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Article

Elaborated Porosity and High-Quality Reservoirs in Deeply Buried Coarse-Grained Sediment: Insight of the Sublacustrine Sandy Conglomerates in the Eocene Shehejie Formation (Es4), Dongying Subbasin, Bohai Bay Basin, China

by
Yongqiang Yang
1,*,
Kouassi Louis Kra
1,*,
Longwei Qiu
1,
Baoliang Yang
1,
Kabir Shola Ahmed
1,
Koua Kadio Aka Donald
2,
Yupeng Qiao
3 and
Yerejiepu Habulashenmu
1
1
School of Geosciences, China University of Petroleum, Qingdao 266580, China
2
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
3
College of Petroleum & Gas Engineering, Liaoning Petrochemical University, Fushun 113001, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(3), 1347; https://doi.org/10.3390/app13031347
Submission received: 7 December 2022 / Revised: 8 January 2023 / Accepted: 12 January 2023 / Published: 19 January 2023
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Abstract

:
Reservoir porosity evaluation is crucial for successful prediction of reservoir quality in deeply buried heterogeneous strata. Recent studies have demonstrated that high-quality reservoirs occur in deeply buried strata. However, little is known about the details of pores related to the good reservoirs. The northern Dongying steep slope sandy conglomerate was investigated to understand the porosity related to high-quality reservoir formation in sandy conglomerate using seismic data, well-logs, SEM-EDS, cathodoluminescence, and optical microscopy. The result reveals three genetic pores: primary, secondary, and mixed pores. The dissolution porosity consists of intergranular pores, intragranular pores, intercrystalline pores, and moldic pores. The intergranular dissolution and enlarged pores are the main contributors to good reservoir quality among the different pores. The index of compaction indicates that compaction is the crucial factor diminishing the reservoir quality in the progradation sequences, while cementation stands as the critical factor for porosity reduction in the retrogradation sequences. Comparing the reservoir properties reveals that the dissolution porosity is more significant in the Es4s than the Es4x, which renders the Es4s reservoir relatively good compared to the Es4x. This study demonstrated that for oil exploration in the fourth member of the Shahejie Formation, the Es4s should be prioritized.

1. Introduction

Porosity and permeability are the greatest evaluation parameters for assessing reservoir quality during oil exploration [1]. However, the prediction of porosity and permeability in the deeply buried strata, especially sandy conglomerates, is more complicated due to the strong heterogeneity [2,3,4,5]. Consequently, a clear understanding of their formation and repartition is cardinal for exploring and exploiting deeply buried coarse-grained deposits. The depositional porosity and permeability are chiefly determined by the depositional environment and hydrodynamic conditions, including grain shape, size, mineralogy, and sorting [6,7,8]. Generally, the porosity and intergranular volume decrease with the augmentation of the burial depth [9]. During the burial, the diagenetic mechanism, including compaction, cementation, and dissolution, can reduce or augment the porosity [10,11,12].
The present focuses on the pore type related to the high-quality formation in Es4s and Es4x and compares the progradation and retrogradation sequences in two areas belonging to the north of the Dongying Subsag. Recent work in the study area has demonstrated an abnormally high porosity in the deep burial coarse-grained strata [13]. Furthermore, the recent work of Kra et al. [4] has revealed that the secondary dissolution pore is a significant controlling factor of good reservoir formation. However, the type of pores and their formation mechanisms are still unclear. Furthermore, the authors demonstrated that the reservoir quality in the progradation is moderately better than the retrogradation. However, no comparison has been made within the whole steep slope belt in the study area. Thus, determining the elaborated porosity is crucial to understand the genesis of good reservoirs in deeply buried coarse-grained sediments. Thus, this paper discusses the fundamental question: what are the elaborated pores related to high-quality reservoir establishment? We address the criteria of pore type in the conglomerate reservoir in the study area. We compare the different fans based on the petrophysical properties and diagenesis characteristics to understand a better place for a high-quality reservoir distribution.

2. Geological Background

The Dongying is a Cenozoic subbasin located in the Bohai Bay Basin (Figure 1A) [3,14]. The Dongying subdepression [15] comprises four subsags: the Lijin area, the Minfeng Sag, the Boxing Subsag, and the Niuzhuang Subsag [3,16,17] (Figure 1B). The structural evolution of the Dongying Depression comprises the syn-rift period (65 to 24.6 Ma) and post-rift stage (24.6 to the present) (Figure 1E) [16,18]. From north to south, the Basin is divided into five tectonic zones: the steep northern slope area, the Minfeng, the central anticline zone, the Niuzhang zone, and the gentle southern slope area (Figure 1D) [3,15,18,19]. The Dongying Subsag is bounded by the Chenjiazhuang Uplift (North), the Qingtuozi Uplift (East), the Binxian and Qingcheng uplifts (West), and the Luxi Uplift (South) [3,18] (Figure 1B,C). The Dongying is one of China’s most prominent hydrocarbon-rich basins, covering an area of 5700 km2 [3].
The formations in the Dongying Depression include the Kongdian (Ek), the Shahejie Formation (Es), Quaternary Pinyuan (Qp), the Minghuanzhen (Mm), Dongying (Ed), and the Neogene Guantao (Ng), (Figure 1E) [20,21]. The Es comprises four members, including the Es1, Es2, Es3, and Es4 (Figure 1E). The depositional strata in the north of the Dongying Depression are dominated by conglomerates, pebbly sandstones, and mudstone [22].

3. Materials and Methods

The database for this study is provided by Shengly oilfield company and comprises 300 m cores, 3D seismic, lithology, wireline, and 1200 petrophysics data from 40 wells from two areas in the northern Dongying subdepression.
The seismic sections were used to define the stratigraphy units. The lithofacies are determined by analyzing the core data set’s rock texture, color, and sedimentary structures. The combination of lithofacies was based on scrutinizing the lithofacies’ thicknesses, types, and vertical patterns. RESFORM software was utilized to determine sedimentary facies and their repartition based on lithology data and wireline. A total of 104 thin sections representing all the lithofacies were made with blue resin for pore type and petrography analysis. Fifty thin polished sections were stained with Alizarin (red) and K-ferricyanide solution to discriminate carbonate cement. Forty gold-coated samples covering all the lithofacies were made to analyze authigenic cements and their spatial association. The experiment was carried out with a ZEISS device connected to an energy dispersive X-ray (EDX) spectrometry at a current of 1.0–1.5 nA and 20 kV. Cathodoluminescence pictures were utilized to identify carbonate minerals and mineral growth phases. The cathodoluminescence images were obtained using LEICA-DM4P microscopy running at 10 kV, and 250 mA on twenty thin sections. A total of 400 to 500 points per thin section were counted to quantify the proportion of rock components and pore types. The petrographical analysis was achieved with the LEICA device linked to a camera for picture obtention. Point counting results were used to gauge the COPL (compaction porosity loss), CEPL (porosity loss by cementation), and the Icp (compaction index), utilizing the equations of Ehrenberg [23].
COPL = OP−((100 × IVG)−(OP × IGV))/((OP−IGV))
CEPL = (OP−COPL) × CEM/IGV
OP = estimated initial porosity.
CEM = volume of intergranular cement.
IGV = intergranular porosity + matrix + cement content [24].
Icp is obtained by applying the equation of Lundegard, 1992 [25].
Icp = COPL/(COPL + CEPL)
where (0 ≤ Icp ≤ 1).
If Icp ≥ 0.5, the mechanical compaction represents the cardinal process in porosity reduction. If Icp ≤ 0.5, cementation represents the key cause in porosity reduction. If Icp = 1, the total porosity destruction is caused by compaction. If Icp = 0, all the porosity reduction is due to cementation.

4. Results

4.1. Depositional Sets

Based on the seismic section, well-log, and lithofacies interpretation, the depositional sets consist of two systems: the transgressive system tract (TST: characterized by retrogradation sequences) and the highstand system tract (HST: characterized by progradational sequences) (Figure 2). The SP and the GR curves of the retrogradational sequences are bell-shaped, and the sand body is thinner upward. However, the spontaneous potential presents as funnel-shaped in progradation sequences, representing the sand body’s upward coarseness (Figure 2). The study area has experienced many complete cycles, and each cycle has undergone the process of lake level fluctuation (falling and rising) and the adjacent fault activities. There are four types of combinations in the SP and GR curves in the study area (Figure 3): (1) Bell-shaped + funnel-shaped combination, (2) Bell-shaped + bell-shaped combination, (3) Funnel + bell-shape combination, (4) Funnel + funnel type combination. The retrogradational and progradational sequences are identified based on the different angles of the seismic profiles.

4.2. Rock Composition

The lithofacies in the study are mostly coarse-grained, subangular to subrounded, and poorly to moderately sorted. The rock types are dominantly felspathic litharenite, lithic arkose, and arkose (Figure 4A). The rock’s composition is dominated by rock fragments, feldspar, and quartz in the study area. The rock fragment in the study Es4 interval comprises sedimentary rock fragments (Figure 5A–C), igneous rock fragments (Figure 5D–E), and metamorphic rock fragments (Figure 5G–I). The Es4s in the Lijin Sag present many sources based on the SRF-IRF-MRF diagram (Figure 4B). The progradation sequence (HST) is only dominated by igneous rock fragments. On the contrary, the retrogradation sequences are mainly dominated by igneous rock fragments and metamorphic rock fragments (Figure 4C), suggesting different sedimentation sources during the deposit. In the Yanjia area, the Es4s of both sequences are dominated by metamorphic rock fragments and igneous rock fragments (Figure 4D). However, the sediments are dominated mainly by metamorphic rock fragments (Figure 4D). This classification indicates different sources of sediment during deposition. In the Es4x, the retrogradation and progradation sequences are majority dominated by igneous fragments.

4.3. Petrophysical Properties

The investigation of the conglomerate of the Es4s and the Es4x in the research area reveals a large range of permeability and porosity. The porosity of the Es4s ranges from 0.1 to 17.4 % (average, 8.21%) in the Lijin area and from 1.07 to 14.54 % (average of 7.89%). The permeability varies from 0.01 to 457.6 mD (average, 10.59 mD) in the Lijin area, while it ranges from 0.01 to 49.71 mD in the Yanjia area with an average of 8.95 mD. The permeability and the porosity of the Es4x in the Yanjia zone range from 0.06 to 13.05 mD (average, 2.31 mD) and from 1.8 to 11.5% (average, 7.29%), respectively (Table 1).

4.4. Pore Characteristics

The thin section and SEM observation reveal three genetic pores, including primary pores (Figure 6A), secondary pores (Figure 6A–I), and one mixed pore (Figure 6B). The main pores are intergranular (Figure 6A–C), intragranular (Figure F–I), fracture (Figure 6D,E), moldic, and intercrystalline (Figure 6I). The mixed pore is a combination of primary and secondary pores. These pores are generally intergranular and mostly occur in the study area. The secondary pores come from the dissolution of rock fragments, feldspar, and carbonate cement.

4.5. Diagenetic Events

4.5.1. Compaction

The burial depth of the Es4 of the study interval varies from 2000 to 5000 m. Therefore, mechanical compaction remains dominant in the study interval. Mechanical compaction is patent by point contact (Figure 7A), line contact (Figure 7B), concave/convex contacts (Figure 7B), squeezing and bending of flexible fragments in the rigid grains (Figure 7C), and grain fracturing (Figure 7D).

4.5.2. Authigenic Cement

Carbonate cement is the furthermost abundant cement in the study area. The carbonate cement comprises dolomite, calcite, ferrocalcite, and ankerite. The carbonate cement occurs as poikilitic pore filling and grain replacement (Figure 7E–H). Calcite appears as poikilotopic blocky in intergranular pores. Cathodoluminescence and thin-section analysis show that ferrocalcite engulfs calcite (Figure 7E). Dolomite appears as rhombohedral crystals (Figure 7F), and micritic dolomite fills large primary pores (Figure 7I). Dolomite is surrounded by calcite, ankerite, and ferrocalcite (Figure 7H). Ferrocalcite and ankerite are widely encountered in the research interval and appear as scattered sparry, clusters, and rhomb crystals (Figure 7G,H).
Anhydrite and gypsum cement are encountered in the Es4x sandy conglomerates (Figure 7I). Gypsum and anhydrite cements mainly occur in pores as blocky, elongated, and lath-shaped crystals. Gypsum is commonly replaced by anhydrite and dolomite (Figure 7I).
Quartz overgrowth appears as syntaxial cement around detrital quartz grains (Figure 7J,L) or quartz crystal (Figure 7J,K). Based on thin-section observation, kaolinite is few in the Es4x in the study area and abundant in the Es4s. Illite is the more significant clay mineral in the Es4x. Illite occurs as fibrous crystals within intergranular pores (Figure 7K). Authigenic illite commonly replaces feldspar particles and mica. Pyrite cement is present in both sequences and mainly occurs as framboids in intergranular and intragranular pores (Figure 7L). Pyrite replaces detrital grains, gypsum, anhydrite, and carbonate cement (Figure 7L).

4.5.3. Dissolution

The secondary pores are new pores created from leached framework grains, cements, and depositional matrix, accompanied by the withdrawal of the by-products from the sediment. Dissolution of feldspar grains, lithic fragments, and cement was observed in the progradation and retrogradation sequences. Secondary porosities are created by incomplete or entire leached feldspar (Figure 7M), lithic grains (Figure 7N), carbonate cement (Figure 7O), mica, and matrix.

5. Discussion

5.1. Diagenesis Sequence

The diagenesis sequences comprise the eodiagenetic and mesodiagenetic parasequences. The eodiagenesis parasequences englobe the diagenetic events, which occur at a temperature less than 70 °C and a depth of less than 2000 km. However, the mesodiagenesis events appear at a depth of more than 2000 km and a temperature greater than 70 °C. Eodiagenetic events in the Es4 comprise mechanical compaction, dolomite, calcite, pyrite, anhydrate, and gypsum precipitation. The mesogenetic parasequence comprises mechanical compaction, feldspar dissolution, quartz overgrowth, anhydrite precipitation, ferroan calcite cementation, ankerite precipitation, pyrite, kaolinite, illite, and chlorite.

5.2. Recognition of Pore in the Es4

5.2.1. Recognition of Primary Pore in the Es4

The primary pore originates before or at the final deposition [26]. The primary pores in the study area are mainly intergranular, isolated, and less common (Figure 8). After intensive compaction, the initial porosity has changed drastically from macropore to mesopore and micropore (Figure 8). The shape also has changed into triangular (Figure 8A,D) and irregular (Figure 8B,C). The primary macropore appears rarely. The content of primary pores presents an average of less than 2% in the study areas (Figure 9). However, the content of the primary pore is higher in the Es4s, especially in the Lijin and Yanjia areas, respectively (Figure 9). The Es4x presents the lower percentage of primary pore in the northern Dongying subdepression (Figure 9).

5.2.2. Secondary Pore Recognition and Characteristics

Secondary porosity plays an important role in reservoir quality because it can improve the reservoir quality and modify the original composition by grain dissolution [10,26,27,28]. The secondary porosity identification in the present work is based on the formation mechanism and characteristics of the pore type. The development of the pore system in both progradation and retrogradation sequences is similar. However, the percentage of dissolution in the progradational sequences is moderately higher than in the retrogradation sequences. The secondary pore consists of several pore systems grouped into five main pore types: interparticle pores (Figure 6A), intraparticle pores (Figure 6G–D), fractures (Figure 6D,E), moldic pores, and intracrystal pore (Figure 6I). Each pore type can be subdivided into several pore types according to their occurrence and characteristic. The intergranular secondary pores are created by the leaching of cement and depositional matrix [26,28]. However, in this study, the intergranular pore is often formed by the partial dissolution of the framework grain, leading to the widening of the existing intergranular pore network (Figure 10A). This type of pore can be a pure secondary pore or an enlarged mixed pore. An enlarged mixed pore in this study resolves from the widening of the primary pore due to the partial dissolution of the adjacent grain edges (Figure 10C–F). This pore type is more common in the study area, particularly in the distributary (Fan delta) and braided channels (Nearshore subaqueous fan).
According to their genetic mechanism, the fracture pore consists of the enlarged fracture pore (Figure 6E), rock fracture pore, and grain fracture (Figure 6D,E). A rock fracture is a fracture pore that cuts through several particles or the whole rock. This type of fracture is probably caused by a tectonic event. Grain fracture is a pore that occurs in individual particles. An enlarged fracture pore is formed by overburden pressure in conjunction with dissolution (Figure 6E). This fracture occurs even in individuals or several grains. These types of fracture pores are important pathways for fluid circulation and can be responsible for creating secondary dissolution porosity. The intragranular pores are frequent in the study interval and sometimes are connected to intergranular pores (Figure 6B). Moldic pores are made by the total dissolution of grain. They are common in the progradation and can be recognized by the remains of clay rims (Figure 10A). The intercrystalline pore in the Lijin Sag mainly occurs in kaolinite cement (Figure 6I). However, in the Yanjia area, the carbonate cement also presents some micropores. The secondary porosity in the progradation is dominated by the enlarged intergranular pore. The thin-section observation shows that the enlarged pores are mostly interconnected with each other in the progradation (Figure 10A). In addition, the intragranular pores are also connected to the intergranular pores (Figure 10B). The conjunction of both pores positively affects the reservoir quality in the progradation sequence, mainly in the braided channel facies.
Furthermore, the progradation contains more grain fracture and enlarged pores, which is important for fluid circulation in sediment. Therefore, the progradation presents a good reservoir. On the contrary, the enlarged pore and interconnected intragranular-intergranular pores are less common in retrogradation. The enlarged pore size is not greater than the progradation sequence. Consequently, the reservoir is less good than the progradation sequence.

5.3. Classification of Pore in Es4 Interval

The classification of the porosity in the Es4 is presented in Figure 11 and Table 2. The classification of the conglomerate reservoir pores in the northern Dongying subdepression is summarized based on the genesis and features of the pore type in both retrogradation and progradation sequences. The Es4 comprises three genetic pores: primary, secondary, and mixed pores. The secondary pore consists of intergranular (Figure 11A–C), fracture pores (Figure 11D–G), intragranular (Figure 11H,I), moldic pores (Figure 11J), and intracrystal (Figure 11K,L). The intergranular secondary dissolution pore comprises the enlarged, intergranular normal secondary pore (cement dissolution). The fracture pore consists of rock fracture pores, grain fracture pores, and enlarged fracture pores (Figure 11D–G). Among the pores type, the intergranular and enlarged pores are the main contributors to good reservoir quality.

5.4. Comparison of Secondary Pore between the Es4s and Es4x

Secondary dissolution pores play a significant role in establishing high-quality reservoirs in the study area. The secondary dissolution has enhanced the reservoir quality. However, the dissolution percentage varies in the different sequences, members, and areas (Figure 12). The secondary porosity in the HST is generally greater than the retrogradation sequences, except for the HST in the Lijin area (Figure 12A), probably due to the limited samples. The retrogradation sequences often present a lower porosity in all the study areas (Figure 12A). Similar results have been demonstrated by Al-Ramadan and El-Khoriby, [29] in the Adedia Formation. The secondary dissolution porosity is higher in the Lijin and Yanjia areas than in the lower Es4x (Figure 12B,C). Figure 12D shows that the dissolution porosity is more significant in the Es4s than the Es4x.

5.5. Comparison of the Es4s and Es4x Conglomerate Reservoirs Based on the Physical Properties

Good reservoir quality means good petrophysical properties, including porosity and permeability. The cross plot of the repartition of the porosity and permeability also indicates that the upper part of Es4s presents good reservoir quality than the lower part (Figure 13). From this evidence, it is clear that the Es4s conglomerate reservoirs are better than the reservoir in the Es4x. The good porosity observed in the Es4s is due to many types of dissolution pores, especially the enlarged pores. The presence of this type of pores, which are connected, is probably the cause of the good reservoir quality. Moreover, the enlarged and fracture pore suggests better fluid circulation in Es4s than in the Es4x. Therefore, it is probable that the Es4s formations are deposited in a relatively open and semi-closed system. This can be explained by the presence of red sandstone, reddish mudstone, and plant debris observed in Es4s in the Lijin area, suggesting a local exposition of the sediments. Therefore, meteoric water intrusion can be one of the causes of the enlarged pore resulting in a good reservoir in the Es4s. On the contrary, the Es4x is probably deposited in a semi-closed to a closed system.

5.6. Comparison of Compaction and Cementation Effects in the Es4 Member

The COPL is generally high in the progradation sequences and low in the retrogradation sequences in the two sections. However, the COPL increase in the HST from Lijin to Yanjia indicates that the porosity loss due to compaction is higher in the Yanjia area (Figure 14A). In both areas, the Icp is generally greater than 5, suggesting that compaction remains the leading factor degrading the reservoir quality (Figure 14B).
Moreover, the low value of Icp encountered in the Es4x (mainly in the TST) indicates that cementation is the major factor in porosity reduction (Figure 14B–D). The range of the clay matrix is more significant in the Es4s in the Lijin area and Es4x (Figure 14). The lower clay matrix is observed in the Es4s in the Yanjia area (Figure 15A). This lower clay matrix is due to the withdrawal of a part of the clay in the upper Es4s member. However, the high clay matrix content in the Es4x reduces porosity (Figure 15B). Moreover, thin-section observation has shown that authigenic kaolinite is more abundant in the Es4s and few or inexistent in the Es4x. The clay content in the lower part is dominantly illite and chlorite clays.

6. Conclusions

The dissolution is essential to developing good reservoirs in the Es4 formation of the Dongying subbasin. The dissolution porosity related to the high-quality formation in the Es4 comprises several types: intergranular pore, intragranular pore, intercrystalline pore, and moldic pores. Three types of pores, including primary pores, secondary and mixed pores, are developed in the study interval. The intergranular secondary pores encompass the enlarged pore and intergranular normal secondary pores. The fracture pores comprise rock fracture pores, grain fracture pores, and enlarged fractures. The comparison of the upper part and lower part of the Es4 revealed that the Es4s have good reservoir quality compared to the Es4x. Mechanical compaction is the primary factor reducing porosity in the Es4 interval. However, cementation greatly influences porosity, especially in the retrogradation sequence in the Es4x interval. The compaction index (Icp) indicates that compaction is the major factor in porosity reduction in the Es4s and progradation sequence in the Es4x. However, cementation stands as the leading factor in porosity reduction in the retrogradation sequence in the Es4x. The clay matrix is higher in the Es4s compared to the Es4x. The Es4s reservoirs are probably deposited in an open to semi-closed system, while the Es4x is in the semi-closed to a closed system. This study demonstrated that for oil exploration in the fourth member of the Shahejie Formation, the Es4s should be prioritized.

Author Contributions

Conceptualization, K.L.K. and L.Q.; methodology, K.L.K.; software, K.L.K. and K.K.A.D.; validation, Y.Y., B.Y.; formal analysis, K.L.K.; investigation, K.L.K. and Y.Y.; resources, L.Q.; data curation, Y.H.; writing—original draft preparation, K.L.K.; writing—review and editing, K.L.K., Y.Y. and K.S.A.; visualization, Y.Q.; supervision, L.Q.; project administration, Y.Y.; funding acquisition, L.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Natural Science Foundation of China (No.441972099).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Localization map of the Bohai Bay Basin, East China. (B) Dongying Depression localization map exposing the study field and uplift. (C) Stratigraphy columns of the Dongying Depression revealing the tectonic evolution. (D) Map of the Lijin area showing the study wells. (E) Map of the Yanjia area showing the study wells. (F) Dongying Basin showing the tectonic units and the stratigraphy divisions.
Figure 1. (A) Localization map of the Bohai Bay Basin, East China. (B) Dongying Depression localization map exposing the study field and uplift. (C) Stratigraphy columns of the Dongying Depression revealing the tectonic evolution. (D) Map of the Lijin area showing the study wells. (E) Map of the Yanjia area showing the study wells. (F) Dongying Basin showing the tectonic units and the stratigraphy divisions.
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Figure 2. Well-log curves and system tract of the study area (A,C), interpreted seismic section (B) and sketch (B′) showing the sediment bodies’ internal features of the TST (retrogradation sequence) and HST (progradation sequence), the TST is differentiated by a retrogradation and the HST by a progradation sets.
Figure 2. Well-log curves and system tract of the study area (A,C), interpreted seismic section (B) and sketch (B′) showing the sediment bodies’ internal features of the TST (retrogradation sequence) and HST (progradation sequence), the TST is differentiated by a retrogradation and the HST by a progradation sets.
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Figure 3. Conglomerate facies response on wireline in the Lijin Sag.
Figure 3. Conglomerate facies response on wireline in the Lijin Sag.
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Figure 4. (A) QLF diagram of the Es4 interval in the northern Dongying Depression. (B) SRF-IRF-MRF ternary diagram of the Es4s (Lijin area). (C) SRF-IRF-MRF ternary diagram of the Es4s (Yanjia zone). (D) SRF-IRF-MRF ternary diagram of the Es4x (Yanjia area). IRF-Igneous rock fragments, SRF-Sedimentary rock fragments, MRF-Metamorphic rock fragments.
Figure 4. (A) QLF diagram of the Es4 interval in the northern Dongying Depression. (B) SRF-IRF-MRF ternary diagram of the Es4s (Lijin area). (C) SRF-IRF-MRF ternary diagram of the Es4s (Yanjia zone). (D) SRF-IRF-MRF ternary diagram of the Es4x (Yanjia area). IRF-Igneous rock fragments, SRF-Sedimentary rock fragments, MRF-Metamorphic rock fragments.
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Figure 5. Type of rock fragments in the Es4. (A,B) Sedimentary rock fragments, (D–F) Metamorphic rock fragments, (G–I) Igneous rock fragments.
Figure 5. Type of rock fragments in the Es4. (A,B) Sedimentary rock fragments, (D–F) Metamorphic rock fragments, (G–I) Igneous rock fragments.
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Figure 6. Pore features in the Es4, (A) primary pore, secondary pore, (B,C) enlarged pore, (D) rock fracture pore, grain fracture pore, (E) rock fracture pore, (F–H) intraparticle pore, (I) intercrystalline pore. pore PP-primary pore, IP-intergranular pore, IntraGP-intraparticle pore, FP-fracture pore, EP-enlarged pore, FP-fracture pore, RFP- rock fracture pore, Icp-intercrystalline pore.
Figure 6. Pore features in the Es4, (A) primary pore, secondary pore, (B,C) enlarged pore, (D) rock fracture pore, grain fracture pore, (E) rock fracture pore, (F–H) intraparticle pore, (I) intercrystalline pore. pore PP-primary pore, IP-intergranular pore, IntraGP-intraparticle pore, FP-fracture pore, EP-enlarged pore, FP-fracture pore, RFP- rock fracture pore, Icp-intercrystalline pore.
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Figure 7. Diagenetic feature in the Es4 in the northern Dongying Depression. (A) Point and line contacts; (B) concave/convex contacts; (C) bending of labile fragments; (D) microfracture; (E) calcite and ferro-calcite, note that nonferroan calcite is submerged by ferroan calcite; (F) dolomite; (G) ankerite and ferroan calcite; (H) calcite and dolomite are engulfed by ferroan calcite and ankerite; (I) anhydrite and gypsum filling primary pore; (J) kaolinite, quartz cement filling the pore; (K) quartz crystal surrounded by illite; (L) pyrite, quartz overgrowth, and anhydrite; (M) feldspar dissolution; (N) carbonate rock fragment dissolution; (O) carbonate cement dissolution.
Figure 7. Diagenetic feature in the Es4 in the northern Dongying Depression. (A) Point and line contacts; (B) concave/convex contacts; (C) bending of labile fragments; (D) microfracture; (E) calcite and ferro-calcite, note that nonferroan calcite is submerged by ferroan calcite; (F) dolomite; (G) ankerite and ferroan calcite; (H) calcite and dolomite are engulfed by ferroan calcite and ankerite; (I) anhydrite and gypsum filling primary pore; (J) kaolinite, quartz cement filling the pore; (K) quartz crystal surrounded by illite; (L) pyrite, quartz overgrowth, and anhydrite; (M) feldspar dissolution; (N) carbonate rock fragment dissolution; (O) carbonate cement dissolution.
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Figure 8. Primary pore in the Es4. (A) Triangular primary pore, (B–D) Irregular primary pores.
Figure 8. Primary pore in the Es4. (A) Triangular primary pore, (B–D) Irregular primary pores.
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Figure 9. Range of primary pore in the Es4 in the northern Dongying depression.
Figure 9. Range of primary pore in the Es4 in the northern Dongying depression.
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Figure 10. Secondary pore development. (A) Remaining clay rims, (B) interconnected intragranular and intergranular pores, (C,E) primary pore during deposition, (D,F) enlarged mixed pore after dissolution. PP-primary pore, EP-enlarged pore, EMP-enlarged mixed pore.
Figure 10. Secondary pore development. (A) Remaining clay rims, (B) interconnected intragranular and intergranular pores, (C,E) primary pore during deposition, (D,F) enlarged mixed pore after dissolution. PP-primary pore, EP-enlarged pore, EMP-enlarged mixed pore.
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Figure 11. Schematic figures of the main type of pore in the Es4 of the northern Dongying Depression.
Figure 11. Schematic figures of the main type of pore in the Es4 of the northern Dongying Depression.
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Figure 12. Range of secondary porosity split by sequence stratigraphy and formation members. (A) Secondary porosity split by HST and TST, (B) Secondary porosity split by study areas and Formation Member, (C) Secondary dissolution porosity split by study areas and Formation, (D) Secondary dissolution porosity in the Es4s and Es4x.
Figure 12. Range of secondary porosity split by sequence stratigraphy and formation members. (A) Secondary porosity split by HST and TST, (B) Secondary porosity split by study areas and Formation Member, (C) Secondary dissolution porosity split by study areas and Formation, (D) Secondary dissolution porosity in the Es4s and Es4x.
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Figure 13. Cross plot of the porosity and permeability split by Formation and different study areas. (A) Porosity split by Formation Member and area, (B) Permeability split by Formation Member and study area.
Figure 13. Cross plot of the porosity and permeability split by Formation and different study areas. (A) Porosity split by Formation Member and area, (B) Permeability split by Formation Member and study area.
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Figure 14. Cross plot of the COPL (A), compaction index (B), cement (C), and CEPL (D) split by sequence stratigraphic of the different formations of the Es4 member.
Figure 14. Cross plot of the COPL (A), compaction index (B), cement (C), and CEPL (D) split by sequence stratigraphic of the different formations of the Es4 member.
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Figure 15. Comparison plot of the clay matrix content in the Es4 members. (A) Clay-matrix content split by Formation Member and study areas, (B) Clay-matrix split by Member in the study area.
Figure 15. Comparison plot of the clay matrix content in the Es4 members. (A) Clay-matrix content split by Formation Member and study areas, (B) Clay-matrix split by Member in the study area.
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Table
Table 1. Porosity and permeability of the Es4s and Es4x in the study area.
Table 1. Porosity and permeability of the Es4s and Es4x in the study area.
Porosity (%)Permeability (mD)
MemberEs4s (Lijin)Es4s (Yanjia)Es4x (Yanjia)Es4s (Lijin)Es4s (Yanjia)Es4x (Yanjia)
Min0.101.071.800.010.010.06
Max17.4014.5411.50457.6049.7113.05
Average8.217.897.2910.598.952.31
Table
Table 2. Classification of porosity in conglomerate.
Table 2. Classification of porosity in conglomerate.
Pore TypeFrequencyDistribution AreaEffect on the Reservoir Quality
HSTTSTHSTTST
Primary poreIntergranularLow and isolatedYesYessmall to moderatemoderate
intragranularnonNoNoNoNo
SecondaryIntergranular poremoderate to largeYesYesmoderate to largeModerate
Enlarged poremoderate to largeYesYesmoderate to largeModerate
Matrix clay dissolution poremoderateYesYessmall to moderatevery small
Grain fracture porecommonYesYessmall to moderatesmall to moderate
Grain-enlarged fracture poreLow to moderate and isolatedYesYessmall to moderatesmall to moderate
Rock fracture poreLow and isolatedYesYessmallsmall
Rock enlarged fracture poreLow and isolatedYesYessmall to moderatesmall
Intragranular poremoderate to largeYesYessmall to moderatesmall to moderate
mixed intragranular-intergranular poreLow to moderateYesYessmall to moderatesmall
Moldic poreLow to moderateYesYessmallsmall
Kaolinite intercrystal porelow to moderateYesYessmall to moderatesmall
Carbonate cement intercrystal porelowYesYessmallsmall
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Yang, Y.; Kra, K.L.; Qiu, L.; Yang, B.; Ahmed, K.S.; Donald, K.K.A.; Qiao, Y.; Habulashenmu, Y. Elaborated Porosity and High-Quality Reservoirs in Deeply Buried Coarse-Grained Sediment: Insight of the Sublacustrine Sandy Conglomerates in the Eocene Shehejie Formation (Es4), Dongying Subbasin, Bohai Bay Basin, China. Appl. Sci. 2023, 13, 1347. https://doi.org/10.3390/app13031347

AMA Style

Yang Y, Kra KL, Qiu L, Yang B, Ahmed KS, Donald KKA, Qiao Y, Habulashenmu Y. Elaborated Porosity and High-Quality Reservoirs in Deeply Buried Coarse-Grained Sediment: Insight of the Sublacustrine Sandy Conglomerates in the Eocene Shehejie Formation (Es4), Dongying Subbasin, Bohai Bay Basin, China. Applied Sciences. 2023; 13(3):1347. https://doi.org/10.3390/app13031347

Chicago/Turabian Style

Yang, Yongqiang, Kouassi Louis Kra, Longwei Qiu, Baoliang Yang, Kabir Shola Ahmed, Koua Kadio Aka Donald, Yupeng Qiao, and Yerejiepu Habulashenmu. 2023. "Elaborated Porosity and High-Quality Reservoirs in Deeply Buried Coarse-Grained Sediment: Insight of the Sublacustrine Sandy Conglomerates in the Eocene Shehejie Formation (Es4), Dongying Subbasin, Bohai Bay Basin, China" Applied Sciences 13, no. 3: 1347. https://doi.org/10.3390/app13031347

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