*Article* **Research on Diagenetic Evolution and Hydrocarbon Accumulation Periods of Chang 8 Reservoir in Zhenjing Area of Ordos Basin**

**Guilin Yang, Zhanli Ren \* and Kai Qi**

Department of Geology, Northwest University, Xi'an 710069, China; yangguilin612724@163.com (G.Y.); kaiqi0913@163.com (K.Q.)

**\*** Correspondence: renzhanl@nwu.edu.cn

**Abstract:** The Mesozoic Chang 8 Section in the Zhenjing area is a typical low permeability-tight sand reservoir and is regarded as the most important set of paybeds in the study area. Guided by the principles of basic geological theory, the diagenetic evolution process and hydrocarbon accumulation periods of the Chang 8 reservoir in the study area were determined through various techniques. More specifically, core observation, scanning electron microscopy (SEM), X-ray diffraction (XRD), and vitrinite reflectance experiments were performed in combination with systematic studies on rock pyrolysis and the thermal evolutionary history of basins, the illite-dating method, and so on. The Chang 8 reservoir is dominated by feldspar lithic and lithic feldspar sandstones. Quartz, feldspar, and lithic fragments are the major clastic constituents. In clay minerals, the chlorite content is the highest, followed by illite/smectite formation and kaolinite, while the illite content is the lowest. The major diagenesis effect of the Chang 8 reservoir includes compaction, cementation, dissolution, metasomatism, and rupturing. The assumed diagenetic sequence is the following: mechanical composition → early sedimentation of chlorite clay mineral membrane → early cementation of sparry calcite → authigenic kaolinite precipitation → secondary production and amplification of quartz → dissolution of carbonate cement → dissolution of feldspar → late cementation of minerals such as ferrocalcite. Now, the study area is in Stage A in the middle diagenetic period. Through the inclusion of temperature measurements, in conjunction with illite dating and thermal evolutionary history analysis technology in basins, the Chang 8 reservoir of this study was determined as the phase-I continuous accumulation process and the reservoir formation epoch was 105~125 Ma, which was assigned to the Middle Early Cretaceous Epoch.

**Keywords:** diagenesis; hydrocarbon accumulation periods; Chang 8 reservoir; Zhenjing area; Ordos Basin

## **1. Introduction**

Zhenjing Block is located at the intersection of the Tianhuan Depression, Northern Shaanxi Slope, Weibei Hump and thrust belt at the west edge of the Ordos Basin, which is in a unique geological position (Figure 1) [1–4]. In the study area, the Mesozoic Chang 8 reservoir forms the major paybed, which is rich in oil and gas resources [3–6]. With continuous developments of the oil field, high yields have been difficult to maintain and may even worsen, largely due to insufficient understanding of the reservoir quality. The formation of the reservoir is a complicated and time-consuming process, with the three essential geological processes—sedimentation, diagenesis, and tectonism—requiring thorough examination [7]. Of these processes, diagenesis plays an important role in reservoir reformation and, as a result, has been widely examined by the scientific community in the field [8–19]. The analysis of the accumulation period is an important part of the accumulation system study and key in the analysis of the accumulation process [20–23].

**Citation:** Yang, G.; Ren, Z.; Qi, K. Research on Diagenetic Evolution and Hydrocarbon Accumulation Periods of Chang 8 Reservoir in Zhenjing Area of Ordos Basin. *Energies* **2022**, *15*, 3846. https:// doi.org/10.3390/en15103846

Academic Editors: Dameng Liu and Mofazzal Hossain

Received: 25 March 2022 Accepted: 19 May 2022 Published: 23 May 2022

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Previous studies on the Chang 8 Member in the Zhenjing area of the Ordos Basin mainly focused on sedimentary facies and reservoir characteristics. However, studies on the diagenesis and accumulation stages were relatively weak. It is therefore of great significance to clarify the diagenetic evolution and hydrocarbon accumulation stage of the Chang 8 Member—the main oil-producing layer—for deepening the theoretical understanding of reservoir evaluation and hydrocarbon accumulation [9–15,20–23]. In this paper, both the diagenetic sequence and hydrocarbon accumulation periods of the Chang 8 reservoir section, which is a low-permeability-tight sandstone reservoir, are discussed systematically. First, a brief analysis of the petrology and physical characteristics of the Chang 8 reservoir was carried out based on insights from the borehole core observation, scanning electron microscopy (SEM), and slice observation. On this basis, the diagenesis of the Chang 8 reservoir section was investigated by combining X-ray diffraction (XRD) measurements, vitrinite reflectance, and rock pyrolysis. Thus, it was divided into various evolutionary sequences. Meanwhile, the hydrocarbon accumulation periods of the Chang 8 reservoir were analyzed comprehensively by conducting a thermal history analysis of basins and applying the illite dating method. The hydrocarbon accumulation times and the specific time were determined both indirectly and directly. Our work provides reliable references to the follow-up exploration and exploitation of the study area [5].

**Figure 1.** Location map of the study area: (**a**) Structural location map of the study area; (**b**) exploration and development area map of the study area.

### **2. Lithological Characteristics and Physical Characteristics of the Reservoir**

Based on the statistical studies of borehole core observation, SEM and slice observation data were analyzed. At the same time, the analysis of lithologic triangulation diagram was carried out (Figure 2a). Region I represents quartz sandstone, Region II denotes feldspathic quartz sandstone, Region III stands for the rock debris quartz sandstone, Region IV signifies arkose, Region V is the rock debris arkose, Region VI represents feldspar rock debris sandstone, and Region VII is the rock debris sandstone. It was found that feldspar rock debris sandstone and rock debris arkose are the dominant lithologies in the Chang 8 reservoir (Figure 2a). With respect to the clastic constituents, quartz accounts for the highest proportion (29%), followed by feldspar (26%), which is mainly composed of potash feldspar and plagioclase. The content of rock debris is the lowest, averaging at 24%. Among them, the magmatic rock debris mainly consists of neural acidity and the metamorphic rock debris is mainly composed of quartzite, followed by phyllite. The sedimentary rock debris is mainly siltstone and silty mudstone, followed by mudstones and flint. In addition, the content of mica is the lowest (Table 1) [24].

According to SEM and XRD analysis, the illite content in sandstones of Chang 8 reservoir in the study area is the lowest, ranging from 4% to 11% and averaging at 6.85%. Under the microscope, the thin-film, schistose, hair, and fiber modes are the major adhesion modes on the particle surfaces. Chlorite content is the highest (21~45%), averaging at 36.3%. Insights from SEM analysis indicate that chlorite mainly presents as thin-film mode and foliated mode, while the cementation mode is mainly presented as the looped lining mode and pore-lining mode, followed by illite/smectite formation and kaolinite. It is interesting to notice that the illite/smectite formation looks like a honeycomb under SEM imaging and the content of kaolinite ranges from 21% to 45%, averaging at 36.3%. It was developed on a large scale as filling in the pores and is presented with good crystal form. Under SEM imaging, the crystals look like book pages and worms (Figures 2b and 4c–g).

**Figure 2.** Comprehensive analysis of rock characteristics of the Chang 8 reservoir: (**a**) Triangular map of rock classification; (**b**) distribution histogram of clay minerals.

According to the core data test and analysis, the porosity of the Chang 8 reservoir distributes between 1.8~17.9%, averaging at 10.9%. The permeability ranges between 0.037~0.79 mD, averaging at 0.45 mD (Figure 3). Thus, it belongs to a low-porosity and low-permeability reservoir.

**Figure 3.** (**a**,**b**) Histogram of the porosity–permeability frequency distribution of the Chang 8 reservoir.

**Table 1.** Reservoir rock composition statistics of the Chang 8 oil layer group in the study area.



**Table 1.** *Cont.*

<sup>1</sup> Well (W); <sup>2</sup> quartz (Q); <sup>3</sup> orthoclase (OC); plagioclase (PC); the total of feldspar (T); <sup>4</sup> magmatic rock (MR); metamorphic rock (MPR); sedimentary rock (SR); the total of debris (T); <sup>5</sup> mica (M); <sup>6</sup> the total of rock composition.

### **3. Diagenetic Evolution Analysis of the Reservoir**

Diagenesis is defined as the evolution process where sediments solidify into rocks through a series of physical, chemical, and biological reactions. This process is affected by many factors, such as burying rate, pressure, local temperature distribution, and sediment composition. Hence, it can greatly influence the physical properties of the reservoir. Therefore, diagenesis is closely related to the hydrocarbon accumulation mechanism [10–14,25].

Zhenjing area is located in the continental facies lacustrine deposit environment. There are many types of diagenesis of the Chang 8 reservoir [5,6]. In this work, both the diagenesis and the diagenetic sequence of the Chang 8 reservoir were studied systematically by using slice authentication, SEM and XRD measurements, as well as vitrinite reflectance and rock pyrolysis. The major diagenesis effects include compaction, cementation, dissolution, metasomatism, and rupturing. Among them, dissolution and rupturing have a positive impact on the improvement of the physical properties of the reservoir, while compaction and cementation facilitate the compactness of the reservoir.

### *3.1. Diagenetic Analysis*

### 3.1.1. Compaction

Due to the pressure of the overlying rocks, the process that makes the reservoir structure tighter and tighter is called compaction. It is regarded as the most influential and the most common diagenesis type in the diagenetic evolution of the reservoir [26–35]. The manifestation of the compaction effect of the Chang 8 reservoir in the Zhenjiang area is obvious, especially as the bending deformation of the plastic mineral particles is concerned, due to compaction in the early diagenesis. For example, minerals such as mica developed deformation of plastic particles after experiencing strong mechanical compactness (Figure 4a). Mineral particles are also compacted and filled into spaces among primary pores, thus decreasing the physical properties of the reservoir significantly. As the compaction continues, the lattices at the particle contact points may be deformed and even be dissolved. As a result, the contact relation among the particles may change from the original point contact to the linear contact and even to the linear-concave-convex contact. Many clastic particles like mica are aligned in a direction that forms texture layers (Figure 4b). Compaction is the major cause of tightening and sharp reduction of the primary pores, and could lead to the deterioration of the physical properties of the Chang 8 reservoir in the Zhenjing area.

### 3.1.2. Cementation

The change during the process of minerals precipitation in the pores of fragmental deposits into authigenic minerals is called cementation, which renders sediments solidified into rocks [36,37]. The role of cementation is to fill the pores and it is considered an important cause of decreasing the porosity in the reservoir layer. However, volumes along particles may not decrease due to cementation, which is obviously different from compaction [28,30–35]. From a general point of view, the inter-granular pores are filled by authigenic minerals, which has a negative impact on changes in the physical properties of the reservoir. However, the early filling of authigenic minerals can inhibit compaction to some extent. In the retained inter-granular pores, solvent-sensitive types of cement from secondary pores are formed as a response to dissolution. Hence, cementation has some positive influence on changes in the physical properties of the reservoir to some extent. In particular, early cementation has dual contributions to the physical properties of the reservoir. As an important factor of compaction of the Chang 8 reservoir in the Zhenjing area, the cementation procedure can be divided into authigenic clay mineral cementation, siliceous cementation, and carbonate cementation according to the types of the cement. It is mainly influenced by the fluid features in pores, the sedimentation environment, and composition [32,35].

**Figure 4.** Thin-section microscopic analysis of the casting of the Chang 8 reservoir in Zhenjing area: (**a**) Plastic deformation of mica, Well JH36, 1376 m, orthogonal light, 100×; (**b**) directional arrangement of mica, forming laminae, Well HH157, 2037 m, Orthogonal light, 50×; (**c**) chlorite film is attached to the surface of the detrital particles and filled inside the intergranular pores, Well HH92, 2267 m, SEM, 1300×; (**d**) leaf-like chlorite film is attached to the surface of the detrital particles, Well HH92, 2267 m, SEM, 430×; (**e**) book-like kaolinite filled in intergranular pores, Well HH78, 2400 m, SEM, 1400×; (**f**) flake-like and hair-like illite filled between detrital particles, Well

HH107, 2436 m, SEM, 1000×; (**g**) honeycomb-like illite mixed layer, Well HH111, 2035 m, SEM, 3000×; (**h**) secondary enlargement of quartz, Well HH193, 2298 m, positive cross light, 100×; (**i**) Calcite pore cementation, Well HH193, 2299 m, orthogonal light, 50×; (**j**) dissolution pores formed by dissolution of feldspar, Well HH193, 2295 m, single polarized light, 50×; (**k**) calcite metasomatic feldspar, Well HH188, 2413 m, single polarized light, 100×; (**l**) intersecting microfractures, Well HH166, 2396 m, single polarized light, 50×.
