The Distribution and Origin of Carbonate Cements in Deep-Buried Sandstones in the Central Junggar Basin , Northwest China

Extremely high porosities and permeabilities are commonly discovered in the sandstones of the Xishanyao Formation in the central Junggar Basin with the burial depth greater than 5500m, from which hydrocarbons are currently being produced. High content of carbonate cements (up to 20%) is also observed in a similar depth range. Our study aimed to improve our understanding on the origin of carbonate cements in the Xishanyao Formation, in order to provide insights into the existence of high porosity sandstones at greater depths. Integrated analyses including petrographic analysis, isotopic analysis, fluid-inclusion, and core analysis were applied to investigate the distribution and origin of carbonate cements and the influence of high fluid pressure on reservoir quality. Textural evidences demonstrate that there are two generations of carbonate cements, precipitated at the temperature of 90C and 120C, respectively. The carbonate cements with low δ13CPDB ranging from −19.07 to −8.95‰ dominantly occurred near the overpressure surface and especially accumulated at approximately 100mbelow the surface.Our interpretation is that high content of carbonate cements is significantly influenced by early carbonate cements dissolution andmigration under overpressure. Dissolution of plagioclase resulted in the development of internal pores and porosities of as much as 10% at 6500m depth presumably.


Introduction
Carbonate cements in sandstones have variable mineralogy, texture, and chemical compositions and therefore exhibit significant effects on reservoir properties because it is commonly concentrated rather than being uniformly distributed.It is challenging to quantify the influence of concretionary carbonate cements on fluid flow in reservoirs because it is difficult to determine the distribution of diagenetic heterogeneity based on subsurface data.If the carbonate cements formed during early diagenetic stage, it could provide a framework that resists burial compaction and retains primary porosity until decarbonatization at greater burial depth [1][2][3].Microlitic carbonate cements formed at early diagenetic stage can undertake partial overburden load that can slow compaction and can be dissolved into secondary pores under favorable geologic conditions.Extensive studies have been performed on carbonate cementation-dissolution reactions from the viewpoints of fluid-rock, organic-inorganic, and sandstone-mudstone interactions in the past 40 years by traditional geochemical methods, such as stable isotope and major and trace elements analysis [4][5][6][7][8][9][10][11][12][13][14][15][16][17]  It is observed that the favorable sandstone reservoirs are developed at the depth of 4500∼6000 m in the central Junggar Basin.The average porosity is approximately 10% and the average permeability is 1 × 10 −3 m 2 .Although many studies have been carried out in this area, including petrographic analysis, formation-water geochemistry, fluid inclusions analysis, and overpressure characterization [13,[18][19][20][21][22][23], there is still a lot of debate on the origins and types of

Geological Setting
The Junggar Basin is one of the most prolific oil basin in China (Jiang and Fowler, 1986), covering an area of 136,000 km 2 .It is an intramontane basin bounded by multiple orogenic belts, including the Qinggelidi Mountains, the Kelameili Mountains, the Yilinheibiergen Mountains, the Bodega Mountains, and the Zhayier Mountains (Figure 1).The Junggar Basin is Late Palaeozoic-Cenozoic in age which is developed on the Junggar terrane, consisting of both Precambrian crystalline basement formed at 800 Ma ago and slightly metamorphic Palaeozoic basement [28][29][30][31][32][33].Our study area in this paper belongs to SINOPEC, located in the central depression of the hinterland of the basin which mainly consists of the west segment of the Changji Sag in the south (Figure 1).
The central depression area is one of the important areas for petroleum exploration.The characteristics of source rocks and reservoirs in the central Junggar Basin have been extensively studied.There were two sets of source rocks, including the Permian shales dominated by lacustrine-facies and the Jurassic mudstones dominated by swamp coalbearing.The deeply buried Jurassic sandstones dominated by fluvial-delta facies were main reservoirs with low porosity and permeability generally.However, relatively high porosity and permeability in sandstones display at some depth.At the same time, extensive development of overpressure is displayed over much of the central Junggar Basin.Much more hydrocarbon generated from Permian and Jurassic source rocks accumulated in the overpressured system (Wu Hengzhi et al., 2006;Li Pingping et al., 2006;Yang Zhi et al., 2007) (Figure 2).
Figure 3 shows the modelling burial history of Y1 well (Yongjin area) in Block 3 (see Figure 1 for its location).The erosion event generated an unconformity between Later Jurassic and Early Cretaceous.Therefore, the heat flow is the only variable that needs to be adjusted to match the presentday vitrinite reflectance data.Our modelling results indicated that the paleotemperature decreased gradually from the Permian to the present.This result is consistent with previous studies [34][35][36][37].Drilling data demonstrated that the vitrinite reflectance (  ) ranges within 0.65∼0.82% in the Middle Jurassic Xishanyao Formation.To match the measured and the predicated vitrinite reflectance data, BasinMod simulation software aims to rebuild the geothermal history.At present, the temperature of Jurassic strata was approximately between 120 ∘ C and 150 ∘ C, giving a gradient of 2.2 ∘ C/100 m.With the results of homogenization temperature, the main oil pools in the third central block formed from the end of the early Cretaceous to the early stage of Paleogene (from 75 Ma to 60 Ma) [38].At the same time, the crude oil was detected in high levels of 25-norhopane from Y1 well, which explained that an early stage of hydrocarbon charging occurred in the late Jurassic [38].

Samples and Methods
125 core samples were obtained from six wells (Y1, Y2, Y3, Y6, Y7, and Y8) at a depth range of 5500-6200 m from Jurassic Xishanyao Formation (Figure 1).The strategy of sample collection is based on the characteristics of lithology.Generally, an interval of one meter is between the two samples, if the cores have homogeneous qualities.The ternary plot indicates that the Xishanyao Formation sandstones are dominantly litharenite and feldspathic litharenite as they have an average framework composition of Q 28 F 13 L 59 (Figure 4).
Carbonate cements were investigated by using epoxyimpregnated thin sections, cathodoluminescence, and SEM/EDX images.Conventional core samples and epoxyimpregnated thin sections analysis was conducted on the MIAS 2000 microscopes by the Experimental Research Center of Wuxi Research Institute of Petroleum Geology of SINOPEC and Research Center of Shengli Oilfield Institute of Geology of SINOPEC.Analysis was performed at the room temperature of 25 ∘ C and the relative humidity of 60%.
Cathodoluminescence analysis was conducted on the CL8200 MK5 cathodoluminescence microscopy by the Experimental Research Center of Wuxi Research Institute of Petroleum Geology of SINOPEC.Analysis was performed at the room temperature of 27 ∘ C and the relative humidity of 40%.SEM/EDX analysis was performed on the sem-xl30 and EDX-INCA scanner by the Experimental Research Center of Wuxi Research Institute of Petroleum Geology of SINOPEC.Analysis was performed at the room temperature of 22 ∘ C and the relative humidity of 60%.
Carbon and oxygen isotope analysis was carried out on the MAT253 Gas isotope mass spectrometer made in Germany Firmigan company.Analysis was performed at the sample tray temperature of 72 ∘ C, chromatography temperature of 40 ∘ C, and helium gas pressure of 100 KPa.
Electron microprobe analysis was conducted on the JXA-8100 electron probe microanalyser at the State Key Laboratory of Geological Processes and Mineral Resource.Analysis was performed at the room temperature of 23 ∘ C and the relative humidity of 65%.

Thin Section Analysis.
Thin section images demonstrate that the dominant cements are carbonates, which consists of a high volume of ferroan calcite and ankerite.At the same time, a small amount of dolomite developed in some wells.Thin section images show that both ferroan and nonferroan calcites commonly occurred as extensive, singlecrystal poikilotopes that filled with the intergranular pores (Figure 5(a)).In contrast, ankerite cements are regular to rhombic, replacing calcites with an undulating extinction characteristic (Figures 5(b) and 5(c)).Dolomite cements were also observed in the form of replacing calcites (Figure 5(d)).Some calcite crystal filled in intergranular pores and postdated quartz overgrowth (Figures 5(e) and 5(f)).Some thin section images show that carbonate cements replaced detrital quartz, feldspar, or rock fragments.The abundance of carbonate can be up to 20% and generally in the range of 1-10% within the six studied wells (Figure 6).At the same time, particle contact modes were different because of maldistributed distribution.Some particles were in point contact where carbonate cements develop, while others were in straight of concavoconvex contact.Moreover, carbonates with different mineral compositions distributed in different wells, such as ankerite cements, generally distributed in wells Y2, Y6, and Y8 with an average concentration of 4.5% and calcite cements generally distributed in wells Y1, Y3, and Y7 with an average content of 4.1%.

Cathodoluminescence.
Under the cathodoluminescent (CL) images, the relative content of manganese (Mn) and iron (Fe) in carbonate cements can be used to provide insights into the redox conditions when the pore fluid formed.Mn in calcites is an activator in CL, while Fe acts as a quencher.Carbonate cements with Mn > Fe show bright luminescence, whereas calcite cements with Fe > Mn exhibit dull-luminesce.In Block 3, the carbonate cements partially show bright luminesce (Figure 5(g)) and others show shade of bright luminesce (Figure 5(h)).Cathodoluminescence of these calcite cements can be interpreted by their origin in sandstones.

EDX Analysis.
The trace element data of 14 core samples, determined by EDX analysis, are presented in Table 1.Carbonate cements are generally rich in Fe with low concent of Mn and Mg, and the concentration of Ca increases with increasing burial depth. 13C and  18 O of the carbonate cements, together with burial and thermal histories, can be used to reveal the origin of the cements.Stable isotopes data in Yongjin area are presented in Table 2. Carbon isotope values range from −19.07 to −5.87‰ (PDB) with average value of −8.95‰ (PDB).The oxygen isotope values range from −21.08 to −13.96‰ (PDB) with average value of −17.5‰ (PDB).The  13 C and  18 O values increase with increasing burial depth and there is a positive correlation between the  13 C and  18 O values.

Origins of Carbonate Cements and Source of Fluid.
Petrographic observations revealed at least two generations of carbonate cements.The first-generation carbonate occurred as blocky crystalline calcite, which filled within the intergranular pores reduced considerably by mechanical and chemical compaction.This kind of cements occurred in the form of coating quartz grains, preventing authigenic quartz from overgrowing (Alaa et al., 2007).And they dominated in the deeply buried sandstones and filled with iron (Fe) content,  7).29 samples from Xishanyao Formation (J2x) and 11 samples from Toutunhe formation (J2t) were collected between 5500 m and 6200 m for the carbon and oxygen stable isotopic analysis.The results were shown in Figure 7 as  a plot of  18 O (PDB) versus  13 C (PDB) that more negative  13 C values are generally accompanied by more negative  18 O values indicating that the carbonate cements were significantly influenced by organic matter alteration during burial (Figure 7) in area III.Rare input of  12 C with increasing burial and temperature from thermal alteration of organic matter is indicated by the strong correlation between  13 C and  18 O [39][40][41].
With limited fluid-inclusion data of carbonate cements and quartz overgrowth, the precipitation was formed at about 100 ∘ C and 80-130 ∘ C, respectively (Table 3).From Figures 5(e) and 5(f), the phenomena that carbonate crystal postdates quartz overgrowth revealed that at least part of carbonate cement deposited at 120 ∘ C (inclusion homogenization temperature of overgrowth concentrating in 120 ∘ C).From Figure 3, the carbonate formation mainly resulted from the later hydrocarbon charging.

Effect on Reservoir Properties.
Reservoir physical properties data indicate that porosity near the overpressure surface is relatively high, mainly concentrating in the depth range +50 m∼−250 m of the overpressure top surface, and the carbonate cements are concentrated in these high porosity zone (Figure 8).Thin section images and microscope analysis were used to investigate the origin that why do the high porosity and carbonate cements superimpose in depth, when the formation water flowed and broke through the overpressure surface, causing the precipitation and concentration of calcite near the overpressure surface because of the unstable temperature and pressure [42,43] (Yang Zhi, 2011).Observation from thin section images indicate that secondary intergranular pores are the dominant pore type (Figure 9).In contrast to the characteristics of minerals by planepolarized and cross-polarized light, the remnant of calcite can be found after dissolution developing in the surrounding pores.Data from electron microprobe analysis indicate that the secondary intergranular pores resulted from dissolving intergranular carbonate cements and feldspar (Figure 10, Table 4), mainly generated by calcite dissolution.Megapores were mostly formed by the dissolution of albite and less by K-feldspar and kaolinite in situ deposit which shows that pore configuration has good connection.These evidences demonstrate that the large-scale dissolution of intergranular carbonate cements can generate more intergranular pores and make the pore more connected.
Although sandstones experienced extensive mechanical compaction and chemical compaction, the point or straight grain contact and the pervasive development of intergranular pores suggest that the formation of carbonate cements predated intensive physical compaction.Firstly, early carbonate cements occupied the intergranular pore space which increased rock mechanical strength and resisting ability to compaction.Therefore, the sandstone reservoir can develop high primary porosity even in deeply buried conditions.Secondly, early carbonate cements provided materials for dissolution which can form a large amount of secondary intergranular porosity and feldspar dissolution further improved    the porosity.In these two ways, the high porosity and high content of carbonate cements developed in the same burial depth [44].Carbonate cements commonly occurred as irregularly distributed concretions even at the same depth, so it is challenging to predict porosity and permeability in the subsurface from spaced wells.Thin sections can provide a continuous image of heterogeneity produced by concretionary calcite cements.Our thin section images indicate a negative correlation between the carbonate cements and porosity development.Perhaps this phenomenon elaborates that intergranular pores, developing in deeply buried sandstones, result from early carbonate cements dissolving and migration.Therefore, early carbonate cements develop widely and secondary porosity is higher at later diagenetic phase.
The late generation of poikilotopic calcite is interpreted as a result of plagioclase and early calcite dissolution, which releases cations into pore water and may also be responsible for the precipitation of clay minerals and the silica cements [45].From Figure 3, Ro were in 0.7-1.0%.Thin section observations show that calcite formation is strongly associated with alteration of plagioclase.Among the common rock-forming minerals, plagioclase (especially calcium-rich plagioclase) is dissolved more rapidly than the other silicate phases [46], indicating that porosity generation may primarily result from plagioclase dissolution in deeply burial location.With the increasing of depth, temperature, and thermal maturity, calcite dissolution will be gradually weakened [5].According to thin sections, primary porosity developed before the depth 3500 m, secondary porosity mainly developed between 3500-6200 m, and cracks begin to develop from 6200 m [47,48].Therefore, during the deep-burial water-rock interaction processes, a lower secondary porosity zone resulting from plagioclase dissolution at depth greater than 6500 m would develop (Figure 11).

Conclusions
In Block 3 of the central Junggar Basin, carbonate cements are the predominant cements.Conventional core samples, epoxy-impregnated thin section analysis, and cathodoluminescence analysis indicate that the growth of carbonate cements has two stages and mostly formed at the late diagenetic stage, generating ferroan calcite and ankerite cements.
Data from the six wells demonstrate that carbonate cements of most of the samples are less than 20% and generally in the range of 1-10%.The concentration of carbonate cements increases with increasing burial depth.Carbonate cements mainly concentrate in the depth range +50 m∼ −200 m to the top overpressure surface.Stable isotopic data shows that  13 C PDB ranges from −19.07 to −5.87‰ and  18 O PDB ranges from −21.08 to −13.96‰.This suggests that the carbonate cements in these sandstones were significantly influenced by organic matter during burial history.
Electron microprobe analysis documents that the secondary intergranular pores primarily resulted from dissolving intergranular carbonate cements and feldspar.The chemical compaction and large-scale cementation restricted the dissolving capability of organic acid on late carbonate cements.
Textural data suggest that the late poikilotopic calcite, near the top overpressure surface, is rich in Fe and high porosity is developed in the same depth interval.This can be interpreted as a result of the dissolution of plagioclase.Therefore, another secondary porosity zone is supposed to develop, resulting from plagioclase dissolution at depth greater than 6500 m.However, because of the chemical compaction and quartz overgrowth, the porosity scale will be smaller than the porosity developed at the depth of 5500 m.

Figure 1 :
Figure 1: Map showing the tectonic units and studied well locations in the central Junggar Basin.

Figure 3 :Figure 4 :
Figure 3: Generalized burial and thermal histories of Y1 well from Block 3 in the central Junggar Basin.Location of the example well is marked in Figure 1.

Figure 5 :
Figure 5: Photomicrographs showing petrographic features of the Xishanyao Formation sandstones.Most common types of mineralogy variations in sandstone cement, including (a) secondary pores were mainly filled by crystal calcite, major pores were filled by asphalt at the edges, and the photomicrograph is a stained red-epoxy-impregnated thin sections of conventional core samples by plane-polarized light with magnification of 40 (Y6 well at 6,048.59 m); (b) single-crystal rhombus ankerite, the photomicrograph is a stained blue-epoxy-impregnated thin sections of conventional core samples by plane-polarized light with magnification of 200 (Y6 well at 6028.52 m); (c) a few ankerites replace calcites in a red-epoxy-impregnated thin sections of conventional core samples with cross-polarized light with magnification of 100 (Y2 well at 5966.02 m); and (d) a few dolomites replace calcites in a stained red-epoxy-impregnated thin sections of conventional core samples with cross0polarized light with magnification of 200 (Y2 well at 5967.02 m); (e) crystal calcite filled in pores, and crystal postdate quartz overgrowth in a stained blue-epoxy-impregnated thin sections of conventional core samples by cross-polarized light with magnification of 40 (Y1 well at 5876 m); (f) dolomites develop in a stained blue-epoxy-impregnated thin sections of conventional core samples by planepolarized light with magnification of 100 (Y2 well at 6000.25 m); (g) calcite cements show bright yellow luminescence in cathodoluminescence photomicrograph with magnification of 40 (Y7 well at 6095 m); (h) calcite cements show saffron luminescence and dolomites cements are disphotic in cathodoluminescence photomicrograph with magnification of 40 (Y8 well at 6099.46 m).

Figure 7 :Figure 8 :Figure 9 :Figure 10 :
Figure 7: Stable isotopic data of carbon and oxygen for carbonate cements of Block 3.

Figure 11 :
Figure 11: Diagenetic reactions, porosity, and hydrocarbon evolution of the different diagenetic phase in Block 3.
13C and 18O in the carbonate cements can be used to unravel the origins of the cements.The main mechanisms of generation of  13 C-depleted CO 2 in large amounts during burial are discussed by Irwin et al. [39], including diagenetic carbonate in area I, carbonate relating to biogas in area II, and carbonate relating to organic acid in area III (Jansa et al., 1990; Wang Darui, 2000; Wilkinson et al., 2006) (Figure which belonged to late diagenesis cements.The secondgeneration carbonate cement was ankerite, which replaced calcite (Figure5(c)).

Table 1 :
Composition from EDX analysis of carbonate cements in sandstone of Block 3.Figure6: Regional abundance of carbonate cements versus depth in the Jurassic Formation.Text data from 125 core samples show that carbonate is abundant from below 5800 m and increases with depth down to about 6100 m burial depth.The contents of calcite have little changes with the depth.However, ankerite has a sharp interface of 5850 m burial depth and the content increases with the depth.

Table 3 :
Fluid-inclusion data from quartz overgrowth and carbonate cements.

Table 4 :
Electron microprobe analysis of residual mineral contents in secondary pores of wells in studied area.