The Effect of Pre–Triassic Unconformity on a Hydrocarbon Reservoir: A Case Study from the Eastern Mahu Area, Northwestern Junggar Basin, China

Abstract

Unconformities are of significant interest to petroleum geologists because of their crucial roles in influencing reservoir quality and controlling oil and gas migration. This study investigates the impact of unconformities on a reservoir within a prolific oil–gas-bearing zone between the Middle Permian and Lower Triassic strata in the northwestern Junggar Basin, utilizing thin sections, well logging data, seismic profiles, and geochemical analyses. The results reveal a well-developed three-layer unconformity structure characterized by a thick weathered clay layer, which acts as an effective caprock for hydrocarbons. The diagenetic evolution of the Lower Wuerhe Formation in the northwestern Junggar Basin consists of an initial stage of compaction followed by a subsequent stage of dissolution and cementation. Four key factors, including low argillaceous content in sandstone and conglomerate, diagenetic compaction, zeolite dissolution and cementation, and clay mineral infill, have played a crucial role in influencing the reservoir characteristics of the Lower Wuerhe Formation. In addition, the development of unconformities promotes atmospheric freshwater leaching, which enhances the dissolution of the underlying reservoir while developing an extensive network of strike-slip faults that improve connectivity within hydrocarbon reservoirs. This process facilitates both vertical and lateral migration of hydrocarbons along hard rock layers, which allows the unconformity to breach into the overlying conglomerate reservoirs. The results of this study suggest that the reservoir in proximity to the unconformity surface often exhibits high porosity and rich hydrocarbon content, offering valuable insights for future oil and gas exploration and development.

1. Introduction

Unconformity refers to an erosional interface found within stratigraphic successions [1,2,3,4]. This interface is crucial for identifying and subdividing stratigraphic sequences [5,6,7] and is significant for paleoenvironmental and paleogeographic reconstructions, as well as for assessing petroleum reservoirs [7,8,9,10,11]. Increasingly, industry professionals have recognized that unconformities act as active zones for the migration of ore-bearing or hydrocarbon-bearing fluids [12,13,14,15,16]. This understanding arises from the observation that many important oil and gas fields, such as the Mahu and Karmay oil fields in the Junggar Basin [17], the Tazhong oil and gas fields in the Tarim Basin [18,19], the Anyue gas field in the Sichuan Basin [20], and the Jingbian gas field in the Ordos Basin [21], are influenced by stratigraphic unconformities.

The effects of unconformities on hydrocarbon reservoirs can be seen in two primary aspects. First, petroleum reservoirs located beneath unconformities often undergo modifications due to weathering and leaching processes, resulting in the formation of secondary zones with high porosity and permeability [11,22,23]. Second, unconformities can act as effective pathways for hydrocarbon migration, potentially forming stratigraphic reservoirs under suitable conditions [24,25,26,27]. In hydrocarbon exploration, an unconformity is more than just a surface; it represents a geological body with a three-layer structure [16,27,28,29,30,31,32]. The upper layer typically consists of a basal conglomerate or a transgressive sedimentary body, primarily composed of conglomerate, sandstone, and mudstone. The middle layer, known as the weathered clay layer, can act as an effective caprock [33,34,35]. The lower layer, referred to as the semi-weathered layer or semi-weathered leaching zone, includes lithologies such as carbonate, metamorphic, volcanic, and clastic rocks. Variations in the thickness of the weathered clay layer and the underlying semi-weathered rock zones, influenced by paleomorphology and tectonic uplift, can affect hydrocarbon sealing and reservoir quality. Currently, the vertical structure of the unconformity is delineated using conventional logging data [28,36]. However, this method mainly relies on qualitative research, making it challenging to determine the precise depth of leached rock from a single logging curve. Therefore, utilizing multiple logging curves is essential for a comprehensive and systematic identification of the thickness of the leached rock [16].

The Junggar Basin is a superimposed basin in China that has experienced multiple phases of tectonic deformation, characterized by the development of a series of unconformities [37,38]. The Mahu area, which includes the Mahu Sag and its surrounding uplifts in the northwestern Junggar Basin, is a major petroleum-producing region. Previous investigations have identified four key unconformities: (1) the unconformity between the Carboniferous and Permian (C/P), (2) the unconformity between the Fengcheng and Xiazijie Formations (P₁f/P₂x), (3) the unconformity between the Lower Wuerhe and Upper Wuerhe–Baikouquan Formations (P₂w/P₃w-T₁b), and (4) the unconformity between the Upper Wuerhe and Baikouquan Formations (P₃w/T₁b) [39,40,41,42]. Among these, the unconformity above the Permian Fengcheng Formation (P₁f/P₂x) in the Wuerhe oil field of the Mahu Sag have been studied [42]. The bottom unconformity of the Upper Wuerhe Formation in the Zhongguai area has also been investigated, focusing primarily on the quantitative evaluation of its transporting capacity [16]. Wang et al. (2019) examined the pre-Triassic unconformity and its influence on hydrocarbon migration within the Wuerhe–Fengnan area, located on the northwestern margin of the Mahu area [27]. However, there have been limited studies of the pre-Triassic unconformity in the eastern part of the Mahu area.

The pre-Triassic unconformities in the northwestern Junggar Basin display two segmentation unconformities in the southern region and a superimposed unconformity in the northern Mahu area [41]. These features arose from significant tectonic compression that caused tectonic inversion during the Late Permian–Early Triassic period [43,44]. Consequently, the Upper Wuerhe Formation is present only in the southern part of the Mahu area, while it is absent in the north. This unique geological setting offers an opportunity to investigate the three-layer structure of segmentation and superimposed unconformities, as well as their effects on the underlying reservoirs, thereby enhancing our understanding of oil and gas accumulation in previously unexplored areas.

This study aims to assess the impact of the pre-Triassic unconformity on a reservoir in the eastern Mahu area. A total of eight well logging datasets were used to determine the logging response characteristic of the unconformity structure. These characteristics were then applied to 104 wells to delineate the thickness of the three-layer structure of the unconformities across the entire Mahu area. The resulting thickness maps were comprehensively examined to determine the sealing range of the weathered clay layer for hydrocarbons. Ultimately, we aim to establish the relationship between different types of unconformity structures and oil accumulation. Furthermore, through petrological characterization, physical property analysis, and comprehensive sedimentology analysis, we evaluate how the pre-Triassic unconformity influences the underlying reservoirs. Our results provide strategic insights for petroleum companies regarding the evaluation of deep reservoir quality and inform future exploration plans.

2. Geological Setting

The Junggar Basin, the second largest sedimentary basin in China, has a triangular shape and covers approximately 134,000 km² [17]. It is surrounded by the Zaier and Hala’alate Mountains to the northwest, the Aletay and Kelameili Mountains to the northeast, and the Tianshan and Bogda Mountains to the south (Figure 1a). Tectonically, the Junggar Basin can be subdivided into seven first-class structural units: the Central Depression, Luliang Uplift, Wulungu Depression, Eastern Uplift, Western Uplift, and North Tianshan Fault Zone, alongside 41 s-class structural units (Figure 1a) [40]. The strata of the basin, in ascending order, consist of the Carboniferous, Middle–Lower Permian, Upper Permian–Triassic, Middle–Lower Jurassic Badaowan Formation–Xishanyao Formation, Toutunhe Formation–Kalazha Formation, Cretaceous System, Paleogene System, and Neogene System. All strata are in unconformable contact with each other [41].

The Mahu Sag is located in the northwestern Junggar Basin (Figure 1). The Permian stratigraphic units, in ascending order, consist of the Jiamuhe (P₁j), Fengcheng (P₁f), Xiazijie (P₂x), Lower Wuerhe (P₂w), and Upper Wuerhe (P₃w) Formations. The Triassic stratigraphic sequences, in ascending order, include the Baikouquan (T₁b), Kelamayi (T₂k), and Baijiantan (T₃b) Formations (Figure 2). The Middle Permian was an extensional tectonic period during which large-scale volcanic activity occurred in the basin [17]. In the western part of the Junggar Basin, the Middle Permian Xiazijie Formation and the Lower Wuerhe Formation are unconformably overlaid on the Lower Permian strata (Figure 2), with some layers also overlapping the Carboniferous strata. The Xiazijie Formation mainly consists of gray and brownish sand–mudstone. The distribution of the Lower Wuerhe Formation is more extensive than that of the Xiazijie Formation, with a thickness of approximately 500–1600 m in the Mahu Sag, primarily composed of conglomerate and gray mudstone (Figure 2). Mudstone layers within the depression are relatively thick, ranging from 200 to 800 m, making them important source rock strata [45]. Before the deposition of the Upper Permian Upper Wuerhe Formation, the Junggar Basin experienced a strong tectonic event, leading to the uplift and erosion [40,46]. The northern part of the Mahu area underwent overall uplift and erosion, resulting in the Upper Wuerhe Formation developing only in the southern region (Figure 3), with a thickness of 200–400 m. The deposition of the Upper Wuerhe Formation overlaps with the Lower Wuerhe Formation in the north, east, and west directions. In the Mahu area, its lithology is characterized by coarse conglomerate in the lower part and finely grained mudstone mixed with sandstone and conglomerate in the upper part. The mudstone layers are relatively thick and may represent weathered clay layers. During the Triassic period, the Junggar Basin became a large-scale depressed basin. From the Upper Wuerhe Formation to the Karamay Formation, the Mahu area continuously overlies northward, covering the Lower Wuerhe Formation and underlying strata, completely concealing the source rocks of the Permian Fengcheng Formation and Lower Wuerhe Formation, creating a favorable base for the enrichment of deep basin oil and gas [17,47].

Figure 1

(a) Location of the Junggar Basin in northwestern China (modified from [42]). The blue dashed box represents the location of the study area; (b) geological setting of the northwestern Junggar Basin showing study sites used for testing unconformity structure thickness. The black line represents the location of the seismic cross section shown in Figure 3.

[Figure omitted. See PDF]

3. Materials and Methods

3.1. Thickness of Unconformity Three-Layer Structure

The identification of the unconformity was conducted through seismic reflection characteristics such as toplap, truncation, and overlap by using two-dimensional and three-dimensional seismic profiles within the northwestern Junggar Basin. The thickness of the unconformity three-layer structure was determined by using well logging curves and lithologic changes. The well logging response characteristics and empirical value ranges above the unconformity, the weathered clay layer, and the underlying semi-weathered or unweathered layer were identified by 20 wells beneath the Triassic Baikouquan Formation (Table 1). Thickness statistics for the three-layer structure of the unconformity body were compiled across 104 wells in the northwestern Junggar Basin (Table 2).

3.2. Evaluation of Reservoir Below the Unconformity

Reservoir evaluation beneath the unconformity was based on petrology observations and analyses of porosity and permeability. All samples of the Lower Wuerhe and Upper Wuerhe Formations were obtained from the drilling cores of the Xinjiang Petroleum Company, China National Petroleum Corporation (CNPC). Thin sections were impregnated with blue resin, stained with alizarin red S dye solution and ferrocyanide, and observed under a polarized light microscope. Dye staining identified prominent pores and cement. Samples were ground into 200-mesh powders with an agate mortar and analyzed with a powder diffractometer. Porosity was measured using the helium porosity method at a test pressure of approximately 200 psi, and permeability was measured using nitrogen. The YG-II high-pressure pore structure meter was used for these tests, which were completed at the Exploration and Development Research Institute of Xinjiang Petroleum Company. Typical sandstone samples were analyzed via X-ray diffraction (XRD), operating at a voltage of 40 kV and a beam current of 30 mA, to ascertain the bulk rock composition. The XRD mineral composition analysis involves grinding the sample to a 200-mesh powder with an agate mortar, followed by experimentation using Bruker’s D8 Focus powder diffractometer.

4. Results

4.1. Logging Response Characteristics of Unconformity Structure

Accurate identification of the three-layer unconformity structure is crucial for evaluating petroleum reservoirs. Typically, the transgressive sandstone or conglomerate above the unconformity surface displays characteristics including low natural gamma ray (GR), medium resistivity, and low acoustic transit time (AC), with the natural gamma curve exhibiting a box-like shape [30,49,50]. The weathered clay layer is enriched with elements such as aluminum and iron, leading to a high natural gamma value, low resistivity, and a high AC curve that presents a funnel shape [49,51]. In contrast, the semi-weathered rock beneath the unconformity surface is characterized by low GR and AC, whereas the resistivity, density (DEN), and borehole diameter (CALI) curves gradually increase with depth [30,49,50,51]. Research conducted in the northern region of the Mahu Sag has revealed a sudden alteration in AC at the unconformity surface, characterized by “two-stage” decline with depth that corresponds with a decrease in porosity [42].

This study identifies the well logging response characteristics of the basal conglomerate and the weathered clay layer (see Figure 4 and Table 1). A distinct box-shaped pattern is observed in the CALI curve as well as in the three resistivity (RXO, RI, and RT) curves within the weathered clay layer across multiple wells in the Xiayan Uplift, northeastern Mahu area (Figure 4). The CALI values are notably high, whereas the RXO, RI, and RT values are very low (Figure 4 and Table 1). Although the GR curve does not exhibit an immediately apparent response, it reveals a slight upward trend from bottom to top (Figure 4a). The AC and DEN logging responses are less pronounced. Above the unconformity, the well logging responses of the basal conglomerates, including CALI, GR, RXO, RI, and RT, are distinctly observable, with AC and DEN also showing minor responses (Figure 4a). Overall, the logging for the weathered clay layer and the rocks above the unconformity displays either an “S-type” or “anti-S-type” pattern (Figure 4a). These characteristics can aid in determining the thickness of the unconformity structure in the Mahu Sag and surrounding uplift areas.

However, discernible well logging response characteristics for semi-weathered rock layers are unclear, making it challenging to ascertain their thickness. While methods such as Principal Component Analysis (PCA) have been used to estimate the thickness of semi-weathered formations [16], the absence of clear logging response characteristics implies that dimensionality reduction via PCA does not yield distinct features. Consequently, the thickness of the semi-weathered rock formation was assessed visually for each well.

4.2. Distribution of Unconformity Structure

Statistical thicknesses of the unconformity’s three-layer structure were determined from 104 wells in the northwestern Junggar Basin based on the described logging response characteristics (Figure 5 and Table 2). The rocks above the unconformity mainly consist of sandstone, mudstone, and sandy conglomerate, with thicknesses ranging from 5 to 100 m (Figure 5a). The thickest regions are located in the Zhongguai and Xiayan Uplift areas, where thicknesses gradually increase from the western part of Mahu Sag to the Penyijingxi Sag. Figure 5b shows that the thickness of the weathered clay layer varies from 0 to 120 m. The weathered leach zone mainly consists of sandstone, fine sandstone, conglomerates, conglomeratic sandstones, and mudstone, with thicknesses ranging from 0 to 150 m. Data indicate that the thicknesses of both the weathered clay layer and the weathered leach zone are greatest in the Xiayan and Zhongguai Uplift areas, as well as in the central region of Mahu Sag (Figure 5b,c).

4.3. Petrographic and Petrophysical Characteristics

The Lower Wuerhe Formation is characterized by a fan delta sedimentary system, with lithology primarily composed of gray and brown-gray conglomerates, followed by conglomeratic sandstones. Based on compositional classification, the rock types are almost exclusively lithic conglomerates. Quartz content averages 3.4%, feldspar averages 1.8%, and lithic fragments average 94.8% (Figure 6a). Among the lithic fragments, tuffaceous fragments are predominant (averaging 72.1%), followed by sedimentary lithic fragments (sandstone and mudstone averaging 9.2%) and andesite (averaging 7.4%) (Figure 6b).

Measurements of porosity and permeability were conducted on approximately 300 samples. Generally, porosity and permeability show a positive correlation. The porosity of sandstone and sandy conglomerate ranges from 2.1% to 19.1%, with an average of 9.25% (Table 3). Permeability ranges from 0.011 mD to 806 mD, with an average of 15.42 mD (Table 3). Approximately 50% of the samples exhibit porosity greater than 10%, while around 50% show permeability greater than 1 mD (Figure 6c). Therefore, the sandstone and sandy conglomerate reservoir of the Lower Wuerhe Formation in the Mahu area is classified as having medium–low porosity and permeability.

The vertical variation in porosity and permeability in the eastern Mahu area is relatively consistent between the Lower Wuerhe and Baikouquan Formations (Figure 7). In well XY2, both porosity and permeability decrease with depth. In contrast, wells M217, YB2, and D18 show a gradual increase in both porosity and permeability from the Baikouquan Formation to the Lower Wuerhe Formation, with average values stabilizing in the high-value section (Figure 7b–d).

4.4. Diagenesis

Based on thin-section observations under a microscope, the reservoir porosity types in the Lower Wuerhe Formation of the Mahu Sag fall into three primary categories: residual primary intergranular pores (35%) (Figure 8a–c), dissolution pores in volcanic materials (34%) (Figure 8d), and zeolite dissolution pores (23%) (Figure 8e). Additionally, a small proportion of microfractures (8%) is observed (Figure 8f). The occurrence of residual primary intergranular pores in the Lower Wuerhe Formation is notably low. This phenomenon correlates with low component maturity, structural maturity, and significant compaction due to the increased burial depth of the formation.

Dissolution pores include those found in volcanic materials and zeolite. The formation of dissolution pores in volcanic materials primarily results from the selective dissolution of volcanic debris during early diagenesis in an alkaline water environment. This process can result in the development of granular and intergranular dissolution pores. Zeolite dissolution pores are further subdivided into analcime and laumontite dissolution pores. The formation of zeolite dissolution pores is primarily due to alkaline fluids influenced by neutralization and inhibition processes (Figure 8e). During the middle diagenetic stage, organic acids enhance the dissolution of zeolite minerals, particularly around fractures and channels impacted by these acids, resulting in the formation of prominent zeolite dissolution pores.

Furthermore, the sand–conglomerate reservoirs of the Lower Wuerhe Formation in the Mahu area display various cementation types. Common cements include calcite (including ferroan calcite), silica, zeolites (such as laumontite, clinoptilolite, and heulandite), pyrite, and authigenic clay minerals. Zeolite cements, primarily consisting of laumontite, clinoptilolite, and heulandite, vary in their distribution (Figure 8g–k). Laumontite was prevalent in the Mahu area, with concentrations ranging from 1% to 15%, occurring as platy crystals between particles. Clinoptilolite often replaces particles but is found in lower concentrations. The distribution of zeolite cements was influenced by volcanic clastic material and depositional facies, with clinoptilolite and laumontite typically located in delta front areas.

4.5. Diagenetic Evolution

In the early diagenetic stage of the Lower Wuerhe Formation, mechanical compaction was predominant due to increased burial depth. Poor sorting and high mud content in the sand–conglomerates of this formation led to a rapid decrease in primary porosity under intense compaction (Figure 9). Furthermore, compaction flow facilitated the migration of pore fluids through thick layers of volcanic clastic materials, which increased salinity and alkalinity. Hydrolysis transformed volcanic clastic materials into clay and zeolite minerals while releasing significant amounts of alkaline metal cations (K⁺, Na⁺, Ca²⁺, Mg²⁺), thus reinforcing the alkaline diagenetic environment. During this period, authigenic clay minerals such as illite and chlorite precipitated and transformed. The early diagenetic stage was essential for forming pores linked to alkaline diagenesis.

At the transition from the early to the middle diagenetic stage, the maturation of organic matter and hydrocarbon generation released organic acids, shifting localized pore fluid environments from alkaline to acidic (Figure 9). These organic acids dissolved volcanic clastic materials and early diagenetic zeolite minerals, leading to the formation of volcanic clastic and zeolite dissolution pores (Figure 8g–k). Thin-section statistics show that dissolution pore porosity ranges from 2% to 5%, with the most developed dissolution pores found in the mid-slope zone (i.e., fan delta front facies) (Figure 10).

In summary, the diagenetic evolution of the Lower Wuerhe Formation is marked by the formation and transformation of secondary minerals, including zeolites, montmorillonite, calcite, illite, chlorite, and laumontite, which reflect the weathering processes [52,53]. These mineral changes significantly influence the reservoir’s porosity and permeability. During the early diagenetic stage, zeolitization, montmorillonitization, calcite and silica cementation, and dissolution effects from alkaline waters occur. From the early to middle diagenetic stages, the maturation of organic matter and hydrocarbon generation release organic acids, leading to a shift in the pore fluid environment from alkaline to acidic conditions. This stage is characterized by the transformation of calcic zeolites and laumontite, as well as the dissolution of volcaniclastic and early diagenetic zeolite minerals by organic acids, which results in the formation of dissolution pores. The late diagenetic stage is primarily marked by calcite cementation [54,55].

5. Discussion

5.1. Factors Affecting Reservoir Formation Below the Unconformity

Several factors influence reservoir formation below the unconformity. Firstly, low matrix content (mud, tuffaceous material, and silt) is crucial for developing favorable reservoirs (e.g., Figure 8l). In addition, the following factors play critical roles.

5.1.1. Mud-Deprived Sand Bodies in Subaqueous Distributary Channels as the Foundation of High-Quality Reservoirs

The Lower Wuerhe Formation exhibits a fan delta sedimentary system (Figure 10), which can be subdivided into microfacies such as debris flows, braided channels, interdistributary bays, subaqueous distributary channels, and mouth bars. Among these, the braided channels on the delta plain, the mouth bars at the delta front, and the subaqueous distributary channels represent the primary reservoir sand body types.

Physical property statistics indicate significant differences in reservoir properties among various sedimentary microfacies [41]. The sand bodies in the subaqueous distributary channels and mouth bars of the fan delta possess the best physical properties. This is attributed to the strong hydrodynamic conditions and relatively long transport distances of clastic materials, resulting in well-sorted and well-rounded particles with low mud content. Such conditions not only favor the preservation of primary porosity but also facilitate the formation of secondary porosity through dissolution processes. Conversely, the braided channel sand bodies on the delta plain exhibit slightly poorer physical properties due to shorter transport distances, weaker sedimentary differentiation, and higher mud content (Figure 8l and Figure 10).

5.1.2. Dual Role of Early Alkaline and Late Acidic Dissolution in Significantly Enhancing Reservoir Properties

The sandstones and conglomerates of the Lower Wuerhe Formation display low compositional and structural maturity, leading to inherently low intergranular porosity at the time of deposition. Intense compaction subsequently further degraded primary porosity. However, the diagenetic environment in the Mahu area during the Permian was predominantly alkaline. During the early diagenetic stage, abundant alkaline formation fluids facilitated the effective dissolution of volcanic clastic materials. This process resulted in the formation of intergranular and intragranular dissolution pores within the volcanic clasts, enhancing initial reservoir properties. Furthermore, the formation of alkaline authigenic minerals, such as zeolites and albite, significantly contributed to cementation, preventing clastic particle slippage during compaction and mitigating the loss of primary porosity (Figure 8g–k).

In the early stage of mesodiagenesis, the maturation of organic matter resulted in hydrocarbon generation and the release of organic acids (Figure 9). These acids partially dissolved volcanic clasts and early diagenetic zeolite minerals (primarily analcime), creating secondary dissolution pores (Figure 8e and Figure 9). Braided channel conglomerates in the plain facies, characterized by high mud content and poor initial properties, were minimally affected by early alkaline dissolution, leading to a rapid decline in reservoir porosity with depth. In contrast, mud-deprived conglomerates in the delta front facies, which had low mud content and better initial properties, experienced significant porosity enhancement due to early alkaline dissolution and associated zeolite cementation. The preservation of primary and dissolution pores, combined with later-stage acidic dissolution of zeolites, results in reservoir porosity diverging from the normal compaction trend, exhibiting abnormally high levels of porosity. Even at burial depths exceeding 5000 m, reservoir porosity may remain above 10% (Figure 7, Figure 8e,g–k, Figure 9 and Figure 10).

The dual roles of early alkaline and late acidic dissolution in enhancing reservoir properties enhance our understanding of diagenesis in clastic rocks. This knowledge can inform future research on the interactions between tectonism, weathering, and diagenesis in reservoir development. Furthermore, understanding the factors that influence reservoir properties can lead to more efficient exploration and development strategies. The identification of primary and secondary reservoir sand bodies, along with their associated porosity and permeability, can guide drilling operations toward more productive zones.

5.2. Impact of Zeolites and Clay Minerals on Reservoirs

The Lower Wuerhe Formation in the Mahu Sag of the Junggar Basin is characterized by zeolite-cemented conglomerates, sandy conglomerates, and sandstones. These reservoirs typically exhibit strong compaction, leading to low porosity and ultra-low permeability (Figure 6c). The flow performance and productivity of these reservoirs are significantly affected by both clay minerals and authigenic zeolite minerals.

Data from eight wells exhibiting significant laumontite and clay mineral development were analyzed for correlation (Figure 11 and Figure 12). Laumontite content in the Lower Wuerhe Formation reservoirs is typically well developed, displaying prominent cleavages, and primarily exists as pore-filling cement (e.g., Figure 8h). Some sandstone rock fragments are partially replaced by laumontite cement (e.g., Figure 8g). Correlation analyses indicate that laumontite content correlates with porosity, permeability, and clay mineral content (Figure 11a–c). When laumontite content is less than 10%, it significantly enhances reservoir porosity and permeability; however, when it exceeds 10%, both porosity and permeability decline (Figure 11a,b). Furthermore, laumontite content exhibits a positive correlation with clay mineral content when it exceeds 5% (Figure 11c). When laumontite content is below 5%, clay mineral content increases. These results indicate that a reciprocal relationship exists between laumontite content and levels of porosity, permeability, and clay mineral content. Additionally, laumontite content can be quantitatively estimated using DEN, CNL, AC, and LgRT due to a positive relationship between DEN and laumontite, as well as a negative relationship between laumontite and the levels of AC, CNL, and LgRT (Figure 11d–g).

Linear fitting analysis of porosity and related parameters indicates that porosity is negatively correlated with clay minerals and positively correlated with permeability (Figure 12a,b). Porosity is positively correlated with AC and CNL but negatively correlated with DEN (Figure 12c–e). Thus, these three parameters can be used to estimate porosity in the Mahu area. Furthermore, clay minerals exhibit a positive correlation with DEN and a negative correlation with AC and CNL (Figure 12f–h). The results above suggest that clay minerals are a key factor restricting the development of high-quality reservoirs.

In summary, both laumontite and clay mineral contents significantly affect reservoir properties. The correlations between these minerals and logging parameters allow for the quantitative estimation of their contents in the Lower Wuerhe Formation reservoirs, leading to more precise assessments of reservoir properties.

5.3. Impact of Unconformity on Reservoir

5.3.1. Unconformity-Controlled Weathering and Leaching

Unconformities are well recognized as regions that have undergone tectonic activity, followed by weathering, erosion, and leaching. These processes significantly enhance the development of secondary porosity [51,56]. Specifically, sandstone or conglomerate reservoirs found within weathered leaching zones often exhibit high porosity and permeability. Prolonged erosion and leaching facilitate the infiltration of meteoric freshwater through fractures and pores, which further promotes the formation of secondary dissolution pores and fractures.

In contrast, the occurrence of weathered clay layers generates an abundance of clay minerals during weathering. Subsequent compaction can adversely affect both porosity and permeability, leading to overall lower porosity values. Prior geochemical analyses have thoroughly discussed paleoclimate and provenance evolution from the late Permian to the Early Triassic in the Mahu area [57]. Notably, the intensity of chemical weathering has shifted from a low degree to a moderate level, indicating significant geological changes over time. Research has revealed numerous weathered paleosol layers in field outcrops along the northwest margin of the Junggar Basin, where the upper and lower strata exhibit unconformity [45]. These observations suggest that the formation of unconformities is closely linked to the processes of weathering and leaching. Furthermore, the degree of weathering and leaching may correlate significantly with the paleogeomorphology of the region.

Figure 5 clearly illustrates that the weathered clay layer is most developed in the Zhongguai Uplift and the eastern slope of the Mahu area. These regions have experienced considerable tectonic activity, weathering, and erosion. Consequently, it can be inferred that leaching in these areas is also substantial, as weathering requires the presence of water [57]. The enhancement of weathering is related to seasonal rainfall patterns, which play a vital role in these processes. Leaching increases the porosity of the underlying semi-weathered rock, making it an excellent reservoir for hydrocarbons. Moreover, certain key elements released through weathering and leaching may promote mineral transformations during diagenesis, thereby enhancing the development of secondary porosity in the Lower Wuerhe Formation (see Figure 7). These findings underscore the complex interactions between geological processes and reservoir quality in the studied region.

5.3.2. Unconformity-Controlled Petroleum Migration and Accumulation

Unconformities influence reservoirs and hydrocarbon traps by linking stratigraphically distant source rocks and reservoir rocks, thereby facilitating hydrocarbon accumulation [11,51,56]. The spatial structure of an unconformity dictates its hydrocarbon migration capacity. If the intermediate weathered clay layer is poorly developed, the structural layers above and below the unconformity are in direct contact, making hydrocarbon migration dependent on lithology [58]. The strata above the unconformity primarily consist of basal conglomerate or transgressive sand bodies; hence, hydrocarbon migration is chiefly influenced by the weathered clay layer and semi-weathered leaching zone. Thick weathered clay layers are generally considered effective seals for hydrocarbons. However, if the layer is thin or absent, hydrocarbons beneath the unconformity can migrate directly to the upper layer along the unconformity surface.

Our isogram map of the three-layer unconformity structure in the Mahu area indicates that thick weathered clay layers are predominantly developed in the eastern slope, the Zhongguai Uplift in the south, and the Ke-Bai Fault Zone in the southwest (Figure 5b). These regions serve as primary zones for hydrocarbon enrichment. In addition, the thickness of the upper layer above the unconformity and the semi-weathered leaching zone reaches its maximum in the eastern Mahu slope and southern Mahu area (southern Mahu Sag combined with Zhongguai Uplift) (Figure 5a,c). Therefore, when the weathered clay layer effectively seals hydrocarbons, those beneath the Lower Wuerhe Formation will migrate through faults and along the unconformity surface. Three-dimensional seismic data reveal numerous northwest–southeast-trending strike-slip faults developed within the Mahu Sag (Figure 13a) [48,59]. These faults are nearly vertical, exhibiting high dip angles of 70° to 90°. In cross sections, the strike-slip faults display complex flower structures or manifest as single faults (Figure 13b), connecting with other secondary faults that divide the strata into small-scale fault blocks (Figure 13c–e). These strike-slip faults intersect strata from the Permian to the Jurassic. Consequently, hydrocarbons generated by the Fengcheng Formation source rocks can migrate both upwards along strike-slip faults and horizontally through well-developed unconformities into the Triassic and Jurassic (Figure 14). The upper Permian and Triassic fan delta front reservoirs thus represent a vast area of hydrocarbon accumulation (Figure 14a,b).

Faults can function as both barriers and migration pathways [60,61]. A fault can only serve as a migration pathway when the lithology on both sides is relatively hard. Conversely, if the lithology is less permeable, the fault typically functions as a barrier. Therefore, the overall characteristics of the eastern Mahu area are influenced by lithology, with pure oil zones typically developing in the updip direction of the strike-slip faults, while oil and water generally flow together in the downdip direction (Figure 14a). The strike-slip fault has both resource channeling and destructive dual functions.

6. Conclusions

This study investigates the effects of the unconformity between the Middle Permian Lower Wuerhe Formation and the Lower Triassic Baikouquan Formation on the eastern slope of the Mahu area. We utilized well logging response characteristics from eight wells in this region, which were then applied to data from 104 wells throughout the entire Mahu area to determine the thickness of the three-layer unconformity structure. The maximum thickness of the weathered clay layer was found to be concentrated in the east slope of Mahu Sag, the Zhongguai Uplift, and the southwestern fault zone of the Mahu Sag.

Through detailed petrological characterization and physical property analysis, we identified two critical stages in the diagenetic evolution of the Lower Wuerhe Formation. The early diagenetic stage is marked by processes such as compaction, zeolitization of volcanic glass and feldspar, montmorillonitization of volcanic glass, calcite and siliceous cementation, and dissolution due to alkaline waters. In contrast, the early middle diagenetic stage is characterized by the albitization of laumontite and heulandite, the conversion of stilbite into heulandite, and organic acid dissolution. The late middle diagenetic stage primarily involves calcite cementation.

Additionally, four key factors influencing reservoir development in the Lower Wuerhe Formation were identified: low argillaceous content in sandstone and sandy conglomerate, diagenetic compaction, zeolite dissolution and cementation, and clay mineral infill. Sand bodies in subaqueous distributary channels and mouth bars exhibit the best physical properties. The content of laumontite, when less than 10%, significantly enhances reservoir porosity and permeability.

The unconformity plays a dual role in controlling the reservoir characteristics of the Lower Wuerhe Formation. Its well-developed nature has led to significant weathering and atmospheric freshwater leaching, which enhances the dissolution of the underlying reservoir and increases secondary porosity. The weathered clay layer, particularly in the Zhongguai Uplift and the eastern slope of the Mahu area, shows the most significant development, indicating potential high porosity and permeability in these regions, which can serve as excellent hydrocarbon reservoirs. Furthermore, the extensive network of strike-slip faults improves the connectivity of hydrocarbon reservoirs, facilitating both vertical and lateral migration.

By integrating these quantitative results with the findings, our study provides a comprehensive framework for understanding and predicting reservoir properties below the unconformity in the Lower Wuerhe Formation. This information is crucial for optimizing hydrocarbon exploration and development in the Mahu area and similar geological settings.

Author Contributions

Conceptualization, Y.T., M.Z. and X.W.; methodology, X.W., D.Y., Z.Y. and L.Z.; validation, X.W.; formal analysis, X.W.; investigation, X.W. and D.Y.; resources, Y.T., X.W., D.Y. and M.Z.; data curation, M.Z. and L.Z.; writing—original draft preparation, X.W.; writing—review and editing, X.W. and Z.Y.; visualization, M.Z.; supervision, M.Z. and L.Z.; project administration, M.Z.; funding acquisition, M.Z., D.Y. and X.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest. Y.T., M.Z., and L.Z. are employees of Exploration and Development Research Instituted of PetroChina Xinjiang Oilfield Co., Ltd. The paper reflects the views of the scientists and not the company.

Footnotes

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Figures and Tables

Figure 2. Generalized stratigraphy of Carboniferous, Permian, and Triassic strata in the northwestern Junggar Basin. The data on stratigraphic thickness were collected from [48].

Figure 3. Seismic interpretation profile showing the unconformity of the Mahu Sag, northwestern Junggar Basin. The location of the profile is shown in Figure 1b. P1j = Jiamuhe Formation; P1f = Fengcheng Formation; P2x = Xiazijie Formation; P2w = Lower Wuerhe Formation; P3W = Upper Wuerhe Formation; T1b = Baikouquan Formation; T2k = Karamay Formation; T3b = Baijiantan Formation; J = Jurassic; TWT = two-way travel time.

Figure 4. Lithologic and geophysical characteristics and vertical structure type of the unconformity between the Lower Wuerhe and Baikouquan Formations in the Xiayan Uplift and Zhongguai Uplift. The shades of orange, yellow, and green represent the bottom conglomerate, the weathered clay layer, and the semi-weathered leaching zone, respectively. The black wavy line marks unconformities. (a) Well Xy2; (b) well Jt1.

Figure 5. Isoline map of thickness showing the distribution of the basal conglomerate (a), weathered clay rock (b), and weathered leached rock (c) of the Permian–Triassic unconformity in the northwestern Junggar Basin.

Figure 6. (a) Mineralogical classification of the sandstones of the Lower Wuerhe Formation. (b) Characteristics of reservoir rocks and minerals of the Lower Wuerhe Formation in the Xiayan Uplift. (c) Plot of the permeability versus the porosity in the Xiayan Uplift, Junggar Basin.

Figure 7. Vertical variation in porosity and permeability of wells XY2 (a), M217 (b), YB2 (c), and D18 (d). The red dotted lines represent the boundary of between strata. The blue dotted lines represent the boundary between subdivided members of each stratum.

Figure 8. Photomicrographs of the reservoir diagenetic characteristics of the Lower Wuerhe Formation in the eastern Mahu area. (a) Residual primary intergranular pores, well M224, 4207.35 m, P2w; (b) residual primary intergranular pores, well M224, 4505.02 m, P2w; (c) residual primary intergranular pores, well M211, 3760.22 m, P2w; (d) dissolution pores in volcanic materials, well Yb4, P2w, sandy conglomerate; (e) zeolite dissolution pores (yellow arrow), well M217, 4003.34 m, P2w; (f) microfractures, well Yb4, 3916.93 m, P2w; (g) conglomerate, laumontite cementation (red arrows), well Yb4, 3870.37 m, P2w; (h) conglomerate, laumontite cementation (red arrows), well Yb4, 3869.37 m, P2w; (i) conglomerate, laumontite cementation (red arrow), well Yb4, 3914.64, P2w; (j) conglomerate, laumontite cementation, heulandite locally, well M218, 3944.26 m, P2w; (k) sandstone, intergranular pores filled with authigenic albite, well YT1, 4872.5, P2w; (l) sand conglomerate, mud heterobase filling, well M202, 3793.17 m, P2w.

Figure 9. Diagenetic evolution history, source rock thermal evolution history, and pore evolution history of the Lower Wuerhe Formation.

Figure 10. (a) Sedimentary facies in the Permian Lower Wuerhe Formation in the eastern Mahu area, Junggar Basin. The red, blue, and black lines represent location of cross section AA’, BB’, and CC’; (b) cross section AA’ showing the distribution of fan delta in the eastern slope of the Mahu area. T1b = Baikouquan Formation; P2w4 = fourth member of the Lower Wuerhe Formation; P2w3 = third member of the Lower Wuerhe Formation; P2w2 = second member of the Lower Wuerhe Formation; P2w1 = first member of the Lower Wuerhe Formation.

Figure 11. Relationship between laumontite content and other parameters in the Lower Wuerhe Formation. (a) laumontite vs. porosity; (b) laumontite vs. permeability; (c) laumontite vs. clay minerals; (d) laumontite vs. DEN; (e) laumontite vs. AC; (f) laumontite vs. CNL; (g) laumontite vs. LgRT.

Figure 12. Relationship between porosity content and other parameters, and between clay minerals and logging values in the Lower Wuerhe Formation. (a) Porosity vs. clay minerals; (b) porosity vs. permeability; (c) porosity vs. AC; (d) porosity vs. DEN; (e) porosity vs. CNL; (f) clay minerals vs. AC; (g) clay minerals vs. DEN; (h) clay minerals vs. CNL.

Figure 13. (a) Strike-slip fault system in the eastern Mahu Sag. The x and y axes are in m; (b) seismic profile showing the nearly vertical main faults and flower structures; (c–e) seismic profiles showing the nearly vertical main fault system in different location. T2k = Karamay Formation; T1b = Baikouquan Formation; TWT = two-way travel time.

Figure 14. (a) Cross section BB’ showing the nearly vertical main faults, flower structures, and hydrocarbon accumulation in the eastern slope of the Mahu area, Junggar Basin. The location of the profile is shown in Figure 10a; (b) cross section diagram of Yb1, Yb4, M202, and M201, showing oil reservoirs below and above the unconformity. The location of the profiles BB’ and CC’ are shown in Figure 10a. The red arrows represent the migration path of oil and gas.

Table 1

Logging response characteristics of unconformity structures.

Three-Layer Structures	Well Number	Resistivity (RXO, RI, and RT)		CALI (cm)		GR (API)		DEN (g/cm3)		AC (us/ft)
		Min–Maximum	Average	Min–Maximum	Average	Min–Maximum	Average	Min–Maximum	Average	Min–Maximum	Average
Rock above the unconformity (m)	8	1.9–24.7	8.8	7.9–11.2	9.05	70–143	106.25	2.4–2.7	2.5225	65–116	81.75
Weathered clay layer (m)	8	1.1–5.5	2.375	8.0–14.0	10.05	63–129	93.975	2.25–2.65	2.5175	64–122	92.75
Effective thickness (m)	8	5.5–24.7	15.1	7.9–11.2	9.05	70–129	99.5	2.4–2.7	2.5225	65–122	93.5

Table 2

Thickness of three-layer structure of the unconformity in the northwestern Junggar Basin.

Well	Upper Layer (m)	Weathered Clay Layer (m)	Lower Layer (m)	Well	Upper Layer (m)	Weathered Clay Layer (m)	Lower Layer (m)
AH204	16	4	6	MH10	30	40	50
AH16	50	50	70	MH106	30	13	12
AK1	47.5	2.7	57.5	MH15	20	30	40
B2040	15	14	22	MH16	24	16	20
B26	50	50	150	MH2	50	16	30
B27	30	35	100	MH20	30	21	70
B75	42	2	4	MH21	50	80	40
BQ1	50	10	80	MH22	60	90	60
BQ2	36	0	26	MH25	50	46	100
BQ5	38	0	30	MH28	50	60	40
DA18	70	60	150	MH3	38	16	56
DT1	38	26	38	MH30	50	80	60
FC1	58	0	64	MH32	35	20	30
FN1	30	0	0	MH34	10	0	0
FN14	50	0	58	MH35	40	10	20
FN3	30	0	0	MH38	50	50	60
FN4	60	0	69	MH39	68	46	100
FY1	30	0	0	MH4	14	10	16
J201	90	80	40	MH40	20	40	140
J202	50	85	80	MH402	50	30	120
J203	20	50	40	MH45	80	10	80
J204	15	65	25	MH48	50	70	100
J205	88	61	60	MH5	46	54	48
J210	15	50	60	MH6	30	70	50
JL17	24	36	75	MH7	45	30	100
JL19	40	60	30	MH8	20	10	30
JL2	20	50	24	MH085	50	30	50
JL23	8	10	32	MH9	40	0	30
JL31	8	8	20	MY1	24	36	30
JL35	52	30	70	MZ1	40	30	42
JL43	60	20	40	MZ10	30	24	25
JL51	70	40	36	MZ2	26	68	58
JT1	100	40	100	MZ4	60	52	40
K009	40	30	80	MZ5	27	90	64
K17	20	30	100	MZ6	40	0	60
K201	34	50	100	MZ7	60	100	100
K204	50	30	75	MZ9	40	70	90
K205	50	55	150	PB1	60	40	30
K207	75	80	130	WU35	68.2	0	72
K208	50	70	60	XIA72	77	4.5	50
K303	35	44	75	XIA88	87	56	78
K82	20	30	80	XY1	76	32	22
K821	16	18	32	XY2	83.5	76.5	74
K841	6	2	12	XY3	45	56	74
K88	50	8	60	XY4	78	58	50
K89	50	25	125	YB1	62	22	30
KE80	37	21	16	YB2	50	80	60
MA102	37.5	1.7	35	YB3	70	80	90
MA18	50	50	30	YB5	76	53	64
MA2	35	2.5	36.2	YT1	74	28	66
MH26	50	30	50	ZJ6	70	130	120
MH1	41	45	24	BAI75	42	2	4

Table 3

Porosity and permeability of the Lower Wuerhe Formation in the eastern Mahu area, northwestern Junggar Basin.

Well	Porosity (%)		Number	Permeability (mD)		Number
	Minimum–Maximum	Average		Minimum–Maximum	Average
D13	3.9–13.9	9.612	11	0.025–90.2	11.208	11
Yt1	3–18.2	11.5	47	0.015–6.74	0.514	45
Yb5	5.2–10.9	8.28	10	0.104–2.84	1.294	7
Yb4	3.4–12	8.154	46	0.012–806	22.874	44
Yb2	2.1–11.6	7.433	33	0.011–125	12.608	30
Yb1	2.7–19.1	9.548	25	0.014–193	19.744	20
Xy2	4.74–17.06	10.61	45	0.03–356.08	43.438	37
M218	2.7–14	7.51	38	0.036–184	9.665	35
M217	4.5–16.2	9.1	30	0.025–10.8	1.429	12
M202-201	4.3–10.8	7.425	16	0.214–283	31.56	16
D18	4.2–18.6	12.62	26	0.016–80	15.34	22

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