Abstract
In order to study the differential diagenesis of sandy conglomerate reservoirs in different tectonic units in eastern Junggar Basin, and establish the differential temporal sequence of burial – diagenesis – hydrocarbon charging – pore evolution, the Upper Permian Wutonggou Formation sandy conglomerate reservoir in the Dongdaohaizi Sag and Baijiahai Uplift, eastern Junggar Basin are studied, based on observation of thin sections under microscope, measurement of scanning electron microscopy (SEM), X-ray diffraction (XRD), physical properties, cathodoluminescence (CL), micro beam fluorescence, and analysis of fluid inclusions, combined with previous research results. The result shows that the reservoir displays a differential diagenetic process as they are situated at different tectonic units of the Dongdaohaizi Sag and the Baijiahai Uplift. The Dongdaohaizi Sag is dominated by continuous subsidence, the reservoir buried in relatively deep depths experienced three stages of hydrocarbon charging, which is more strongly affected by compaction, fracturing, dissolution, and late-stage cementation, the effect of middle-stage cementation is relatively weak. The pore evolution experienced four stages, including decreased porosity by shallow burial compaction, decreased porosity by moderate burial compaction and middle-stage cementation, decreased and increased porosity by moderate to deep burial middle-stage cementation and dissolution, and increased and decreased porosity by deep burial dissolution and late-stage cementation. The diagenetic stage had reached A2 sub-stage of mesodiagenesis, and the present porosity is relatively low; however, the developed structural fractures in the reservoir has played a good role in improving the reservoir seepage capacity. By contrast, the Baijiahai Uplift is characterized by multi-stage uplift, the reservoir buried in relatively shallow depths experienced two stages of hydrocarbon charging, which is more strongly affected by middle-stage cementation, with relatively weak compaction and dissolution, fracturing, and late-stage cementation was limited. The pore evolution experienced three stages, including decreased porosity by shallow burial compaction, decreased porosity by moderate burial compaction and middle-stage cementation, and decreased and increased porosity by moderate-deep burial middle-stage cementation and dissolution. The diagenetic stage has reached A1 sub-stage of mesodiagenesis, and the present porosity is relatively high. Two types of favorable reservoirs are developed in the study area. The first one is matrix pore favorable reservoir, which is mainly located in the area of the Baijiahai Uplift of the reservoir with relatively high porosity and permeability. The second one is structural fracture developed favorable reservoir, which is mainly located in the fault development area of the Dongdaohaizi Sag.
1 Introduction
With the continuous progress in oil and gas exploration and development technology, research related to unconventional reservoirs have become increasingly important and have been the focus of general attention. However, research on the unconventional reservoirs of clastic rocks are mostly focused on sandstone reservoirs [1,2,3,4,5,6,7], while the research on sandy conglomerate reservoirs are relatively limited. In recent years, sandy conglomerate reservoirs have been found in major basins all over the world [8,9,10,11,12,13,14,15,16], and they have become an important target of oil and gas replacement reserves in the world. At present, experts and scholars worldwide mainly focus on three aspects of research on sandy conglomerate reservoirs: (1) study of sedimentary characteristics of the sandy conglomerate reservoirs, such as distribution of the sand body and its control on the reservoirs [17,18], the influence of lithofacies and their assemblages on reservoir quality [19,20], and the characteristics and identification methods of the inter-layers in the sandy conglomerate reservoirs [21]; (2) research on the factors controlling high-quality sandy conglomerate reservoir development, such as the influence of fractures, overpressure, and diagenesis on the formation of a high-quality reservoir [22,23,24]; (3) study of microscopic characteristics of the sandy conglomerate reservoirs, such as micro-heterogeneity characteristics of the reservoir and the pore throat developing mechanism of the reservoir [25,26]. Compared with sandstone reservoirs, sandy conglomerate reservoirs are mostly formed in fan delta sedimentary environments with rapid accumulation near their provenance [27]. Thus, the reservoir lithology changes rapidly, argillaceous content is high, and the syndepositional interstitial components filled between gravels and detrital grains are diverse which resulted in a relatively low mineral and textural maturity. Furthermore, various cements formed during diagenesis make the mineral composition and texture of sandy conglomerate reservoirs more complex and the heterogeneity more strong. Thus, diagenesis and pore evolution are important factors affecting the reservoir quality of sandy conglomerates. However, the exquisite and profound research on the burial – diagenesis – hydrocarbon charging temporal sequence and the corresponding pore evolution process occurred in sandy conglomerate reservoirs is still very limited.
The Junggar Basin in China has a long history of oil and gas exploration [28,29], large area of the sandy conglomerate body developed under fan delta deposition, and forms large scale sandy conglomerate reservoir [27]. The early oil exploration in the basin was mainly concentrated in the structural high part of the uplift, due to the limitations of the previously accepted theory of “oil-bearing anticlines.” However, with the establishment of hydrocarbon accumulation theory in hydrocarbon-rich sags in recent years, and the successful discovery of large oilfields in the Mahu Sag, the target of oil exploration has gradually shifted from the structural high part of the uplift to the hydrocarbon-rich sag [30,31,32]. The Dongdaohaizi Sag, located in the eastern Central Depression of the Junggar Basin, has become another main target after the key oil and gas exploration area discovered in the southern part of the Baijiahai Uplift. Many high-yield industrial oil and gas wells have been successfully discovered in the sandy conglomerate reservoir of the Upper Permian Wutonggou Formation. However, due to the differential tectonic evolution, the different burial – diagenesis – hydrocarbon charging – pore evolution paths of the Wutonggou Formation reservoirs existed in the Dongdaohaizi Sag and the Baijiahai Uplift, which restricted the development and types of the high-quality reservoirs. Previous research on differential diagenesis of the Wutonggou Formation reservoir in the Dongdaohaizi Sag and Baijiahai Uplift is very weak, the differential temporal sequence of burial – diagenesis – hydrocarbon charging – pore evolution is essentially unknown, which influences the reservoir evaluation and prediction for “sweet spots” in this area. For the above reason, the Wutonggou Formation reservoir in the Dongdaohaizi Sag and Baijiahai Uplift is compared in detail based on the analysis of reservoir characteristics. The emphasis is focused on the diagenetic characteristics and differences of the two areas. The influence of differential diagenesis on reservoir pore evolution is discussed, the hydrocarbon charging stage of the reservoir is investigated, and the relationship between diagenetic events and the hydrocarbon-charging temporal sequence of the reservoir is determined. Meanwhile, the history of burial – diagenesis – hydrocarbon charging – pore evolution in the reservoir is established corresponding to the burial thermal history of the basin, the pore evolution of the reservoir is discussed, and the type and genesis of favorable reservoir are also clarified. These research results can provide references and ideas for sandy conglomerate reservoir evaluation and favorable reservoir prediction in the Junggar Basin.
2 Geological setting
Located in northwestern China, the Junggar Basin is composed of six first-grade tectonic units, including the Wulungu Depression, Luliang Super-Uplift, Central Depression, Eastern Super-Uplift, Western Super-Uplift, and Northern Tianshan Overthrust Belt, and each first-grade tectonic unit contains several second-grade tectonic units, showing a checkerboard structural framework [33,34]. The study area is located in the second-grade tectonic units of the Dongdaohaizi Sag and Baijiahai Uplift, in the Central Depression. The northern part of the study area is adjacent to the Dinan Uplift and the eastern part is near the Wucaiwan Sag, showing a pattern of “two uplifts and one sag” [35], the total area is 2,336 km2 (Figure 1a). The strata developed from the bottom to the top in the study area are mainly Carboniferous, Permian, Triassic, Jurassic, and Cretaceous (Figure 1b). Among them, the lacustrine-facies mudstone of the middle Permian Pingdiquan Formation (P2p) is the main source rock, and the fan delta sandy conglomerate of the upper Permian Wutonggou Formation (P3wt) is the main reservoir in the area [36]. According to the depositional cycle, the Wutonggou Formation, from the bottom to the top, can be divided into three members: first member (P3wt1), second member (P3wt2), and third member (P3wt3). The first member of the Wutonggou Formation in the Baijiahai Uplift has been eroded, while the same member of the Wutonggou Formation in the Dongdaohaizi Sag developed completely [37] (Figure 1c). The thick sandy conglomerates in the subaqueous distributary channels-interbedded sandstones of the fan delta front are developed in the first and second members, which are the main oil and gas reservoirs of the Wutonggou Formation. The thick mudstones-interbedded sandstones of the fan pro-delta developed in the third member are the main cap rock of the Wutonggou Formation.
Tectonic location, stratigraphic column, and sedimentary microfacies profile of the study area in Junggar Basin: (a) tectonic location; (b) strata developed in the study area; and (c) sedimentary microfacies profile of the target layer.
3 Samples and methods
3.1 Samples and data
In this research, 85 core samples of sandy conglomerate and sandstone from 15 key wells (Figure 1a) were collected in the Dongdaohaizi Sag and the Baijiahai Uplift (Table 1). These samples were mainly used for thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), microbeam fluorescence, fluid inclusions and laser Raman spectrographic analysis. All experiments were carried out at the State Key Laboratory of Continental Dynamics, Northwest University. Additionally, data from 35 geological logs (N Sag = 20 and N Uplift = 15), 35 cuttings logs (N Sag = 20 and N Uplift = 15), 150 image granularity analyses of the Wutonggou Formation reservoir (N Sag = 98 and N Uplift = 52), 200 core physical property analyses of the Wutonggou Formation reservoir (N Sag = 153 and N Uplift = 47), 320 SEM photos of the Wutonggou Formation reservoir (N Sag = 215 and N Uplift = 105), and 15 logging imaging photos (N Sag = 12 and N Uplift = 3) were collected from the Zhundong Production Plant of Xinjiang Oilfield Company Ltd, PetroChina for this study.
Statistics of samples collected from the Wutonggou Formation in different tectonic units in Junggar Basin
| Tectonic unit | Well | Number | Sample Depth/m | Formation | Lithology |
|---|---|---|---|---|---|
| Dongdaohaizi Sag | DN1 | 4 | 2667.0–2670.7 | P3wt1 | Sandy conglomerate and sandstone |
| DN2 | 7 | 2609.7–2614.2 | P3wt2 | Sandy conglomerate | |
| DN081 | 8 | 4018.3–4024.2 | P3wt2 | Sandy conglomerate | |
| DN13 | 8 | 4103.6–4108.1 | P3wt2 | Sandy conglomerate and pebbly sandstone | |
| DN12 | 6 | 3447.7–3450.9 | P3wt2 | Sandy conglomerate | |
| DN8 | 8 | 3955.3–3958.2 | P3wt2 | Sandy conglomerate | |
| DN10 | 7 | 3396.9–3399.9 | P3wt2 | Sandy conglomerate | |
| DN11 | 6 | 4698.4–4701.0 | P3wt2 | Pebbly sandstone | |
| Baijiahai Uplift | C36 | 7 | 3704.5–3708.9 | P3wt1 | Sandy conglomerate and pebbly sandstone |
| C34 | 5 | 3153.7–3154.7 | P3wt2 | Sandy conglomerate | |
| C31 | 2 | 3165.1–3165.5 | P3wt2 | Sandy conglomerate and sandstone | |
| C16 | 3 | 3208.5–3209.5 | P3wt2 | Sandy conglomerate | |
| C521 | 6 | 3134.5–3136.9 | P3wt2 | Sandy conglomerate | |
| C524 | 6 | 3299.7–3302.9 | P3wt2 | Sandstone | |
| CC2 | 2 | 3168.8–3169.1 | P3wt2 | Sandstone |
3.2 Casting thin sections
After vacuumizing, blue epoxy was injected into the pore space of sandstones and sandy conglomerates, and the samples were consolidated under standard temperature and pressure. After that, the sandstone samples and the sandy conglomerate samples were cut and ground into thin rock slices with diameters of 25 and 45 mm, respectively, with a thickness of 0.03 mm. The thin sections were stained with mixed solutions of alizarin-S and potassium ferrocyanide. All the prepared thin sections (N Sag = 54 and N Uplift = 31) were analyzed by a multi-functional Carl Zeiss Axio Scope A1 polarizing microscope for point-counting analysis of gravel, detrital components, authigenic minerals, and pore types. At least 300 points were observed in each thin section to obtain quantitative statistics for the content of each component in the thin sections. Additionally, the rock sorting, roundness, grain size, and contact between grains and cementation type were also observed. The analysis methods were based on the oil and gas industry standard of the People’s Republic of China SY/T 5368-2016.
3.3 XRD
Rock samples were crushed and milled in ethanol, and then dried at 60°C according to the sample preparation and analytic methods of Moore et al. and Hillier [38,39]. After that, 21 powder samples (N Sag = 12 and N Uplift = 9) with particle sizes smaller than 10 and 2 μm, respectively, were extracted by the centrifugal separation method. XRD analysis was carried out with a D8A X-ray diffractometer (made by Brooke Company, Germany) at 18°C and 1,043 hPa. The samples with grain sizes between 2 and 10 μm were used to determine the composition and content of the whole rock, and the samples with grain sizes smaller than 2 μm were used to determine the composition and content of clay minerals. The analysis methods were based on the oil and gas industry standard of the People’s Republic of China SY/T 5163-2010.
3.4 CL
The collected samples were polished on one side to make 35 thin sections (N Sag = 20 and N Uplift = 15), each approximately 0.05 mm in thickness and 40 mm × 20 mm in size, and washed with oil. A Gatan MonoCL3 + CL spectrometer and polarizing microscope were used for the CL analysis. When the sample is bombarded with the electron beam, the trace elements in different authigenic minerals or in the structural defects in the mineral lattice will produce different fluorescence after absorbing the cathode ray. This technique is used to reveal the formation stages of authigenic minerals and diagenetic evolution in thin-section samples. The experimental conditions include working temperatures of 15–30°C, storage temperatures of −20 to 50°C, and a relative humidity of less than 70%. The analysis methods were based on the oil and gas industry standard of the People’s Republic of China SY/T 5916-2013.
3.5 Homogeneous temperature of fluid inclusions
The 30 samples collected (N Sag = 17 and N Uplift = 13) were washed with oil and polished on two sides to make thin sections, each approximately 0.05 mm in thickness and 40 mm × 20 mm in size. First, a Leica DMLP polarizing microscope was used to analyze the petrographic characteristics of inclusions in order to identify the inclusions that best reflect the diagenetic environment, such as on quartz overgrowths, in quartz interiors, and quartz microcracks. The sizes and gas–liquid ratios of inclusions were recorded. Second, the inclusions with certain shapes, large volumes, and clear phase boundaries were selected and marked (N Sag = 56 and N Uplift = 33). After completion of the above work, the marked samples were segmented into 22 mm × 20 mm in size for the homogenization temperature measurement at a temperature range of −196 to 600°C. The sample area was 22 mm, the minimum working distance for the objective lens was 4.5 mm, and the minimum working distance for the condenser was 112.5 mm. The homogenization temperature of two-phase (gas phase and liquid phase) inclusions in the sample was measured by a LINKAM-600 Cooling–Heating Stage. The initial heating rate was 10–20°C/min, and observations and records were made for every 5–10°C, until the temperature increase when the phase change occurred. When the gas phase or liquid phase disappeared, the heating rate was reduced at the rate of 0.5–1°C/min. When the inclusion reached the homogenization temperature, the 5–10°C temperature increase was needed so as to observe and verdict if the inclusion completely homogeneous. The analysis methods were based on the oil and gas industry standard of the People’s Republic of China SY/T 6010-2011.
3.6 Microbeam fluorescence
Thirty thin sections from the hydrocarbon contained samples (N Sag = 18 and N Uplift = 12) were observed by a Carl Zeiss Axio Scope A1 fluorescence polarizing microscope under the conditions of accelerating voltage of 10 keV and beam current of 250 μA to measure the fluorescence color of hydrocarbon materials in pores and inclusions to determine the charging period of hydrocarbons. The analysis methods were based on the oil and gas industry standard of the People’s Republic of China SY/T 7309-2016.
4 Results
4.1 Reservoir characteristic
4.1.1 Reservoir lithology
The lithology of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is mainly composed of sandy conglomerate (accounting for 60.2% of the total reservoir thickness), with secondary amount of pebbly sandstone (accounting for 28.4% of the total reservoir thickness). The lithology of the Wutonggou Formation reservoir in the Baijiahai Uplift is mainly composed of fine-grained sandstone (accounting for 44.8% of the total reservoir thickness), with secondary amount of sandy conglomerate and pebbly sandstone (accounting for 29.8 and 24.6% of the total reservoir thickness, respectively), and lack conglomerate sediments (Figure 2a), based on statistics of identification of casting thin sections under microscope, combined with core observation and statistics and the logging data from cuttings. In the sandy conglomerate and pebbly sandstone, the gravel content ranges from 6.0 to 65.0%, with an average of 44.0%, including igneous rocks (average of 24.8%), sedimentary rocks (average of 13.3%), and metamorphic rocks (average of 5.9%). Sand interstitial material range from 1.0 to 69.9%, with an average of 28.0%, and is mainly composed of debris (average of 21.8%); Argillaceous interstitial material range from 2.0–25.0%, with an average of 7.8%. According to the classification standard provided by Folk [40], sandstone is mainly litharenite (Figure 2b), quartz ranges from 1.0 to 10.0%, with an average of 3.7%, feldspar ranges from 0 to 25.0%, with an average of 7.1%, debris content ranges from 33.0 to 87.0%, with an average of 66.9%, including tuff (average of 31.7%), igneous rocks (average of 21.2%), a small amount of sedimentary rocks (average of 7.6%), and metamorphic rocks (average of 6.4%).
Reservoir lithology histogram (a) and triangle classification diagram of sandstones (b) of the Wutonggou Formation from different tectonic units in Junggar Basin.
By contrast, the average grain size of the reservoir in the Dongdaohaizi Sag is slightly coarser than that in the Baijiahai uplift. The former mainly ranges from 2.0 to 4.0 mm (accounting for 54.4% of the total), and the latter mainly ranges from 0.2 to 0.5 mm (accounting for 88.6% of the total). The grain sorting and roundness of the reservoir in the Dongdaohaizi Sag is slightly poorer than that in the Baijiahai uplift. The former is mainly poorly sorted (accounting for 70.3% of the total) and with roundness of subangular (accounting for 62.2% of the total). The latter is dominated by moderate to well sorted (accounting for 67.2% of the total) and with subangular to subrounded and subrounded (accounting for 48.8 and 32.6% of the total, respectively). Therefore, textural maturity of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is relatively lower compared with that of the Baijiahai Uplift.
4.1.2 Reservoir pore types
According to the identifications from casting thin sections under microscope, the pore types of the Wutonggou Formation reservoir in the Dongdaohaizi Sag are mainly intergranular dissolved pores (average of 2.3%), followed by intragranular dissolved pores (average of 2.0%), and microfractures (average of 1.0%), intergranular pores are not developed (average of 0.2%). The pore types of the Wutonggou Formation reservoir in the Baijiahai Uplift are mainly intergranular pores (average of 3.7%) and intergranular dissolved pores (average of 1.5%), followed by intragranular dissolved pores (average of 1.2%), microfractures are not developed (average of 0.3%) (Figure 3a). By contrast, the content of secondary pores and microfractures of the Wutonggou Formation reservoir in the Dongdaohaizi Sag are higher than that of the Baijiahai Uplift, but the content of primary pores are lower than that of the Baijiahai Uplift, and the surface porosity (average of 5.5%) was less than that of the Baijiahai Uplift (average of 6.7%).
Histogram of pore types (a) and physical property distribution (b and c) and scatter diagram of physical property correlation (d) of the Wutonggou Formation from different tectonic units in Junggar Basin.
4.1.3 Reservoir quality
Statistics of the reservoir quality show that, in the Dongdaohaizi Sag, the measured porosity ranges from 0.06 to 15.87%, with an average of 7.02%, and permeability ranges from 0.01 to 53.40 mD, with an average of 1.69 mD; while in the Baijiahai Uplift, the measured porosity ranges from 0.08 to 20.30%, with an average of 11.24%, and permeability ranges from 0.01 to 42.46 mD, with an average of 4.11 mD. Quality of the Wutonggou Formation in the Dongdaohaizi Sag is lower than that in the Baijiahai Uplift. According to the evaluation methods of oil and gas reservoirs following the oil and gas industry standard of the People’s Republic of China (SY/T6285-1997) [41], the Wutonggou Formation reservoir in the Dongdaohaizi Sag belongs to extremely low porosity, extremely low permeability, and tight reservoir. The Wutonggou Formation reservoir in the Baijiahai Uplift belongs to medium-low pore and low-extremely low permeability reservoir (Figure 3b and c). Correlation of physical properties show that porosity and permeability of the Wutonggou Formation are positively correlated (Figure 3d), but the correlation is low (correlation coefficient: R 2 = 0.17). Some reservoirs show low porosity but high permeability. Images of logging with FMI indicate that a reservoir with fractures usually has high permeability (Figure 4), indicating that the percolation ability of the reservoir is affected not only by pore size and their connectivity but also by fractures.
Well logging image of the Wutonggou Formation reservoir from the Dongdaohaizi Sag in Junggar Basin: (a) sandy conglomerate, vertical fractures developed, measured porosity is 8.60%, permeability is 2.03 mD (Dongdaohaizi Sag, DN12 well, 3,462–3,463 m); (b) fine grained sandstone, high angle fractures developed, measured porosity is 7.76%, permeability is 1.95 mD (Dongdaohaizi Sag, DN12 well, 3,503–3,504 m); (c) sandy conglomerate, high angle fractures developed, measured porosity is 5.70%, permeability is 2.00 mD (Dongdaohaizi Sag, DN14 well, 4,000–4,008 m); (d) sandy conglomerate, vertical fractures developed, measured porosity is 7.90%, permeability is 4.40 mD (Dongdaohaizi Sag, DN8 well, 3,957–3,958 m); (e) fine grained sandstone, vertical fractures developed, measured porosity is 6.00%, permeability is 1.10 mD (Dongdaohaizi Sag, DN8 well, 4,002–4,003 m).
4.2 Reservoir diagenesis
4.2.1 Compaction
According to the microscopic observation, the main compaction phenomenon of the Wutonggou Formation reservoir is that plastic debris and mica are compressed and deformed to fill pores (Figure 5a and b). The contact relationship between grains of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is mainly line contact (accounting for 55% of the total) with line-concave convex contact and point-line contact (accounting for 27 and 12% of the total, respectively) at the second place, and a small amount of point contact (accounting for 6% of the total), implying a relatively intense compaction. The contact relationship between grains of the Wutonggou Formation reservoir in the Baijiahai Uplift is mainly point-line contact (accounting for 58% of the total) with line contact (accounting for 29% of the total) at the second place, and a small amount of point contact and line-concave convex contact (accounting for 8 and 5% of the total, respectively), representing a relatively weaker compaction.
Microscopic characteristic of diagenesis of the Wutonggou Formation reservoir in different tectonic units in Junggar Basin. (a) Mudstone debris is compressed and deformed, plane polarized light of casting thin section (Dongdaohaizi Sag, DN12 well, P3wt2, 3450.9 m); (b) mica is compressed and deformed, plane polarized light of casting thin section (Baijiahai Uplift, C34 well, P3wt2, 3153.7 m); (c) stage-I calcite cementation, plane polarized light of thin section (c1) and CL (c2) (Baijiahai Uplift, C16 well, P3wt2, 3209.3 m); (d) stage-II calcite cementation, plane polarized light of thin section (d1) and CL (d2) (Baijiahai Uplift, C16 well, P3wt2, 3209.3 m); (e) stage-I calcite cementation, plane polarized light of thin section (e1) and CL (e2) (Dongdaohaizi Sag, DN11 well, P3wt2, 4698.4 m); (f) stage-II calcite cementation, plane polarized light of thin section (f1) and CL (f2) (Dongdaohaizi Sag, DN1 well, P3wt2, 2667.0 m); (g) stage-III calcite (ferrocalcite) cementation, plane polarized light of thin section (g1) and CL (g2) (Dongdaohaizi Sag, DN081 well, P3wt2, 4018.3 m); (h) early-stage chlorite attached to intergranular pore surfaces in the form of thin film linings, plane polarized light of thin section (Baijiahai Uplift, C36 well, P3wt2, 3704.5 m); (i) globular chlorite attached to laumontite surface, SEM (Dongdaohaizi Sag, DN081 well, P3wt2, 4025.8 m); (j) honeycomb mixed-layer illite/smectite cemented pore, SEM (Dongdaohaizi Sag, DN12 well, P3wt2, 3448.1 m); (k) fibrous illite cemented pore, SEM (Baijiahai Uplift, C521 well, P3wt2, 3135.5 m); (l) booklet kaolinite cemented pore, SEM (Dongdaohaizi Sag, DN1 well, P3wt2, 2666.5 m); (m) colorless laumontite is dissolved to form laumontite dissolved pore after it cements pore and microfracture, plane polarized light of casting thin section (Dongdaohaizi Sag, DN1 well, P3wt2, 2668.0 m); (n) columnar laumontite is dissolved to form laumontite dissolved pore, after that it is filled by chlorite, SEM (Dongdaohaizi Sag, DN8 well, P3wt2, 3957.3 m); (o) stage-II quartz overgrowths, plane polarized light of casting thin section (Baijiahai Uplift, C31 well, P3wt2, 3172.7 m); (p) intergranular dissolved pore, plane polarized light of casting thin section (Dongdaohaizi Sag, DN1 well, P3wt2, 2674.5 m); (q) intergranular dissolved pore, plane polarized light of casting thin section (Baijiahai Uplift, C16 well, P3wt2, 3026.7 m); (r) debris dissolved pore, plane polarized light of casting thin section (Dongdaohaizi Sag, DN8 well, P3wt2, 3909.8 m); (s) debris dissolved pore, plane polarized light of casting thin section (Baijiahai Uplift, C16 well, P3wt2, 3209.3 m); (t) feldspar dissolved pore, plane polarized light of casting thin section (Dongdaohaizi Sag, DN10 well, P3wt2, 3398.1 m); (u) calcite dissolved pore, plane polarized light of casting thin section (Dongdaohaizi Sag, DN1 well, P3wt2, 2665.0 m); (v) microfracture is developed in gravel, plane polarized light of casting thin section (Dongdaohaizi Sag, DN13 well, P3wt2, 4103.5 m); (w) microfracture is developed in gravel, plane polarized light of casting thin section (Dongdaohaizi Sag, DN13 well, P3wt2, 4107.7 m). Note: Ms: Muscovite; Cal-I, Cal-II, and Cal-III: Calcite cementation occurred in the first, second, and third stage, respectively; Chl: chlorite; Ill: illite; I/S: mixed-layer of illite/smectite; Kln: kaolinite; Lmt: laumontite; Qtz: Quartz; Qovg-II: quartz overgrowth formed in the second stage.
4.2.2 Cementation
The primary cement in the Wutonggou reservoir is authigenic clay minerals, with a small amount of carbonate, laumontite, and siliceous cement.
Statistics indicate that the content of carbonate cement of the Wutonggou Formation reservoir in the Dongdaohaizi Sag (average of 4.12%) is lower than that in the Baijiahai Uplift (average of 5.46%). Carbonate types also have some differences between the two tectonic units. In the Dongdaohaizi Sag, carbonate cements are mainly calcite (average of 2.44%) and ferrocalcite (average of 1.68%). Whereas in the Baijiahai Uplift, calcite (average of 5.46%) is dominated in the carbonate cements. Under CL, two stages of calcite cementation can be recognized in the Baijiahai Uplift. The microcrystalline calcite with orange–yellow light in the first stage and the sparry calcite with orange–yellow light in the second stage is mainly filled in the intergranular pores (Figure 5c and d). Calcite cementation formed in three stages can be found in the Dongdaohaizi Sag. The first and second calcite cement types are same as those at the Baijiahai Uplift (Figure 5e and f), the third stage is ferrocalcite, which is not luminous and mainly filled in dissolved pores (Figure 5g).
Authigenic clay minerals make up the highest content of cements in the Wutonggou Formation reservoir, including chlorite, mixed-layer illite/smectite, illite, and kaolinite. Authigenic clay mineral content of reservoir in the Dongdaohaizi Sag (average of 5.55%) is lower than that in the Baijiahai Uplift (average of 6.37%). Under the SEM, two types of chlorite can be found: (1) attached to intergranular pore surfaces in the form of thin film linings (Figure 5h), making up an average of 1.08% in the Dongdaohaizi Sag and 2.01% in the Baijiahai Uplift; and (2) cemented in pores and attached to laumontite surfaces in the form of globular and rosette textures (Figure 5i), making up an average of 1.73% in the Dongdaohaizi Sag and which is not present in the Baijiahai Uplift. Mixed-layer illite/smectite is mostly cemented in pores with a honeycomb texture (Figure 5j), dividing large pores into small and micro-pores and coarse throats into fine throats, with an average content of 1.28% in the Dongdaohaizi Sag and 1.97% in the Baijiahai Uplift. Illite is mostly cemented in the pores in the form of fibrous or pore-bridging textures (Figure 5k), with an average content of 1.01% in the Dongdaohaizi Sag and 1.86% in the Baijiahai Uplift. Kaolinite is mostly cemented in pores in the form of anhedral-pseudohexagonal plates, producing vermicular and booklet textures (Figure 5l), with an average content of 0.45% in the Dongdaohaizi Sag and 0.53% in the Baijiahai Uplift.
Fluorescence photos of hydrocarbon charging of the Wutonggou Formation reservoir in different tectonic units in Junggar Basin. (a) hydrocarbon charging of the first and third stages (Dongdaohaizi Sag, DN12 well, P3wt2, 3450.9 m); (b) hydrocarbon charging of the first, second, and third stages (Dongdaohaizi Sag, DN10 well, P3wt2, 3399.4 m); (c) hydrocarbon charging of the first and third stages (Dongdaohaizi Sag, DN1 well, P3wt2, 2649.7 m); (d) hydrocarbon charging of the second and third stages (Dongdaohaizi Sag, DN13 well, P3wt2, 4103.5 m); (e) hydrocarbon charging of the first and second stages (Baijiahai Uplift, C34 well, P3wt2, 3153.7 m); (f) hydrocarbon charging of the second stage (Baijiahai Uplift, C521 well, P3wt2, 3136.5 m); (g) hydrocarbon charging of the first and second stages (Baijiahai Uplift, C34 well, P3wt2, 3153.7 m); (h) hydrocarbon charging of the first and second stages (Baijiahai Uplift, C36 well, P3wt2, 3704.5 m). Note: Hydrocarbon I, Hydrocarbon II, and Hydrocarbon III: Hydrocarbon charging occurred in the first stage, the second stage, and the third stage, respectively.
Laumontite is a common cement in the Wutonggou Formation reservoir, which is lower in the Dongdaohaizi Sag (average of 2.83%) than in the Baijiahai Uplift (average of 3.07%). In thin sections, colorless laumontite cemented in pores could be seen (Figure 5m). Under the SEM, laumontite was usually cemented in pores with a columnar style (Figure 5n).
The content of siliceous cement of the Dongdaohaizi Sag is similar to that of the Baijiahai Uplift (average of 0.98 and 0.87%, respectively), and siliceous cements have two types: (1) quartz overgrowths cemented in pores, mostly consisting of quartz overgrowths formed in the second stage (stage-II) (Figure 5o), occasionally the quartz overgrowths formed in the first stage (stage-I) can be seen. (2) Authigenic quartz cemented in pores.
4.2.3 Dissolution
Microscopic observation shows that the most common dissolution phenomenon of the Wutonggou Formation reservoir in the Dongdaohaizi Sag and Baijiahai Uplift are the dissolved expansion of intergranular pores (Figure 5p and q), with lesser amount of easily dissolved grains (debris and feldspar) dissolving to form intragranular dissolved pores (Figure 5r–t). In addition, laumontite and calcite dissolved to form laumontite and carbonate dissolved pores also can be seen in the reservoir (Figure 5m and u). Statistics indicate that the surface porosity of the dissolved pores of the Wutonggou Formation reservoir in the Dongdaohaizi Sag (average of 4.30%) is greater than that in the Baijiahai Uplift (average of 2.70%), which indicate that the dissolution occurred in the Dongdaohaizi Sag reservoir is stronger than that in the Baijiahai Uplift.
4.2.4 Fracturing
Fracturing of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is relatively developed. Three groups of structural fractures, including high angle, inclined, and horizontal structural fractures, can be seen in the reservoir [42]. According to the microscopic observation, microfractures are usually developed in rigid gravel and debris grains (Figure 5v and w), with the high content (average of 1.30%). At the same time, it can also be seen that the early microfracture is cemented by laumontite cement (Figure 5m). Fracturing of the Wutonggou Formation reservoir in the Baijiahai Uplift is not developed, and there are basically nearly no microfractures in the reservoir, with the low content (average of 0.10%).
4.3 Hydrocarbon charging stage
The observation under microbeam fluorescence microscope shows that in the Dongdaohaizi Sag, three kinds of fluorescent color reflecting hydrocarbon charging can be seen in the Wutonggou Formation reservoir. The first is yellowish–brown fluorescence, which is usually filled in intergranular pores; the second is blue fluorescence, which is usually filled in dissolved pores and microfractures; the third is blue–white fluorescence, which is often filled in dissolved pores. In the Baijiahai Uplift, two kinds of fluorescent color can be seen in the Wutonggou Formation reservoir. The first is yellowish–brown fluorescence, which is usually filled in intergranular pores; The second is blue fluorescence, which is usually filled in dissolved pores and microcracks. Large differences are found in the hydrocarbon composition characteristics and degrees of thermal evolution of the Wutonggou Formation reservoir. The reservoirs in the Dongdaohaizi Sag show evidence of three stages of hydrocarbon charging: an early stage that produced hydrocarbons with yellowish–brown fluoresce, a middle stage that formed hydrocarbons with blue fluoresce, and a late stage that brought about hydrocarbons with blue–white fluoresce. The reservoirs of the Baijiahai Uplift show evidence of two stages of hydrocarbon charging: an early stage that resulted in yellowish–brown fluorescing hydrocarbons, and a late stage produced blue fluorescing hydrocarbons (Figure 6).
The petrographic study of fluid inclusions in the Wutonggou Formation reservoir shows that the most common types of inclusions are gas–liquid two phase aqueous inclusions, hydrocarbon-bearing aqueous inclusions, and hydrocarbon inclusions. In the Dongdaohaizi Sag reservoir, inclusions occur in quartz interior, quartz microcrack, and quartz overgrowth, with circular, elliptical, elongated, and irregular shapes. Inclusion sizes range from 8 to 72 μm2, with an average of 27 μm2. Gas–liquid ratio ranges from 10 to 30%, with an average of 20% (Figure 7a–c). In the Baijiahai Uplift reservoir, inclusions occur in quartz interior and quartz microcrack, with elliptical and irregular shapes. Inclusion sizes range from 12 to 48 μm2, with an average of 24 μm2. Gas–liquid ratio ranges from 10 to 20%, with an average of 16% (Figure 7d–f).
Petrographic characteristics of inclusions of the Wutonggou Formation reservoir in different tectonic units in Junggar Basin. (a) Two phase of gas–liquid aqueous inclusions developed in quartz interior, with size of 17 μm2 and gas–liquid ratio of 20% (Dongdaohaizi Sag, DN081 well, P3wt2, 4020.1 m); (b) two phase of gas–liquid aqueous inclusions developed in quartz overgrowth, with size of 9 μm2 and gas–liquid ratio of 20% (Dongdaohaizi Sag, DN2 well, P3wt2, 2611.4 m); (c) two phase of gas–liquid aqueous inclusions developed in quartz interior, with size of 24 μm2 and gas–liquid ratio of 25% (Dongdaohaizi Sag, DN12 well, P3wt2, 3447.1 m); (d) two phase of gas–liquid aqueous inclusions developed in quartz microcracks, with size of 21 μm2 and gas–liquid ratio of 15% (Baijiahai Uplift, C521 well, P3wt2, 3136.5 m); (e) two phase of gas–liquid aqueous inclusions developed in quartz interior, with size of 19 μm2 and gas–liquid ratio of 20% (Baijiahai Uplift, C34 well, P3wt2, 3153.7 m); (f) two phase of gas–liquid aqueous inclusions developed in quartz interior, with size of 25 μm2 and gas–liquid ratio of 30% (Baijiahai Uplift, C16 well, P3wt2, 3209.3 m).
Homogenization temperature test results of inclusions from different reservoirs show that homogenization temperature of inclusions in the Dongdaohaizi Sag ranges from 70 to 205°C, mainly concentrated in 80–140°C and with three peak values of 80–100°C, 110–120°C, and 130–140°C (Figure 8a). While homogenization temperature of inclusions in the Baijiahai Uplift range from 69 to 175°C, mainly concentrated in 70 to 110°C and with two peak values of 70–80°C and 90–110°C (Figure 8b).
Homogenization temperature distribution of fluid inclusions from the Wutonggou Formation reservoir in different tectonic units: (a) Dongdaohaizi Sag and (b) Baijiahai Uplift.
In summary, three stages of hydrocarbon charging have occurred in the Wutonggou Formation reservoir in the Dongdaohaizi Sag. Stage-I is characterized by yellowish–brown fluorescence, homogenization temperature of aqueous inclusions corresponding to this stage are between 80 and 100°C. Stage-II is characterized by blue fluorescence, homogenization temperature of aqueous inclusions corresponding to the stage-II are between 110 and 120°C. Stage-III is featured by blue–white fluorescence, homogenization temperature of aqueous inclusions in the same stage are between 130 and 140°C. However, only two stages of hydrocarbon charging of the Wutonggou Formation reservoir are recognized in the Baijiahai Uplift. Stage-I is characterized by yellowish–brown fluorescence, homogenization temperature of aqueous inclusions formed in the stage-I are between 70 and 80°C. Stage-II is characterized by blue fluorescence, homogenization temperature of aqueous inclusions corresponding to the stage-II are between 90 and 110°C.
5 Discussion
5.1 Diagenetic stage and sequence
Classification of diagenetic stage for the Wutonggou Formation reservoir in this study is based on the Professional Standard of the division of diagenetic stages for clastic rocks, Ministry of Petroleum, China (SY/T5477-2003) [43]. The evidences and records in the reservoir are as follows: in the Dongdaohaizi Sag, the contacts between grains of the reservoir are mainly line contacts. The cements in the reservoir include ferrocalcite, late-stage chlorite and stage-II quartz overgrowths, and mixed-layer illite/smectite (average mixed-layer ratio is 30%). The pores are mainly intergranular dissolution pores. The reflectance of vitrinite in the strata ranges from 0.7 to 1.4% [44]. The paleotemperature might have reached 140°C when the reservoir reached the maximum burial depth. While in the Baijiahai Uplift, the contacts between grains of the reservoir are mainly point-line contacts, cements are mainly early stage chlorite, stage-II quartz overgrowths, and mixed-layer illite/smectite (average mixed-layer ratio is 46%), while ferrocalcite is not found. The pore type is mainly intergranular dissolution pores. The reflectance of vitrinite ranges from 0.5 to 1.2% [45]. The paleotemperature might have reached 110°C when the reservoir reached its maximum burial depths. Thus, the diagenetic stage of the Wutonggou Formation in the Dongdaohaizi Sag can be classified as A2 sub-stage of mesodiagenesis, and the Wutonggou Formation in the Baijiahai Uplift is A1 sub-stage of mesodiagenesis.
On this basis, the diagenetic sequence of the Wutonggou Formation reservoir in two tectonic units are established, based on the observation of diagenetic sequence under the microscope and the above diagenetic stage classification.
The diagenetic sequence of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is as follows: Compaction → Fracturing → Stage-I calcite → Stage-I chlorite → Mixed-layer illite/smectite → Siliceous → Laumontite → Kaolinite → Illite → Stage-I hydrocarbon charging → Stage-I dissolution → Stage-II calcite → Stage-II hydrocarbon charging → Stage-II dissolution → Stage-III hydrocarbon charging → Stage-III dissolution → Stage-II chlorite → Stage-III calcite (ferrocalcite). The middle-stage cementation (before the end of dissolution) and late-stage cementation (after the end of dissolution) are developed.
While in the Baijiahai Uplift, the diagenetic sequence of the Wutonggou Formation reservoir is as follows: Compaction → Stage-I calcite → Chlorite → Siliceous → Laumontite → Mixed-layer illite/smectite → Kaolinite → Illite → Stage-II calcite → Stage-I hydrocarbon charging → Stage-I dissolution → Stage-II hydrocarbon charging → Stage-II dissolution. Only the middle-stage cementation (before the end of dissolution) is developed.
5.2 Differential analysis of diagenesis and pore evolution
5.2.1 Differentiation of the initial porosity
The Dongdaohaizi Sag was a proximal deposit compared with the Baijiahai uplift under the control of the provenance of Kelameili Mountain in the northeast, during the deposition period of Wutonggou Formation. As a result, the Dongdaohaizi Sag was affected by the near-source relatively strong hydrodynamic force, and the reservoir sediments had larger grain size, low rounding, poor sorting, and high argillaceous content, leading to an overall low textural maturity. According to the research result of Beard, the initial porosity of a reservoir before compaction is related to the sorting coefficient of grains [46] (formulas (A) and (B)). The initial porosity of the Wutonggou Formation reservoir in the Dongdaohaizi Sag was calculated to be in the range of 30.31–42.02% (average of 36.48%), while in the Baijiahai Uplift, it was in the range of 36.69–42.54% (average of 40.10%). Thus, the initial porosity of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is lower than that in the Baijiahai Uplift.
5.2.2 Differential reduced porosity by compaction
The maximum buried depth (3,344–4,775 m) is deeper and average stratum thickness (298 m) of the Wutonggou Formation is thicker in the Dongdaohaizi Sag than that in the Baijiahai Uplift (the maximum buried depth ranges from 3,100 to 4,177 m with average stratum thickness of 134 m), resulting in the different lithostatic pressure produced by the overlying strata in the two tectonic units, which makes the overlying strata pressure of the Wutonggou Formation reservoir greater and compaction stronger in the Dongdaohaizi Sag, compared to those of the Baijiahai Uplift. Previous studies noted that the residual porosity after compaction is related to the surface porosity of residual intergranular pores, surface porosity of cement dissolved pores, and cement content [47] (formula (C)). It is calculated that the residual porosity after compaction of the Wutonggou Formation reservoir in the Dongdaohaizi Sag ranges from 9.60 to 19.56% (average of 14.35%), and porosity is reduced by 17.41–27.59% (average of 22.13%). The residual porosity after compaction of the Wutonggou Formation reservoir in the Baijiahai Uplift ranges from 19.60 to 26.80% (average of 22.22%), and porosity is decreased by 10.89–21.36% (average of 17.88%). Thus, porosity loss caused by compaction in the Dongdaohaizi Sag is higher than that in the Baijiahai Uplift.
5.2.3 Differential reduced porosity by cementation
Diagenetic evolution sequence shows that the two stages of cementation occurred in the Wutonggou Formation reservoir during middle-stage (before the end of dissolution) and late-stage (after the end of dissolution) in the Dongdaohaizi Sag. The content of middle-stage cements (stage-I calcite, stage-I chlorite, stage-II calcite, mixed-layer illite/smectite, illite, kaolinite, laumontite, and siliceous) ranges from 2.67 to 16.53% (average of 10.07%). The content of late-stage cements (stage-III calcite and stage-II chlorite) ranges from 0.72 to 11.96% (average of 13.41%). However, only middle-stage cementation (before the end of dissolution) occurred in the Baijiahai Uplift, but the content of cements is higher, which ranges from 10.93 to 21.95% (average of 15.77%). Previous studies noted that the porosity reduced by cementation is basically the same as the volume percentage of cement in the reservoir. Residual porosity after cementation is approximately equal to the residual porosity after compaction minus the volume percentage of cement. [48]. It is calculated that the middle-stage cementation reduced porosity by 4.38–15.96% (average of 10.07%) and the late-stage cementation reduced porosity by 0.72–11.96% (average of 3.41%) in the Dongdaohaizi Sag. While in the Baijiahai Uplift, the middle-stage cementation reduced porosity by 10.53–21.95% (average of 15.77%). Thus, cementation of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is weaker and the reduced pores lesser than that in the Baijiahai Uplift.
5.2.4 Differential increased porosity by dissolution
Previous studies have shown that three hydrocarbon charging stages occurred in the Dongdaohaizi Sag, which led to more secondary dissolved pores (average of 4.30%) formed during the organic acid dissolution. Whereas in the Baijiahai Uplift, only two stages of hydrocarbon charging occurred, which resulted in relatively weak dissolution by organic acid and the lesser secondary dissolved pores (average of 2.70%) increased after dissolution. Previous studies noted that increased porosity after dissolution is mainly related to the surface porosity of dissolved pores [48] (formula (D)). It is calculated that the increased porosity after dissolution ranges from 0.84 to 9.32% (average of 6.28%) in the Dongdaohaizi Sag and increased porosity after dissolution ranges from 0.51 to 7.50% (average of 4.15%) in the Baijiahai Uplift.
5.2.5 Differentiation of the calculated present porosity
Two different calculation methods are used for calculating the present porosity, due to the differential compaction, cementation, and dissolution of the Wutonggou Formation reservoir in the two tectonic units. The calculated present porosity in the Dongdaohaizi Sag is the porosity reduced by compaction and middle-stage and late-stage cementations, and pores increased by dissolution at the late-stage (formula (E)). While in the Baijiahai Uplift, the calculated present porosity is the porosity reduced by compaction and middle-stage cementation, and pores increased by dissolution (formula (F)). As a result, the calculated present porosity ranges from 3.98 to 10.70% (average of 7.15%) in the Dongdaohaizi Sag, with an error of 0.12% compared to the measured porosity. The calculated present porosity ranges from 6.16 to 15.63% (average of 10.60%) in the Baijiahai Uplift, with an error of −0.64% compared to the measured porosity.
In addition to the above diagenesis, the quality of the reservoir is also controlled by fracturing. Some microfractures are developed inside the reservoir. The previous research shows that fracturing is mainly developed in the Dongdaohaizi Sag, permeability of the fracture developed reservoirs ranges from 1.03 to 53.40 mD (average of 6.92 mD), whereas permeability of the non-fracture developed reservoirs ranges from 0.01 to 1.61 mD (average of 0.25 mD). The development scale of reservoir fracture in the Baijiahai Uplift is small, permeability of the non-fracture developed reservoirs range from 0.01 to 42.56 mD (average of 4.06 mD). It is obvious that due to the differential diagenetic pore evolution process, the calculated present porosity of the Wutonggou Formation reservoir in the Dongdaohaizi Sag is lower than that in the Baijiahai Uplift, which leads to the permeability of non-fracture developed reservoir in the Baijiahai Uplift being higher than that in the Dongdaohaizi Sag. However, the fracture developed in the Dongdaohaizi Sag greatly improves the seepage capacity of the reservoir, and permeability of the fracture developed reservoir is greater than that in the Baijiahai Uplift.
where S 0 – Trask sorting coefficient; P 25 – grain diameter corresponding to 25% of the probability cumulative frequency of sandstone grain size, mm; P 75 – grain diameter corresponding to 75% of the probability cumulative frequency of sandstone grain size, mm; P 1 – surface porosity of residual intergranular pore, %; P 2 – surface porosity of cement dissolved pores, %; P 3 – surface porosity of dissolved pores, %; P M – measured porosity, %; P T – total surface porosity, %; W T – total cement content, %; W M – middle-stage cement content, %; W L – late-stage cement content, %; φ 1 – initial porosity, %; φ 2 – residual porosity after compaction, %; φ 3 – increased pores after dissolution, %; φ 4 – present calculated porosity in the Dongdaohaizi Sag, %; φ 5 – present calculated porosity in the Baijiahai Uplift, %.
5.3 Differential process of burial – diagenesis – hydrocarbon charging – pore evolution
On the basis of analysis of differential diagenesis and pore evolution, differential time sequence of burial – diagenesis – hydrocarbon charging – pore evolution of the Wutonggou Formation reservoir in two tectonic units are analyzed, combined with the diagenetic sequence, the burial thermal history, hydrocarbon charging periods, and regional tectonic evolution background.
From deposition of the Wutonggou Formation during the Carboniferous to late Permian, multistage tectonic movement occurred in the study area [36,45], which controlled the depositional patterns of the basin in different degrees and which in turn influenced the diagenetic processes of the reservoir. In the late Carboniferous, the Dongdaohaizi area had not yet formed a sag, while the Baijiahai area, located in the south of the Dongdaohaizi area, had formed an uplift. In the early Permian, the Baijiahai fault, located in the south of the Dongdaohaizi area, began to develop. The Dongdaohaizi Sag began to form and was separated from the Baijiahai Uplift. The Middle Permian was the key period for tectonic evolution in the study area. During this period, lacustrine mudstones of the Pingdiquan Formation were widely deposited in the Dongdaohaizi Sag. However, under the influence of late Hercynian tectonic movement, the Pingdiquan Formation in the Baijiahai Uplift was continuously uplifting and denuded, so lacustrine mudstones were not preserved there [34]. In the late Permian, Kelameili Mountain, located in the northeast of the study area, became the main source area [37,44], providing large amount of sediments for the study area, as a result, the Wutonggou Formation stratum was deposited in the Dongdaohaizi Sag. The thickness of the stratum was large, but the textural maturity of the reservoir was slightly lower than that in the Baijiahai Uplift, and the initial porosity was low (average of 36.48%). In the Baijiahai Uplift, the thickness of the Wutonggou Formation was relatively thin, and the strata deposited in some high parts were eroded, but the textural maturity of the reservoir was slightly higher than that in the Dongdaohaizi Sag, and the initial porosity was high (average of 40.10%).
After entered the syndiagenetic to A stage of eodiagenesis from Early Triassic to late Triassic, both the Dongdaohaizi Sag and the Baijiahai Uplift were dominated by continuous subsidence (Figure 9a and b), reservoir was mainly affected by mechanical compaction. Under the influence of Indosinian tectonic movement in the late Triassic, the Baijiahai Uplift was generally uplifted, and overlying strata of the Wutonggou Formation experienced erosion to some degree, leading to the reduction in pressure of overlying strata and weakening of the compaction strength. The Dongdaohaizi Sag did not uplift, but in this process, the EW low angle compressive-torsional fault, NE-SW high angle compressive-torsional fault, and NW-SE thrusting fault developed under the influence of extension faulting during the Hercynian-Indosinian [36,49], which mainly controlled the Permian and Triassic strata and led to fracturing, began to develop, and a certain scale of structural fractures was formed in the reservoir in a short time. In the B stage of eodiagenesis from Late Triassic to late Jurassic, cementation began to occur as burial deepened. Under the influence of the second phase of Yanshan tectonic movement in late Jurassic, both the Dongdaohaizi Sag and the Baijiahai Uplift were uplifted in different degrees (Figure 9a and b), compaction basically stopped. Finally, the influence of compaction on the reservoir in the Dongdaohaizi Sag (the decreased porosity was 22.13%) was stronger than that in the Baijiahai Uplift (the decreased porosity was 17.88%). During the A1 sub-stage of mesodiagenesis from late Jurassic to late Cretaceous, tectonic activities were relatively weakened, and the basin entered a stage of slow subsidence (Figure 9a and b). The process of water–rock reaction was accelerated due to burial deepening and temperature increasing. As the residual porosity of the reservoir in the Baijiahai Uplift was higher than that in the Dongdaohaizi Sag after compaction, it was easier for diagenetic fluids in the reservoir to flow, and a large number of authigenic minerals precipitated and cemented pores in the Dongdaohaizi Sag, making the medium-stage cementation influence on the reservoir in the Dongdaohaizi Sag (the decreased porosity was 10.07%) stronger than that in the Baijiahai Uplift (the decreased porosity was 15.77%). During this time, the lacustrine mudstone of the Pingdiquan Formation began to generate hydrocarbon [36,44]. Hydrocarbon fluid along the opening fractures and pores migrated from the Dongdaohaizi Sag to the Baijiahai Uplift, resulting in the Dongdaohaizi Sag experiencing hydrocarbon charging in three stages, while only two hydrocarbon charging stages occurred in the Baijiahai Uplift, due to the lack of the Pingdiquan Formation in the Baijiahai Uplift. Thus, hydrocarbon charging accompanied by organic acid dissolution influence on the reservoir in the Dongdaohaizi Sag (the increased porosity was 6.28%) was stronger than that in the Baijiahai Uplift (the increased porosity was 4.15%). After that, the basin further inclined to the southwest under the nearly north-south horizontal tectonic compression, attributed to the strong collision between Indian plate and Eurasian plate during Himalayan tectonic movement after late Cretaceous [49]. Both the Dongdaohaizi Sag and the Baijiahai Uplift, in the east of the basin, were wholly uplifted, resulting in a declined rate of subsidence in the Dongdaohaizi Sag, the average maximum burial depth of the Wutonggou Formation reached 4,026 m (Figure 9a), thus the diagenetic evolution stage entered the A2 sub-stage of mesodiagenesis, and late-stage cementation occurred (the decreased porosity was 3.41%). Whereas subsidence rate of the Baijiahai Uplift basically stopped, the average maximum burial depth of the Wutonggou Formation reached 3,261 m (Figure 9b), diagenetic evolution stage stayed in A1 sub-stage of mesodiagenesis with no cementation occurring in the late-stage.
![Figure 9
Integrated pattern of burial – diagenesis – hydrocarbon charging – pore evolution of the Wutonggou Formation reservoir in the Dongdaohaizi Sag (a) and Baijiahai Uplift (b), Junggar Basin. (Burial thermal history modified from reference [44,45]).](/document/doi/10.1515/geo-2022-0389/asset/graphic/j_geo-2022-0389_fig_009.jpg)
Finally, the Wutonggou Formation reservoir in the Dongdaohaizi Sag has experienced four stages of pore evolution process, including decreased porosity by shallow burial (0–1,400 m) compaction, decreased porosity by moderate burial (1,400–2,500 m) compaction and middle-stage cementation, decreased and increased porosity by moderate-deep burial (2,500–3,600 m) middle-stage cementation and dissolution, and increased and decreased porosity by deep burial (3,600–4,026 m) dissolution and late-stage cementation. The porosity decreased from 36.48% at first to 7.15% at present, with an average reduction rate of 80.40%. The Wutonggou Formation reservoir in the Baijiahai Uplift has experienced three stages of pore evolution process, including decreased porosity by shallow burial (0–1,100 m) compaction, decreased porosity by moderate burial (1,100–1,700 m) compaction and middle-stage cementation, and decreased and increased porosity by moderate-deep burial (1,700–3,261 m) middle-stage cementation and dissolution. The porosity decreased from 40.10% at first to 10.60% at present, with an average porosity reduction rate of 73.62%.
In summary, the Wutonggou Formation reservoir in the Dongdaohaizi Sag is of a low maturity and low initial porosity. During the burial – diagenesis – hydrocarbon charging – pore evolution, the porosity reduction rate of the reservoir was high, and the present porosity is low. However, the development of structural fracture enhances the seepage capacity of the reservoir and improves the reservoir quality. The development of favorable reservoir is mainly controlled by fracturing. While the Wutonggou Formation reservoir in the Baijiahai Uplift has a high maturity and high initial porosity. During burial – diagenesis – hydrocarbon charging – pore evolution, the average porosity reduction rate of the reservoir was low, and the present porosity is high. The development of favorable reservoir is mainly controlled by sedimentation and burial – diagenesis – hydrocarbon charging – pore evolution (Figure 10).
Diagenesis and pore evolution model of the Wutonggou Formation reservoir in different tectonic units in Junggar Basin.
The favorable reservoir types in the study area are divided into two types. The first type is matrix pore favorable reservoir, which has the high initial porosity, relatively shallow burial depth, and the relatively low degree of diagenetic – pore evolution. On the basis of retaining a large number of primary pores, some secondary pores are produced. Seepage capacity of the reservoir is mainly affected by matrix porosity. It is mainly located in the area of the Baijiahai Uplift with relatively high porosity and permeability (Figure 10). The second type is structural fracture developed favorable reservoir, which has the low initial porosity, relatively deep burial depth, and experienced the relatively intense diagenetic – pore evolution. Primary pores in the reservoir basically lost and only part of the secondary pores developed. But fracturing caused by strong tectonic movement makes the reservoir develop a certain scale of structural fractures, thus improving the seepage capacity of the reservoir. It is mainly located in the fault development area of the Dongdaohaizi Sag (Figure 10).
6 Conclusion
Differences in the distance from source to sink, sedimentation and maturity of rock texture controlled by the provenance led to the initial quality of the reservoir in the Dongdaohaizi Sag being lower than that in the Baijiahai Uplift. The different tectonic subsidence and burial modes make the diagenesis -hydrocarbon charging - pore evolution process of the Wutonggou Formation conglomerate reservoir in the Dongdaohaizi Sag and Baijiahai Uplift exist differences. In the Dongdaohaizi Sag, the reservoir experienced strong compaction → fracturing → weak middle-stage cementation → three stages hydrocarbon charging → strong dissolution → late-stage cementation. The diagenetic evolution degree is relatively high. While the reservoir in the Baijiahai Uplift experienced weak compaction → strong middle-stage cementation → two stages hydrocarbon charging → weak dissolution. The diagenetic evolution degree is relatively low.
Influenced by burial – diagenesis – hydrocarbon charging – pore evolution, the Wutonggou Formation reservoir in the Dongdaohaizi Sag has experienced four stages of pore evolution process. The porosity decreased from 36.48% at first to 7.15% at present, with low physical properties, but the development of structural fractures enhanced the seepage capacity and improved the quality of the reservoir. While the Wutonggou Formation reservoir in the Baijiahai Uplift has experienced three stages of pore evolution process. The porosity decreased from 40.10% at first to 10.60% at present, with high physical properties.
The reservoir quality in the Dongdaohaizi Sag is mainly controlled by fracturing, the structural fracture developed favorable reservoir is formed in the fault development area, which is the most important goal for hydrocarbon exploration in this area. The reservoir quality in the Baijiahai Uplift is mainly controlled by sedimentation and burial – diagenesis – hydrocarbon charging – pore evolution, the matrix pore favorable reservoir is formed in the area of the reservoir with relatively high porosity and permeability, which is the second primary goal for hydrocarbon exploration.
Acknowledgments
This study was financially supported by The National Natural Science Foundation of China (No. 41972129) and The National Science and Technology Key Project of China (No. 2017ZX05008-004-004-001; 2016ZX05008-003-005). The authors appreciate the Zhundong Production Plant of Xinjiang Oilfield Company Ltd, PetroChina for offering basic data, core samples, and permission to publish the article.
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Funding information: This work was jointly supported by The National Natural Science Foundation of China (No. 41972129) and The National Science and Technology Key Project of China (No. 2017ZX05008-004-004-001, 2016ZX05008-003-005).
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Conflict of interest: We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of the manuscript entitled.
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