and salinities (from 0.5 to 24 wt% NaCl), with salinities
decreasing with decreasing temperatures (Fig. 6). Referring
to the burial histories in NW Tarim and Central Tarim
areas, meteoric water most likely charged the Ordovician
strata in the NW Tarim area during the significant uplift
stage from the late Permian to the early Triassic (Fig. 2B).
Meteoric water then flowed westward to the Central Tarim
and mixed with hydrothermal fluid and connate water
along its pathways (Cai et al. 2001a; Cai et al. 2008).
The petrological appearance (Fig. 5) and geochemical
characteristics (Fig. 6) indicate that hydrothermal minerals
(e.g., dolomite, calcite, fluorite, and barite) were closely
related to regional igneous rocks (Yang et al. 2012; Pan
et al. 2009). The dating of U-Pb zircon from the diabase
in the Ordovician strata suggests the regional igneous
rocks intrusion occurred in Early Permian
(390.5 � 2.9 Ma) (Dong et al. 2013). The above findings
led to the conclusion that hydrothermal event in the
Upper Cambrian to Lower Ordovician strata in Tarim
Basin most likely postdates the extensive magmatic
emplacement and volcanic eruption event during early
Permian and the regional magmatic event most likely has
provided heat source, fluid pathway, fluids flow tensional
stress fields, and even a small part of hydrothermal fluids
for the consequent hydrothermal event (Davies & Smith
2006; Dong et al. 2013). Therefore, the influx of freshwa-
ter and hydrothermal fluid in the Ordovician strata and
resulting fluid mixing were most likely nearly simultaneous
(probably during late Permian to early Triassic).
From the homogenization temperature and salinity data
in late-stage diagenetic minerals, we therefore conclude
that there are two end-member fluids named hydrothermal
fluid (hot and high saline brine) and meteoric water (cool
and low-salinity water), which have been added to the
paleo-water in the Ordovician strata in Tarim Basin during
the period.
TSR origin of 12C-rich Calcite, 34S-rich Sulfide and
elemental Sulfur
In the Central Tarim Basin, carbon isotope values of the
carbonate samples both in this study and in previous stud-
ies (Cai et al. 2001b, 2008; Li et al. 2011) suggest that
Fig. 8. 87Sr/86Sr isotopic ratios of hydrothermal calcite, fluorite, dolomite,
anhydrite, and barite in the Ordovician unit in Tarim Basin.
© 2014 John Wiley & Sons Ltd
10 L. JIANG et al.
calcite cement, saddle dolomite, CO2, and formation water
contain carbon sourced from isotopically light carbon
sources. Isotopically light carbon (down to �2&) could be
achieved from mantle-originated hydrothermal fluids (Xing
& Li 2012; Dong et al. 2013) or from involving meteoric
water (Allan & Mathews 1982). However, the rather nega-
tive carbon isotope values of the calcite (down to
�7.11&) and formation water (down to �13.8&) suggest
that there are other isotopically light carbon inputs (Li
et al. 2011). An exotic source of carbon or atmospheric
CO2 (e.g., in meteoric-influenced systems) is potentially
supported by the low salinity water recorded in inclusions
within calcite and fluorite (Fig. 5); however, it could not
explain the isotopically lighter carbon with d13C values
< �2& VPDB. Bacterial sulfate reduction (BSR) can be
excluded by the high precipitation temperatures (>80°C)of most of these burial diagenetic minerals (Machel 2001).
Therefore, the only other possible explanation for the low
carbon isotope values of the later diagenetic calcites is ther-
mochemical sulfate reduction (TSR) (Cai et al. 2001b,
2008; Li et al. 2011). This assertion is further supported
by the high sulfur isotopic values ranging from 21.4& to
29.7& (VCDT), being close to the sulfur isotopic values
of coeval or slightly older seawater (Strauss 1997) and
anhydrite vein in this formation (Cai et al. 2008).
Thermochemical sulfate reduction has not been previ-
ously reported in NW Tarim. Thus, it is of great impor-
tance to know whether TSR happened in the Ordovician
strata in NW Tarim, as different styles of diagenesis may
lead the different patterns of reservoir quality and thus dif-
ferent exploration strategies between NW Tarim and Cen-
tral Tarim areas. In this study, evidence to support the
interpretation of TSR in Ordovician strata in NW Tarim is
(i) the relatively lighter carbon isotopic values of the late
diagenetic calcites (down to �7.11& VPDB, and carbon
isotope values decreasing with decreasing oxygen isotopes
in calcite cements; Fig. 7); (ii) late diagenetic, coarse crys-
talline pyrite, and elemental sulfur being closely grown
with the isotopically light carbon calcite; (iii) the higher
temperatures recorded by calcite fluid inclusions (Fig. 6;
Xing & Li 2012), which exceed the reported global lowest
temperature for TSR (>100°C; Worden et al. 2000; Ma-
chel 2001); and (iv) very high salinity of the FIs in the late
diagenetic minerals (Fig. 6), suggesting abundant SO2�4
was most likely transported from underlying Cambrian
anhydrite bearing strata into the Ordovician strata by
deep-seated faults (Cai et al. 2001a; L€u et al. 2008).
Collectively, both in the Central Tarim and in the NW
Tarim, the petrological and isotopical evidence from diage-
netic calcite, sulfide, and elemental sulfur in the Ordovician
strata suggest that they were most likely derived from ther-
mochemical sulfate reduction (TSR) (Worden et al. 1995,
2000; Cai et al. 2001b, 2003, 2004, 2008, 2009b; Jiang
et al. 2014a). Hot brine from the deeper Cambrian strata
during the hydrothermal event may have contributed both
heat and sulfate as required by TSR.
Isotopic evidence for fluids mixing
The d18O values of calcite cements from the Upper Cam-
brian to Ordovician strata (Table 1) are between �17.39
and �7.49& V-PDB, with most of the value ranging from
�7.5 to �15& V-PDB (Fig. 7; Li et al. 2011; Dong et al.
2013). Early diagenetic calcite with high oxygen isotope
values lies between �7 and �5& V-PDB (average at about
�6&; Li et al. 2011; Table 1), within the reported late
Cambrian to early Ordovician carbonate oxygen isotope
value range (from �10 to �5& V-PDB) (Veizer et al.
1999). These calcites contain numerous single phase inclu-
sions, which probably formed below 50°C (Goldstein &
Reynolds 1994), during early diagenesis. Using the O’Neil
et al. (1969), calcite-water oxygen isotope fractionation
equation, the d18O value of the water present during the
growth of isotopically heavy early diagenetic calcite was
between 0& and 2& V-SMOW. However, the isotopically
lighter deep burial diagenetic calcites were precipitated
between the temperature of 60°C and 180°C (Fig. 5).
Using the same equation from O’Neil et al. (1969), the
isotopically lighter calcite grew from water with d18O rang-
ing from �6.7 to 12.5& V-SMOW, while the connate sea-
water should has d18O lies between 2.6 to 14.1& V-
SMOW, in the temperature range from 60°C to 180°C.Hydrothermal dolomites (HTD) in the Upper Cambrian
to Ordovician strata show d18O values lie between �5.88
and �10.24& V-PDB (Table 1; Dong et al. 2013), these
dolomites formed in a temperature range from 70°C to
140°C, by employing the dolomite-water oxygen isotope
fractionation equation from Land (1983), the water
responsible for the growth of saddle dolomite has d18Ovalues ranging from 1.0 to 9.4& V-SMOW. Invasion of
meteoric water is precluded by the high salinity data from
fluid inclusions in saddle dolomite. Therefore, the high-
salinity hydrothermal dolomitizing fluid indicated the con-
tribution of halite and evaporites from the Cambrian
strata.
As calculated above, the fluids responsible for the forma-
tion of deep burial diagenetic calcite in the Ordovician
strata show a relatively wider range of d18O values (from
�6.7 to 12.5) than the calculated fluids from hydrothermal
dolomite (1.0 to 9.7& V-SMOW). Moreover, the calcu-
lated connate seawater has d18O values between 2.6 and
14.1& V-SMOW, and the coeval brine has d18O value of
about 15& SMOW during Cambrian and Ordovician
(Claypool et al. 1980). Hence, the isotopic lighter values
of the late diagenetic calcites (as low as �6.7& V-SMOW)
suggest that meteoric water was most likely involved in this
diagenetic period (Cai et al. 2001a), and this conclusion is
supported by the very low salinity data (down to 0.5 wt%
© 2014 John Wiley & Sons Ltd
Hydrothermal fluid in ordovician carbonates in Tarim Basin 11
NaCl) obtained from the calcite and fluorite fluid inclu-
sions (Fig. 6B,H).
The Sr isotope ratios of the calcite cements, anhydrites,
barites, dolomites, and fluorites lie in a range from
0.708823 to 0.710445 (Fig. 8, Table 1). Except for some
early diagenetic calcite cements within the Sr isotope ratios
range of early Ordovician seawater (Veizer et al. 1999;
McArthur & Howarth 2004), all the other deep burial dia-
genetic minerals show significantly higher Sr isotopic values
(up to 0.7010), possibly suggesting that the influxed mete-
oric water or invaded hydrothermal fluid may have leached
other rocks on their pathways (Cai et al. 2008; Li et al.
2011). The Sr isotope values of calcite cements increase
with decreasing oxygen isotopes (Fig. 9) suggesting an
increasing influence of meteoric water or the increasing
temperature sourced from the hydrothermal activity. Salin-
ity data obtained from fluid inclusions in some fluorite and
calcite cements show very low values (down to 0.5 wt%
NaCl), but high Sr isotope values up to 0.7010 (Table 1;
Fig. 8), suggesting that these calcite and fluorite must have
precipitated from meteoric water that obtained 87Sr from
clastic sediments during fluid flow. In contrast, calcite
cement, fluorite, saddle dolomite, and barite samples have
relatively high salinity in fluid inclusions (from 15 to 25 wt
% NaCl) and high 87Sr/86Sr ratios (up to 0.7010),
suggesting that hydrothermal fluid derived from the Cam-
brian also leached Sr from potassium-bearing rocks (likely
clastic sediments or K-rich igneous rocks). Although no
salinity data have been obtained from anhydrite cement,
the relatively high Sr isotopes and REE distribution similar
to the Cambrian anhydrite may suggest that the anhydrite
may have been related to the halite- and evaporate-bearing
Cambrian strata (Cai et al. 2008).
In summary, the key attributes of meteoric water and
hydrothermal fluid are summarized in Table 2. There are
differences in the two end members of fluid in terms of
diagenetic minerals, their fluid inclusion temperatures,
Fig. 9. Cross-plot of d18O and 87Sr/86Sr ratio in the fracture- and vug-fill-
ing calcites in the Ordovician unit in Tarim Basin.
Tab
le2Inferred
salinities,
temperatures,
d18O,an
d87Sr/8
6Sr
ratiosofen
d-m
ember
fluids.
end
mem
bers
occurren
cein
rock
crystal
form
diagen
etic
minerals
d18O
fluid
V-SMOW
d13Ccarbonate
V-PDB
87Sr/8
6Sr
temperature
salinity
Meteo
ricwater
vugfillormicro-fracture
fill
(subcm
tom-scale
fractures)
blocky
fabric
calcite,
fluorite
�6.7
to0&
downto
�2&
upto
0.7010
60°C
to100°C
0to
<3.5
wt%
NaC
l
Hyd
rothermal
fluids
vugfillormicro-fracture
fill
(subcm
tom-scale
fractures)
blocky
fabric
calcite,
saddle
dolomite,
barite,
anhyd
rite,
fluorite,an
dquartz
1to
12.5&
downto
�13.8&
upto
0.7104
140to
>200°C
upto
>24wt%
NaC
l
© 2014 John Wiley & Sons Ltd
12 L. JIANG et al.
salinities, d13C, and 87Sr/86Sr isotope values, as well as the
calculated d18O values of the fluids.
Hydrothermal model and its implication to exploration
Previous studies have shown that the hydrothermal fluid
was related to regional Permian magmatic activity (Chen
et al. 1997; Tang et al. 2004; Yang et al. 2007, 2012;
Tian et al. 2010; Zhang et al. 2010; Xing & Li 2012;
Dong et al. 2013). The regional Permian magmatic activity
could have led to rock fracturing and thermal diffusion,
driving hydrothermal fluid to migrate into the overlying
Ordovician strata (Fig. 10). On the other hand, induced
fractures, right up to the surface, may also have been
related to the Permian igneous activity (Fig. 10; L€u et al.
2008), allowing meteoric water to penetrate into the
Ordovician strata and mix with hydrothermal fluid and
connate formation water (Cai et al. 2008; Xing & Li
2012). This scenario is supported by fluid inclusion salini-
ties and homogenization temperatures, as well as the d13C,d18O, and 87Sr/86Sr values of the hydrothermal minerals
(Figs 7–9; Table 2).
In the Tarim Basin, hot hydrothermal fluid with high
d18O value (up to 12.5& V-SMOW), high temperature
(>200°C), high salinity (up to 24 wt% NaCl), and high87Sr/86Sr ratio (up to 0.7010) was most likely expelled from
Pre-Cambrian and Lower Cambrian strata into the Ordovi-
cian strata (Cai et al. 2008; Jia et al. in press) by means of
tectonic movement and thermal convection (Davies &
Smith 2006). Hydrothermal fluid was likely to flow either
westward or eastward along the Ordovician strata. However,
cooler meteoric water with low d18O value, low tempera-
tures (<60°C), very low salinity (0.5–3.5 wt% NaCl), but
also high 87Sr/86Sr ratios (up to 0.7010) was most likely
introduced into the Ordovician strata by gravity in NW Ta-
rim area. Both hydrothermal fluid and meteoric water flowed
and then mixed with the formation water in the Ordovician
strata along regional conduits (e.g., fault, fracture, uncon-
formity). The mixed fluid probably resulted in decrease in
homogenization temperatures, salinities, and 87Sr/86Sr
ratios of hydrothermal minerals, but increase in d18O values
of saddle dolomites and calcites along its pathway (Figs 9
and 10). The Ordovician strata likely acted as a deeply bur-
ied regional porous and permeable conduit system for focus-
ing and channeling basinal fluid flow in the Tarim Basin.
Hence, the fluid flow and mixing scenario in the Ordovician
in the Tarim Basin is similar to the case in the Middle Devo-
nian in West Canada Sedimentary Basin (WCSB), where the
hot, radiogenic hydrothermal fluids moved eastward up-dip
along the Presqu’ile barrier and mixed with cooler ambient
formation waters during tectonic movements (Qing &
Mountjoye 1992). The basinal hydrothermal fluid flow and
mixing in the WCSB also precipitated hydrothermal minerals
and led to dolomitization of the host carbonate in the Mid-
dle Devonian, which locally improved reservoir quality
(Qing & Mountjoye 1992, 1994).
Both in this study and in previous studies, it has been
shown that contact metamorphic limestones, close to gran-
ite intrusions and diabase dikes, commonly have very low
porosity in outcrops in the NW Tarim Basin (Pan et al.
2009; Zhang et al. 2014). In contrast, carbonate reservoirs
away from granite intrusions and diabase dikes have rela-
tively high porosity (locally >10%). Significantly, good
quality carbonate reservoirs contain abundant fractures
filled by hydrothermal minerals (Figs 5 and 7–9). Collec-tively, these findings suggest that cross-formational hydro-
thermal fluid flow induced by the Permian magmatic
activity, rather than the volcanic hydrothermal fluid, may
have improved the reservoir quality (Pan et al. 2009).
Mixtures of hydrothermal fluid and meteoric water with
the connate formation water can lead to either precipitation
of hydrothermal minerals decreasing reservoir porosity or
Fig. 10. The hydrothermal model for fluids
mixing induced by Early Permian magmatic
activity in the Ordovician unit in Tarim Basin.
© 2014 John Wiley & Sons Ltd
Hydrothermal fluid in ordovician carbonates in Tarim Basin 13
dissolution of the host carbonate rocks increasing the reser-
voir porosity; the dissolution and precipitation process was
mainly controlled by water chemistry (e.g., Ca, carbonate,
and other chemical composition concentrations), pH, and
fluid flux (Corbella et al. 2004). The formation of faults
during the hydrothermal activity could increase the reservoir
quality and result in fractured hydrocarbon reservoirs (Smith
2005; L€u et al. 2008). Hydrothermal dolomitization has
been reported to significantly enhance carbonate reservoir
quality either by replacing or dissolving calcite, or by brecci-
ation (Qing & Mountjoye 1994; Machel & Lonnee 2002;
Lavoie et al. 2005; Smith 2005; Davies & Smith 2006;
Wendte et al. 2009; Saller & Dickson 2011). Some authors
have presented evidence that shows a decrease in porosity
due to excess dolomitization and sulfate cementation (e.g.,
Lucia 2004; Jones and Xiao, 2005). However, in this study,
point counting indicates that the hydrothermal dolomitiza-
tion is more likely to be a mole-for-mole replacement pro-
cess and the replaced highly crystalline dolomite can resist
compaction and retain much of the original porosity than
limestone (Lucia 2004). Hence, hydrothermal dolomitiza-
tion in this study seems to have enhanced reservoir quality
(Fig. 4; Qing & Mountjoye 1994; Saller & Dickson 2011).
Thermochemical sulfate reduction (TSR) in the deep
burial carbonate reservoirs may have potential in enhancing
porosity by dissolving either sulfates or carbonates during
and after this physical and chemical process (Cai et al.
2014). Moreover, the best carbonate reservoirs were clo-
sely related to the invasion of hydrothermal fluids and the
associated TSR as well as the influx of meteoric water
(Yang et al. 2012). Therefore, fluid mixing, dolomitiza-
tion, TSR, and faulting caused by the hydrothermal event
have significantly improved the Ordovician reservoirs in
Tarim Basin (Fig. 5; Qing & Mountjoye 1992, 1994; Cor-
bella et al. 2004; Lavoie et al. 2005; Smith 2005; Davies
& Smith 2006; L€u et al. 2008; Pan et al. 2009; Yang
et al. 2012).
CONCLUSIONS
(1) Calcite, saddle dolomite, fluorite, barite, quartz, and
anhydrite, both from a number of outcrops in NW Ta-
rim Basin and from a number of subsurface cores in
Central Tarim Basin in the Ordovician in Tarim Basin,
show FIs homogenization temperatures 10°C higher
than the formation temperatures and thus are of
hydrothermal origin.
(2) In Tarim Basin, mixing of hot and high-salinity hydro-
thermal fluids with cool and low-salinity meteoric
water was likely occurred in the Ordovician strata,
which is supported by the homogenization tempera-
tures and salinities of fluid inclusions as well as d13C,d18O, and 87Sr/86Sr isotope values of the deep burial
diagenetic minerals.
(3) TSR most likely occurred both in NW Tarim Basin
and in Central Tarim Basin, when sulfate-rich hydro-
thermal fluid migrated into the Ordovician during the
Permian, leading to the formation of 12C-rich calcites,
and pyrite and elemental sulfur with d34S values close
to the Cambrian to Ordovician seawater.
(4) The structurally controlled hydrothermal activity
induced by Permian magmatism in Tarim Basin
resulted in the formation of deep-seated fracture, and
less commonly of up-reaching fractures, leading to
the migration of hydrothermal fluids into the Ordo-
vician, and locally mixed with meteoric water in Ta-
rim Basin.
(5) Hydrothermal fluid mixing, faulting, dolomitization,
and TSR in the Ordovician strata in Tarim Basin can
locally dissolve and/or replace the host carbonate min-
erals to increase the reservoir porosity, which has sig-
nificant implications for hydrocarbon exploration.
ACKNOWLEDGEMENTS
This work is financially supported by Special Major Project
on Petroleum Study (Grant No. 2011ZX05008-003),
China National Funds for Distinguished Young Scientists
(Grant No. 41125009), Natural Science Foundation of
China (Grant No. 41402132), and China Postdoctoral Sci-
ence Foundation (Grant No. 2014M550835). Construc-
tive comments on the previous version of this paper by
Chief Editor Professor Richard H. Worden and two anon-
ymous reviewers were very helpful.
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