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Abstract

The Tengger Formation in the Baiyinchagan sag of the Erlian Basin has behaved as a low-permeability petroleum system during its diagenetic history. Through the observation and examination of thin section, scanning electron microscopy, and X-ray diffraction data, this study found the existence of alkaline diagenesis, as indicated by the dissolution of quartz, the precipitation of authigenic illite and chlorite, and the formation of carbonate and authigenic albite cements. There were two types of alkaline diagenetic conditions: the early alkaline diagenetic conditions were controlled by the syndepositional environment, and the multiphase burial alkaline diagenetic conditions were controlled by the evolution of organic acids and thermal fluids. The quantitative model of the evolution of porosity over geologic time developed in this study indicates that quartz dissolution increased the porosity by a total of 0.85%. This work may provide significant advances in the understanding of the low-permeability reservoirs in the Erlian Basin and lays the scientific foundation for oil and gas exploitation.

Keywords

Alkaline diagenesis low-permeability reservoir Lower Cretaceous Baiyinchagan sag Erlian Basin

Introduction

Due to their poor reservoir quality, low-permeability reservoirs with permeability of less than 20 mD are challenging to exploit and to achieve economic production (Bernard and Horsfield, 2014; Bjørlykke, 1998; Loucks et al., 2012; Ma et al., 2018; Milliken and Curtis, 2016; Nelson, 2009; Pittman, 1979; Polito et al., 2006). During the burial process, diagenesis plays the key role in the development and distribution of secondary pores, which provide space for oil and gas in the reservoir and paths for hydrocarbon migration (Basan et al., 1997; Bjørlykke and Jahren, 2012; Ellwood et al., 1986; Farrell et al., 2013; Kominz et al., 2011). The dissolution caused by organic acids that create acidic diagenetic environments is thought to contribute to the improvement in reservoir properties (Beig and Lüttge, 2006; Hansley and Nuccio, 1992; Mørk, 2013; Schmidt and McDonald, 1979; Stoessell and Pittman, 1990; Surdam et al., 1989; Taylor et al., 2010). However, alkaline diagenesis changes the mineral composition and affects the reservoir properties (Qiu et al., 2002; Rampe et al., 2016; Tuttle and Goldhaber, 1993).

Evidence of alkaline diagenesis can be obtained on the basis of petrographic characteristics (Bowser and Jones, 2002; Jones and Mumpton, 1986; Reimer et al., 2009; Wright, 2012). One positive indicator is the phenomenon of quartz dissolution, which has attracted research on the effects of pH, temperature, pressure, alkaline concentration of the solution, and dissolution rate (Bennett, 1991; Bjorkum, 1996; Blake and Walter, 1999; Brady and Walther, 1990; Crundwell, 2014; Dove et al., 2005; Gautier et al., 2001; Kim and Olek, 2014; Knauss and Wolery, 1988). Studies in the literature suggest that higher pH and temperature conditions play an important factor in quartz dissolution. For example, Knauss and Wolery (1988) noted that when the pH value is more than 8, the etch pits of quartz were strongly developed, and the dissolution rates reached their peak. Bennett (1991) noted that quartz dissolution can be accelerated in the presence of dissolved organic compounds in natural waters at near-neutral pH values and low temperatures. Crundwell (2017) noted that the dissolution rates of quartz increase as the pH increases into the alkaline region. Therefore, quartz dissolution developed during the burial diagenesis can be regarded as an indicator of an alkaline diagenetic environment (Belaid et al., 2010; Lou et al., 2002; Pichat et al., 2016; Yaoqi et al., 2011). In addition, the mineralogical, geochemical, and isotopic characteristics of authigenic clay minerals can provide excellent information about fluid events (Clauer and Chaudhuri, 2012; Dai et al., 2016; Uysal and Golding, 2003). According to the experiments of Uysal and Golding (2003), the availability of potassium and the supply of acidic fluids determine the amount of kaolinite precipitation and the amount of illite dissolution. White et al. (1942) noted that chlorite is more stable in more alkaline solutions. Furthermore, the formation conditions of carbonate cements are closely related to alkaline fluids (Emerson et al., 1980; Meister, 2013; Owen et al., 2008; Swart, 2015).

Based on the data of reservoir physical properties, the Tengger Formation reservoirs of the Baiyinchagan sag in the Erlian Basin present an average porosity and permeability of 10.2% and 16 mD, respectively (Deng et al., 2013; Xu et al., 2005) (Figures 1 and 2). Reservoir quality has become a critical issue because it affects the productivity of oil and gas wells. However, most studies have concentrated on sedimentary and tectonic activity, and few studies have documented the impact of diagenesis, especially alkaline diagenesis. In this paper, the effect of alkaline diagenesis on reservoir properties is studied, and this work provides a basis for reservoir evaluation and oil and gas exploration. Moreover, the characteristics of alkaline diagenesis are identified, the alkaline fluid source is discussed, and the porosity under the control of the alkaline diagenetic environment is estimated. This work may provide crucial information to the knowledge of reservoir quality and the recovery following oil and gas exploration.

Figure 1.

Location of the study area. (a) The tectonic unit division of the Baiyinchagan sag. (b) The tectonic unit division of the Erlian Basin, China. In the western Erlian Basin is the Baiyinchagan sag. (c) Cross section from the northern steep slope to the southern gentle slope during the Tengger Formation time of the Baiyinchagan sag.

Figure 2.

Generalized stratigraphy of the Baiyinchagan sag showing the sedimentary and tectonic evolution stages and the major petroleum system elements. The sedimentary facies type of the K1ba-K2er Formations include alluvial fan, fan delta, braid delta, lacustrine, turbidite fan, and fluvial. The sedimentary cycle featured multiple phases. The tectonic evolution stages of the K1ba-K2er Formations include the initial rift stage, the three-episode synrifting stage, and the postrifting stage.

Geological settings

The Baiyinchagan sag, with an area of 3200 km², is located in the western Erlian Basin in China and represents a continental rift lacustrine basin (Ku et al., 2005; Zhang et al., 2003) (Figure 1). The western Baiyinchagan sag, with an area of 2200 km², contains the main oil and gas exploration fields and consists of the Tala fault structural belt, the western subdepression belt, and the Baiyinwengte fault belt (Gao et al., 2015; Jiang et al., 2000; Lin et al., 2009) (Figure 1).

The basement lithology consists of lower Paleozoic metamorphic rocks (Figure 2). The sedimentary strata, with a total thickness greater than 5000 m, are composed of the Lower Cretaceous Baiyanhua Group, which includes the Alshan, Tengger, Duhongmu, and Saihantala Formations. During the Late Cretaceous, the Erliandabusu Formation was deposited (Gao et al., 2015; Lin et al., 2009; Xu et al., 2005) (Figure 2).

The Tengger Formation is one of the sources and reservoirs of oil and gas and is dominated by interbedded mudstone, sandstone, and sandy conglomerate (Gao et al., 2015) (Figure 2). During the depositional period of the Tengger Formation, the study area featured a delta sedimentary environment and featured the deposition of fan delta, lacustrine, braid delta, and turbidity fan facies (Gao et al., 2015; Jiang et al., 2000) (Figure 2).

Methods

Nineteen sandstone samples from nine wells in the Tengger Formation in the western Baiyinchagan sag were chosen for mineral petrography and space composition analysis. Based on the physical data, the evolution of porosity under the control of alkaline diagenesis was reconstructed.

Petrographic analysis

A polarized-light microscope was used to analyze the comprehensive petrographic characteristics of the 19 samples. The compositional and textural characteristics of the samples were observed to confirm the existence of alkaline diagenesis and to interpret the porosity evolution. The typical features of alkaline diagenesis include the dissolution of quartz, the precipitation of carbonate cements and feldspar, and the development of clay minerals (Qui et al., 2002; Savage et al., 2010; Turner and Fishman, 1991) (Figures 3 and 5).

Figure 3.

Different types of siliceous mineral dissolution in the Tengger Formation in the Baiyinchagan sag. (a) Da34, 1328.57 meters. Optical photomicrograph showing the dissolved edges of quartz grains and quartz overgrowth, forming concave, embayed, and serrated shapes. (b) Ch1, 1526.25 meters. Optical photomicrograph showing the dissolution of quartz grains and quartz overgrowth. The interior of the quartz grain features intragranular pores. (c) Ch37, 1635.43 meters. SEM image showing several pits on the surface of a quartz overgrowth. (d) De1, 1720.95 meters. SEM image showing pits on the surface of authigenic quartz. (e) De1, 1720.95 meters. Optical photomicrograph showing carbonate cements replacing both the edges and the interior of quartz grains. (f) Ch9, 1126.09 meters. Optical photomicrograph showing quartzose lithic fragment dissolution within a grain, forming intragranular pores. Ank: ankerite cement; Dol: dolomite cement; MI: micrite carbonate cement; QLF: quartziferous lithic fragment; Qo: quartz overgrowth; Qzt: quartz grain.

All samples were examined by scanning electron microscopy (SEM) (JMS-5500-lv with 20–200,000× magnification), which can provide qualitative information on the rock intergranular texture and show the relationships between grains and pores (Desbois et al., 2011; Houben et al., 2013; Pittman and Thomas, 1979). The SEM data were chiefly used to identify and characterize the crystalline structure and orientation (Bloch et al., 2002; Hurst and Nadeau, 1995; Morad et al., 2010; Peltonen et al., 2009) (Figures 3 and 5).

All samples were also subjected to glycolated X-ray diffraction (XRD) analysis. The XRD data acquired from the glycolated clay samples were used to calculate the clay types in order to assess changes in fluid events (Barré et al., 2008; Di Maio et al., 2004; Moore and Reynolds, 1989; Quirein et al., 2010; Sheldon and Tabor, 2009; Srodon et al., 2001; Wintsch et al., 1995) (Figures 3 and 5).

2. Porosity evolution analysis

According to the formula for calculating the original porosity of unconsolidated sandstone (Table 1), the sandstone sorting coefficient of the Tengger Formation is between 1.58 and 2.76, with an average value of 2.18. The original porosity of the Tengger Formation was 29.21–35.40%, with an average value of 31.55%. For example, for sample Well Ch2, 1326.13 m, the sorting coefficient is 1.84, the current porosity is 10.40%, and the pore face ratio is 4.39%. Based on the evolutionary sequence of diagenesis, the porosity of the Tengger Formation has been restored at the key phases controlled by cementation and dissolution (Li et al., 2016; McCreesh et al., 1991; Tobin et al., 2010; Wang and Chen, 2007; Wang et al., 2013; Zhang et al., 2016) (Figures 3 and 5). A function was established to transform the two-dimensional face ratio in thin sections into three-dimensional porosity (Table 1) (Figure 7).

Table 1.

Quantitative calculation formula of porosity during the burial diagenetic process of the Tengger Formation in the Baiyinchagan sag (Wang et al., 2013; Yuan et al., 2017).

Porosity evolution parameters	Computational formula	Parameters
Relationship between areal rate and porosity function /%	$Fd = (φ_{p} - φ_{y}) φ_{d} / φ_{t}$	$P_{25}$ and $P_{75}$ are the particle diameters corresponding to 25% and 75% of the particle probability accumulation curve respectively, mm $φ_{o}$ -Original porosity, % $φ 1$ -Porosity of sandstone after compaction, % $φ 2$ -Porosity of sandstone after cementation, % $φ 3$ -Secondary porosity of sandstone, % $φ_{L}$ -Porosity of compaction loss, % $φ c$ -Porosity of cementation loss, % $φ_{p}$ -Porosity of physical analysis, % $φ t$ -The total areal rate, % $φ pm$ -The intergranular areal rate, % $φ_{ca}$ -The carbonate cement areal rate, % $φ_{y}$ - The clay minerals areal rate, % $φ_{d}$ -The dissolution areal rate, %Ct-Cement content, % $F_{L}$ -The loss rate of compaction porosity, % $Fc$ -The loss rate of cementation porosity, % $Fa$ -The increase rate of secondary porosity, % $F_{d}$ -The increase rate of dissolution porosity, %
Sort coefficient	$So = P_{25} / P_{75}$
Original porosity of unconsolidated sandstone /%	$φ_{o} = 20.91 + 22.90 / So$
Porosity of sandstone after compaction /%	$φ_{1} = Ct + [(φ_{pm} + φ_{ca}) φ_{p} / φ_{t}]$ $φ_{L} = φ 0 - φ 1$ $F_{L} = (φ_{L} / φ_{0}) \times 100 %$
Porosity of sandstone after cementation /%	$φ_{2} = (φ pm / φ t) \times φ p$ $φ c = φ 1 - φ 2$ $Fc = (φ_{c} / φ_{1}) \times 100 %$
Porosity produced by secondary porosity /%	$φ_{3} = φ d \times φ p / φ t$ $Fa = (φ_{3} / φ_{1}) \times 100 %$
Calculation of current porosity /%	$φ n = φ_{2} + φ 3$
Error rate /%	$E = \| φ_{p} - φ_{n} \| \times 100 %$

Results

Dissolution of siliceous minerals

Four types of quartz grain dissolution were identified in the study area: (1) Dissolution occurred along the edges of quartz grains or quartz overgrowths, forming intergranular dissolution pores. Most of the dissolved edges exhibit concave, embayment, and serrated shapes (Figure 3(a) and (b)). (2) The dissolution occurred in the interior of quartz grains and formed some intergranular pores (Figure 3(b)). (3) The dissolution occurred on the surface of quartz grains or authigenic quartz and formed pits that were filled with clay minerals or carbonate cement (Figure 3(c) and (d)). (4) After the edges of quartz particles were dissolved, precipitated clay minerals or carbonate cement occupied the intragranular pores and even further replaced the quartz grains (Figure 3(e)).

Dissolution of quartzite fragments, such as metamorphic quartz fragments, occurred at the edges or interior, which formed intergranular or intragranular dissolution pores (Figure 3(f)).

The cementation of authigenic clays mineral

The XRD mineralogical data show the distribution of authigenic clay minerals in the Tengger Formation reservoirs and indicate that the clay mineral composition can be characterized as illite rich and kaolinite poor (Figure 4). Authigenic illite is present in the form of filament-like shapes filling intergranular pores or covering the surfaces of grains (Figure 5(a)). Authigenic chlorite is present in the shape of pompoms or petals in the intergranular pores, and some of the chlorite covers the quartz grains in the shape of a diaphragm (Figure 5(b)).

Figure 4.

XRD data of the Tengger Formation reservoirs in the Baiyinchagan sag. (a) The content of illite appears a decreasing trend at the interval depths of 1400 -1600 and 1800 -2000 meters, and increasing trend at the interval depths of 1000 -1400, 1600 -1800, and 2000 -2200 meters. (b) The content of Kaolinite mainly ranges from 1 to 20%. At the depths of 1600–1800 and 2000–2200 meters, decreasing trend appears. (c) The content of chlorite shows increasing trend at the depths of 1000–1600 and 2000–2200 meters, and decreasing trend between 1600 and 1800 meters. (d) The ratio of illite and smectite shows a decreasing trend with the depth.

Figure 5.

The characterization of authigenic diagenetic minerals. (a) Ch3, 1332.65 meters. SEM image shows authigenic illite filled in the intergranular pores or coated on the surface of the grains. (b) Ch2, 1326.12 meters. SEM image shows authigenic chlorite like petal shaped filled in the intergranular pores. (c) Ch21, 1193.38 meters. SEM image shows feldspar overgrowth. (d) Ch21, 1193.38 meters. Optical photomicrograph shows feldspar overgrowth and dissolution of quartz grains. (e) Xi5, 2161.30 meters. SEM image shows authigenic albite. (f) Ch2, 1495.40 meters. Optical photomicrograph shows quartz overgrowth, feldspar overgrowth, dolomite, and ankerite cements. AL: albite; Ank: ankerite cement; CH: chlorite; Dol: dolomite cement; Fd: feldspar; Fd-o: feldspar overgrowth; I: illite; Qtz: quartz.

Carbonate cement, feldspar overgrowth, and authigenic albite

The carbonate cementation can be divided into two phases, namely an early micritic calcite phase distributed along the margins of grains (Figure 3(a)) and a late carbonate cement presenting blocky to poikilotopic textures that is mainly developed in the early formed intergranular and intragranular dissolution pores, some of which replaced the edges of quartz and feldspar (Figures 3(e) and 5(c)). Feldspar overgrowths are commonly developed around the detrital feldspar grains and have edge widths of 0.02–0.13 µm (Figure 5(d) to (f)). Authigenic albite with a slate-like texture filling intergranular pores is also present (Figure 5(e)).

Discussion

The formation mechanism of alkaline conditions

As the pH value increases, SiO₂ becomes extremely active, and it easily migrates and easily dissolves in alkaline fluids. The dissolution of quartz grains is relatively common and is associated with feldspar overgrowth or carbonate cement, indicating that the Tengger Formation reservoirs experienced an alkaline diagenetic environment (Cappelle and Behrends, 2008; Dove, 1999; Lindgård et al., 2012) (Figure 3).

The formation of carbonate cement in the Tengger Formation occurred in two stages. The type of carbonate cement in the first stage was mainly composed of micritic calcite, and a small amount of dolomite formed mostly along grain edges. The characterization of the early carbonate cement indicated that the sedimentary water included a relatively high content of Ca²⁺ and formed in the alkaline syndepositional environment (Figure 6). This conclusion is supported by the study of lithofacies paleogeography and trace element data (Liu and Sun, 2010), which suggest that the lacustrine delta facies of the Tengger Formation developed under an arid and hot climate and in relatively close proximity to carbonate rocks. The content of Ca²⁺ in the formation water was relatively high, and the water chemistry was alkaline and relatively highly saline, especially in the center of the Baiyinchagan sag. In addition, the logging data showed that dolomitic mudstone and dolomitic shale are present in the Tengger Formation, indicating an alkaline sedimentary environment (Gao et al., 2015; Jiang et al., 2000).

Figure 6.

Diagenetic evolution events of the Tengger Formation in the Baiyinchagan sag. A and B—During the syndepositional stage to the early stage of diagenesis, the sediments were gradually compacted, and siderite formed small crystals filling in intergranular pores or coating the grains. C—During the early diagenesis stage, quartz overgrowth developed. With the maturation of organic matter, some locations were charged with some oil and gas. D—During the early to middle diagenesis stages, carbonate cements filled the pores, and feldspar overgrowth appeared. The second phase of oil and gas charged in some location which mainly showed the characterization of quartz overgrowth and feldspar dissolution. E—During the middle diagenesis, the third phase of oil and gas charged in some location, a few quartz grain developed overgrowth edge, some feldspar, carbonate cements, and lithic fragment dissolved.

The type of carbonate cement in the second stage is mainly composed of blocky dolomite. During the late Jurassic, the extension of the crust caused massive volcanic eruptions and formed volcanic rocks and pyroclastic rocks. During the early Cretaceous, the entire sag entered a period of intense rifting. One of the material sources of the late carbonate cements may be related to carbon dioxide-rich high-temperature thermal fluids migrating along tectonic faults, resulting in the dissolution of feldspar and the early carbonate minerals and producing a large amount of dissolved Ca²⁺, Fe²⁺, Mg²⁺, and CO₃²⁻. Another source may be related to the influence of organic acids, which also affected the dissolution of carbonate minerals. Additionally, the transformation of clay minerals during the burial diagenetic process provided some alkali metal ions. Therefore, an alkaline diagenetic environment formed, and the late carbonate cements began to precipitate. The carbonate cementation and the dissolution of both quartz grains and quartz overgrowths are indicative of alkaline conditions (Cappelle and Behrends, 2008; Liu et al., 2018) (Figure 6).

Feldspar overgrowth and authigenic albite are also indicative of an alkaline environment (Martinez-Ramirez and Palomo, 2001; Živica, 2007). Some feldspar particles developed enlarged edges with no dissolution, indicating the transition from an acidic diagenetic environment to an alkaline diagenetic environment.

Kaolinite is unstable under alkaline conditions (Belver et al., 2002; Komadel, 2016; Komadel and Madejová, 2006; Panda et al., 2010; Yukselen and Kaya, 2003) and is converted to illite in the presence of K⁺ or chlorite or smectite in the presence of Fe²⁺, Mg²⁺ as the pH increases. Smectite in the presence of K⁺ can transform into illite. At depths of 1600–1800 and 2000–2200 m, the content of kaolinite decreased, whereas the content of illite increased, indicating the role of alkaline diagenesis (Elert et al., 2008; Jones and Mumpton, 1986; Yuretich and Cerling, 1983) (Figure 4).

The early diagenetic environment was controlled by the alkaline sedimentary conditions, as proved by the micritic carbonate cementation and quartz dissolution (Martinez-Ramirez and Palomo, 2001) (Figure 6). The burial diagenetic environment experienced multistage alternation between acidic and alkaline conditions. The dissolution consumed the organic acids, and CO₂ from deep formations entered the Tengger Formation reservoirs along the fracture system, increasing the number of alkaline cations and resulting in enhancement of the alkaline fluid properties (Figure 6).

The effect of diagenesis on reservoir properties

The diagenetic environment of the Tengger Formation was constantly changing, thus forming the complex diagenetic phenomena in the present reservoirs. The characteristics of the alkaline diagenesis include quartz dissolution, feldspar overgrowth, and precipitation of carbonate cements, authigenic illite and chlorite. The characteristics of the acidic diagenesis include dissolution of feldspar and carbonate cements and formation of quartz overgrowths.

In addition to compaction, cementation and dissolution have different influences on reservoir quality (Mingjie et al., 2016; Tobin et al., 2010; Wang and Chen, 2007). Carbonate cements formed in the early diagenesis stage can fill residual primary pores and reduce the reservoir quality. However, the presence of carbonate cements can effectively reduce the compaction of rocks and provide a material basis for large-scale secondary corrosion in later stages. When the organic matter in the source rock starts to generate organic acid, the carbonate cements dissolve to form secondary dissolution pores, which provide storage space for oil and gas accumulation. Late carbonate cements often fill in the pores formed by the dissolution of feldspar, lithic debris, and calcite in the early stage, and these cements are not easily removed by dissolution and seriously reduce the reservoir pore space. The silicon released by the dissolution of unstable clastic particles, such as feldspar grains, was the main material source for the authigenic quartz in the Tengger Formation. The formation of quartz overgrowths generally blocked the primary intergranular pores and reduced the porosity. However, the siliceous cements experienced dissolution in the alkaline environment, which was conducive to the formation of secondary pores (Figure 3). Fibrous-like illite and petal-like chlorite filled in the intergranular pores, which reduced the pore radius and blocked the pore throats (Figure 5(a) and (b)).

The quantitative recovery results of the porosity evolution of Well Ch2 are shown in Table 2 and Figure 6. Models were constructed to show the different stages of diagenesis. The porosity evolution of the Tengger Formation has been recovered (Figure 7).

Table 2.

Quantitative recovery results of porosity evolution in sandstone sample of the Tengger Formation from the Well Ch2.

Well	Typical diagenesis	Ancient time till now (Ma)	Ancient buried depth (m)	Inversion and back stripping porosity (%)	Compaction loss porosity (%)	Cementation loss porosity (%)	Secondary porosity (%)	Porosity of physical analysis (%)
Ch2	Compaction/Early carbonate cementation	110	1100	18.07	12.37	3.00	0.10	12.90
	Compaction/Feldspar dissolution/Quartz growth	107	1500	17.59	2.60	1.38	3.50	8.30
	Compaction/Late carbonate cementation/Quartz dissolution	103	2000	9.62	3.34	5.08	0.45	7.80
	Compaction/Carbonate dissolution/Quartz growth	93	1400	10.16	0.20	1.01	1.74	8.90
	Compaction/Pyrite cementation/Fracture development	0	1326	10.24	0.10	0.12	0.30	10.40

Figure 7.

The characterization of pore evolution during the burial diagenetic process. The inversion and back stripping methods were used to reconstruct the porosity evolution caused by the principal diagenesis. (a) Model A shows the present diagenetic characterization was created by the oil charge, the quartz overgrowth, the quartz dissolved pore, the feldspar overgrowth, the feldspar dissolved pore, the carbonate cementation, the carbonate cement dissolved pore, and lithic fragment dissolved pore. (b) Model B shows the stage before carbonate cement dissolved. The main diagenesis acted as the quartz overgrowth, the quartz dissolved pore, the feldspar overgrowth, the feldspar dissolved pore, the carbonate cementation, and some lithic fragment dissolved pore. (c) Model C shows the stage before carbonate cemented. The main diagenesis showed the quartz overgrowth, the quartz dissolved pore, and the feldspar dissolved pore. (d) Model D shows the stage before quartz dissolved. The main diagenesis showed the quartz overgrowth and the feldspar dissolved pore. (e) Model E shows the stage before quartz growth, which mainly showed the feldspar dissolution. (f) Model F shows the stage before feldspar dissolution. The major type of pore was the primary intergranular pores. C: carbonate cement; F: feldspar; LF: lithic fragment; Qtz: quartz.

During the early diagenesis stage, from deposition to 110 Ma, compaction and early carbonate reduced the porosity by 12.37 and 3.00%, respectively. The alkaline diagenetic environment provided the conditions for the dissolution of quartz grains, and the pore space increased by 0.1%. From 110 to 107 Ma, organic acids began to form, creating an acidic diagenetic environment. Compaction and quartz overgrowth formation reduced the porosity by 2.60 and 1.38%, respectively. The dissolution of carbonate cements and feldspar increased the porosity by 3.50%.

During the middle diagenesis stage, from 107 to 103 Ma, the concentration of organic acids gradually decreased, and the pH value of the pore fluid increased. The reservoir quality was controlled by carbonate cementation and compaction, which reduced the porosity by 5.08 and 3.34%, respectively. The dissolution of quartz and kaolinite increased the porosity by 0.45%. From 103 to 93 Ma, compaction and cementation reduced the porosity by 0.20 and 1.01%, respectively. However, feldspar dissolution increased the porosity by 1.74%. From 93 Ma to the present, the late carbonate cements are evidence of the effects of an alkaline diagenetic environment and reduced the porosity by 0.12%. Furthermore, compaction reduced the porosity by 0.10%, whereas quartz dissolution increased the porosity by 0.30%.

Conclusions

The existence of alkaline diagenesis in the Tengger Formation reservoirs is evidenced by the dissolution of quartz, the precipitation of authigenic illite and chlorite, and the formation of carbonate and authigenic albite cements.

The early alkaline depositional background and the evolution of organic acids and thermal fluids from deep formations during burial controlled the formation and evolution of the alkaline diagenetic environment.

According to the reconstruction of porosity, the processes of compaction, cementation, and dissolution had different effects on the reservoir quality. Quartz dissolution relatively increased the porosity, while the carbonate cement formation reduced the porosity during the multiple stages of the alkaline diagenetic environment.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This work was supported by National Science and Technology Major Project of China (Grant No. 2016ZX05007-004-02),National Natural Science Foundation of China (Grant No. 41002032),and Innovation Team of Sedimentary Geology (Chengdu University of Technology) (Grant No. KYTD201703).

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Effect of alkaline diagenesis on sandstone reservoir quality: Insights from the Lower Cretaceous Erlian Basin,China

Abstract

Keywords

Introduction

Geological settings

Methods

Results

Dissolution of siliceous minerals

The cementation of authigenic clays mineral

Carbonate cement, feldspar overgrowth, and authigenic albite

Discussion

The formation mechanism of alkaline conditions

The effect of diagenesis on reservoir properties

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References