Quantitative impact of diagenesis on reservoir quality of the Triassic Chang 6 tight oil sandstones, Zhenjing area, Ordos Basin, China

Accepted Manuscript Quantitative impact of diagenesis on reservoir quality of the Triassic Chang 6 tight oil sandstones, Zhenjing area, Ordos Basin, China Yang Li, Xiangchun Chang, Wei Yin, Tingting Sun, Tingting Song PII:

S0264-8172(17)30260-X

DOI:

10.1016/j.marpetgeo.2017.07.005

Reference:

JMPG 2987

To appear in:

Marine and Petroleum Geology

Received Date: 3 May 2017 Revised Date:

6 July 2017

Accepted Date: 7 July 2017

Please cite this article as: Li, Y., Chang, X., Yin, W., Sun, T., Song, T., Quantitative impact of diagenesis on reservoir quality of the Triassic Chang 6 tight oil sandstones, Zhenjing area, Ordos Basin, China, Marine and Petroleum Geology (2017), doi: 10.1016/j.marpetgeo.2017.07.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Quantitative impact of diagenesis on reservoir quality of the Triassic Chang 6 tight oil sandstones, Zhenjing area, Ordos Basin, China Yang Lia, Xiangchun Changa * b , Wei Yinc, Tingting Suna, Tingting Songa,

College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao

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a

266590, China;

Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and

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b

Technology, Qingdao 266590, China;

Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China

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c

* Corresponding author. E-mail address: [email protected] (X. Chang).

Abstract

The Upper Triassic Chang 6 sandstone, an important exploration target in the Ordos Basin, is a typical

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tight oil reservoir. Reservoir quality is a critical factor for tight oil exploration. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), stable isotopes, and fluid inclusions, the

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diagenetic processes and their impact on the reservoir quality of the Chang 6 sandstones in the Zhenjing area were quantitatively analysed. The initial porosity of the Chang 6 sandstones is 39.2%, as calculated

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from point counting and grain size analysis. Mechanical and chemical compaction are the dominant processes for the destruction of pore spaces, leading to a porosity reduction of 14.2% to 20.2% during progressive burial. The porosity continually decreased from 4.3% to 12.4% due to carbonate cementation, quartz overgrowth and clay mineral precipitation. Diagenetic processes were influenced by grain size, sorting and mineral compositions. Evaluation of petrographic observations indicates that different extents of compaction and calcite cementation are responsible for the formation of high-porosity and low-porosity reservoirs. Secondary porosity formed due to the burial dissolution of feldspar, rock fragments and 1

ACCEPTED MANUSCRIPT laumontite in the Chang 6 sandstones. However, in a relatively closed geochemical system, products of dissolution cannot be transported away over a long distance. As a result, they precipitated in nearby pores and pore throats. In addition, quantitative calculations showed that the dissolution and associated

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precipitation of products of dissolution were nearly balanced. Consequently, the total porosity of the Chang 6 sandstones increased slightly due to burial dissolution, but the permeability decreased significantly because of the occlusion of pore throats by the dissolution-associated precipitation of authigenic minerals.

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Therefore, the limited increase in net-porosity from dissolution, combined with intense compaction and

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cementation, account for the low permeability and strong heterogeneity in the Chang 6 sandstones in the Zhenjing area.

Key words: tight oil sandstones, diagenesis, reservoir quality, Ordos Basin.

1. Introduction

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During the last two decades, energy firms have had an increasing interest in unconventional resources (Shanley and Cluff 2015), and many of the successful exploration efforts confirm that sufficient porosity and permeability control commercial development (Taylor et al., 2010). Therefore, it is crucial to

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understand the factors affecting their development. Depositional factors and diagenetic alterations should

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be fully considered to predict reservoir quality. Depositional factors control the initial porosity and permeability of the rocks and affect the subsequent types and extent of diagenetic alterations, while diagenetic alterations commonly accentuate the variations in depositional porosity and permeability (Morad et al., 2010).

Generally, sandstone reservoir quality is significantly altered by various diagenetic processes after deposition. A consensus emerged that compaction and cementation are the two most important processes responsible for the deterioration of reservoir properties (Worden and Burley 2003; Yan et al., 2010; Yang et

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ACCEPTED MANUSCRIPT al., 2013; Yang et al., 2014; Lai et al., 2015, 2016; Zhang et al., 2016; Rahman and Worden, 2016; Wang et al., 2017a, b). However, the genetic models for porous reservoirs remain a topic of debate. Some researchers emphasize the importance of burial dissolution in the development of high-quality reservoirs

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and advocate that dissolution could significantly improve reservoir quality (Schmidt and McDonald 1977; Swarbrick 1999; Zeng 2001; Franca et al., 2003). In contrast, other researchers argue that secondary pore formation appears to be nearly balanced with the byproducts of dissolution if the dissolution occurs in

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relatively closed diagenetic systems and, thus, dissolution contributes little to the formation of high-quality

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reservoirs (Higgs et al., 2007; Yuan et al., 2015a).

As the second largest sedimentary basin in China, the Ordos Basin has abundant unconventional oil and gas resources (Jia et al., 2012; Zou et al., 2012). The Chang 6 oil layer of the Upper Triassic Yanchang Formation is an important tight oil reservoir that is characterized by low permeability and strong

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heterogeneity. Several studies have been performed to assess the diagenetic processes, such as compaction and carbonate cementation (Zhong et al., 2012; Lai et al., 2016; Liu et al., 2016), precipitation of clay minerals (Huang et al., 2009), and dissolution (Liu, 2011) on the reservoir quality of the Yanchang

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sandstones in the Zhenjing area. Although some valuable information on the relationship between

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diagenesis and reservoir properties was documented, they commonly lack a quantitative evaluation of the relative importance of diagenesis on reservoir quality. In addition, the generation of secondary porosity and its effect on reservoir porosity and permeability is a complicated process because burial dissolution is commonly accompanied by the precipitation of products of dissolution. Therefore, the secondary porosity volume should not simply be considered the reservoir porosity net increase (Huang et al., 2009; Bjørlykke and Jahren, 2012; Yuan et al., 2015b). In this study, the petrography and diagenesis of the Chang 6 sandstones were investigated in detail to quantitatively evaluate their impact on reservoir quality and to

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ACCEPTED MANUSCRIPT understand the development of high-quality reservoirs. Based on the chemical reaction equations and the quantitative analysis of the byproducts of dissolution, the dissolution contribution to porosity was calculated to determine how dissolution affects the reservoir properties.

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2. Geological setting The Ordos Basin is a multicyclic superimposed basin in North China (Liu, 2006), and it is commonly subdivided into six first-order structural units, namely, the Yimeng uplift, Weibei uplift, Jinxi fault-fold belt,

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Yishan slope, Tianhuan depression and Western thrust belt. The Zhenjing area is structurally located south

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of the Tianhuan depression with an area of approximately 2510 km2 (Fig. 1). By systematic analysis of the sedimentary facies of Chang 6 sandstones, the main sedimentary environment was identified as a delta front depositional system. The micro-facies mainly indicate subaqueous distributary channel and distributary bay environments. Sandbodies of subaqueous distributary channel micro-facies are

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characterized as having good reservoir properties, with coarse sediment and good sorting due to the high sedimentation rate and frequent washing by water (Liu et al., 2011; Yin et al., 2012). The horizontal distribution of main micro-facies is shown in Figure 1. The Upper Triassic Yanchang Formation can be

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subdivided into ten oil layer groups from top to bottom, called Chang 1 to Chang 10, respectively. Due to

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the unbalanced uplift fluctuation during the late stage Indosinian Movement, the palaeogeomorphic features of the Ordos Basin were highly elevated in the west and low-lying in the east during the Mesozoic Era (Liu, 2006). Consequently, denudation was much more intensive in the west compared to the east, which caused the Chang 1 to Chang 3 oil layers to be eroded and only the Chang 4 to Chang 10 layers were preserved in the Zhenjing area. The Chang 6 sandstones, which are underlain by the Chang 7 source rocks, are an important reservoir in the area (Fig. 2). The burial and thermal histories reconstructed from the H109 well in the study area show that the

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ACCEPTED MANUSCRIPT Chang 6 sandstones had a maximum burial depth of approximately 3000 m and experienced a maximum temperature of approximately 120°C. Due to the subsequent uplift and transient subsidence, the present-day depth is approximately 2100 m with a corresponding temperature of approximately 80°C (Fig. 3).

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3. Materials and methods All samples used in this study were collected from cores of the Chang 6 sandstones in the Zhenjing area. A total of 230 thin sections, half-stained with Alizarin Red S for distinguishing calcite, from 12 wells

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were analysed to determine the sandstone compositions. Among them, 135 thin sections were impregnated

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with blue epoxy to determine the sandstone pore types. In addition, 73 thin sections were selected to analyse the rock constituents, such as grain size, cement and pore volume using Adobe Photoshop software. Grain sorting and pore networks were determined following the method of Wang et al. (2015a). Porosity and permeability data were collected from the SINOPEC Petroleum Exploration and Production Research

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Institute, Beijing. A scanning electron microscope (SEM) was used to confirm the pore structure, distinguish the clay mineral types, and determine the mode of clay occurrence within the pore spaces. X-ray diffraction (XRD) was performed on 82 samples to identify the clay-mineral contents. Fluid

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inclusions and carbon and oxygen isotope analyses were performed to assist in the diagenetic sequence

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determination, and the procedures used here are the same as those described by Wang et al. (2015b).

4. Results

4.1 Detrital composition and rock fabric The Chang 6 sandstones are primarily feldspathic litharenites and lithic arkose (Fig. 4) according to the classification by Folk (1970). The quartz grain contents range from 38.1% to 91.2% (53.4% on average). The feldspar contents range from 1% to 35.3% (27% on average), among which the percentage of potassium feldspar is 43%, and plagioclase is 57%. The lithic fragment contents range from 5% to 47.1%

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ACCEPTED MANUSCRIPT (19.6% on average). The Chang 6 sandstones consist primarily of fine, subangular to subrounded grains. The standard deviation values of grain size vary from 0.43 to 0.87 with an average value of 0.55, indicating moderately

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to well-sorted sandstones. The average grain size of Chang 6 sandstones ranges from 0.005 mm to 0.27 mm and can be divided into two categories: fine-grained sandstones (average grain size ranges from 0.05 mm to 0.27 mm) and siltstone (average grain size ranges from 0.005 mm to 0.05 mm).

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4.2 Diagenetic events 4.2.1 Compaction

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The compaction processes are subdivided into mechanical compaction and chemical compaction. In the early stage of deposition, mechanical compaction is dominant. After deposition, the sediment overburden pressure increases with progressive burial, and the distance between the rock particles is reduced, enlarging the contact surface. The detrital grains tend to have stable packing accompanied by the inevitable

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destruction of intergranular pores, as evidenced from the rearrangement, deformation (Fig. 5A) and fracture (Fig. 5B) of framework grains in the Chang 6 sandstones.

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Chemical compaction occurs primarily at temperatures higher than 70°C when rocks are deeply buried (Bjorkum, 1996). With increasing pressures during progressive burial, chemical compaction progresses

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further, as evidenced by the detrital grain boundaries dominated by a line of concave-convex contacts and occasional stylolite (Fig. 5A).

4.2.2 Cementation

Calcite is the dominant carbonate mineral in the Chang 6 sandstones. It primarily occurs as cement filling intergranular pores. They occur variably in most of the samples. Generally, two types of calcites can be distinguished according to the paragenetic sequences (referred to as early calcite and late calcite), which will be discussed in the following section. Point counting analysis showed that the contents of calcite vary 6

ACCEPTED MANUSCRIPT significantly in the Chang 6 sandstones. Calcite cements were divided into two types according to the contents and their distribution. One type was characterized by calcite content that ranged from 8.1% to 18.3% and extensively occluded the intergranular pores (Fig. 5C). Another type was characterized by

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calcite content that ranged from 0.5% to 5.0% and filled some of the intergranular pores or directly covered the chlorite coatings (Fig. 5D). Laumontite cements are also pervasive in the Chang 6 sandstones. Microscopic observation showed that most of the laumontite cements have undergone a range of

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dissolution (Fig. 5E).

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In the study area, the development of silica cementation is primarily caused by the formation of quartz overgrowth and authigenic microcrystalline quartz. Various degrees of quartz overgrowth can be observed with content ranges from 0.5% to 1%.The dissolution of quartz grains cannot be widely observed in thin section. Additionally, the quartz overgrowth primarily occurs near the dissolution pores (Fig. 5F).

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The clay mineral types in the Chang 6 sandstones are primarily chlorite, kaolinite, and illite, with varying clay contents from 0.8% to 15.1% measured by XRD. The authigenic chlorites occur predominantly as rimming grains (Fig. 5D, G), and possibly formed due to the alteration and dissolution of

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unstable volcanic rock fragments (Wang et al, 2017a). Kaolinite often occurs in the form of pore-filling

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aggregates and fibrous illite (Fig. 5H) distributed in grain boundaries.

4.2.3 Dissolution

Secondary pores in the study area are common, and mainly attributed to mineral dissolution. As temperature increases, organic matter gradually matures, and the kerogen in this diagenetic stage can produce sufficient organic acid and CO2 to partially or completely dissolve framework grains and some cements, leaving behind secondary pores (Surdam et al., 1989). In the study area, the dominant minerals dissolved are feldspars, followed by laumontite as well as other rock fragments. The dissolution of

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ACCEPTED MANUSCRIPT carbonate cements and clay minerals are rare. Quantitative analysis showed that the percentage of intragranular pores developed from dissolution of feldspar, debris and laumontite cements are 80.18%, 10.21% and 8.22%, respectively.

4.3 Porosity and permeability

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Chang 6 sandstone porosity in the Zhenjing area ranges from 0.7% to 18.6% with an average of 11%. The sandstone permeability ranges from 0.002 mD to 9.11 mD with an average of 0.585 mD. A relative

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proportion of porosity less than 15% accounts for 85.84% of the sandstone analysed, and a relative proportion of permeability less than 1 mD accounts for 77.26% of the sandstone analysed (Fig. 6). The

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permeability has an approximately exponential, positive correlation with the porosity. The core observation and experimental measurements showed that the lower limit of effective porosity (oil soaked) was 10%, therefore the Chang 6 sandstones were defined as either high-porosity or low-porosity by taking 10% porosity as the critical porosity value (Fig. 7). However, there are several anomalous data points where the

trend line (Fig. 7).

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porosity is over 10% and the permeability is less than 0.1 mD, which is much lower than predicted from the

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4.4 Pore textures and classification

Based on the observations from thin sections, pores in the Chang 6 sandstones were classified into four

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categories: (a) intergranular pores (Fig. 8A), (b) intragranular pores by partial dissolution of grains (Fig. 8B), (c) moldic pores (Fig. 8C) and (d) intercrystalline micropores (Fig. 8D). Intergranular pores in the Chang 6 sandstones account for approximately 70% of the total pores with diameter ranging from 15 µm to 150 µm (55 µm on average). Some of the primary intergranular pores amalgamated with intergranular pores by dissolution, making it difficult to distinguish the respective proportions of them. According to the nomenclature and classification of secondary pores by Schmidt (1979), the hybrid pores of primary intergranular pores and intergranular pores by dissolution can be 8

ACCEPTED MANUSCRIPT defined as enlarged intergranular pores and are mainly distributed in high-porosity sandstones. Intragranular pores account for approximately 25% of the total pores and are mainly due to dissolution of feldspar, debris or laumontite cements. Moldic pores were characterized as pores that keep the outlines of

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the precursor grains and they are most likely developed from the complete dissolution of grains. Intercrystalline micropores can be found in areas where authigenic kaolinite is abundant.

4.5 Fluid inclusions and homogenization temperatures

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Homogenization temperatures of the fluid inclusions measured from calcite cements (Fig. 9A) range

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from 90°C to 111°C with an average of 96 °C. Additionally, homogenization temperatures of the fluid inclusions in quartz overgrowths (Fig. 9B) range from 89°C to 105°C with an average of 90 °C.

4.6 Carbon and oxygen isotopes

Bulk rock stable isotope compositions show that the calcite cements have a wide range of oxygen

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isotope (δ18O) values varying from -23.4‰ to -12.8‰ with an average of -1.8‰, and carbon isotope(δ13C) values varying from -1.1‰ to -0.7‰, except one sample with a value of -8.1‰ (Fig. 10). The oxygen and

5. Discussion

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carbon isotope values can help determine the diagenetic stage of the calcite cements.

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5.1 Sequence of diagenetic events

Diagenetic processes are subdivided into two stages, namely eogenesis and mesogenesis. The boundary between the eogenetic and mesogenetic stages is defined by burial depths of approximately 2000 m, equivalent to a temperature of approximately 70°C (Morad et al., 2000). Based on observations from thin sections and the scanning electron microscope, combined with data from stable isotopes and fluid inclusions, the interpretation of the sequence of diagenetic events are summarized and shown in Figure 11. Due to the difference in diagenetic processes between sandstones of different porosity, the sequences of

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ACCEPTED MANUSCRIPT diagenetic events of high-porosity and low-porosity sandstones are discussed separately in this study. During the eodiagenesis stage, the main diagenetic processes that high-porosity sandstones experienced include mechanical compaction, laumontite cementation and chlorite grain-coating, followed

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by chemical compaction, late calcite cementation, dissolution, kaolinite, and illite cementation in the mesodiagenesis stage.

Mechanical compaction is simultaneous with sediment deposition, and considered to dominate under

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temperatures ranging from 70°C to 80°C (Bjørlykke, 2014), which mainly correspond to the eodiagenetic

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stage. When rocks are deeply buried and temperatures become higher than 70°C, chemical compaction primarily occurs (Bjorkum, 1996), which mainly corresponds to the mesodiagenetic stage. Worden and Morad (2003) proposed that chlorites develop at temperatures approximately 60-70°C, indicating the end of the early diagenetic stage. The distribution of laumontite is strictly controlled by sedimentary facies and it

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is primarily distributed in the subaqueous distributary channel sandstone. The homogenization temperatures of the laumontite fluid inclusions range from 60°C to 70°C (Yang and Qiu, 2002), indicating cementation during eogenesis.

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Based on thin section observations, the calcite can be divided into two precipitation stages. Generally,

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the early calcite cements have a higher volume (up to 18.3% volume), and mostly or completely fill the sandstone intergranular pores (Fig. 5C). These features, together with limited compaction and the absence of quartz and chlorite cements, suggest calcite precipitation prior to or during compaction. This is also supported by the fact that nearly all the calcite fluid inclusions are single phase, which indicates a formation temperature of less than approximately 50°C (Goldstein, 2001). Although it is hard to precisely determine the relative amount of each calcite type, the samples dominated by early calcites commonly have positive δ18O values (Fig. 10), suggesting their formation at relatively low temperatures due to

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ACCEPTED MANUSCRIPT temperature-dependent oxygen isotopic fractionation. In addition, the occurrence of calcites that sporadically fill intergranular pores with chlorite rims (Fig. 5D) indicates that calcite cementation occurred after chlorite cementation. The wide range of δ18O sample values is related not only to calcite precipitation

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during progressive burial but also to the fact that the data are from bulk rock analyses that commonly are mixtures of early and late calcites. Most samples have high δ13C compositions, suggesting that the carbon for early calcite precipitation was likely derived from inorganic sources. The homogeneous temperatures

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measured from the fluid inclusions in the calcite cements are higher than 90°C, indicating that late stage

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calcite cementation occurred during mesogenesis.

Additionally, the quartz overgrowth primarily occurs near the dissolution pores (Fig. 5F), indicating that the quartz cements are most likely a result of feldspar alteration. Fluid inclusion homogenization temperatures range from 89°C to 105°C, suggesting a mesogenetic origin for the quartz cements.

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The formation of kaolinite and illite may be due to the transformation of other clay minerals during diagenesis and the dissolution of feldspar (Zhong et al, 2013). The acidic conditions required for kaolinite formation were reached with the maturation of organic matter in the Chang 7 source rocks (Huang et al,

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2009). Worden and Morad (2000) proposed that feldspar dissolution can result in the growth of illite or

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kaolinite at high formation water temperatures. Kaolinite and illite are more abundant where there are more secondary pores near the altered detrital feldspar grains. To interpret the diagenetic processes that low-porosity reservoirs experienced, the primary texture and mineral compositions of high-porosity and low-porosity sandstones were quantitatively analysed and compared (Table 1). The results indicate that the low-porosity sandstones experienced stronger mechanical compaction due to the greater plastic debris content, smaller grain size and relatively poor sorting. In addition, the mineral compositions together with the stable isotope results showed that early calcite

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ACCEPTED MANUSCRIPT cementation dominates in the low-porosity sandstones. However, early calcite cementation was absent in the high-porosity sandstones.

5.2 Quantitative analysis of the porosity evolution

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5.2.1 Initial porosity recovery Based on the results of the wet packing experiment (Scherer 1987; Beard and Weyl 1973), the initial porosity (Ф1) recovery formulas are as follows:

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and

(1)

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Ф1= 20.91+20.90/So

S0 = (P25/P75)1/2

(2)

where Ф1=initial porosity, P25=diameter of the particle whose cumulative content is 25% in the cumulative curve, and P75=diameter of the particle whose cumulative content is 75% in the cumulative

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curve.

Four micrographs from each thin section were chosen to calculate the Trask sorting coefficient (So) and initial porosity (Ф1), and the mean value of the results for each thin section was used as its initial

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porosity. Based on more than 100 thin sections, the initial porosity values in the Chang 6 reservoir vary

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from 36.3% to 41.5% with an average value of 39.2%.

5.2.2 Porosity after strong compaction The porosity after compaction can be calculated using formula 3: Ф2 = [(Q1+Q2)/Q3]×Ф3+C

(3)

ФL= Ф1-Ф2

(4)

where Ф2=porosity after strong compaction, Q1= areal porosity of the intergranular pores, Q2=areal porosity of the cement dissolution, Q3=total areal porosity, Ф3=core analysis porosity, C=cement content,

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ACCEPTED MANUSCRIPT and ФL= the porosity reduction due to compaction. The porosity value reduction due to compaction ranges from 14.2% to 20.2%.

5.2.3 Porosity after cementation and replacement

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The residual porosity after compaction, cementation and replacement is Ф4. Ф4= (Q1/Q3)×Ф3

(5)

ФCL= Ф2-Ф4

(6)

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where ФCL=the porosity reduction due to cementation and replacement.

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Using Adobe Photoshop software, the area of the cements and the different porosity types in the microscopic images were selected with the selecting tools. The ratio of the selected pixels to the whole image equals the percent area of the selected component. Subsequently, the different cement and porosity types can be quantitatively determined (Zhang et al., 2014). Using the above method, the porosity reduction

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due to cementation and replacement is calculated and ranges from 4.3% to 12.4%.

5.3 Does dissolution play an important role in the enhancement of porosity? Many studies have analysed the dissolution effect on sandstone porosity. Some scholars believe that

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when acidic fluids exist in the reservoir, there will be increased feldspar, rock fragment, and laumontite

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dissolution, which improves reservoir quality (Surdam et al., 1989; Zeng et al., 2001, 2006; Wang and Chen, 2007; Wang et al., 2011). To quantitatively analyse the dissolution contribution to the reservoir porosity, many factors should be considered (Lai et al., 2017; Wang et al., 2017b). This article, fully considers whether the dissolution and precipitation systems are open or closed based on the reaction equations of feldspar and laumontite under acidic conditions, and the locations of products of dissolution are analysed. Feldspar dissolution is the primary cause of secondary porosity in the Zhenjing area. With different

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ACCEPTED MANUSCRIPT temperatures, pH values and fluid types in the reservoir, the solubility of the potassium feldspar, albite and anorthite may vary (Worden and Morad 2000; Yuan et al., 2013). Laumontite cements can be extensively observed under the microscope. Additionally, laumontite dissolved easily along its cleavage fractures,

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forming intragranular pores (Fig. 5E). Moldic pores are generated when dissolution is strong (Fig. 8C). In an open system, feldspar products of dissolution such as kaolinite and liquid SiO2 can be removed from the reservoir successfully, and the secondary pores can contribute to the total porosity (Wilkinson et

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al., 1997; Yuan et al., 2015a). However, if the reservoir is in a closed system, kaolinite and liquid SiO2

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cannot be transported out of the reservoir effectively. With continuing chemical reactions, the products of dissolution will precipitate in the form of kaolinite aggregate and quartz overgrowth in the intergranular pores, grain boundaries and pore throats (Chuhan et al., 2001; Higgs et al., 2007). In this case, we should consider the difference between the secondary pore volume due to dissolution and the volume of the

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products precipitated after dissolution to determine the net porosity contribution due to dissolution in a closed system. Notably, a small difference indicates that the porosity contribution due to dissolution is minor. The Ordos Basin is a typical cratonic basin developed on the Archean granulites and Lower

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Proterozoic greenschists of the North China block (Yang et al., 2005). After deposition of the Triassic

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Chang 6 sandstones, the Ordos Basin continued to steadily subside before uplift and, therefore, the Chang 6 sandstones experienced continuous compaction during progressive burial. Thus, it is likely that the Chang 6 sandstone reservoir was in a closed environment during the mesogenetic stage. In this environment, the pore water flow is primarily driven by sediment compaction and clay-mineral dehydration, and the pore water moves at a slow rate (Bjorlykke and Jahren, 2012). Due to the reservoir closure, products of dissolution cannot migrate over long distances (Giles and De Boer, 1990). The products such as kaolinite, illite, and SiO2 precipitated as aggregates and quartz overgrowths near the secondary pores.

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ACCEPTED MANUSCRIPT In the relatively closed and high-temperature (approximately 120°C) geochemical system, the illitization of kaolinite can occur. With the illitization of kaolinite, potassium ions are consumed, promoting potassium feldspar dissolution (Huang et al., 2009). At 75°C, the liquid SiO2 in the fluid reaches alpha

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quartz saturation within a migration distance of less than 0.5 m (Giles and De Boer, 1990). When the flow rate is reduced to 0.01 m/a, the liquid SiO2 precipitates in situ, or the migration distance is so short that it precipitates near the secondary pores in the form of quartz overgrowth.

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The dissolution reaction equations for different types of feldspar and laumontite in acidic reservoirs

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under a closed system are shown in Table 2.

The porosity produced by dissolution is defined as Ф5. Ф5 can be calculated using formula 7: Ф5= (Q4/Q3) ×Ф5×σ

(7)

where Q4=the areal porosity of all the secondary pores and σ= the net increase rate after the products of

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dissolution are precipitated.

According to formula 7, the areal porosity of feldspar dissolution in the study area ranges from 0.1% to 5%, and the increased porosity values due to dissolution varies from 0.006% to 0.70%. The areal

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porosity in the ZJ18 well was measured and the statistical results are shown in Table 3.

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As shown in Table 3, quantitative analysis of the areal porosity of secondary pores and the total areal porosity of kaolinite, illite, and quartz overgrowth at the same depth show that the difference between the areal porosity of secondary pores and the areal porosity of products of dissolution is 0.70%. Therefore, the net increase of porosity is 19.66% after dissolution. This is consistent with the characteristics of the feldspar, rock fragments, and laumontite dissolution in a closed system. Volumes of secondary pores in the Chang 6 sandstones measured from thin sections have a positive correlation with the total content of kaolinite, illite and quartz overgrowth (Fig. 12), suggesting that some

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ACCEPTED MANUSCRIPT of the kaolinite, illite and quartz overgrowth are the products of feldspar. Some of the statistical porosity evolution data are shown in Table 4. After error analysis, the errors (E) between the calculated final porosity (ФF) and the core analysis porosity (Ф3) range from 1.31% to 10.87%

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with an average value of 4.96%, indicating that the quantitative analysis of the porosity evolution is valid. Error analysis shows that the sorting coefficient (So) is the main factor which leads to the error. Inevitably, there are human errors when the So is counted. Another significant factor may be multiple chemical

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reactions at different temperatures and times, which make it difficult to accurately determine the

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proportions of each reaction. Additionally, systematic error exists due to the superimposition of different diagenetic events.

5.4 Controls on reservoir quality

5.4.1 Primary texture and mineral compositions controls on reservoir

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quality

Reservoir quality is controlled by factors such as grain size, sorting and extent, and type of diagenetic events (Lai et al., 2015; Zhang et al., 2016). Grain size and sorting which reflect the primary texture of

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Chang 6 sandstones may control the extent of compaction and thus reservoir quality. Thin section porosity

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has a positive correlation with average grain size (Fig. 13), showing that finer-grained sandstone may be strongly influenced by mechanical compaction. Both porosity and permeability decrease with increasing standard deviation, indicating that reservoir quality improves as sorting improves (Fig. 14A, B). However, the correlation between standard deviation and permeability is stronger than that between standard deviation and porosity. It is well known that matrix grain content will decrease by strong hydrodynamics and thus with improved sorting the matrix grain content will decrease. Decreasing of matrix grain content may have little effect on the porosity but has great influence on permeability. As a result, permeability is

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ACCEPTED MANUSCRIPT obviously related to standard deviation. Additionally, the relatively high content of plastic debris in low-porosity reservoirs showed that mineralogical compositions may accounts for differential compaction in Chang 6 sandstones.

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5.4.2 Diagenetic controls on reservoir quality From analysis of the observed diagenesis, the main diagenetic events which controlled the reservoir property in the Chang 6 sandstones are compaction, calcite cementation and clay minerals cementation.

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Quantitative analysis of the porosity evolution showed that porosity reduction by compaction and

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cementation can reach 36.6%, indicating that the key factors that control the reservoir quality are compaction and cementation. As shown in Figure 15, the porosity was predominantly reduced due to compaction.

Other than compaction, calcite cementation is the key diagenetic process responsible for the formation

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of high-porosity and low-porosity sandstones. The relationship between calcite content and porosity shows that porosity decreases with increasing calcite content (Fig. 16A), indicating that calcite cementation is one of the key factors controlling reservoir quality. However, the relationship between calcite content and

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permeability shows that the permeability decreases with increasing calcite content when permeability is

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greater than 1 mD. Nevertheless, there was no linear correlation between the permeability and the calcite content when permeability was less than 1 mD (Fig. 16B), indicating that calcite cementation may not be the main reason for ultra-low-permeability reservoirs. There is a consensus that grain-coating chlorites play an important role in preserving porosity by retard or inhibiting the precipitation of quartz overgrowth during burial (Ehrenberg, 1993; Ajdukiewicz and Larese, 2012; Nguyen et al., 2013). However, with the increase of the thickness of grain-coating chlorites, the reservoir quality may be poorer because chlorite coatings will partially fill the pores and significantly

17

ACCEPTED MANUSCRIPT obstruct the pore throats (Pittman et al., 1992; Bloch et al., 2002;Wang et al., 2017a).In the study area, there is a positive correlation between the chlorite content and porosity in Chang 6 sandstones (Fig. 16C), indicating that the chlorite coatings have preserved the porosity. Similarly, the permeability also increases

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with increasing chlorite contents in Chang 6 sandstones (Fig. 16D). The positive correlation between the chlorite content and permeability may due to the preserved primary intergranular pores, which commonly have relatively large pore-throat diameters, and the fact that the grain-coating chlorite neither completely

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fill nor significantly obstruct the pore throats.

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Dissolution of feldspars and laumontite cements is invoked to the generation of secondary porosity. The total content of products of dissolution such as kaolinite, illite and quartz overgrowth does not have a linear correlation with porosity but has a negative correlation when permeability is less than 1 mD (Fig. 17A, B). It is likely that some of the secondary pores produced by dissolution reactions were occupied by

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the products of dissolution, indicating that dissolution has little effect on porosity. However, kaolinite, illite, and quartz overgrowth precipitated at the margin of the primary pores and the pore throats will lead to significant permeability reduction. Quantitative analysis shows that the net increase of porosity due to

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dissolution can only reach 0.70% in a closed system in the Chang 6 sandstones. However, the simultaneous

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precipitation of products of dissolution resulted in further heterogeneity of the reservoirs which may be responsible for the samples with porosity higher than 10% but permeability less than 0.1 mD.

6. Conclusions

(1) The Chang 6 sandstones are primarily feldspathic litharenites and lithic arkose with fine, subangular to subrounded grains. High-porosity and low-porosity reservoirs can be distinguished in the Chang 6 sandstones. Grain sorting and size of sandstone influence the extent of diagenetic processes and rock properties. Compaction is the key factor for sandstone porosity reduction, and two stages of calcite

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ACCEPTED MANUSCRIPT cementation are the main reason for the development of low-porosity sandstone reservoirs. The absence of early stage calcite cementation and the preservation of the porosity by grain-coating chlorites may be response for the evolution of low-porosity sandstone reservoirs.

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(2) The porosity evolution history is as follows: the initial porosity was 39.2%, and the compaction process reduced the porosity from 14.2% to 20.2%; the porosity further decreased from 4.3% to 12.4% due to carbonate cementation, silica cementation, and clay mineral cementation, the dominant dissolved mineral

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was feldspar and the net increase of porosity due to dissolution varied from 0.006% to 0.70%.

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(3) The dissolution of feldspar, laumontite and rock fragments in a closed diagenetic environment with high temperatures and low flow rates would cause the precipitation of products of dissolution immediately near the dissolution pores. Feldspar dissolution in a closed system can only result in a 0.70% net increase of porosity in the Chang 6 reservoirs, which produced little improvement in reservoir quality. Instead, due to

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the precipitation of the products of dissolution, the primary intergranular pores and pore throats were filled, leading to the significant deterioration of permeability.

Acknowledgments

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This study was supported by the National Natural Science Foundation of China (Grant No. 41272139),

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Shandong Province Natural Science Fund for Distinguished Young Scholars (Grant No. JQ201311), the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (Grant No. 2016ASKJ13) and the Graduate Scientific and Technological Innovation Project Financially Supported by Shandong University of Science and Technology (Grant No. SDKDYC170317).

References

19

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ACCEPTED MANUSCRIPT Figure Captions Figure 1. Simplified structure units and location map of the Zhenjing area in the southwest Ordos Basin, China, and the sedimentary micro-facies in the Chang 6 sandstones.

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Figure 2. Lithology section and sequence stratigraphic column of study area showing location of Chang 6 (Ch6) tight oil reservoirs. Fm. =Formation; Ch=Chang; Sed. =sediment.

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Figure 3. Burial and thermal histories of the Chang 6 and Chang 7 sandstones for well H109 in the Zhenjing area. See Figure 1 for the well location. St=Strata.

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Figure 4. Ternary diagram showing the framework compositions of the Chang 6 sandstones in the Zhenjing area. Q=quartz; F=feldspar; RF=rock fragments.

Figure 5. Optical and scanning electron microscopy images showing petrographic features of Chang 6 sandstones in the Zhenjing area. (A) Well H76, 2217.75 m, plastic minerals were deformed by

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mechanical compaction (yellow arrow), and concave-convex contact of two grains (red arrow) cross-polarized light; (B) Well H111,1812.02 m, quartz grain was broken by mechanical

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compaction (yellow arrow), plane-polarized light; (C) Well H111,1738.08 m, calcite cements significantly filling the intergranular pores, plane-polarized light; (D) Well H76, 2224.19 m,

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calcite cements sporadically filling intergranular pores with chlorite rims, plane-polarized light; (E) Well Y1, secondary pores after laumontite dissolution, plane-polarized light, P=secondary pores; (F) Well H6, 1552.22 m, quartz overgrowth located near the secondary pores (yellow arrow), cross-polarized light, P=secondary pores; (G) Well Z19, kaolinite aggregates and quartz overgrowth located near secondary pores by feldspar dissolution, SEM; (H) Well Z18, 2090.97 m, quartz overgrowth, illite and kaolinite cements distributed in grain boundaries, SEM.

Figure 6. The frequency distribution histogram and the line chart of the cumulative frequency of the Chang 28

ACCEPTED MANUSCRIPT 6 reservoir property in the Zhenjing area. Figure 7. Plot of porosity versus permeability of the Chang 6 sandstones in the Zhenjing area. Note the several anomalous data points where the porosity is over 10% and the permeability is less than

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0.1 mD. Figure 8. Optical microscopy images showing features of dissolution in the Chang 6 sandstones. (A)Well H111,1812.2 m, intergranular pores (P in yellow), plane-polarized light; (B)Well H12,1921.6 m,

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intragranular pores (P in blue) by dissolution, plane-polarized light; (C) Well H12,1916.6 m,

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quartz overgrowths located near the moldic pore (P in red) by dissolution, plane-polarized light; (D) Well H6, 1552.22 m, intragranular pores (P in blue) by dissolution of feldspar and kaolinite with intercrystalline pores (P in black)filling the intergranular pore, plane-polarized light. Figure 9. Photomicrographs showing the occurrence of the fluid inclusions of the Chang 6 sandstones. (A)

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Well H76 2249 m, fluid inclusions on the calcite cements in light brown or light grey. (B) Well H35 1755.8 m, salt water inclusions in the quartz overgrowths in light brown.

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Figure 10. Carbon and oxygen isotopic compositions of carbonates in the Chang 6 sandstones. Figure 11. Paragenetic sequence of the primary diagenetic features for (A) high-porosity reservoirs; and

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(B)low-porosity reservoirs.

Figure 12. Variations in secondary porosity with the total content of kaolinite, illite and quartz overgrowth. Kal+ill+QO= the total content of kaolinite, illite and quartz overgrowth.

Figure 13. Thin section porosity versus petrographically defined average grain size. Figure 14. Cross-plots showing the relationship between standard deviation by particle analysis of Chang 6 sandstones and (A) porosity, and (B) permeability.

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ACCEPTED MANUSCRIPT Figure 15. Plot of intergranular volume versus cement showing the relative importance of compaction and cementation in the reduction of porosity in Chang 6 sandstones (diagram after Houseknecht, 1987; Ehrenberg, 1989).

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Figure 16. Variations in porosity and permeability with the abundance of calcite cements (A and B) and chlorites (C and D).

Figure 17. Variations in porosity (A) and permeability (B) with contents of clay minerals cements in the

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Chang 6 sandstones

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Table Captions

Table 1. Grain size, mineral compositions and pore types in high-porosity and low-porosity reservoirs.

Table 2. Chemical equations of potassium feldspar, albite, anorthite, andesine and laumontite dissolution

L=laumontite.

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under acidic conditions. K-F=potassium feldspar; Na-F= albite; Ca-F= anorthite; A=andesine;

Table 3. Statistics of the areal porosity of dissolution pores and the products of dissolution of the Chang 6

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sandstones in the Zhenjing area, Well ZJ18. P1=areal porosity of dissolution pores; P2=total areal

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porosity of kaolinite, illite and quartz overgrowth. Table 4. Statistics of the porosity evolution. Ф1= initial porosity; S0= Trask sorting coefficient; Ф2=porosity after strong compaction; Ф3= core analysis porosity; ФL= the porosity reduction due to compaction; Ф4=the residual porosity after compaction, cementation and replacement; ФCL= the porosity reduction due to cementation and replacement; Ф5= the porosity produced by dissolution; ФF=the calculated final porosity; E=the error rate between core analysis porosity and calculated final porosity.

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Content of rigid debris(%) Content of calcite(%) Content of chlorite(%) Content of kaolinite(%)

0.51-0.87（0.76 on average）

12.1-15.3（14.5 on average）

14.3-18.5（16.8 on average）

7.3-9.6（8.7 on average）

5.2-8.2（6.9 on average）

2.5-8.1（5.3 on average）

3.4-15.5（8.2 on average）

0.3-3.1（2.5 on average）

0.2-3.0（2.4 on average）

0.3-3.2（2.0 on average） Primary intergranular pores were dominant with relatively large diameter, secondary pores were developed, and moldic pores can be seen occasionly.

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Pore types

0.35-0.72（0.53 on average）

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Grain size(mm) Standard deviation Content of plastic debris(%)

Low-porosity feldspathic litharenites/lithic arkose 0.05-0.15（0.08 on average）

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Petrography

High-porosity feldspathic litharenites/lithic arkose 0.09-0.27（0.16 on average）

0.5-2.8（1.7 on average）

Most of the primary intergranular pores disappeared, residual intergranular pores were dominant, and moldic pores were rare.

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Table 1. Reservoirs

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Table 2 after reaction

before reaction

+

+

Reaction

2KAlSi3O8(K-F)+H2O+2H = Al2[Si2O5][OH]4(K)+4SiO2(Q)+2K

Solid

KAlSi3O8

Amount（ （mol） ）

Al2[Si2O5][OH]4

+

2NaAlSi3O8(Na-F)+H2O+2H = Al2[Si2O5][OH]4(K)+4SiO2(Q)+2Na+

SiO2

NaAlSi3O8

2

1

4

2

Molar mass（ （g/mol） ）

278

258

60

262

Density（ （g/cm3）

2.555

2.615

2.65

2.61

Volume（ （cm ）

217.61 3

Volume change (cm ）

98.66

90.57

-28.38

Net increase （%） ）

13.04

Time

before reaction

Reaction

3KAlSi3O8(K-F)+2H+ = KAl3[Si3O10][OH]2(I)+6SiO2(Q)+2K+

Solid

KAlSi3O8

Amount（ （mol） ）

3

1

Molar mass（ （g/mol） ）

278

398

2.555

2.75

326.42

144.73

3

Volume（ （cm ） 3

-45.84

Net increase （%） ）

14.04

Time

before reaction

SiO2 6

60

2.615

2.65

98.66

90.57

-11.54 5.75 after reaction

Na[AlSi3O8]

KAl3[Si3O10][OH]2

SiO2

1

6

60

262

398

60

2.65

2.61

2.75

2.65

135.85

301.15

144.73

135.85

6.83 before reaction

after reaction

CaAlSi3O8(Ca-F)+H2O+2H = Al2[Si2O5][OH]4(K)+ Ca

Na[AlSi3O8]-Ca[Al2Si2O8](A)+2H +k = KAl2[AlSi3O10][OH]2(M)+2SiO2(Q)+Ca2++Na+

Solid

CaAlSi3O8

Na[AlSi3O8]-Ca[Al2Si2O8]

KAl2[AlSi3O10][OH]2

1

1

2

540

398

60

2.88

2.65

1

Al2[Si2O5][OH]4 1

Molar mass（ （g/mol） ）

278

258

Density（ （g/cm3）

2.685

2.615

2+

-20.57

Reaction

Amount（ （mol） ）

+

258

3

after reaction

AC C

Volume change（cm ）

4

3Na[AlSi3O8](Na-F)+K++2H+ = KAl3[Si3O10][OH]2(I)+6SiO2(Q)+3Na+

TE D

Density（ （g/cm ）

KAl3[Si3O10][OH]2

SiO2

1

before reaction

EP

3

after reaction

200.77

Al2[Si2O5][OH]4

M AN U

3

after reaction

RI PT

before reaction

SC

Time

+

2.66

+

SiO2

ACCEPTED MANUSCRIPT

103.54 3

Volume change（cm ）

98.66

203

-4.88

Net increase （%） ）

9.62

after reaction +

+

before reaction 2+

Reaction

3CaAlSi3O8(Ca-F)+2K +4H = 2KAl3[Si3O10][OH]2(I)+ 3Ca

Solid

CaAlSi3O8 3

2

Molar mass（ （g/mol） ）

278

398

2.685

2.75

310.61

289.45 -21.16

Net increase （%） ）

6.81

+

KAl3[Si3O10][OH]2 2

SiO2 6

398

60

2.25

2.75

2.65

552

289..45

135.85

M AN U

Volume change（cm ）

TE D

3

414

EP

Volume（ （cm ）

CaAl2Si4O12 ·4H2O 3

AC C

3

after reaction

3CaAl2Si4O12 ·4H2O(L)+2K +4H = 2KAl3[Si3O10][OH]2(I)+6SiO2+12H2O+ 3Ca2+

KAl3[Si3O10][OH]2

Amount（ （mol） ）

Density（ （g/cm ）

+

SC

before reaction

3

45.28

-19.53

4.71

Time

138.19

RI PT

Volume（ （cm3）

-126.7 22.95

ACCEPTED MANUSCRIPT Table 3

P2(%) 3.1 2.1 2.2 4.3 3.1 2.0 3.2 2.86

RI PT

P1(%) 4.1 5.2 5.1 2.3 2.1 1.1 5.0 3.56

AC C

EP

TE D

M AN U

SC

Depth（ （m） ） Layer Ch6 2088.62 Ch6 2089.96 Ch6 2090.97 Ch6 2091.71 Ch6 2092.61 Ch6 2093.33 Ch6 2097.72 Mean value

ACCEPTED MANUSCRIPT Table 4. Wells

Depths (m)

S0

Ф1(%)

Ф2(%)

Ф3(%)

ФL(%)

Ф4(%)

H18(1)

1900.47

1.11

39.8

25.0

12.5

14.8

12.8

12.3

0.46

11.8

5.60%

H18(2)

1900.68

1.30

36.9

21.6

13.3

15.3

8.2

13.4

0.53

12.9

3.01%

H18(3)

1900.85

1.26

37.6

21.1

13.3

16.5

6.6

14.4

0.44

14

5.26%

H18(4)

1901.08

1.08

40.2

22.9

9.6

17.3

12.6

10.4

0.35

10

4.17%

H18(5)

1901.18

1.08

40.3

26.1

9.8

14.2

16.6

9.6

0.053

9.5

3.06%

H18(6)

1901.33

1.03

41.1

26.5

11.5

14.6

15.7

10.8

0.006

10.8

6.09%

H18(7)

1901.44

1.11

39.7

24.9

11.5

14.8

13.7

11.2

0.19

11

4.35%

H18(8)

1913.34

1.18

38.7

22.9

14.5

15.8

7.0

15.9

0.57

15.3

5.52%

H18(9)

1913.44

1.02

41.5

22.2

15.3

19.3

6.9

15.3

0.67

14.6

4.58%

H18(10)

1913.6

1.08

40.2

20.1

15

20.1

4.6

15.5

0.7

14.8

1.33%

H18(11)

1915.54

1.32

36.8

16.6

12.1

20.2

4.6

12.0

0.66

11.8

2.48%

H18(12)

1915.79

1.10

39.9

20.4

11.7

19.5

8.6

11.8

0.56

H18(13)

1.12

39.7

21.1

1.13

39.4

20.9

12.5 12.1

18.6

H6(1)

1916.09 1550.48

H6(2)

1551.56

1.21

38.2

19.7

H9(1)

1936.66

1.13

39.3

20.9

H9(2)

1936.72

1.12

39.6

22.3

H9(3)

1936.87

1.20

38.3

22.9

H9(4)

1940.25

1.13

39.4

20.2

H9(5)

1942.06

1.27

37.3

17.5

H9(6)

1952.3

1.18

38.6

18.5

H10(1)

1733.94

1.10

39.9

23.7

H10(2)

1734.66

1.36

36.2

18.7

H10(3)

1735.11

1.34

36.5

H10(4)

1735.51

1.31

H101(1)

1925.16

1.12

H101(2)

1925.4

1.12

H101(3)

1926.06

1.20

H101(4)

1926.3

H101(5)

1926.42

1.11

H101(6)

1926.6

1.08

H101(7)

1926.93

1.06

H101(8)

1927.13

1.06

H101(9)

1927.3

1.12

H101(10)

1927.46

1.11

H101(11)

1927.76

1.15

H101(12)

1932.04

1.13

H101(13)

1932.18

1.12

H103(1)

1861.155

1.07

H103(2)

1862.09

H103(3)

1862.8

1.11

H103(4)

1863.425

1.10

H103(5)

1864.33

1.10

H103(6)

1864.895

1.13

H103(7)

1865.27

1.14

H103(8)

1906.24

1.16

RI PT

E

11.2

4.27%

12.7

1.60%

7.8

13.2

0.54

9.1

11.9

0.66

11.5

4.97%

5.4

14.4

0.65

13.7

6.16%

11.2

9.8

0.76

9.8

6.68%

17.3

10.6

11.6

0.64

11

3.77%

11.4

15.4

10.5

12.4

0.54

11.9

4.39%

4.2

19.2

15.4

4.9

0.45

4.4

4.76%

5.4

19.8

11.1

6.5

0.57

5.9

9.26%

10.5

20.1

6.9

11.6

0.68

10.9

3.81%

11.5

16.2

11.0

12.7

0.66

12

4.35%

10.3

17.5

7.1

11.6

0.63

11

6.80%

18.2

10.9

18.3

5.8

12.4

0.35

11.1

1.83%

36.9

17.8

12.4

19.1

4.2

13.5

0.33

13.2

6.45%

39.6

21.3

14.4

18.3

7.1

14.2

0.59

13.6

5.56%

39.6

20.4

14.8

19.2

5.0

15.4

0.78

14.6

1.35%

38.4

19.0

13.4

19.4

4.4

14.6

0.55

14

4.48%

22.2

10.7

17.3

10.5

11.7

0.66

11

2.80%

39.7

21.8

15.3

17.9

5.6

16.2

0.68

15.5

1.31%

40.3

21.9

15.1

18.4

5.3

16.6

0.64

16

5.96%

40.6

21.3

13.6

19.3

8.2

13.1

0.65

12.4

8.82%

40.7

23.3

14

17.4

8.0

15.3

0.68

14.6

4.29%

39.6

23.1

13.1

16.5

9.5

13.6

0.66

12.9

1.53%

39.8

25.5

13.2

14.3

10.8

14.7

0.65

14

6.06%

39.1

22.6

14

16.5

7.4

15.2

0.54

14.7

5.00%

39.4

22.1

12.8

17.3

8.6

13.6

0.56

13

1.56%

39.6

21.4

12

18.2

9.8

11.6

0.55

11

8.33%

40.4

21.1

11.5

19.3

8.4

12.7

0.58

12.1

5.22%

20.0

13.3

19.4

4.8

15.3

0.67

14.6

9.77%

39.7

21.9

12.4

17.8

8.3

13.6

0.65

12.9

4.03%

39.9

20.8

13.3

19.1

6.0

14.8

0.66

14.1

6.02%

40.0

24.6

15.8

15.4

7.4

17.2

0.7

16.5

4.43%

39.4

23.1

15.1

16.3

6.4

16.7

0.69

16

5.96%

39.2

22.0

13.4

17.2

7.0

15.0

0.68

14.3

6.72%

39.0

21.6

12.1

17.4

7.9

13.7

0.67

13

7.44%

M AN U

18.4

10.6

TE D

AC C 1.13

18.5

ФF(%)

10.5

39.5

EP

1.13

18.5

Ф5(%)

SC

14.6

ФCL(%)

39.4

ACCEPTED MANUSCRIPT 1906.65

1.10

39.9

21.8

12.1

18.1

8.8

13.0

0.65

12.3

1.65%

1907.25

1.13

39.5

20.3

9.2

19.2

9.4

10.8

0.64

10.2

10.9%

H103(11)

1907.72

1.10

40.0

22.8

12.5

17.2

9.3

13.4

0.63

12.8

2.40%

H103(12)

1908.28

1.12

39.5

22.8

9.3

16.7

12.0

10.8

0.64

10.2

9.68%

AC C

EP

TE D

M AN U

SC

RI PT

H103(9) H103(10)

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 1

1

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 2

2

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 3

3

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 4

4

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 5

5

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 6

6

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 7

7

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 8

8

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 9.

9

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 10

10

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 11

11

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 12

12

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 13

13

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure14

14

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure15

15

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure16

16

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure17

17

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

18

ACCEPTED MANUSCRIPT Highlights (1) Diagenetic processes in the Chang 6 sandstones were analyzed. (2) The Chang 6 sandstones can be divided into high-porosity and low-porosity reservoirs.

RI PT

(3) Compaction is the key factor for deterioration of physical properties in Chang 6 sandstones. (4) Two stages of calcite cementation response for the evolution of high-porosity and low-porosity reservoirs.

SC

(5) Redistribution of burial dissolution products may slightly increase porosity, but significantly

AC C

EP

TE D

M AN U

reduce permeability less than 1 mD.