<sup>3</sup> Carbonate cementation

In the study area, calcite is regarded as the major carbonate cement in the Chang 8 reservoir, with an average content of 9.56%. Carbonate types of cement produced substrate cementation, which were formed after the pore-lining chlorite in early diagenesis. Calcite is cemented among clastic particles (Figure 4i), thus forming a compacted reservoir. According to an intersecting analysis of the porosity and carbonate cement, a negative correlation was discovered (Figure 5b). Hence, it is believed that carbonate cement is the major cause of the compactness of the Chang 8 reservoir in the study area.

### 3.1.3. Dissolution

Secondary pores, which are produced upon dissolution of mineral components and cement in the reservoir, facilitate the large-scale expansion of the reservoir spaces. It is the most important diagenesis process to improve the physical properties of the reservoir [5,26,31,34]. After experiencing tectonic lifting and oil–gas emplacement in the Chang 8 reservoir section of the Zhenjing area, feldspar and rock debris will be dissolved and eroded by organic acids (Figure 4j), thus forming mold pores, intragranular pores, and inter-granular pores. This effect mainly occurs at the end of early diagenesis and Phase-A of middle diagenesis.

### 3.1.4. Metasomatism

The occurrence of metasomatism is closely related to the local temperature and pressure distributions, as well as the fluid properties in pores and it occurs in all diagenetic periods [29,32,35]. Metasomatism is a process of mutual replacement of minerals. It is accompanied by the dissolution and sedimentation effects. Hence, the influence of the metasomatism process on the physical properties of the reservoir is very small and even can be ignored. Calcite metasomatism is regarded as the most common type of metasomatism in the study area (Figure 4k), with local metasomatism of kaolinite and clay minerals.

### 3.1.5. Rupturing

According to the core observation and analysis of slice data under SEM imaging, microcracks that are produced by rupturing in the Chang 8 reservoir were developed greatly (Figure 4l). These microcracks connect pores in the reservoir effectively, which play an important role in the improvement of the physical properties of the reservoir (decompaction), especially the migration and settlement of oil and gas [5,30].

### *3.2. Diagenetic Sequence and Diagenetic Periods*

The various diagenesis types in each diagenetic period are different and the duration of diagenesis also varies. Based on the SEM analysis and the casting slicing observation, a comprehensive analysis of the diagenetic sequence of sandstones in the Chang 8 reservoir of the Zhenjing area was carried out by combining the diagenesis theoretical knowledge [29–31].

In the early diagenetic period A, the Chang 8 reservoir section in the study area becomes more and more compact, thus resulting in the plastic deformation of minerals such as mica. The contact relation among particles changes from the original point contact into the linear contact. According to SEM observation, clastic particles such as mica align toward oriented array to form a texture layer. In this period, the porosity of the reservoir declines sharply due to compaction. In the early diagenesis period B, compaction continues to increase and chlorite film begins to produce in pores. In this period, the production of cement-like kaolinite and sparry calcite decreases the porosity of the reservoir continuously. Compared with the early diagenetic period, the influence of compaction in the middle diagenetic period A on the physical properties of the reservoir has been very small and the secondary quartz development expands. In the reservoir, production of organic acids occurs, while carbonate cement and feldspar are dissolved, thus forming multiple secondary pores. The dissolution that consumes acid water and pore water becomes increasingly alkaline, thus facilitating changes in illite/smectite formation toward illite. During this period, diagenesis plays an important role in improving the physical properties of the Chang 8 reservoir. In the middle diagenetic period B, illite/smectite formation continues to change toward illite. Cementation and dissolution occur alternatively, while the porosity tends to be stable gradually. To sum up, the diagenetic sequences of the Chang 8 reservoir in the study area are determined as follows: mechanical compaction → early sedimentation of chlorite clay mineral → early cementation of sparry calcite → authigenic kaolinite precipitation → quartz secondary expansion → dissolution of carbonate cement → dissolution of feldspar → late cementation of minerals like ferrocalcite (Figure 6).


**Figure 6.** Diagenetic evolution of the Chang 8 reservoir in the Zhenjing area.

In this work, the J&M TIDAS PMT IV&MSP200 vitrinite reflectance test system was applied to test the vitrinite reflectance of ten rock samples (including mudstone, oil shale and coal) in the study area. The minimum and maximum reflectance values are 0.67% and 1.34%, respectively, averaging at 0.90% (Table 2). The maximum paleogeotemperature in the rock burying process was tested by acoustic emission, which ranges between 109.3~125.3 ◦C, averaging at 120.32 ◦C (Table 3). The extracted Tmax of mudstone in the Chang 8 reservoir of the study area ranges between 445~463 ◦C, averaging at 454 ◦C [38]. According to the XRD analysis, the montmorillonite content in the illite/montmorillonite formation is between 15~30%, averaging at 20% (Table 4). Moreover, according to the comprehensive analysis of the diagenetic sequence, both vitrinite reflectance and paleogeotemperature tests were performed based on the acoustic emission. With references to the relevant literature and industrial standards, it is believed that the Chang 8 reservoir in Zhenjing area is currently in the middle diagenetic period A (Figure 6).


**Table 2.** Table of vitrinite reflectance in Zhenjing area.

**Table 3.** Maximum paleo-geotemperature (acoustic emission) of Chang 8 reservoir.



### **4. Determination of Hydrocarbon Accumulation Periods**

In this work, hydrocarbon accumulation periods of the Chang 8 reservoir in the study area were studied systematically by applying the indirect limiting method of the thermal evolution history-inclusion temperature measurement and the direct dating method of illite [39–44]. First, the freezing-point and homogenization temperatures of rock samples from 12 wells (including HH188 and HH156) in the Chang 8 reservoir section were tested. The LINKM600 cold-heat table in the Thermal Chronology Laboratory of Northwestern University was used under the enforcement of 10.5 V of voltage, 26 ◦C of indoor temperature, and 65% of humidity. Meanwhile, the corresponding salinity was calculated. In this section, the saline inclusion, which coexists with the hydrocarbon inclusion was chosen as the test object. It is mainly reserved at the quartz expansion edges or inside of fracture and quartz. According to the analysis of the experimental results, the homogeneous temperature of inclusion has a wide range of 69~155 ◦C, with peaks ranging between 100~125 ◦C. This result and the reproduction diagram of the burying history were used together for mutual calibration, through which the accumulation period of the Chang 8 reservoir was 110~120 Ma (Figure 7).

**Figure 7.** Analysis of thermal evolution history of Well HH155.

When both oil and gas enter the reservoir, the growth of authigenic illite stops. Hence, this period is viewed as the time for hydrocarbon accumulation [44–50]. In this work, illite test analysis was accomplished in the Geochemistry Laboratory of China University of Petroleum (Beijing). The 38AR diluent was put in accurately by using the VG3600 mass spectrometer while melting the samples under 1500 ◦C. Later, the isotope ratios of (38Ar/36Ar) and (40Ar/38Ar) were tested. The radioactive factor 40Ar of the samples was calculated and the corresponding age was calculated according to the K content [44,47–50]. Based on the above principle, a dating analysis was carried out on the oil-containing sandstone illite in HH192 and HH198. The acquired results showed that the accumulation period of the Chang 8 reservoir was 105~115 Ma (Figure 7). Consistent with the inclusion analysis outcomes, it is concluded that the Chang 8 reservoir in the study area is a phase-I continuous accumulation process and the accumulation period is 105~125 Ma, which is the middle Early Cretaceous Epoch (Figure 7).

### **5. Conclusions**


**Author Contributions:** Supervision, Z.R. and K.Q.; Writing—review & editing, G.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China (No. 41630312), National Key R&D Projects (No. 2017YFC0603106) and National Major Projects (No. 2017ZX05005002-008).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Acknowledgments:** We appreciate encouragement and guidance from Zhanli Ren, during the formulation and drafting of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

