Microscale Mineral and Pore Structure Characterization of the Low-Permeability Sandstone in the Ordos Basin, China

School of Civil Engineering & Geodesy, Shaanxi College of Communication Technology, Xi’an 710018, China No. 6 Gas Production Plant, Changqing Oilfield Company, PetroChina, Yanan 716000, China Exploration and Development Research Institute, PetroChina Changqing Oilfield Company, Xi’an 710018, China Xi’an Key Laboratory of Tight Oil (Shale Oil) Development, Xi’an Shiyou University, Xi’an 710065, China School of Human Settlement and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China


Introduction
e recent waterflooding and chemical flooding advancements have significantly increased the low-permeability sandstone reservoirs [1,2]. e low-permeability sandstone reservoir in the Heshui area, located in the Ordos Basin in China, is now one of the China's most significant hydrocarbon reservoirs [3]. In recent years, through the exploration and evaluation of the Heshui area, scholars have found that the yield of Chang 2 reservoir in oil tests was relatively high. is finding is vital for increasing China's oil production and reserve, demonstrating the vast potential for future prospection and development [4,5]. However, compared with massive studies of other areas, the Heshui area study is still limited and is in the primary stages [4].
Previous studies in this area were primarily concerned with the oil groups from Chang 8 to Chang 6. A lack of systematic investigations of the characteristics, formation, and controlling factors in Chang 2 significantly restrained the target evaluation and optimization analysis [6,7].
Consequently, the mineralogical, diagenetic, tectonic, and pore structure characterization of Chang 2 sandstones are still poorly understood. Mineral composition, physical properties, and pore size distributions are the most significant parameters in sandstones that affect the mechanical, storage, and transport properties of the sandstones [8][9][10]. Compared to conventional reservoirs, the pore networks in sandstone reservoirs are more complicated because of the abundance of microscale heterogenetic distributions of minerals [11].
Composition, proportion, and distribution are the essential characteristics of the minerals [12]. Pores could be divided into different groups based on various category criteria, and the criterion proposed by the IUPAC is the most common one [13]. Many scholars have applied this criterion to analyze the pore networks. In contrast, this criterion is not a generally accepted way for each research, and the pore network classification needs to base on actual demand. in section (TS) and scanning electron microscopy (SEM) are the image analysis methods to determine the mineralogical characteristics qualitatively and quantitatively [14]. Pressure-controlled mercury intrusion and rate-controlled mercury intrusion (PCMI/RCMI) are the standard techniques to quantify the pore size distributions [15,16]. Combining PCMI and RCMI can give us information about the whole pore size distributions of more than 3.6 nm [17].
Based on the development status of low-(ultralow-) permeability reservoirs and the characteristics of Chang 2 in the Heshui area, the present research aims to provide a comprehensive and systematic study of its physical properties and the microscopic pore structure, utilizing copious experimental methods, including core observation, TS, SEM, PCMI, and RCMI methods. Besides, the present research also examined the main controlling factors affecting the reservoir, which is of great significance for future research on the Yanchang Formation and further expansion of the domains in oil-gas exploration in the central basin [18,19].

Geological Background.
Heshui area is located in the southern part of the Shaanbei Slope (Northern Shaanxi Slope) and the Tianhuan Depression in the Ordos Basin. Its tectonic evolution is inseparable from the overall tectonic background of the Ordos block. e general structure of the Heshui area features a gentile monocline oriented on an east-west axis, without any faults or large-scale basement uplifts. e monoclinic slope is generally less than 1°, and the average gradient is about 6 m/km. Small nose-shaped uplifts with an amplitude of 10 to 20 m were formed in local areas due to differential compaction [20]. ese uplifts tend to have graduate tilts at the two ends and are largely axisymmetric. e closed areas generally do not exceed 10 km 2 in size. e angle is directional in specific ways and is regularly distributed [21]. Although the structures above in the study area affect the distribution of oil pools, their influence is only marginal.
e Chang 2 oil group has undergone a certain degree of denudation on the southern and southwestern margins of the basin, but it is still preserved. e lithological features of the oil group tend to be roughly coarse-grained in the north and fine-grained in the south, with an average thickness of 110 to 120 m ( Figure 1). e overall petrological characteristics are as follows: light gray and brown and fine-and medium-fine-grained sandstone intercalated with dark shales in typology. Typically, the rocks are massive, with trivial and thin stratification, dominated by argillaceous and calcareous cementation.

Experimental
Procedures. An optical microscope is applied to identify the mineral types and quantify the mineral's composition and proportion. Before the tests, the rocks were sliced to pieces and covered by glass. e E-c system was applied to analyze the mineral characteristics quantitatively.
Zeiss GeminiSEM 500 was used to analyze the mineralogical distributions of the samples. Before the tests, the samples were first polished as a cube, milled via argon ion. After those steps, the samples were finally coated with carbon.
Micromertrics AutoPore 9420 mercury porosimeter was used to perform the PCMI experiments. e samples were first dried for 12 h and then saturated by lipophilic remover. Finally, the samples were put into the apparatus and the relationship between capillary pressure and mercury saturation was tested.
Automated System for Pore Examination ASPE-730 was used to conduct the RCMI tests. e pre-exam procedures were the same as that of PCMI. However, in this test, the maximum pressure was not over 6.2 MPa, corresponding to the pore radius of 0.12 μm.

Petrological Characteristics.
According to the statistics collected from core observation, thin-section casting, and other analytical processes, rocks in the study area are mainly medium gray, fine-grained sandstones, fine-grained sandstones, and fine siltstones, with an average particle size between 0.1 and 0.35 mm and a maximum of 0.5 mm. e rocks are dominated by lithic arkose and feldspathic detritus quartz sandstones ( Figure 2). e total amount of debris is between 74.6% and 90%, with an average of 84.4%. Among the debris, the content of quartz is 24.6% to 60%, with an average of 44.9%; the range of feldspar is 12% to 39%, with an average of 22.8%; the content of cuttings is 10% to 34.2%, with an average of 19.2% (Table S1). e degree of sorting is good to medium. e degree of rounding is mainly subangular to subcircular; the main types of cementing are enlarged-porous cementing, porous cementing, pore-increasing cementing, and pore-film cementing. e overall content of interstitials is not high, mainly composed of cement ranging from 10% to 24.5%, with an average of 16.2%. Most of the cementations are authigenic clay materials, followed by feldspathic components and pyrites. Matrix only takes up a deficient composition (Table S2).

Diagenesis and Diagenetic
Stages. Chang 2 reservoir in the study area contains relatively high plastic debris such as phyllite, schist, and mica. Feldspar dissolved pores and lithic dissolved pores are fairly developed, in which diagenetic fractures can be found occasionally. Kaolinite, illite, and chlorite in the authigenic clay minerals are ubiquitous, with high carbonate content. Quartz is secondarily enlarged and contains authigenic quartz grains. is shows that the effect of compaction and pressure solution was strong in this period.
e cement mainly comprises authigenic clay minerals and carbonate minerals, and the content of kaolinite in sandstone is relatively high. Carbonate and siliceous cementation are common, while metasomatism and fracturing are relatively common (Figure 3). Among the aforementioned diagenetic processes, the dissolution of feldspar and debris and the metasomatism of kaolinite are beneficial to forming good reservoirs. In contrast, other kinds of processes are likely to cause damage to the reservoirs. e diagenetic sequence of the reservoir is compaction ⟶ chloriteization ⟶ quartz enlargement ⟶ calcite precipitation ⟶ organic acid generation ⟶ feldspar, debris, and calcite dissolution ⟶ iron calcite precipitation ⟶ oil emplacement. e above characteristics indicate that the diagenetic stage of the sandstones in the study area is amidst stages A to B of the middle diagenetic stage.

Physical Characteristics of the Reservoir.
e statistical analysis (Figure 4(a)) show that the reservoir porosity is mainly between 7% and13% with an average of 10.5%, the   [22].

Advances in Civil Engineering
main frequency is distributed in the 5% to 15% section, accounting for 76% of the total distribution. e permeability is mainly distributed between 0.15 and 3 mD, and the average is 2.2 mD. e frequency distribution is mainly between 0.5 and 5 mD, accounting for 56.7% of the total distribution. e above results suggest that the target reservoir in Heshui area belongs to low-porosity-ultralowpermeability reservoirs [23]. Figure 4(c) shows a positive correlation between the porosity and permeability (R 2 � 0.556), indicating good physical characteristics.    Advances in Civil Engineering

Microstructure and Characteristics of the Reservoir.
e sandstones in the study area mainly develops intergranular pores with an average content of 1.8%, followed by feldspar dissolved pores with an average content of 0.9%, then by lithic dissolved pores of 0.2%, and then by intercrystalline pores of 0.1%. Microcracks can occasionally be found. e average aperture ratio is 3.2%. e average pore diameter mainly falls between 30 and 80 μm. e main types of pore combination are dissolved pore-intergranular pore type, microporous type, intergranular pore-dissolved pore type, and intergranular pore type ( Figure 5). e main types of throats are lamellar throats, and a small number of constricted throats also exist. e tortuosity of the mainstream throat is relatively low-relatively high (Figure 6), and the average throat radius is 0.93 μm. e sorting coefficient is 1.49, and the relative sorting coefficient is 0.14. e type of pore-throat combination is mainly mesopore-fine pore-fine throat type, and the coordination number of throats mainly falls between 2 and 6, and the connectivity between throat and pore is poor. e high-pressure mercury intrusion experiment (see Table S3 for mercury intrusion parameters) shows that the pore-throat radius of the reservoir is relatively small, and the pore-throat volume is relatively large. Besides, the throat is relatively thin and small. e degree of sorting is moderate to low. e residual mercury saturation is relatively high, and the displacement pressure is relatively low. e maximum mercury saturation is average. is shows that the storage ability of the study area is average, the filtration capacity is relatively poor, and the connected storage space is relatively small. Besides, we also conducted an oil-water phase permeability experiment, which shows that rocks in the study area are neutral-weakly hydrophilic, and the oil-water displacement efficiency is low.

Major Controlling Factors.
Based on the formation and distribution characteristics of the reservoirs, it is believed that the development and spatial distribution of Chang 2 reservoir in the study area are comprehensively controlled by tectonics, sedimentation, and diagenetic effects. e study area is in the Shaanbei Monocline. Since it was affected by denudation and differential compaction, smallscale nose-shaped uplifts are developed in local areas. ese nose-like uplifts and the belt-like sandstone reservoirs distributed in the up-dip direction near NE-SW (Figure 7), together with the interriver argillaceous lithology, are mutually configured to form a good trap [24]. Chang 2 reservoir in the study area is controlled by lithology and tectonism, and the types of reservoirs are mainly lithology-tectonism composite reservoirs. In the up-dip direction of the structure, a good trap is easily formed due to the compaction of the sand body. However, we also need to point out that not all reservoirs in high structural positions can become good oil layers. e reasons for this need to be analyzed in conjunction with sedimentation and diagenesis.
Sedimentation controls the spatial distribution of various sedimentary environments and their products on the macroscopic level and the physical properties of rocks on the microscopic level. According to the sedimentary facies analysis and sand distribution characteristics of Chang 2 reservoir in the study area, it is found that the target layer in this area belongs to the delta front deposit, and the sand body extends far, running through the role area [25,26]. Large-area underwater distributary channels, braided channels, and estuary bar sand bodies with larger thickness and better physical properties provide the possibility of forming oil layers. We also found that the microfacies sand bodies of the estuary bar have more refined grains and uniform structure, with the primary intergranular pores accounting for about 80%. e dissolved pores were developed, which accounts for about 15%. e physical properties data show that the study area is a favorable reservoir facies zone, but the spread area is small and cannot be used as the main force zone. e sand body in the microfacies at the estuary dam is fine-grained, well-sorted, with primary intergranular pores accounting for 60% to 80%. Dissolved pores area developed, accounting for about 20% of pores. Porosity is greater than 10%. Permeability value is greater than 1 mD, and the oil-bearing characteristic is generally good. ese regions can serve as favorable reservoir facies belts. Due to strong reservoir heterogeneity, the physical properties change, resulting in poor oil content and storage performance in the flanks of the underwater distributary channel and the distributary bays.
We were taking two samples from different sedimentary facies belts as an instance (Figure 8). ere are two platforms and a low missing zone for the samples from underwater distributary channels, while single long platforms and short, a missing zone for the samples from estuary bar. A heterogeneous hydrodynamic environment led to high heterogeneous grain distributions and then caused the complex pore-throat structures. However, due to the strong hydrodynamic condition, the larger pores were relatively developed (Figures 8(a) and 8(c)). Compared to underwater distributary channels, the sands from the estuary bar have more homogeneous grain distributions. e long platform indicates the relatively uniformly distributed grain sizes, and the short missing zone suggested the stable hydrodynamic condition. However, weak hydrodynamic streams generally could not carry large grains, leading to poor physical properties and simple porethroat networks with low average radius (Figures 8(b) and 8(d)).
Diagenesis controls the evolution direction of pores in the process of burial diagenesis and is of great significance to reservoir evaluation and prediction [27,28]. e diagenetic type of the target layer in the study area is very complicated. According to microscopic observation and statistics, the diagenetic processes exhibit strong, compact compaction, dissolution, and cementation effects (Figure 9). Among those processes, the dissolution of feldspar and debris and the metasomatism of kaolinite are beneficial to forming good reservoirs. In contrast, other types are likely to cause damage to the reservoirs.

Conclusions
(i) e rock types of research area are mainly lithic arkose and feldspathic detritus quartz sandstone. Clay materials are mainly kaolinite, chlorite, and illite. Detrital components are dominated by quartz, followed by feldspar. e diagenetic stage is amidst stages A to B of the middle diagenetic stage. (ii) e aimed sandstones belong to low-porosity-ultralow-permeability reservoirs. e main types of pore combination are dissolved pore-intergranular pore type, microporous type, intergranular poredissolved pore type, and intergranular pore type. e main types of throats are lamellar throats, and a small number of constricted throats also exist. e type of pore-throat combination is mainly mesopore-fine pore-fine throat type. (iii) e development of Chang 2 reservoir in the Heshui area of the Ordos Basin is influenced by tectonism, sedimentation, and diagenesis. Small-scale noseshaped uplifts formed by tectonism form a good trap. e sorting of the sedimentary process has significant control over the porosity of the reservoir. e compaction, dissolution, and cementation effects during the diagenetic processes are relatively strong, which controls the evolution direction of the reservoir pores.

Data Availability
e experimental data used to support the findings of this study are included in the manuscript.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this study.  Figure 9: e relationships between diagenetic minerals and reservoir quality. 8 Advances in Civil Engineering

Supplementary Materials
Supplementary material contains Tables S1 to S3, which show the data of debris components, cement composition, and pore-throat structure parameters, respectively. (Supplementary Materials)