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Diagenesis of an evaporite-related carbonate reservoir in deeply buried

Cambrian strata, Tarim Basin, Northwest China

LEI JIANGa,b*, RICHARD H. WORDENc, CHUN FANG CAIa, AN JIANG SHENd, STEPHEN F.

CROWLEYc,

aKey Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese

Academy of Sciences, Beijing 100029, China

bBureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin,

Austin, TX 78713, United States

cDepartment of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, University

of Liverpool, Liverpool, L69 3GP, UK

dPetroChina Hangzhou Geology, Hangzhou 310023, China

*Corresponding author: Dr. Lei Jiang

Email: [email protected]

Telephone: 86-10-82998128; Fax: 86-10-62010846

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ABSTRACT

Many hydrocarbon shows have been reported from potential evaporite-related carbonate reservoirs in

the Lower to Middle Cambrian units in the Tarim Basin. Petrography, facies, and geochemical

analyses from outcrop and core samples were integrated to document the impact of diagenetic

evolution on these evaporite-related carbonate reservoirs. A simplified depositional model has been

developed that reveals a restricted carbonate platform dominated by an evaporatic inner platform

lagoon, with shoal and reef facies developed around the platform margins. Grainstones and packstones

were deposited at the platform margin and on inner platform shoals, whereas carbonate mudstones and

wackestones were deposited in the lagoon associated with evaporites. Early dolomitization and

dissolution occurred related to the reflux of evaporated seawater. Breccia-associated fracture porosity

due to a short period of meteoric water-induced dissolution is the predominant type of effective

porosity in the anhydrite-bearing dolomudstone reservoir. Thermochemical sulfate reduction (TSR)

occurred in the deep subsurface diagenetic environment characterized by high temperature (100-160

°C) and high salinity (16-26 wt.%) aqueous fluid inclusions in TSR calcite that has relatively negative

δ13C values (as low as -12‰ VPDB) and elevated 87Sr/86Sr values. The porous dolostones contain

solution-enlarged pores due to dissolution of anhydrite and carbonate as a secondary consequence of

TSR. This study shows that deeply buried, evaporite-related, ancient carbonate rocks may be effective

reservoirs due to a combination of fracture porosity and TSR-induced secondary porosity. Evaporitic

Cambrian carbonates buried to over 5,000 m (16,400 ft) can thus be considered as targets for future

exploration.

Keywords: Evaporites, Carbonate reservoir, Depositional model, Facies, Dolomitization, Meteoric

water, Thermochemical sulfate reduction, Fluid inclusions, C/O/Sr isotopes, Cambrian

INTRODUCTION

Sabkha carbonate sequences can host hydrocarbon resources with dolomitized subtidal sediments being

the most productive reservoir intervals. The associated evaporites typically act as lateral and top seals.

Some well-documented and economically important carbonate reservoirs that are associated with

evaporites have been reviewed by Moore and Wade (2013), examples from North America include:

Devonian strata in the Western Canada Basin (Vandeginste et al., 2009), the Ordovician Red River

Formation (Ruzyla and Friedman, 1985) and Mississippian Mission Canyon Formation marginal

marine sabkha reservoirs (Lindsay and Kendall, 1985); the Ordovician Ellenburger Formation (Loucks

and Anderson, 1985) and Upper Permian Guadalupian units (e.g., San Andres, Grayburg) (Saller and

Henderson, 1998; Sarg, 1981); Upper Jurassic Smackover Formation evaporite-related dolomite

reservoirs along the northern Gulf of Mexico USA (Moore and Heydari, 1993). Examples from the

Middle East include Permian-Triassic Khuff Formation in the Middle East (Worden et al., 2004;

Worden et al., 2000), the Eocene Wafra Field in Saudi Arabia and Kuwait (Saller et al., 2014), Lower

Triassic carbonate reservoirs in the Persian Gulf (Peyravi et al., 2014), and the Jurassic Hith Arab

Formation evaporite-related reservoirs (Wilson, 1985). Recent studies have also documented that huge

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hydrocarbon accumulations have been found in evaporite-related dolostone reservoirs in the Upper

Permian to Lower Triassic gas reservoirs in the Sichuan Basin (Jiang et al., 2013; Jiang et al., 2014b;

Ma et al., 2008).

In the examples listed, shoaling-upward carbonate sequences are typically capped with evaporites, and

carbonates are commonly dolomitized due to reflux of evaporated brines (Moore and Wade, 2013). An

arid climate has been reported to be an important factor in the development of many major dolomite

bodies (Sibley, 1980). In arid environments, primary and early diagenetic calcite can be replaced by

dolomite due to the reflux of evaporated, hypersaline seawater through a restricted platform (Adams

and Rhodes, 1960; Saller and Henderson, 1998). These hypersaline brines flow through underlying

porous formations, including platform-margin facies, and dolomitize these strata under shallow-burial

conditions (Jiang et al., 2014b; Jones and Xiao, 2005). Dolomitization has been shown to be

fundamental to the enhancement and preservation of reservoir quality of these carbonate units (Saller

and Henderson, 1998). However, other diagenetic processes, such as meteoric dissolution, solution-

collapse brecciation, sulfate reduction and associated dissolution, and organic acid- and hydrothermal

fluid-induced mesogenetic dissolution, have also been shown to be important for the porosity of these

reservoirs (Davies and Smith, 2006; Ehrenberg et al., 2012; Hill, 1995; Jiang et al., 2015b; Loucks,

1999; Loucks and Anderson, 1985; Qing and Mountjoy, 1994; Saller et al., 2014; Smith, 2006; Worden

et al., 1996). Partial plugging of pores by authigenic minerals, such as gypsum, anhydrite, dolomite,

calcite, and quartz, is also common.

In the Tarim Basin in northwest China, it has recently been documented that deeply buried (3,500 -

8,000 m; high reservoir quality, evaporite-bearing, Lower to Middle Cambrian dolostones, with

thickness up to 450 m (Fig.1), are potentially important reservoirs for large quantities of petroleum

(Huang et al., 2016; Shen et al., 2016; Song et al., 2014; Tian et al., 2015; Wang et al., 2014; Zhao et

al., 2012; Zhao et al., 2014). The localized occurrence of source rocks in the lowest Cambrian strata

(Cai et al., 2009; Cai et al., 2015) and the thick, regional anhydrite seals (Wang et al., 2014) that are

present in the Lower to Middle Cambrian strata, help to de-risk these play system elements. Recent

exploration has proved the existence of petroleum shows in these deeply buried dolomite reservoirs

(Cai et al., 2015; Wang et al., 2014). The wide distribution area of these dolomites (nearly 30×104 km2)

has therefore strengthened the confidence for exploration in the deeply buried Cambrian strata of the

Tarim Basin (Wang et al., 2014).

Up to this time, despite great efforts (Wang et al., 2014), there has been no major discoveries with good

reservoir quality and effective caprocks. The biggest breakthrough thus far has been the discovery of a

reservoir that flowed oil at 691.88 bbl/day and gas at 158,545 m3/day (5.61 MMSCFD) in a Lower to

Middle Cambrian dolostone reservoirs (Wang et al., 2014). Elements crucial for future exploration

success in deeply buried Cambrian reservoirs include: better understanding of the distribution of source

rocks, secondary migration patterns and its link to structural evolution (especially the location of

palaeohighs), seal, and, most importantly, the origin, characteristics, and distribution of the porosity

and reservoirs.

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This investigation presents new data from recently cored wells that encountered potential Cambrian

dolomite reservoirs across the Tarim Basin, as well as data from outcrop samples of equivalent

Cambrian strata in the western part of the basin. Techniques employed include conventional core

description, wireline-log analysis, transmitted-light petrography, cathodoluminesence (CL) analysis,

and scanning electron microscope (SEM) petrography, fluid-inclusion analysis, and C, O and Sr

isotopic data to help define the diagenesis of the dolostone reservoirs that are associated with the

evaporites, and how the diagenesis impacted pore evolution. Specifically, we address the following

questions:

(1) What depositional model explains the Lower to Middle Cambrian strata in the Tarim Basin?

(2) What is the paragenetic sequence for the formation of dolomite in these reservoirs and what role did

evaporites play?

(3) How has diagenesis impacted reservoir quality in these evaporite-related dolomite reservoirs?

GEOLOGICAL SETTING

The Tarim Basin is an intracratonic basin surrounded by the Tianshan Mountains to the north, the

Kunlun Mountains to the southwest, and the Algyn Mountains to the southeast (Fig. 1). From bottom to

top, the Cambrian stratigraphic section has been subdivided into six formations (Fig. 2): Yuertusi

Formation (Є1y), Xiaoerbulake Formation (Є1x), Wusongger Formation (Є1w), Shayilike Formation

(Є2s), Awatage Formation (Є2a), and Qiulitage Formation (Є3q). Є1y is characterized by deep basin

facies, whereas Є1x is dominated by platform facies and platform-margin reef and shoal facies (Fig. 3).

Strata associated with sabkha and other evaporitic subenvironments (e.g., a hypersaline lagoon) are the

predominant facies in Wusongger Formation and Awatage Formation. Shayilike Formation is

characterized by platform-interior shoal and bank facies. The uppermost Qiulitage Formation is

dominated by restricted carbonate platform facies.

During the Cambrian, the Tarim plate evolved into three isolated carbonate platforms (Western Tarim

platform, Western Lop Nor platform, and Kuruktag platform) with relatively deep water sedimentation

in between (Zhao et al., 2011). Six third-order sequences have been recognized in the Cambrian

section, based on a combination of detailed core and outcrop observation, facies analysis, and seismic-

based sequence stratigraphic interpretation (Fig. 2) (Liu et al., 2012; Zhao et al., 2011). The most

representative and largest Western Tarim platform evolved from an isolated open platform to an

isolated restricted platform during the early Cambrian. From the end of the early Cambrian to the

middle Cambrian, this platform was dominated by evaporites, and it evolved again into a restricted

open platform, from the end of the middle Cambrian to the late Cambrian (Fig. 3). The burial and

geothermal histories of different tectonic units of the Tarim Basin have been reported previously (Qiu

et al., 2012; Ye, 1994). The Cambrian strata were rapidly buried to depths greater than 5,000 m to

8,000 m (Figs. 5A, B, C). The basin was then uplifted between 2,000 m and 3,500 m before it

continued subsiding to its current depth. In contrast, the burial history of the Keping outcrop area in

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the northwest Tarim Basin shows that the Cambrian strata were buried to about 4,000 m during the

middle Permian. Subsequently, the whole section was uplifted to the surface in the early Triassic (Fig.

4D).

The widespread, lowermost Cambrian basinal facies mudstones and black shales are ubiquitous over

the whole Tarim Basin. These rocks are considered to be the main source rock for Cambrian reservoirs,

with total organic carbon (TOC) values ranging from 1.0 to 3.0%. Organic matter is dominated by type

I and II1 kerogen, with derived vitrinite reflectance equivalences (VRE) ranging between 1.5 and 2.4%.

Peak oil generation from the Cambrian source rocks occurred during the late Caledonian to early

Hercynian (~ 439-362 Ma) (Cai et al., 2015; Li et al., 2010). Hydrocarbons migrated upward to the

overlying Cambrian and Ordovician dolomite reservoirs utilizing open faults and fractures (Jiang et al.,

2015a).

METHODS

Approximately 200 core and outcrop samples of Cambrian carbonate (mostly dolomite) were collected

from ten exploration wells and two outcrop sites (Keping and Xiaoerbulake) across the Tarim Basin

(Fig. 1). A representative suite of samples was then selected for detailed petrological (optical

transmitted light and cathodoluminescence microscopy, and scanning electron microscopy), fluid

inclusion and isotopic (carbon, oxygen and strontium) analysis. A total of 120 polished thin sections

were prepared from dyed resin impregnated samples and a small subset was stained with alizarin red S

to assist with identification of calcite and dolomite. All sections were subsequently examined by optical

transmitted light and cathodoluminescence (CL) microscopy. CL observations were made using a

Relion III ‘cold’ cathode device. Operating conditions for CL were set to 15 kV and 500 A.

Additional mineralogical and textural observations were made on carbon-coated thin sections using a

Philips XL30 scanning electron microscope (back-scattered electron mode) coupled with an energy

dispersive X-ray analyser.

Fluid inclusion microthermometic measurements were undertaken on individual types of dolomite

using double-polished wafers. Populations of inclusions were carefully characterized (size, distribution,

liquid:vapour ratio, and presence of hydrocarbons) by transmitted light and UV fluorescence

microscopy, and last ice melting (Tm) and homogenization temperatures (Th) of primary or pseudo-

secondary inclusions were measured with a Linkam THMSG 600 fluid inclusion stage. Ice melting

temperatures were converted to salinity values (equivalent wt.% NaCl) using standard equations

(Bodnar, 2003; Oakes et al., 1990). No pressure corrections were applied to Th values because the

pressure state of the fluid system at the time of mineral precipitation is unknown.

Samples of ‘bulk’ limestone, discrete dolomitization fabrics, and dolomite cements were extracted from

clean rock surfaces for carbon δ13C, oxygen δ 18O, and strontium (87Sr/86Sr) isotope analysis using a

tungsten-tipped dental burr. Carbon and oxygen isotope ratios were measured from samples of calcite

and dolomite by reacting 4 to 5 mg of powder with ‘anhydrous’ phosphoric acid at 25 C (~16 h) and

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50C (~48 h) respectively. Produced CO2 was recovered cryogenically and mass ratios were measured

using a dual-inlet VG SIRA 10 mass spectrometer. Oxygen isotope ratios were corrected for 17O effects

and adjusted for temperature-dependent fractionations associated with the carbonate-phosphoric acid

reaction using fractionation factors () of 1.01025 for calcite (Friedman and O'Neil, 1977) and 1.01066

for dolomite (Rosenbaum and Sheppard, 1986). Isotopic ratios are reported as delta values with respect

to the VPDB carbon and oxygen isotope scales. Analytical precision (1) for both calcite and dolomite

is better than ±0.1‰, based on replicate analysis of in-house, quality control materials. Samples for

strontium isotope ratio measurements were prepared by dissolving approximately 60 mg of powder

using ultra-pure acids. Strontium released by acid decomposition was separated by conventional ion

exchange techniques and isotope ratios were measured using a Finnigan MAT-262 thermal ionization

mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences

(IGGCAS), Beijing. All sample 87Sr/86Sr values were normalized to an NBS-987 ratio of 0.710253.

Analytical precision (2) was monitored by repeated measurement of NBS-987 and is better than

0.000015.

RESULTS

DESCRIPTION OF DIAGENETIC PRODUCTS

Cambrian strata from the Tarim Basin are dominated by dolomites and evaporites, with only a few

limestone and siliceous units. Some shales are present in the lowermost Cambrian (Fig. 2). The detailed

diagenetic sequences are summarized in Figure 5. The authigenic minerals are discussed below in order

of decreasing relative abundance.

Dolomite

Dolomite is the dominant mineral in these strata. Several different types of dolomite can be recognized,

with a range of crystal sizes between 10 and 1,000 μm. Three types of dolomite are: dolomudstone,

fabric-retentive dolomite and fabric-destructive dolomite (Fig. 6). Large limpid and saddle shaped

dolomite cement crystals locally filled the open pores in fabric-destructive sucrosic dolomite. Saddle

dolomite has orange cathodoluminescence (Fig. 6F); however, all the other dolomites show very dull

red cathodoluminescence (Figs. 6D, F).

Anhydrite

Anhydrite occurs as a volumetrically significant mineral in the upper Early to Middle Cambrian (from

0% at the platform margin to up to 60% at the paleo-lagoon center); the total thickness of anhydrite-

rich strata reach up to 450 m (Figs. 1 and 7). Anhydrite occurs as beds (up to tens of centimetres thick),

nodules (range in diameter from about 500 μm to several centimetres), and cements with a wide range

of crystalline sizes and shapes from less than 50 μm to centimetres (Fig. 7). These anhydrites probably

transformed from gypsum during burial. Where abundant, anhydrite significantly decreases core plug

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porosity. However, breccia and associated fracture porosity are common in anhydrite-bearing sections

(Figs. 7G, H).

Chalcedony and Quartz

There are two main types of silica minerals in the Cambrian strata. The first type is chalcedony, which

is associated with dolomicrite and anhydrite nodules. Chalcedony commonly occurs as small patches

(<20 μm) and is typically irregular and fibrous (Fig. 8A). The second type is vug/fracture-filling quartz

with euhedral crystal size from 50 to >1,000 μm (Fig. 9B).

Halite

Sedimentary halite has been found in well H6 and well F1, which is located in the platform interior

where evaporites are mostly abundant (up to 450 m) (Fig. 2). In some cores, halite also occurs as

fracture fillings (Fig. 8G).

Calcite

Calcite is volumetrically minor in the Lower to Middle Cambrian units. However, some late stage

vuggy and fracture-filling calcite is present (Fig. 8B). Calcite occurs as a coarsely crystalline spar fill in

fractures and vuggy pore spaces.

Pyrite

Rare pyrite occurs as cubic crystals up to a millimetre in size, and as framboidal aggregates composed

of micrometer crystals. Most pyrite occurs as a trace mineral in the dolostone (Figs. 8C, F), and

commonly as the late cements after fabric-destructive dolomite formation. Individual crystals are

euhedral to subhedral, and are generally several micrometres in size.

Celestite

In the Lower to Middle Cambrian units, celestite commonly occurs as a trace mineral and is typically

associated with anhydrite nodules or anhydrite-filling in early moldic pores (Fig. 8D). Celestite occurs

as up to 200 m crystals (Fig. 8D) and it appears to have formed relatively late (later than moldic pore

development).

Fluorite

Fluorite occurs as a pore-filling mineral (Fig. 8E) and is associated with late stage dolomite cement in

the Lower to Middle Cambrian units. Fluorite is a trace mineral, occurring as up to 100 m crystals

(Fig. 8E). Given that fluorite occurs in association with saddle dolomite, it is here interpreted to have

formed late in the paragenetic sequence.

K-feldspar

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K-feldspar is locally present in dolomudstone as small (<50 μm) grains or crystals with euhedral to

subhedral shapes (Fig. 8F). Given the extremely low solubility of the main components within this

aluminosilicate, K-feldspar may originally have had a detrital origin although the euhedral to subhedral

shape suggest diagenetic recrystalisation (Worden and Rushton, 1992). The presence of K-feldspar

along with quartz (see previous) may represent a minor amount of clastic deposition.

Bitumen

Bitumen occurs as fracture fillings both in core samples that have abundant evaporite and carbonate

breccias (Fig. 7H), as well as in outcrops (Fig. 8H). Bitumen is commonly present both in primary

pores and in secondary dissolution pores in the microbial carbonates (Fig. 9D).

PORE SYSTEMS

We have adopted the pore type classification terminology employed by Lucia (1995) and Choquette

and Pray (1970). The Lower to Middle Cambrian evaporite-related carbonate reservoirs contain several

different types of pores. Secondary porosity potentially developed in a wide range of diagenetic

environments, from the eogenetic to mesogenetic stages (shallow to moderate burial depths). The

following pore types were observed, in order of importance, are: dissolution enlarged interparticle

pores and reef framework pores, primary framework pores, irregular vugs, interparticle pores, anhydrite

moldic pores, intercrystalline pores, breccia fracture pores, and saddle dolomite intracrystalline pores.

Solution-Enlarged Interparticle

Solution-enlarged interparticle pores, between dolomitized microbial and ooids grains, occur in both

platform margin and interior shoal facies in the Lower to Middle Cambrian units. The host dolostones

are comprised of fine- to medium-crystalline, fabric-destructive dolomite. This pore type is

characterized by large pore sizes (up to 500 μm across) between the partially dissolved dolomitized

grains (Figs. 9A, B, and C). Dissolution-enlarged pores are locally filled with pyrite, quartz, fluorite,

and bitumen. Visual porosity estimation of this pore type is up to 15% in ZS-5 well. Dolostone

reservoir rock dominated by solution-enlarged interparticle porosity in other parts of the Tarim Basin

ranges from 0.9% to 12.6%, with average porosity value of about 4% and permeability values are in a

narrow range from 2.8 to 3.3 mD, with average value of 3 mD (Table 1). This is volumetrically the

most important pore type in these sediments, and it show good connectivity (51.8% interconnection

reported by Shen et al. (2016); Table 1).

Framework

Microbial reefs and mounds are common in the Lower to Middle Cambrian units. Epiohyton and

cyanobacteria-enriched clots are the two main reef-building organisms. The dendrolites, thrombolites

and stromatolites in the reefs and mounds represent potential reservoirs; they are characterised by

framework pores up to 3 mm in size (e.g., spongiostromata stromatolite bed; Figs. 9D, E). Visual

characterisation shows that this pore type ranges from about 500 μm to several mm in diameter.

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Dolostone reservoir rock dominated by framework porosity ranges from 1.3% to 10.6%, with average

porosity value of about 5% (Table 1). Permeability values span a wide range from 0.02 to 50 mD, with

average value of 4.7 mD (Table 1). This is the second most volumetrically important pore type, and it

show good connectivity (56.2% interconnection reported by Shen et al. (2016); Table 1).

Irregular Vug

Irregular solution-enlarged vugs have three main occurrences. The first two types of solution-enlarged

vugs occur in the reef-shoal facies where interparticle and reef framework pores have been enlarged by

dissolution (Figs. 9A, D). The third type of irregular solution-enlarged vug occurs in tidal-flat facies

that are enriched in evaporites and are associated with subaerial exposure surfaces.

Anhydrite Moldic

This pore type is commonly observed in supratidal dolomudstones which contain anhydrite cement in

vugs and fractures. These anhydrite-dissolution pores formed late after burial, likely later than the

formation of vugs and fractures. Anhydrite dissolution pores are relatively large being up to 1 mm in

diameter. Visual porosity estimates of this type of pore based on point counting can be as high as 12%.

Anhydrite dissolution pores provide a significant contribution to porosity in the evaporite-bearing

carbonate strata. Evaporite enriched dolostone reservoir dominated by anhydrite moldic porosity ranges

from 0.14% to 6.2%, with average porosity value up to 5.3% (Table 1). Permeability values are in a

wide range from 0.004 to 202 mD, with average value of 11.6 mD (Table 1). This is the third most

volumetrically important pore type, and it show good connectivity with 78.8% interconnection

reported (Shen et al. (2016); Table 1).

Intercrystalline Pores

Most porous dolostones in the Lower to Middle Cambrian have intercrystal pores (Figs. 9G). The host

dolostones are predominantly dolomudstones or finely crystalline, fabric-retentive dolomite. Visual

estimation of this pore type based on point counting is locally as high as 10%. These pores are locally

filled with anhydrite cement, suggesting that they most likely formed during early dolomitization. This

pore type also has significantly contributed to porosity in the evaporite-bearing carbonate.

Breccia and Fractures

In the anhydrite nodule-bearing sections, e.g., the Lower Cambrian Wusongger Formation (Є1w) and

the Middle Cambrian Awatage Formation (Є2a), breccias and fractures porosity (Fig. 7E, Fig. 9H) are

abundant and widespread. Parts of these pores are filled by anhydrite cement, but some remain open.

Visual estimation of this pore type based on point counting can reach to about 10%. Bitumen is

commonly present in these fractures. Evaporite enriched dolostone reservoir dominated by breccia and

fracture porosity from the other part of the Tarim ranges from 1.5% to 9.3%, with an average value of

about 4.2% (Table 1). This is a volumetrically relatively unimportant pore type, and shows relatively

poor connectivity with 15.8% interconnection reported (Shen et al. (2016); Table 1).

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Saddle Dolomite Intercrystalline

Some relatively late saddle dolomite cements occur in vugs and fractures; secondary porosity due to the

dissolution of saddle dolomite (Fig. 9I) represents the latest dissolution stage. The saddle dolomite

dissolution pores are up to 200 μm. Although commonly observed, this pore type is not a dominant

pore type and does not contribute significantly to porosity.

GEOCHEMICAL RESULTS

Stable Carbon and Oxygen Isotopes

Stable isotope analyses (O and C) were performed on limestone and dolomite samples. The analytical

results are presented in Figure 10. Five limestone samples (micrite) display a relatively narrow isotopic

range, lying between -2.1 and 0.0 ‰ VPDB for δ13C, and between -7.3 and -5.9 ‰ VPDB for δ18O

which lie within the range expected for calcite precipitated in equilibrium with Cambrian seawater

(Fig. 10) (Montañez et al., 2000; Veizer et al., 1999).

δ13C and δ18O values of different group of carbonate minerals show distinct ranges (Fig. 10).

Specifically, bulk rock dolomite samples show δ13C values ranging from -6.5 to 2.9 ‰, and δ18O values

ranging between -8.5 and -5.6 ‰. The samples taken from medium- to coarsely crystalline- dolomite

cement and saddle dolomite cement have a narrow δ13C and δ18O range (-4.3 to 0.2 ‰ and -11.8 to -7.7

‰ respectively). In comparison, late stage calcite cements have relatively negative δ13C and δ18O

values in ranges between -11.6 ‰ and -2.2 ‰, as well as -14.5 ‰ and -9.6 ‰, respectively.

Strontium Isotope

Four limestone samples yielded a narrow range of 87Sr/86Sr from 0.7088 to 0.7094 (Fig. 11), lying

within the reported range of Cambrian seawater strontium isotope values (McArthur et al., 2001;

Montañez et al., 1996; Montañez et al., 2000; Veizer et al., 1999). The bulk dolomite samples,

including dolomudstone, fabric-retentive dolomite, fabric-destructive dolomite have narrow, broadly

similar ranges of 87Sr/86Sr mainly from 0.7087 to 0.7093, except one dolomicrite sample that has a

relatively high 87Sr/86Sr value of 0.7098 (Fig. 11). However, dolomite cements (including saddle

dolomite) have higher 87Sr/86Sr values, falling between 0.7096 and 0.7100 (Fig. 11).

Fluid-Inclusion Studies

Selected samples were used for fluid inclusion study. Fluid inclusion assemblages (FIAs) are defined

following the rules presented by Goldstein and Reynolds (1994), and Goldstein (2012). A consistent

FIA is defined as one in which more than 90% of homogenization temperature (Th) data from a given

authigenic mineral fall within a range of less than 10-15°C, suggesting it is therefore unlikely that

thermal re-equilibration has occurred.

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Transmitted-light, CL and SEM imaging were used to identify the origin of inclusions. Single phase

aqueous, 1 to 5 μm in size, primary inclusions were found in fabric retentive dolomite and early

crystalline dolomite cement. Single phase liquid inclusions imply that the growth of the host mineral

occurred below about 50°C (Goldstein and Reynolds, 1994).

Two-phase, liquid-gas, primary fluid inclusions were found in coarsely crystalline, fabric destructive

dolomite, medium- to coarse-crystalline dolomite cements, and saddle dolomite cements, as well as in

vug/fracture filling calcite and anhydrite cements. These fluid-inclusions sizes varied from about 3 μm

to 15 μm in the longest dimension. Measured homogenization temperatures fall predominantly between

90°C and 190°C for saddle dolomite, whereas fabric destructive dolomite and medium- to coarse-

crystalline dolomite cements show a relatively lower and narrower temperature range from 90°C to

130°C (Fig. 12A). Salinity data from the fluid inclusions, derived from observed measurements of last

ice melting temperatures, indicate that water present at the time of growth of fabric destructive

dolomite and medium- to coarse-crystalline dolomite cements ranged from 6 wt.% to less than 14 wt.

%, although a few samples have reached 20 wt.% (Fig. 12B). In contrast, saddle dolomite samples

precipitated from much more saline water with salinities ranging from 10 wt.% to 26 wt.%, with most

salinity values greater than 18 wt.% (Fig. 12B).

Vug/fracture calcites and anhydrite cements show the overlapped temperature ranges between 110°C to

150°C, although anhydrite has a wider temperature ranges, down to 110°C and up to 160°C (Figs. 12

C, E). Salinity data indicate that water present at the time of growth of calcite and anhydrite was as

saline as the water that precipitated saddle dolomites, with salinity ranging from 16 wt.% to 26 wt.%

(Figs. 12 D, F).

DISCUSSION

FACIES AND DEPOSITIONAL MODEL

The Lower to Middle Cambrian units in the Tarim Basin are composed of ten carbonate and evaporite

lithofacies and one siliciclastic lithofacies that collectively represent four major types of

paleoenvironment: platform margin reef and shoal, restricted inner platform, evaporated lagoon, and

inner platform shoal (Fig. 13; Table 1).

The reconstructed paleogeography of the facies/environments of the Tarim plate during the Cambrian

suggests that the Tarim Basin had an isolated platform configuration, rimmed by platform margin

microbial shoals and reef barriers (Figs. 9D, E), and with microbial mounds in the inner platform (Shen

et al., 2016; Zhao et al., 2011). Epiphyton and cyanobacteria-enriched clots are two main reef-building

organisms, which commonly have growth associated with dendrolites, thrombolites and stromatolites

(Song et al., 2014). These rocks contain well-developed framework and interparticle pore spaces; they

constitute the primary reservoir interval of the Lower Cambrian in the basin. Their extensive and easily

correlatable units make up the Xiaoerbulake Formation in the Lower Cambrian.

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During the Early to Middle Cambrian, the Tarim Basin was dominated by restricted to hypersaline

platform environments where a giant evaporated lagoon developed due to the arid climate (Shen et al.,

2016; Zhao et al., 2011). Thin section analyses revealed that inner platform shoal sub-facies coexisted

with, and are located between, the platform-margin facies and the evaporite lagoon center during the

Lower to Middle Cambrian (Fig. 3). A depositional model of this Lower to Middle Cambrian unit is

presented in Figure 13. Rocks interpreted to represent deposition in an inner platform shoal setting

include microbial grainstones and packstone (Figs. 9A, B, C). These deposits are composed of

abundant algae and microbial deposits and contain well-developed interparticle and vug pore spaces.

These pores represent good potential reservoir in the basin.

The isolated inner platform was dominated by a hypersaline lagoon with a restricted inner platform

facies. This setting allowed the precipitation of thick gypsum- (now transformed into anhydrite)

enriched carbonate deposits. These are composed of breccia with well-developed fractures that are

interbedded with mudstone and skeletal wackestone from the restricted inner platform facies. From the

cross-section correlation in Figure 3, the thicknesses of evaporite-bearing carbonate rocks decrease

from the platform center (up to 450 m) to both of the bounding platform margin rims (no evaporite

deposits) (Fig. 3). Breccias and fractures (Figs. 7G, H) are well-developed in evaporated lagoon facies,

whereas anhydrite moldic (Fig. 9F) and intercrystalline (Fig. 9G) pore spaces are well-developed in

restricted inner platform facies. These types of porosity likely represent great potential reservoir

interval in the basin (Shen et al., 2016). Halite (Fig. 8G) has been observed associated with anhydrite

and dolomudstone in some core samples in the platform center area; halite probably represents the

evaporitic center of the lagoon.

INTERPRETATION OF DIAGENETIC SEQUENCE

Dolomitization, Evaporites and Chalcedony Precipitation

The earliest diagenetic events in the Lower to Middle Cambrian unit in the Tarim Basin were

dominated by dolomitization, gypsum precipitation, and local chalcedony precipitation. In most of the

platform facies, anhydrite is closely associated with dolomudstone and dolowackestone (Fig. 3),

indicating that anhydrite precipitation and dolomitization were nearly simultaneous. In restricted and

evaporitic environments, evaporation of seawater and precipitation of sulfate minerals (e.g., gypsum

and anhydrite) resulted in increasing of Mg:Ca ratio and promoted dolomite precipitation.

Dolomitization then decreased the Mg:Ca ratio of the pore fluid in the sediment and increased the Ca2+

concentration which, in turn, promoted gypsum formation. Growth of gypsum subsequently resulted in

an increase in the Mg:Ca ratio. Hence, dolomitization and gypsum precipitation in sabkha and

restricted lagoon settings were sustainable as long as there was periodic input of seawater.

The lithology of the Lower to Middle Cambrian, as shown in Figure 3, and the sedimentary facies,

suggest that restricted inner platform and other evaporitic subenvironments (e.g., hypersaline lagoon)

existed in the backshore adjacent to the shoreline within an isolated carbonate platform. The

interpretation of reflux dolomitization by evaporitic brine is supported by relatively heavy oxygen

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compositions (highest measured δ18O values, from -5 to -6.5 ‰ with even higher published values δ18O

values; Fig. 10). Refluxing brine could percolate into the underlying porous carbonate deposited in the

high-energy facies, and promote dolomitization (Jiang et al., 2014b; Jones and Xiao, 2005).

δ13C values (Fig. 10) of all limestone samples and most of the bulk dolomite rock samples lie within the

reported range of coeval seawater δ13C values (Montañez et al., 2000; Veizer et al., 1999). Furthermore, 87Sr/86Sr values of these limestone and dolomite bulk rock samples lie in the narrow range from 0.7080

to 0.7094, typical of Cambrian seawater 87Sr/86Sr values (Montañez et al., 1996; Veizer et al., 1999).

Therefore, the isotopic compositions of δ13C and 87Sr/86Sr support the conclusion that dolomitization

fluids for the most of dolomite bulk rocks were caused by evaporation of Cambrian seawater. However,

some dolomudstone samples have relatively high 87Sr/86Sr value of ~ 0.7098 (Fig. 11). These

anomalous dolomite samples are locally enriched in grains of K-feldspar identified in the SEM (Fig.

8F). We speculate that the exotic K-feldspar associated with dolomudstone may have been sourced

from wind-borne sediment derived from the Cambrian Tarim plate .

The negative δ13C values (less than -6 ‰; Fig. 10) for some dolomudstone from lagoonal facies, the

abundance of dolomitized burrows especially in the Middle to Upper Cambrian unit, and the presence

of extracellular polymeric substances (EPS) in the Upper Cambrian stromatolites (You et al., 2013),

demonstrate that some microbially-mediated dolomite could have precipitated. Many dolomudstones

and fabric-retentive dolomite samples have well-developed intercrystalline porosity (Fig. 14), which is

most likely related to the above early dolomitization events. However, signs of continued

dolomitization and growth of pore-filling cements (e.g., dolomite, calcite, quartz, anhydrite) are also

common in these dolostones and they significantly decreased porosity in areas near the source of

dolomitizing brines, for example in the hypersaline lagoon. (Fig. 14) (Saller and Henderson, 1998). In

addition, chalcedony has been observed closely associated with anhydrites comprised of small (<20

μm) and irregular crystals (Fig. 8A) suggesting that it precipitated from locally SiO 2-saturated water at

low temperatures. No two phase fluid inclusions, and only a few single phase fluid inclusions have

been found in dolomudstone, fabric-retentive dolomite, and in the associated anhydrite and chalcedony.

The presence of only single phase aqueous inclusions indicates that the diagenetic growth temperature

for these minerals are lower than 50°C (Goldstein and Reynolds, 1994).

Early Dissolution, Breccias and Fractures

Some breccias and their associated fractures are common in anhydrite-bearing dolostones (Fig. 14).

Brecciation and fracturing may have occurred immediately below subaerial exposure surfaces and were

probably caused by meteoric water percolation during subaerial exposure. However, the lack of

freshwater cements and the presence of anhydrite nodules, suggest that meteoric diagenesis was

unlikely to have been persistent for long periods of time (Loucks, 1999). In addition, the dissolution of

carbonate minerals may also occur in dolomitizing water because it is undersaturated with respect to

calcium carbonate (Saller and Henderson, 1998; Saller et al., 2014). Breccia-associated fractures are

open or infilled by anhydrite and/or bitumen, suggesting that these fractures were formed before the

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precipitation of fracture-filling anhydrite and oil charging. It also implies that these breccia fractures

may have served as high permeability pathways for fluid migration, including oil and gas.

Sulfate Genesis

Anhydrite precipitated during multiple stages from the time of deposition through to early and then

burial diagenesis. Core-based observations and detailed petrographic analysis lead to the interpretation

that at least some of the bedded anhydrite precipitated in saline lagoons during deposition. In contrast

other sulfate minerals formed during, or after, early dolomitization, brecciation and fracture formation.

The lithological association of anhydrite layers with interlayers of anhydrite cement and nodule-

enriched dolomudstone (Fig. 7), suggests these sedimentary anhydrite and/or gypsum units precipitated

on the floor of a saline lagoon and/or sabkha environment on an arid restricted platform (Warren and

Kendall, 1985). Many isolated anhydrite nodules and patches of cement occur in the lower part of the

anhydrite-enriched sections. These isolated anhydrite nodules and cements have a range of crystal sizes

suggesting that anhydrite grew in a range of diagenetic environments from shallow to deeper burial

(Saller et al., 2014). Fracture-filling anhydrite has relatively high homogenization temperatures (from

100 to 160 °C) measured from the fluids inclusions (Fig. 12) which confirms its deep diagenetic in

origin. Calcite rim cements have been observed on the outer edge of some anhydrite nodules. Celestite

also occurs in some nodules, with crystals typically being bigger than co-existing anhydrite crystals in

the same nodule, indicating that celestite might have formed later than the anhydrite nodule (Saller et

al., 2014).

Burial Dolomitization

Coarsely crystalline (sizes from ~ 200 μm to >1000 μm) dolomite and pore-filling dolomite cement,

including saddle dolomite cement, commonly occur in many thin sections. These dolomites commonly

have larger crystal sizes and are planar-s to non-planar-a, and grew in pores on top of early diagenetic

dolomites (Fig. 6), suggesting that they probably precipitated during burial diagenesis. δ13C and 87Sr/86Sr isotope values of these dolomites are comparable to the early dolomites and remaining

limestones (Figs. 10 and 11), possibly demonstrating that connate Cambrian seawater was the

predominant fluid agent of burial dolomitization. However, negative δ18O values, -9.0 to -7.5 ‰ VPDB

for coarsely crystalline dolomites and -12.0 to -9.0 ‰ VPDB for saddle dolomites, the relatively higher

growth temperatures 90 to 130°C for coarsely crystalline dolomite and 90 to 170°C for saddle

dolomite, confirm that these types of dolomite formed during burial at a depth range from about 2,000

to >3,500 m (Fig. 4), and some may be related to a regional hydrothermal event (Dong et al., 2013).

Elevated 87Sr/86Sr ratio imply the addition of deeper, hotter basinal fluids that leached Sr from

potassium-bearing rocks (e.g., clastic sediments or K-rich igneous rocks or K-feldspar) (Cai et al.,

2008; Jiang et al., 2015a). The local presence of fluorite and quartz cement in the open, dissolution-

enlarged pores is a possible further indication of hydrothermal fluids.

Deep Burial Dissolution Pores and Calcite Cements

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Abundant dissolution pores (Fig. 9) are present in vug/fracture filling anhydrite in the Lower to Middle

Cambrian unit, suggesting that these dissolution pores developed after anhydrite precipitation. The

homogenization temperatures measured from anhydrite demonstrate that they precipitated at high

burial temperatures ranging from 100 to 160°C (Fig. 12). Last ice melting temperature data obtained

from fluid inclusions in these anhydrites reveal highly saline fluid (20-26 wt.%) was responsible for

anhydrite growth (Fig. 12). Vug/fracture filling calcite (infilling the voids that resulted from anhydrite

dissolution) with relatively low δ13C values (down to -12 ‰ VPDB, Fig. 10) is present in a few

samples. These calcites precipitated at high temperatures (from 110 to 150 °C) and from high salinity

(from 16 to 24 wt.%) fluids (Fig. 12). Hence, the above findings support an interpretation that

thermochemical sulfate reduction (TSR), an abiological oxidation of hydrocarbons by sulfate at

elevated temperatures, occurred and was responsible for anhydrite dissolution and calcite precipitation

(Bildstein et al., 2001; Cai et al., 2001; Cai et al., 2003; Jiang et al., 2015b; Machel, 1987; Worden et

al., 2000).

A general reaction can simply be written as follows:

sulfate + petroleum calcite + H2S ± H2O ± CO2 ± S ± altered petroleum (R1)

Fluid inclusion data from the present study show that TSR calcites grew from slightly lower salinity

water compared to the water from which anhydrite precipitated (Fig. 12), the latter represented water

salinity that TSR started. Therefore, some low salinity water was likely produced and added to the

formation water in the Lower to Middle Cambrian unit in the Tarim Basin during TSR. Similar cases

have shown that significant amounts of fresh water (up to five time of the original formation water

before TSR) had been generated and added in the Feixianguan Formation from Sichuan Basin (Jiang et

al., 2015b), in the Permian Khuff Formation from Abu Dhabi (Worden et al., 1996), and in the

Devonian fields from Western Canada Sedimentary Basin (WCSB) (Yang et al., 2001). Moreover,

recent research on TSR modelling also suggests the generation of some water (Fu et al., 2016). Pyrite

has commonly been observed growing at the edge of solution-enlarged pore spaces and vugs. This

pyrite could be formed and generated some acidic fluids during and/or after TSR via either reaction

(R2) or (R3) (Jiang et al., 2014a; Liu et al., 2013); siderite (FeCO3) represents the Fe-rich component in

dolomite.

Fe2+ + 2H2S FeS2 + H2 + 2H+ (R2)

FeCO3 + 2H2S FeS2 + CO2 + H2O + H2 (R3)

Hence, at least near the site of pyrite precipitation, the acidity of diagenetic fluids must have been

increased by releasing H+. As a consequence, carbonate dissolution may have occurred during and after

pyrite precipitation, which contributed to the formation of dissolution-enlarged pore spaces (Fig. 9).

Alternatively, Cai et al. (2014) proposed the following TSR reaction in which anhydrite and dolomite

may have been dissolved or replaced by TSR calcite to explain new generation or rearrangement of

porosity.

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CaSO4 + CaMg(CO3)2 + hydrocarbons + H+ →

CaCO3 + H2S + Mg2+ + Ca2+ + altered hydrocarbons (R4)

Our petrological data suggest that there are substantial amounts of pore spaces (e.g., solution-enlarged

interparticle pores and vugs (Figs. 9 and 15) that are due to TSR diagenesis. Based on detailed

petrological and geochemical study of the WCSB, Hutcheon et al. (1995) modelled the effect of TSR

on the reservoir quality and concluded that three quarters of the porosity created by dissolution of

anhydrite, in a closed system, was lost by calcite precipitation, and a net additional 1 to 2% porosity

unit was gained because of TSR.

Potentials of Deeply Buried Evaporite-related Dolostone Reservoirs in The Tarim Basin

The total porosity (VR) can be calculated by multiplying the total rock volume (VTR) by fractional

porosity (P):

VR = VTR × P (Equation 1)

The total rock volume (VTR) can be obtained by the multiplying of the effective area (SEA) of the

reservoir by the reservoir's thickness (HT):

VTR = SEA × HT (Equation 2)

The effective area (SEA) of the potential reservoirs in the Lower to Middle Cambrian units in the Tarim

Basin is about 3 × 105 km2 (Wang et al., 2014). The thickness of the potential evaporite and carbonate

reservoir in the upper part of the Middle Cambrian Awatage Formation and upper Cambrian

Wusonggeer Formation lies in a range of 200-340 m and 110-150 m, respectively (Wang et al., 2014).

The thickness of the potential evaporite-related carbonate reservoir in the lower part of the Middle

Cambrian Sayilike Formation and lower part of the Lower Cambrian Xiaoerbulake Formation lies in a

range of 15-50 m and 50-70 m, respectively (Wang et al., 2014). Hence the total potential reservoir

thickness (HT) of the Lower to Middle Cambrian is between 375 m and 610 m, with average value of

about 500 m. Based on the porosity data in Table 1, the average porosity value (P) is about 5% for

these deeply buried potential reservoirs. Therefore, the total pore volume (VR) for petroleum resources

is about 7.5 × 1012 m3. Assuming 1 to 2% of the current porosity (P-TSR) in the Cambrian reservoirs

was gained during diagenesis (mainly by TSR), then the calculated reservoir volume due to TSR

processes (VR-TSR) is from about 1.5 × 1012 to 3 × 1012 m3.

Four reservoir types, that are representative of the Lower to Middle Cambrian carbonate and evaporite

reservoirs, are summarized in Table 1. They are shoal and reef complex, isolated shoal and mound,

evaporite-related carbonate, and evaporite enriched breccias. Each reservoir type is characterized by a

different pore type, porosity, permeability, pore interconnection percentage. Generally, reservoir

quality has been controlled by sedimentary facies, within which primary porosity was greatly modified

during diagenesis.

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Breccia, fracture and intercrystalline porosity tend to be preserved in anhydrite-bearing dolomudstone

in the evaporated lagoon facies in the Lower and Middle Cambrian units. This reservoir type has

relatively low pore interconnection (15.8%; Table 1) which suggests that it is may not a good reservoir

type. Evaporite-related carbonate that is enriched in anhydrite moldic and intercrystalline porosity has a

high degree of pore interconnection (78.8%; Table 1). The Lower to Middle Cambrian reservoirs in the

Tarim Basin have a total thickness of about 310-490 m. Hence, there is great potential for evaporite-

related carbonate reservoirs in the future exploration in the Tarim Basin.

Shoal and reef complex and isolated shoal reservoirs in the Lower to Middle Cambrian unit (Fig. 13)

are characterized by dissolution-enlarged porosity and vug porosity, which were predominantly derived

from primary porosity (e.g., framework, interparticle). These two reservoir types commonly have the

best reservoir quality and have relatively good interconnection (from 51.8 to 56.2%; Table 1). The total

thickness for these two reservoirs is from about 65 to 130 m. Hence, these two reservoir types,

especially the shoal and reef complex reservoir, also deserve more attention during future exploration.

Intercrystalline porosity and breccia and fracture porosity (Fig. 14) were probably generated by a

relatively early stage of hypersaline dolomitization and/or very short period of meteoric water

dissolution during a time of an arid climate (Jiang et al., 2014a; Machel, 2004; Saller et al., 2014; Sun,

1995) (Fig. 14). Interparticle and framework porosity (Fig. 14) in high energy, shoal and reef facies in

the Lower to Middle Cambrian strata are the result of the preservation of the primary pores (Sun,

1995). Subsequent dissolution, e.g., bacterial sulfate reduction or thermochemical sulfate reduction

(Cai et al., 2014; Jiang et al., 2015b; Saller et al., 2014) (Fig. 14), may be responsible for an increase in

porosity during burial diagenesis. Vug, fracture, and late diagenetic dissolution pores also locally occur

and may be related to the burial corrosion due to sulfate reduction reactions (e.g., BSR and TSR). This

study shows that, despite being deeply buried (> 8,000 m), evaporite-related dolostone reservoirs can

be preserved and/or created by diagenetic processes and could host significant petroleum resources.

CONCLUSIONS

(1) An isolated platform existed during the Early to Middle Cambrian; sedimentary facies in this

restricted platform are dominated by evaporitic lagoon and restricted inner platform facies, where thick

gypsum-dominated evaporite-bearing sediments (up to 450 m) were deposited.

(2) The thicknesses of evaporites decrease from the inner platform center to the edge. Inner platform

shoal facies that are characterized by grainstones are also present. Mudstone and wackestone are

associated with evaporites in the giant evaporative lagoon within the inner platform facies. Packstone

and wackestone are the predominant rock type in areas between platform-margin and inner platform

shoal facies.

(3) Diagenesis has significantly modified the depositional mineralogy and pore systems of the Lower to

Middle Cambrian unit in the Tarim Basin. The most important diagenetic events that affected reservoir

quality were: (a) reflux dolomitization that occurred during shallow burial, (b) breccia and fracture

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formation related to meteoric water dissolution during short subaerial exposure periods, and (c)

thermochemical sulfate reduction (TSR) that occurred in deep burial environments.

(4) Intercrystalline pores and breccia-associated fractures comprise the dominant pore network in the

anhydrite-bearing dolomudstones. However, porous dolo-grainstone and dolo-packstone are enriched

in solution-enlarged pores/vugs.

(5) Late diagenetic, solution-enlarged pores are interpreted to be related to TSR that occurred at high

temperatures (100-160°C). The newly generated H2S reacted with the siderite component in earlier-

formed dolomite, as evidenced by pyrite, with the resulting protons causing localised dissolution. Small

volumes of TSR calcite with relatively low δ13C values down to -12 ‰ VPDB precipitated and

occupied some of the dissolution pores.

(6) Cambrian, evaporite-related carbonate rocks in the Tarim Basin host 7.5× 1012 m3 total reservoir

volume for potential petroleum resources, from which TSR has generated up to 3× 1012 m3. Hence, this

study suggests that these rocks can have considerable reservoir potential despite being buried to as

much as 8,000 m, and deep burial diagenesis dominated by TSR has significant contribution to the

reservoir development.

ACKNOWLEDGEMENTS:

This work has been financially supported by the Natural Science Foundation of China (Grant No.

41402132) and a China Postdoctoral Science Foundation award (Grant No. 2014M550835).

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Liu, Q. Y., R. H. Worden, Z. J. Jin, W. H. Liu, J. Li, B. Gao, D. W. Zhang, A. P. Hu, and C. Yang, 2013, TSR versus non-TSR processes and their impact on gas geochemistry and carbon stable isotopes in Carboniferous, Permian and Lower Triassic marine carbonate gas reservoirs in the Eastern Sichuan Basin, China: Geochimica et Cosmochimica Acta, v. 100, p. 96-115.

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McArthur, J. M., R. J. Howarth, and T. R. Bailey, 2001, Strontium isotope stratigraphy: LOWESS version 3: best fit to the marine Sr isotope curve for 0–509 Ma and‐ accompanying look up table for deriving numerical age: The Journal of Geology, v.‐ 109, p. 155-170.

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Montañez, I. P., J. L. Banner, D. A. Osleger, L. E. Borg, and P. J. Bosserman, 1996, Integrated Sr isotope variations and sea-level history of Middle to Upper Cambrian platform carbonates: Implications for the evolution of Cambrian seawater 87Sr/86Sr: Geology, v. 24, p. 917-920.

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Zhao, Z. J., J. H. Luo, Y. B. Zhang, X. N. Wu, and W. Q. Pan, 2011, Lithofacies paleogeography of Cambrian sequences in the Tarim Basin: Acta Petrolei Sinica, v. 32, p. 937-948.

TABLE CAPTION

Table 1. Four main reservor types and their characteristics in the Lower to Middle Cambrian units in

the Tarim Basin. Data are collected from a: Shen et al. 2016; b: Song et al., 2014; c: Wang et al., 2014;

d: Tian et al., 2015; e: Huang et al., 2016; -- data not measured or unavailable

FIGURE CAPTION

Figure 1. Map of the Tarim Basin showing tectonic units, locations of sampled wells and outcrops.

Orange filled area showing the thickness of anhydrite-bearing dolostone decreasing from the center to

the eage.

Figure 2. Sequence, lithology, facies, and reservoir description chart of the Cambrian strata in the

Tarim Basin. Sequences and facies analysis data are modified from Zhao et al. (2011) and Liu et al.

(2012).

Figure 3. The cross section (from A to B in Figure 1) shows various sediment facies of the Cambrian

strata in the Tarim Basin. Modified from Zhao et al. (2011) and Liu et al. (2012).

Figure 4. Burial histories of the Camrbian sediments in different tectonic units: (A) from TZ1 well in

Tazhong Uplift; (B) from TD1 well in Tadong Uplift; (C) from H4 well in the Bachu Uplift; and (D)

from Keping outcrop and Yakela subsurface in Keping Uplift. Burial histories were modified from Qiu

et al. (2012) for A, B, C, and Ye (1994) for D, respectively.

Figure 5. Schematic diagram of the dolostone paragenesis in the Lower to Middle Cambrian unit in the

Tarim Basin.

Figure 6. Thin-section and CL photomicrographs show different kinds of dolomites in the Cambrian

strata in the Tarim Basin. A) stromatolitic laminae of alternating dark dolomudstone (red arrow) and

light microsparite layers (yellow arrow), pyrite and EPS fossils (You et al., 2013) are presented in the

dark dolomudstone, white arrow points to up direction, outcrop Xiaoerbulake, ∈3q. B) Very finely

crystalline dolostone (dark color), and the associated anhydrite nodule (white color; red arrow) from

restricted lagoon facies, white arrow point to up direction, well ZS5, 6,557.2m, ∈1w. C) Very fine-

crystalline, fabric-retentive dolomite with dolomite cement; the algae fabric and intercrystalline and

dissolution pores (in blue) are preserved, well H4, 5,355.5, ∈2s. D) CL photomicrograph for photo C

(in different orientation); fabric-retentive dolomite shows very dull red luminescence (yellow arrow),

and dolomite cement shows slightly lighter but also dull red luminescence (red yellow), well H4,

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5,355.5, ∈2s. E) Medium-coarse crystalline, planar-s to nonplanar-a, fabric-destructive dolomite,

intergranular pores (blue) are well developed partially filled with saddle dolomite. The primary fabric

has been obliterated due to intense dolomitization and recrystallization, well ZS5, 6,280.0m, ∈2s. F) CL

photomicrograph for photo E; fabric-destructive dolomite shows very dull red luminescence whereas

saddle dolomite shows orange luminescence.

Figure 7. Photographs (cross polarized light) showing different types of anhydrite in breccia and

fracture enriched evaporites and carbonates in the Lower to Middle Camrbian unit from the Tarim

Basin. A) anhydrite layer (white color; red arrow) interbeded with dolomudstone (dark), well ZS5,

6,552.7m, ∈2s. B) Anhydrite nodule (red arrow) with various size up to several centimeters in

dolomudstone, well ZS5, 6,193.9m, ∈2a. C) Granular anhydrite cement (red arrow) with crystal size

from 20 to 50 μm, well ZS5, 6,435.2 m, ∈1w. D) Tabular-like anhydrite cements (red arrow) with

crystal lenth up to 1 mm, well ZS5, 6,535.9 m, ∈2s. E) Finely crystalline fracture filling anhydrite (red

arrow), well ZS5, 6434.3 m, ∈1w. F) Isolated anhydrite nodule (red arrow) with relatively coarse

crystalline sizes in dolomudstone, indicating that it probably precipitated during burial diagenesis, well

ZS5, 6,709.5 m, ∈1x. G) Breccia that consists of anhydrite and carbonate are commonly seen in cores,

well ZS5, 6,194.4 m, ∈1a. H) Fractures are commonly present in breccias and sometimes filled by

bitumen (red arrow), well ZS5, 6,191.5, ∈1a.

Figure 8. Thin-section and SEM in backscattered electron imaging mode (BSEM) photographs of non-

carbonate minerals present in the Cambiran strata of the Tarim Basin. Minerals composed of high-

atomic-mass elements are brighter in BSEM than those composed of low-atomic-mass elements. A) A

thin-section photo showing the intergrowth of anhydite-enriched dolomudstone with chalcedony, well

ZS5. B) A BSEM image showing calcite partially replacing anhydrite nodule in dolomudstone, well

ZS5, 6,708.4 m, ∈1x. C) A BSEM image showing the growth of pyrite in the intergranular pores in

fabric-destructive dolomite, well ZS5, 6280.0m, ∈2s. D) A BSEM image showing a mineral assemblage

of anhydrite and celestite in dolomicrite, well ZS5, 6,434.3m, ∈1w. E) A BSEM image showing the

growth of fluorite in the intergranular pores in fabric-destructive dolomite, well ZS5, 6,281.3m, ∈2s. F)

A BSEM image showing the intergrowth of pyrite and K-feldspar in dolomudstone, well ZS5,

6,708.4m, ∈1x. G) A core sample photograph showing fracture filling halite (arrow), the length of this

core is about 20 cm, well H6, ∈2a. H) Outcrop showing vertical fracture crosscut the formation.

Fracture is filled by calcite at the edge and bitumen (red arrow) in the middle. The hammer was used as

a scale bar with length of about 30 cm, outcrop Xiaoerbulake, ∈2s.

Figure 9. Photomicrographs of different pore types in evaporite-related dolostone reservoir rocks from

the Lower to Middle Cambrian unit in the Tarim Basin. A) Interparticle pore (IP) preserved in

microbial dolograinstone, with some solution-enlarged pores (SE) locally present (blue; white arrow),

well YH 7X-1, 5832.0 m, ∈2a. B) Dissolution-enlarged pores (SE) are well developed in fabric-

destructive dolomite, with bitumen (black) and quartz cement (white), well ZS5, 6,280.0 m, ∈2s. C)

Dissolution-enlarged pore (SE) rimmed by bitumen suggests the oil charging occurred in these

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dolostone reservoirs; well ZS 5, 6,281.3 m, ∈2a. D) Algae reef in microbal mound facies with preserved

framework pores (FW) up to 2 mm aross, well Fang 1, 4599.0 m, ∈1x. E) Spongiostromata stromatolite

beds showing framework pores (FW). Pore sizes range from 0.5 mm to about 3 mm (blue; red arrow).

Solution-enlarged pore (SE) locally developed (white arrow), outcrop Sheairike, ∈1x. F) Anhydrite

moldic pores (AM) with size up to 1 mm across that are well developed in evaporite-bearing

dolomustones in evaporite lagoon facies, well YH 10, 6171.0 m, ∈2a. G) Intercrystalline pore (IC)

preserved in fine-crystalline fabric retentive dolomite, well He 4, 5,353.3 m, ∈2s. H) Open fracture

pores (F) due to the dissolution of fracture-filling anydrite (white) are abundant in anhydrite-enriched

dolomudstone. In some cases all the anhydrite cements have been removed during diagenesis resulting

in an open fracture. Breccia and associated fractures are locally present which were likely formed due

to meteoric water leaching during subaerial exposure surfaces, well ZS5, 6,193.9 m, ∈2a. I) Late

diagenetic dissolution pores (D) in saddle dolomite, outcrop Xiaoerbulage, ∈1w.

Figure 10. Carbon and oxygen-isotope compositions and ranges of bulk rock dolomite, limestone,

dolomite and calcite cements from the Lower to Middle Cambrian unit in the Tarim Basin.

Figure 11. 87Sr/86Sr ratios of bulk rock dolomite, limestone, dolomite and calcite cements in the Lower

to Middle Cambrian unit, Tarim Basin. Coeval seawater 87Sr/86Sr field (0.7086-0.7093) in blue is from

Montañez et al. (1996), Montañez et al. (2000) and Veizer et al. (1999). Intrusive rocks 87Sr/86Sr data

(0.7072-0.7076), marked with green bar, are from Dong et al. (2013).

Figure 12. Homogenization temperatures and salinity data obtained from fluid inclusions in fabric-

destructive dolomite, medium to coarse dolomite cement, and saddle dolomite cement, vug/fracture

calcite, as well as anhydrite cement in the Lower to Middle Cambrian unit, Tarim Basin.

Figure 13. Schematic diagram showing the depositional model for the Early to Middle Cambrian in the

Tarim Baisn.

Figure 14. Schematic diagram showing the origin of various types of pore spaces, in different

sedimentary facies, from the Lower to Middle Cambrian unit in the Tarim Basin. Representive wells

(Fig. 1) have been added showing the reservoir characteristics.

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Table 1

Reservoir type Facies Pore type Porosity (%)Average

Porosity (%)Permeability

(mD)Average

Permeability (mD)Interconnection

(%)Thickness

(m) Formation Location

1.32-10.55a 4.81a -- --

1.90-9.39a 5.5a -- --

2-13.32b 4.32b 0.02-50b 4.66b

up to 15% -- -- -- 15-50c Є2s ZS 5 well

0.85-6.97a 4.6a -- -- 200-340c Є2a YH 7X-1

1.29-9.7d 2.15d -- -- He 4 well8.36-12.6c -- -- -- ZS1 well

2.4-6.5c -- 2.8-3.3c -- ZS1C well

4.35-5.74c -- -- -- Fang 1, He 4 well0.76-3.57a 1.55a -- -- Fang 1 well1.08-7.80e 3.93e 0.008-2.127e 0.489e Keping-Bachu up to 12% -- -- -- 310-490c Є1w & Є2a ZS 5 well

1.5-5.0a -- -- -- 210-390cЄ2a & Є2s ZS 5 well

3.7-6.2a 5.3a-- -- 50-80c Є1x Fang 1 well

1.19-2.43d 1.77d -- -- 200-340c Є2a He 4 well

0.14-3.71e 1.52e 0.004-202.010e 11.615e15-50c Є2s Bachu area

up to 10% -- -- -- 310-490c Є1w & Є2a ZS 5 well1.50-9.27a 4.2a -- -- 200-340c Є2a YH 10 well

50-80c Є1x Keping-Bachu

Inner platform mound

Framework, irregular vug

56.2a

50-80c

Shoal and reef comlex

Platform margin reef and shoal

Solution-enlarged interparticle, framework, Irregular vug

Inner platform shoal

Solution-enlarged interparticle, Irregular vugIsolated shoal

and mound

Evaporite enriched breccia

Evaporite lagoon Breccia, fracture 15.8a

Є1x

Є1x

Evaporite-related carbonate

Evaporite lagoon, Restricted inner platform

Anhydrite moldic, intercrystalline 78.8a

30-40e

51.8a

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