Coal Measures Sandstone Reservoir in Xishanyao Formation

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1		Dazhen Tang	--	1699	2022-08-19 06:00:07	\|
2	Chinese part has been changed in to English	Dazhen Tang	+ 1797 word(s)	3496	2022-08-31 15:28:44	\|

This entry is adapted from the peer-reviewed paper 10.3390/en15155499

The reservoir physical properties, pore types, diagenetic characteristics and reservoir quality controlling effect of the Xishanyao formation coal measure sandstone in the southern margin of the Junggar basin were discussed in this study based on thin section observation, high pressure mercury injection, low-temperature nitrogen adsorption and scanning electron microscope observation. The result shows that the porosity and permeability of the sandstone are generally low with a medium-high texture maturity and low compositional maturity. The sandstone storage space is mainly composed of residual intergranular pores, secondary dissolution pores, inter-crystalline pores and micro-fractures. The diagenetic stage of coal measure sandstone is in the mesodiagenesis A1-A2 stage, and their diagenetic interaction types mainly include compaction, cementation and dissolution. The reservoir quality of the coal measure sandstone deteriorates by compaction due to high matrix content and plastic debris content. Because of the large amounts of organic acids generated during the thermal evolution of the coal measure source rock, the coal measure sandstone suffers from strong dissolution. The secondary dissolution pores formed by the massive dissolution of feldspar, lithic fragments and early carbonate cementation in the sandstone significantly improved the reservoir quality. In the coal measure sandstone, clay mineral cementation is the most developed cementation form, followed by quartz cementation and carbonate cementation. Although kaolinite cementation and dolomite cementation can generate a small number of inter-crystalline pores, cementation deteriorates the reservoir quality. The Xishanyao formation coal measure sandstone formed in a lacustrine-delta environment, and its composition and texture make it susceptible to the influence of compaction and dissolution during diagenesis.

tight sandstone coal measure southern margin of Junggar basin diagenetic process reservoir quality control reservoir forming

1. Introduction

The Junggar basin is abundant in coalbed methane (CBM) resources, of which the predicted resource of CBM with a burial depth of less than 2000 m is 3.11 × 10¹² m³, and the southern margin of the Junggar basin (SJB) has the best preservation conditions for CBM[1]. The main coal-bearing strata in the SJB are lower-middle Jurassic, and the sedimentary environment mainly includes alluvial fans, fan deltas, braided river deltas, and shore shallow lakes[1，2，3]. The SJB has characteristics of thick and multiple coal seams developed with a laterally widely distribution and frequent interbedded sandstone, shale, and coal seams in the vertical direction which is favorable for the co-accumulation of coal measure gases[4，5，6，7，8，9，10](CMG, including CBM, coal measure tight sandstone gas and coal measure shale gas). The precedent of successful development in the Ordos basin [5，11，12]and the maturation of CMG symbiotic accumulation theory [7，8] have made the industry aware of the great potential in CMG development. The reservoir quality is crucial for the efficient development of coal measure tight sandstone gas, and regional differences in diagenetic environment and tectonic events lead to different factors affecting reservoir quality [13，14，15，16，17，18，19]. In order to efficiently develop the abundant tight sandstone that is contained in the coal measures in the SJB, it is necessary to conduct research on the petrological characteristics and reservoir quality of coal measures sandstone.

The tight sandstone reservoirs generally undergo a complex diagenesis process, resulting in progressive tightness during burial and thermal evolution [13，14，15，16，17，18]. The effects of reservoir quality include sedimentary, diagenesis and epigenesis, and the specific influencing factors include clastic composition, depositional environment, sedimentary facies, burial temperature and pressure [19，20]. Sedimentation controls the grain size, sorting, and arrangement of particles, as well as the mineral transformation and sedimentary structure [16，21，22，23]. The influence of diagenesis on the sandstone is mainly in terms of temperature, pH, pressure, etc., and is presented in the form of compaction, dissolution, cementation, and metasomatism [19，21，24]. Compaction is one of the main factors leading to the reduction of porosity and permeability [25], and the reservoir quality is particularly affected by the degree of compaction and the content of plastic particles (such as lithic fragments, mica, argillaceous debris) and matrix [19，26]. Cementation generally leads to the plugging of pore throats and seriously reduces permeability [27、28、29、30、31，32], but at the same time, it can also restrain the deterioration of reservoir quality to a certain extent: The inter-crystalline pores formed by the clay minerals cementation can provide storage space for sandstones [31，33，34]; the clay film covering the particle surface resists compaction and inhibits the development of other types of cement [13，35], thereby preserving residual pores; the clay film coating the particle surface can resist compaction and inhibit the development of other types of cement; the quartz cementation and carbonate cementation in the eodiagenesis can improve the compaction resistance ability of sandstone [15，36，37]. By analyzing the sedimentary environment, composition, and diagenesis characteristics of the coal measure sandstone, the reservoir quality control factors of the coal measure sandstone in the Xishanyao formation can be clarified and the formation and evolution process of the coal measure sandstone reservoir can be explained.

The coal seams of the Xishanyao formation on the southern margin of the Junggar basin are mainly in the low-medium coal rank, with frequent interbedding of sandstones, mudstones and coal seams [1]. The diagenesis of coal measure sandstone is strongly influenced by the thermal evolution of coal seams and dark shale [7，8]. Previous studies on sandstone reservoirs have mainly focused on the mechanism of pore throat formation, quantitative characterization of pore throats, and the relationship between oil and gas intrusion and reservoir diagenesis of sandstone reservoirs in different zones or layers within the basin [38，39，40，41，42，43]. However, studies on the coal-bearing sandstones of the Middle Jurassic Xishanyao formation have also focused on the evolution of the sedimentary environment, the classification of reservoir types, and the classification and evaluation of reservoirs [2，4，44，45，46，47]. As the understanding of the diagenetic sequence of the coal measure sandstone reservoirs in the Xishanyao formation, and the influence on reservoir quality is not comprehensive enough, it is urgent to carry out relevant work. This paper combined techniques including casting thin section observation, scanning electron microscope (SEM), XRD experiment, high-pressure mercury intrusion, low-temperature nitrogen adsorption, and Ro test of coal measure shale to conduct research on the following contents: (1) Determine the characteristics of coal measure sandstone in the Xishanyao formation; (2) clarify the diagenetic evolution sequence of coal measure sandstone in the Xishanyao formation; (3) clarify the controlling effect of sedimentation and diagenesis on the reservoir quality of coal measure sandstone. This study is expected to provide a reference for the exploration and development of tight sandstone gas in the Xishanyao formation coal measures.

2. Geology Background

The Junggar basin is located in the northern part of Xinjiang; it is a large-scale Mesozoic-Cenozoic depression basin with an area of about 13.4 × 10⁴ km² and an approximately triangular shape in the plane, with a width of 1120 km from east to west and 800 km from north to south [1，2，3]. Since the Late Paleozoic, the Junggar basin has experienced Hercynian, Indosinian, Yanshan and Himalaya tectonic movements, and the superposition of multi-phase stress fields has led to its structural complexity [2，44，48]. The Junggar basin can be divided into six structural units: the Luliang uplift, the Ulungu depression, the northern Tianshan piedmont thrust belt, the western uplift, the central depression, and the eastern uplift [3，49] (Figure 1A). The SJB is located in the northern Tianshan piedmont thrust belt, which elongates east-west direction with a north-south width of nearly 200 km (Figure 1B). The northern Tianshan piedmont thrust belt is a multi-phase superimposed and inherited structural belt, and is usually divided into five secondary structural units, including the Sikeshu sag, Qigu fault–fold belt, Huomatu anticlinal zone, Huan anticlinal zone, and Fukang fault zone [50].

Figure 1. Maps showing: (A) Location of the Junggar basin in China (modified by [4]). (B) Structural outline map (modified by [10，51]) of the Southern margin of Junggar basin (SJB). (C) The stratigraphic column shows the Middle-lower Jurassic Strata in SJB (modified by [51], Xishanyao formation is marked in red).

A major part of the surface of the SJB is covered by quaternary strata. The strata in the SJB area are Permian, Triassic, Jurassic and Palaeocene from old to new (Figure 1C). Among them, the Badaowan formation and Xishanyao formation are the main coal-bearing strata in the study area, which are widely distributed in the SJB (Figure 1C). The coal-bearing strata of the Xishanyao formation were formed in a lacustrine–deltaic sedimentary system [2，3]. The Xishanyao formation strata had multiple coal accumulation centers that were greater than 40 m thick between the present-day Horgos River and Sigong River controlled by tectonic and sedimentary conditions [1].

3. Paragenetic Sequence of Coal Measure Sandstone Diagenesis

3.1. Determination of Diagenetic Stage

The clarification of diagenetic stages should be based on paleo-temperature (T), vitrinite reflectance (Ro), maximum pyrolysis peak temperature (Tmax), and smectite mixed layer ratio (S%) of I/S, spatial and temporal assemblages of authigenic minerals and the contact relationships between particles [5，22，67]. The Ro of coal measure shale in the Xishanyao formation in the study area ranges from 0.63–0.89%. The clay minerals of the coal measure sandstone are dominated by I/S, kaolinite and illite, with the mixed layer ratio (S%) of I/S ranging from 10–30% (avg on 18%). The compaction strongly influences the coal measure sandstone, with contact relationships composed of line contact, concave-convex contact, and suture contact. The reconstructed thermal history of the Jurassic strata, which is conducted by Wang et al.[68], found that the maximum burial depth of the Xishanyao formation exceeded 3000 m and the paleotemperature exceeded 100 °C. According to the diagenetic stage classification standard of clastic rocks (China’s oil and gas industry standard SY/T5477-2003), it can be comprehensively determined that the diagenetic stage of the Xishanyao formation coal measure sandstone is in the mesodiagenesis A1 and A2 stage (Figure 2).

3.2. Diagenetic Sequences of Coal Measure Sandstone

Combining with the thermal and burial history of the SJB, referring to the division of diagenetic stages in clastic rocks of China’s oil and gas industry standard SY/T5477-2003, the diagenetic evolution of the Xishanyao formation coal measure sandstone in the area is summarized as follows:

Syndiagenesis

In this stage, the diagenetic environment has not been completely disconnected from the overlying water. The organic matter deposited in the peat swamp generates CO₂ under the action of microorganisms and dissolves in water, making the diagenetic environment weakly acidic. Due to the acidic nature of the sedimentary water, the surface of the detrital particles is rarely coated by a chlorite clay film. Due to the strong hydrodynamic environment during the sedimentary period, the coal measure sandstone contains a certain amount of plastic micrite carbonate and carbonaceous fragments will be blended into the sandstone.

Eodiagenesis (at the depth < 2 km, temperature < 70 °C, and Ro < 0.5%)

The type and degree of diagenesis in the eodiagenesis stage will affect the type of diagenesis and the physical properties of the reservoir in the later diagenetic stage [22]. At this stage, the diagenetic environment is weakly acidic, and the diagenesis types include mechanical compaction, cementation (mainly argillaceous cementation) and dissolution, and the diagenetic environment gradually changes to a closed system. The compaction significantly reduces primary porosity [69，70], due to the shallow burial depth and the high subsidence rate. The rock is in a weakly consolidated and semi-consolidated state, and the contact relationships of the clastic particles change from point contact to point-line contact. Pore types of the coal measure sandstone in this stage are mainly primary pores, and a small amount of micrite carbonate cement fills the pores.

With the increase in temperature, the coal seams and dark shale gradually released organic acid and CO₂ [64], which makes the diagenetic environment gradually change to a weak acid. The feldspar and detrital grains began to dissolve slightly and form dissolution pores (Figure 2).

Figure 2. The paragenetic sequence and pore evolution of the Xishanyao formation in the SJB. The thermal evolution history is based on the study of Wang et al. [68] with some modifications. Solid blue lines represent probable timing based on observed diagenetic and mineralization phases. While the dashed blue lines represent inferred or not well-constrained diagenetic and mineralization phases. J₂x = Xishanyao formation; I/S = illite/smectite; Ro = vitrinite reflectance.

Mesodiagenesis (at the depth > 2 km, temperature > 70 °C, and Ro > 0.5%)

The effect of compaction in the mesodiagenesis A stage was extremely strong, and the compaction type gradually shifted from mechanical compaction to chemical compaction. The contact relationships change from point contact and line contact to line contact, concave–convex contact and suture contact during this stage, along with a large proportion of brittle minerals broken up. The dissolution pores formed in the eodiagenesis stage are damaged under the compaction, and the damage of compaction to the quality of the reservoir is further strengthened. The mesodiagenesis A stage can be further divided into two substages, A1 and A2, according to temperature and Ro. These two substages have certain differences in diagenetic environment and diagenetic reaction:

In the mesodiagenesis A1 stage, the temperature was 70–90 °C, and the Ro was 0.5–0.7%, in an acidic diagenetic environment. The amount of organic acid generated in coal measures reaches the highest level at this temperature [71], the diagenetic environment is acidic, and the tensity of dissolution reached the highest level (Figure 2). The massive dissolution of feldspar, aluminosilicate lithic fragments and early micrite carbonate cement formed a large number of dissolution pores and mould pores. Because of the relatively closed diagenetic system, the dissolution products cannot be discharged. This has led to the kaolinite filling pores and quartz overgrowth (part of the SiO₂ comes from the chemical compaction).

In the Mesodiagenesis A2 stage, the temperature was 90–130 °C, and the Ro was 0.7–1.3%. The acidic diagenetic environment is gradually weakened due to the decomposition of organic acids. The effect of dissolution decreases gradually, while the cementation gradually increases. The quartz overgrowth gradually decreases, which is manifested as second level quartz cementation. Late period carbonate cementation (dolomite and ankerite) gradually filled the pore space (Figure 2). The clay minerals appear abundantly and fill the pores, further reducing the porosity.

4. Diagenetic Control of Coal Measure Reservoir Quality

Reservoir quality is influenced by diagenetic activities, such as compaction, cementation, dissolution, recrystallization, and metasomatism [20,72]. For the Xishanyao formation coal measure sandstone, compaction, dissolution and cementation are the main diagenetic controlling factors for reservoir quality.

4.1. The Influence of Compaction on Reservoir Quality

Compaction is one of the main factors that cause the reduction of intergranular pore volume [73] and the densification of coal measure sandstone reservoirs. Plastic particles in the sandstone have a weak compaction resistance, while the rigid particles have a high compaction resistance ability [61,74]. The coal measure sandstones of the Xishanyao formation contain a high content of plastic grains, such as volcanic lithic fragments, muddy gravel and carbonaceous fragments, which makes its compaction resistance ability weak.

The coal measure sandstone reservoirs are strongly affected by mechanical compaction, which is characteristic of the close contacted clastic particles, the ruptured rigid particles, and the deformed plastic particles. Chemical compaction commonly happens in SJB due to the high intensity of compaction, which may lead to particle suture contact and reduced interparticle space. The intense mechanical and chemical compaction has a great negative effect on the physical properties of the sandstone reservoir.

4.2. The Influence of Cementation on Reservoir Quality

The coal measure sandstone of the Xishanyao formation in the SJB is mainly clay mineral cement, followed by quartz cement and carbonate cement. The development of these cementations is one of the main factors leading to the significant decline in reservoir quality [74,75,76,77].

The closed diagenetic system of coal measure sandstone prevents the dissolution products from being discharged in time, which also leads to the development of authigenic kaolinite cementation and quartz cementation. The developed kaolinite cement fills the pore space in large quantities, but its loose inter-crystalline structure contains inter-crystalline pores that can resist compaction [78]. In addition, illite and I/S mainly fill intergranular pores and pore throats, thereby reducing the reservoir quality of the sandstone [65,79,80]. In summary, different types of clay minerals occur widely and fill or divide the pore space, resulting in tortuous pore throats, reducing pore connectivity, and weakening seepage capacity [13,37,65,79,80].

The content of volcanic lithic fragments in coal measure sandstone is relatively high, which may inhibit the quartz overgrowth to some extent [77]. It can be observed that the overgrowth level of the coal measure sandstone is shown in the second stage. The quartz overgrowth in the eodiagenesis stage can help the sandstone better resist compaction. In the mesodiagenesis, pressure solution, feldspar dissolution and clay mineral transformation produced many SiO₂ (aq), which developed into quartz cement [13,81,82].

The carbonate cement is locally developed in coal measure sandstones. In the eodiagenesis stage, several micrite carbonate particles blended in the coal measure sandstone, which blocked and filled the intergranular pores. In the mesodiagenesis A2 stage, the late period dolomite and ankerite cements appeared in the sandstone, which blocked the intergranular pores of the sandstone and reduced the reservoir quality.

4.3. The Influence of Dissolution on Reservoir Quality

The dissolution in diagenesis has a positive effect on reservoir physical properties [13]. The coal seam and dark shale begin to produce organic acids through the decomposition of plant residues during the eodiagenesis stage [71] and reach a peak in the mesodiagenesis stage (temperature range from 80 °C to 140 °C, [37,83]). The amount of organic acid generated in the coal measure is much higher than in other source rocks [5,7,84].

The sedimentary environment makes the sandstone have good primary physical properties and pore connectivity, and the weak dissolution in the eodiagenesis stage provides a seepage channel for the migration of acidic fluids in the mesodiagenesis stage. The large proportion of aluminosilicate minerals, such as feldspar and volcanic in coal measure sandstone provides a good material basis for the dissolution [5]. Dissolution is most intense in mesodiagenesis. During this period, the thermal evolution of organic matter releases a large number of organic acids into the diagenetic system of the sandstone, causing the soluble minerals to selectively dissolve and form a large number of intragranular dissolved pores and intergranular dissolution pores [35]. Overall, the diagenetic stage of the coal measure sandstone in this area is mainly in the A1 and A2 stages of the mesodiagenesis, which happens to be the most intense stage of organic acid dissolution. The dissolution has brought a great positive effect on the physical properties of the reservoir.

5. The Influence of Sedimentation on the Reservoir Quality of Coal Measure Sandstone

The influence of the sedimentation on reservoir quality cannot be well evaluated only by sedimentary facies, which need to be analyzed from the aspects of composition, texture, and diagenetic alteration [19,72,89,90,91].

The parameters, such as the average particle size, sorting, roundness, and matrix content of the sandstone determine the initial intergranular volume [15,21,32,92,93], and the initial porosity of sediment can reach 40% [94]. Permeability is a function of the matrix and grain size [39,95]. For sandstones in the same diagenetic stage, the larger the average grain size, the better the sorting and roundness; the lower the matrix content, the stronger the compaction resistance, and the better the physical properties of the reservoir will be [96,97]. Reservoirs with good physical properties are conducive to the exchange of fluids, thereby enhancing the strength of cementation and dissolution during diagenesis [67,98].

The coal measures sandstone in the Xishanyao formation is formed in a lacustrine-braided river delta environment [1,3]. The sandstone particles are poor to medium sorted, sub-angular to sub-round roundness, have high matrix content, and have a medium-high texture maturity. Coal measure sandstones with medium-high texture maturity are prone to grain rearrangement (rotation and slippage) during compaction, thereby enhancing the effect of compaction [99].

Differences in the composition of particles lead to differences in physical and chemical diagenesis reactions during diagenesis [67,98,100]. When the quartz content is low, the reservoir quality increases with the increase in the quartz content. If the content of detrital quartz is higher than 75%, the lower content of feldspar will lead to the reduction of dissolved pores, and at the same time, the quartz cementation in the reservoir will be more developed, thereby reducing the porosity and permeability of the reservoir [101,102].

The compositional maturity of coal measure sandstone in the SJB is relatively low, and its provenance is composed of pyroclastic rocks, intermediate-acid magmatic rocks, and metamorphic rocks derived from the Tianshan Mountains [46,54]. Because the provenance is closer to the depositional area, the proportion of feldspar and volcanic lithic fragments in the coal measure sandstone is relatively large. Due to the weak compaction resistance of feldspar, lithic fragments and plastic particles (such as muddy gravel and carbonaceous fragments) [103,104], the coal measure sandstone reservoirs are greatly affected by mechanical compaction. In addition, the abundant organic matter and the high content of feldspar and lithic fragments in the coal measure strata also lead to strong dissolution during the diagenetic stage.

Due to the warm and humid environment during the deposition period of the Xishanyao formation, the coal measures are generally developed. The abundant organic matter generated CO₂ under the action of microbial fermentation and made the water weakly acidic [9]. The weakly acidic water medium condition makes it difficult to coat clay minerals on the grain surface and form chlorite clay films. The absence of the clay film on the grain surface makes the reservoir easily affected by compaction and makes the observation of quartz overgrowth more difficult. In addition to the dissolution during the diagenesis, a part of kaolinite is also formed during the depositional period, and their origins were mainly because of the mineral transformation, illite and chlorite under acidic conditions, and feldspar alteration [5,65,85].

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Aobo Zhang

Shida Chen

Dazhen Tang

Shuling Tang

Taiyuan Zhang

Yifan Pu

Bin Sun

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Update Date: 31 Aug 2022

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