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Article

Development of Braided River Delta–Shallow Lacustrine Siliciclastic–Carbonate Mixed Sedimentation in the Upper Ganchaigou Formation, Huatugou Oilfield, Qaidam Basin, China

by
Yuxin Liang
1,
Xinmin Song
1,
Youjing Wang
1,* and
Wenjie Feng
2
1
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
2
School of Geosciences, Yangtze University, Wuhan 430100, China
*
Author to whom correspondence should be addressed.
Minerals 2026, 16(1), 92; https://doi.org/10.3390/min16010092
Submission received: 8 December 2025 / Revised: 1 January 2026 / Accepted: 4 January 2026 / Published: 17 January 2026
(This article belongs to the Topic Reservoir Genesis and Quality Evolution in Hydrocarbon Systems)
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Abstract

This study systematically investigates the lithofacies, sedimentary microfacies, vertical evolution, and spatial distribution of the braided river delta–shallow lacustrine carbonate mixed sedimentary rocks of the Upper Ganchaigou Formation in the Huatugou Oilfield of the Qaidam Basin, China. This study integrates data from field outcrops, core observations, thin section petrography, laboratory analyses, and well-logging interpretations. Based on these datasets, the sedimentary characteristics are identified, and a comprehensive sedimentary model is constructed. The results reveal that the study area contains five clastic facies, three types of mixed sedimentary facies, and ten sedimentary microfacies. Two distinct modes of mixed sedimentation are recognized: component mixing and stratigraphic mixing. A full lacustrine transgression–regression cycle is formed by the two types of mixed sedimentation characteristics, which exhibit noticeable differences in vertical evolution. Component mixing, which occurs in a mixed environment of continuous clastic supply and carbonate precipitation during the transgression, is the primary characteristic of the VIII–X oil formation. The mixed strata that make up the VI–VII oil formation show rhythmic interbedding of carbonate and clastic rocks. During the lacustrine regression, it shows the alternating sedimentary environment regulated by frequent variations in lacustrine levels. The planar distribution is affected by both intensity of sediment from the west and the changes in lacustrine level. During the lacustrine transgression, it is dominated by littoral-shallow lacustrine mixed beach bar and mixed sedimentary delta. On the other hand, during the lacustrine regression, it is dominated by laterally amalgamated sand bodies in the braided-river delta front. Based on this, a mixed sedimentary evolution model controlled by the coupling of “source–lacustrine level” is established. It offers a guide for reconstructing the sedimentary environment in basins that are similar to it and reveals the evolution path of mixed sedimentation in the short-axis source area of arid saline lacustrine basins.

1. Introduction

Based on established sedimentological concepts and published models, carbonate rocks and siliciclastic sediments are deposited in fundamentally distinct environments. Carbonate rocks, primarily products of biochemical processes, are typically deposited in shallow, warm, and clear-water settings [1,2,3]. In contrast, siliciclastic sedimentation is predominantly controlled by clastic sediment supply [4], occurs in a wider range of environments, and is characterized by an inverse relationship with biochemical activity, as terrigenous input tends to suppress carbonate production. Mount (1984) introduced the concept of “mixed sediments,” defining them by the textural intermixing of siliciclastic and carbonate components, and subsequently established a four-end-member classification and nomenclature scheme for such deposits [5,6]. There are two ways to define mixed sediments. Mixed sediments, when used narrowly, refer to the blending of elements within the same rock layer. In general, alternating or interbedded layers of carbonates and terrigenous clastics are also considered mixed sediments [7,8,9].
As a special type of sedimentary sediment, mixed sediments of terrigenous clastics and carbonates are of great significance for understanding paleo-climate, sea (lacustrine) level changes, sedimentation rates, and the impacts of tectonics in sedimentation within basins [10,11,12,13,14,15,16]. In recent years, significant oil and gas discoveries have been made in mixed sedimentary rocks in the Qaidam Basin, China, Bohai Bay Basin, China, Albemarle Basin, America and Transvaal Basin in South Africa [17,18,19,20], showing good exploration and development potential.
Numerous studies have demonstrated that the development of mixed sedimentation in lacustrine basins is closely coupled with lake-level fluctuations, and its pattern—whether continuous mixing or discontinuous interbedding—can serve as a key indicator of sequence stratigraphic responses [21,22,23,24,25]. Particularly under arid to semi-arid climatic conditions, intense evaporation leads to a significant increase in lake water salinity, which greatly enhances both chemical and biochemical carbonate precipitation [19,26]. This process creates a prerequisite for the widespread coexistence of siliciclastic and carbonate components [19,26]. Recent studies from locations such as the Bohai Bay Basin and the Red Sea Rift have confirmed that hypersaline lacustrine water bodies under arid climates serve as a key controlling factor for the development of lacustrine carbonate and mixed sedimentary systems [27,28]. However, current understanding of mixed sedimentation in saline lacustrine basins remains subject to several limitations. Current models are predominantly established under scenarios characterized by long-axis sediment supply and relatively stable lake levels [4,29]. In contrast, there remains a paucity of systematic case studies and mechanistic explanations regarding how high-frequency lake-level fluctuations couple with intermittent short-axis sediment supply in arid settings to control mixing patterns and sedimentary architectures from deltaic to shore-shallow lacustrine environments [30].
Studies on the paleoclimate and paleoenvironment of the Upper Ganchaigou Formation in the western Qaidam Basin indicate that during its deposition, the paleosalinity averaged 12.7‰ and reached a maximum of 26.6‰, classifying it as a typical (hyper-) saline lake [31,32]. The Huatugou Oilfield is situated within the delta front to shallow lacustrine zone, which represents a facies belt with high carbonate productivity. The Upper Ganchaigou Formation (N1) is a significant oil-producing interval in the Huatugou Oilfield in the Qaidam Basin, which is a typical continental arid saline lacustrine oilfield [33]. During this time, the lacustrine level changed regularly, creating a distinctive braided-river delta front-shallow lacustrine sedimentary system. Carbonate and clastic mixed sedimentary rocks were extensively formed. However, existing research focuses on the generation mechanism and static characteristics of mixed sedimentary rocks [27,28,29].
Because of this, the Upper Ganchaigou Formation (N1) in the Huatugou Oilfield of the Qaidam Basin is the focus of this study. This study systematically identifies lithofacies, sedimentary microfacies, mixed sedimentary characteristics, and their spatial distribution using field outcrops, cores, thin-section analysis, grain-size measurements, and well-logging data. The goal is to develop a sedimentary model, provide a new geological reference for the study of mixed sediments in terrestrial saline lacustrine basins, and clarify the vertical evolutionary sequence and planar distribution of mixed sediments in the study area.

2. Regional Geological Background

The Qaidam Basin is on the northern edge of the Qinghai–Tibet Plateau. Its boundary is the Altun Mountains to the northwest, the Qilian Mountains to the northeast, and the Kunlun Mountains to the south. It is a large Mesozoic–Cenozoic terrestrial sedimentary basin in western China [34,35,36,37] (Figure 1a). The Huatugou Oilfield is located within the Shizigou–Youshashan anticline zone of the Mangya Depression in the western Qaidam Basin. It lies above the Shizigou major reverse fault and is bounded by the Ganchaigou structural belt to the northeast, the Youshashan Oilfield to the southeast, and the Hongliuquan and Qigequan structural units to the west [38] (Figure 1b).
Upper Neogene Pliocene deposits cover the surface of the Huatugou Oilfield, with the Shizigou Formation (N23) on the outskirts and the Upper Youshashan Formation (N22) in the center. According to drilling data, the Upper Youshashan Formation (N22), Lower Youshashan Formation (N21), Upper Ganchaigou Formation (N1), and the top of the Lower Ganchaigou Formation (E3) are the strata found in the large reverse fault from top to bottom. The Upper Ganchaigou Formation (N1) and Lower Youshashan Formation (N21) are recurrent within the fault zone. The Upper Ganchaigou Formation (N1), which is part of the Huatugou Oilfield’s VI–X oil formations, is the study’s target interval. The study area is located on the margin of a depression lacustrine basin with gentle topography. Depositional systems, such as piedmont alluvial fans, axial river systems, deltas, and littoral shallow-lacustrine bar systems, developed in succession from the western end of the basin at the base of the Kunlun Mountains to the Huatugou region. The study area had an arid paleoclimate and high paleowater salinity during the Upper Ganchaigou Formation’s deposition [31,32].
The thick sedimentary sequence gradually changed from a mixed sedimentary system at the base to a braided-river delta system toward the top due to periodic variations in lacustrine water levels.

3. Materials and Methods

This study utilized field geological outcrops, core samples, microscopic thin sections, analytical tests, and well logging data to conduct a systematic sedimentological analysis of the Upper Ganchaigou Formation in the Huatugou Oilfield. Core samples from five cored wells (C1, C2, C3, C4, and C5) (Figure 1b) in the study area were thoroughly observed and described, with a focus on the characteristics of rock color, composition, structure, and sedimentary structures. 213 thin sections prepared from the core samples were analyzed in detail using a polarizing microscope to examine the mineral composition, structural features, and interstitial material types. Conventional well logging curves (GR, AC, RT, SP, etc.) from 200 wells in the study area were interpreted, establishing the relationship between well logging facies and sedimentary microfacies. Typical core samples were selected for grain size analysis, obtaining grain size parameters (mean grain size, sorting factor, skewness, kurtosis). Sedimentary dynamic maps such as the C-M diagram were also plotted.
A lithofacies classification system based on rock type, texture, and sedimentary structures was developed for the study area using core observations, outcrop macroscopic features, and thin section identification. Component mixing and stratigraphic mixing are two fundamental mixed sedimentary characteristics. Their formation processes and sedimentary environmental significance were also examined in this study. Different types of sedimentary microfacies were identified and categorized by combining well logging facies characteristics with lithofacies analysis. Each microfacies’ sedimentary environments and identification markers were identified.
Finally, by integrating well profile comparisons, planar facies distribution, and grain size characteristics, a sedimentary evolution model for the study area was established.

4. Results

4.1. Field Outcropping Characteristics

For the analysis of sedimentary systems in the study area, field outcrop observations offer the most direct regional macroscopic evidence. The key lithofacies, mixing patterns, and sedimentary sequences in the braided-delta to shallow lacustrine environment are clearly visible in the study area’s well-exposed Upper Ganchaigou Formation section (Figure 2).
In outcrops, the algal limestone forms stable, thick-bedded units, with individual beds reaching up to 5 m in thickness and exhibiting extensive lateral continuity (Figure 2a). Its weathered surface typically exhibits a dark reddish-brown color, and the beds protrude due to their high resistance to weathering (Figure 2b). The algal limestone commonly forms a distinct vertical association with the underlying thinly bedded marl (individual beds ~20 cm thick) (Figure 2c,g).
Thick-bedded, medium-grained sandstone exhibiting massive or cross-bedding is in direct contact with the overlying marl (Figure 2d), constituting a classic macroscopic expression of clastic–carbonate sequence-scale mixing.
Furthermore, outcrops clearly reveal coarsening-upward sandstone bodies (Figure 2e) reflecting the progradational process of the delta front, and pebbly deposits (Figure 2f) representing episodic high-energy events. The combination of these features provides key macro-sedimentological evidence for the existence and evolution of the braided river delta front sedimentary system.

4.2. Lithofacies Characteristics

The lithofacies classification in this study adheres to fundamental sedimentological principles, using the primary composition, texture, and genesis of the rocks as the basis for classification. The clastic lithofacies refers to rocks predominantly formed by the mechanical transport and deposition of terrigenous detrital grains (gravel, sand, silt, clay), where carbonate components are present only as cement or in trace amounts (typically <10%). The Mixed Lithofacies refers to rocks formed by the syn-depositional involvement of both terrigenous siliciclastic and intrabasinal carbonate components (e.g., ooids, bioclasts, micritic matrix), with their combined content exceeding 10% [1]. Their formation reflects the contemporaneous interaction between sediment supply and in-lake chemical/biological depositional processes. Specific lithofacies types and their characteristics are detailed in Table 1.

4.2.1. Clastic Lithofacies

  • Gravelly sandstone facies (F1):
The facies is primarily composed of coarse sand and fine gravel, with a gravel content of approximately 5%–30% and grain sizes ranging from 2 mm to 25 mm. The gravel consists mainly of rigid clasts, such as quartz and flint, which are sub-rounded to sub-angular and exhibit medium to poor sorting. Bedding is predominantly massive or parallel, with scour surfaces commonly observed at the base (Figure 3a). It was formed in a strongly hydrodynamic environment and is mostly located in the lower part of distributary channels or in mouth bars.
  • Medium-to coarse-grained sandstone facies (F2):
Medium-to coarse-grained sandstone (Figure 3b), which is well-sorted, rounded, and has a low mud content, predominates in the facies. Cross-bedding and massive bedding are frequently created. These deposits are usually found in high-energy zones at the delta front, which correspond to mouth bars or distributary channels, and are formed under strong hydrodynamic traction flows.
  • Fine sandstone facies (F3):
The main component is fine sandstone (Figure 3c), which is well sorted and rounded, with low mud content. It has developed massive bedding and low-angle cross-bedding. It is a distal-bar deposit under relatively weak hydrodynamic conditions at the delta front, corresponding to the delta front-end channel and mouth bar.
  • Siltstone facies/muddy siltstone facies (F4):
The main components are siltstone (Figure 3d) or muddy siltstone (Figure 3e), with a high mud content. Siltstone and muddy siltstone are often inter-bedded, with massive bedding, low-angle cross bedding, and deformable bedding. It is a low hydrodynamic intensity, suspended sediment, with weak wave modification, corresponding to overflow sand or sheet sand.
  • Mudstone facies (F5):
The main component is mud, with a small amount of silt. It has massive bedding, wavy bedding, and horizontal bedding. It was formed in a low-energy still water environment and corresponds to interdistributary bay or prodelta mudstone (Figure 3f).

4.2.2. Mixed Sedimentary Facies

Algal Limestone Facies (F6): Stromatolites and laminated algal limestone are the two types of this facies. Stromatolitic lamellae and irregular interfaces are the primary sedimentary structures found in stromatolites, which are mainly made up of bioclastics, ooids, and mud. They correspond to algal mound microfacies and are formed by stromatolite and algal growth. Mudstone, bioclastics, oolitic grains, and silt make up the majority of lamellar algal limestone, which usually has horizontal or wavy bedding. The edges of algal mounds are formed by the regular alternations of mudstone precipitation, silty suspension deposition, and algal growth (Figure 3g).
  • Mixed sandstone facies (F7):
Calcareous fine sandstone and calcareous siltstone are two subtypes (Figure 3g). Fine sand, silt, oolitic grains, and intraclasts are its principal constituents. The facies exhibit massive structures, cross-bedding, and parallel bedding, which are indicative of a mix of carbonate deposits and braided-delta front sand bodies. Oolitic grains surround and mix with clastic grains. Mixed sedimentary channels, mouth bars, or beach bars are represented by the primary sedimentation, which is sandy with varying degrees of calcareous material development (Figure 3h).
  • Marl facies (F8):
Primary components are micritic, silt, and mud, with horizontal bedding. Some gray mudstone contains a small amount of siltstone interlayers, with calcareous bands/laminae and deformations. It was formed by the combined action of suspension deposition and chemical deposition under still water conditions. It is in the prodelta mixed sedimentary zone (Figure 3i).

4.3. Sediment Grain Size and Dynamics

Hydrodynamic mechanisms and the types and distributions of sedimentary facies can be inferred from sediment grain size and sedimentary dynamics. The VIII–X oil formations exhibit significantly less traction current influence, according to CM diagram analysis, indicating that their sedimentary facies were deposited in a distal delta front–shallow lacustrine bar environment, which is very different from the formations above. On the other hand, channel-fill sandstones with high C values, which indicate stronger hydrodynamic conditions, are found in the VI–VII oil formations.
These characteristics suggest that there were significant facies zone migration caused by rapid advance and retreat of the delta front in this sedimentary interval. Large-scale distributary channels possibly formed from the progradation of the distal delta plain to the proximal delta front.
According to a grain size analysis of the oil formations, the VIII–X formations are composed of relatively fine sediments with high kurtosis, which are consistent with a depositional environment dominated by beach bars. These formations are characterized by multiple superimposed thin layers at the millimeter scale. The typical grain size variation from the proximal to distal ends of a delta front is reflected in the VI–VII formations, which have coarser sediments, a wider grain size distribution, and more noticeable differentiation. A gradual increase in hydrodynamic energy is suggested by the kurtosis trends, which show an increase in sediment grain size variability. This pattern suggests that base-level cycles have an increasingly greater impact on the distribution of facies and the sedimentary system over time.
Combining the characteristics of the CM diagram (Figure 4) and the grain size analysis (average grain size, standard deviation, kurtosis, etc.) (Figure 5), the differences between the VI-VII oil formations and the VIII-X oil formations in terms of sedimentary environment can be clearly identified, which together indicate the evolution law of “high-to-low” lacustrine level.

4.4. Sedimentary Microfacies Classification and Description

The thorough description and interpretation of conventional cores serve as the foundation for the sedimentary microfacies analysis [34]. The Upper Ganchaigou Formation (N1) in the study area is a braided-river delta–littoral-shallow lacustrine beach-bar system, according to core observation, well logging response, and grain size analysis. The Upper Ganchaigou Formation (N1) can be further classified into ten sedimentary microfacies, including distributary channel, mouth bar, overflow sand, sheet sand, beach bar, mixed channel, mixed mouth bar, mixed overbank sands, mixed beach bar, and algal mound microfacies, based on variations in lithological composition and sediment grain size and dynamics.
Distributary channels are high-energy, strip-shaped channels that form in deltaic environments. Gravelly sandstone (F1) dominates the lithofacies at the base, grading upward into medium- to coarse-grained sandstone (F2) and fine sandstone (F3), showing a fining-upward sequence from bottom to top. The sand bodies have a lateral width of about 100 m and a thickness of 2 to 8 m. Well-log features include high values on acoustic (AC) curves, medium to high resistivity on resistivity (RT) curves, and “box-shaped” or “bell-shaped” patterns on spontaneous potential (SP) and natural gamma (GR) curves (Figure 6).
At the distributary channel outlets, mouth bars develop on their own. Incision and filling may take place in regions with active channels. A vertical “lower bar–upper channel” combination may form if the mouth bar deposits are not entirely eroded by distributary channels. From bottom to top, deposits of muddy siltstone/siltstone facies (F4), fine sandstone facies (F3), and medium- to coarse-grained sandstone facies (F2) cover the base of the mouth bar, which comes into contact with underlying mudstone (F5). With coarser sediments at the top and finer sediments at the base, these deposits show a coarsening-upward sequence. Each sandstone body is about 250 m wide and 2 to 5 m thick. Funnel-shaped SP and GR curves, medium to high resistivity on RT curves, and high AC values at the top are examples of well-log signatures (Figure 6).
Overbank sands developed on both sides or at the end of distributary channels. When the energy of the river flow dropped or flooding occurred, finer-grained sediments (fine sand to silt) overflew the channel and were deposited in depressions between channels or along the edges of natural bars. The main lithofacies are siltstone or muddy siltstone (F4), indicating low-energy water bodies and are generally 1–2 m thick. The spontaneous potential (SP) and natural gamma curve (GR) show a “finger-like” pattern, and the resistivity curve (RT) shows medium resistance (Figure 6).
Sheet sands were formed in areas dominated by wave or lacustrine currents, gradually transitioning into prodelta mudstone or shallow lacustrine mudstone towards the lacustrine. The lithofacies were dominated by argillaceous siltstone (F4), and high natural gamma (GR) finger-like or spike-like responses can be seen on the well logging curves. The thickness is usually 0.2–1.0 m, and there are usually multiple layers of sheet sands and mudstone interbedded vertically (Figure 6).
Beach Bars form in the littoral-shallow lacustrine zone and are persistently reworked by wave action. With thicknesses ranging from 0.2 to 1 m and a lateral extent of roughly 300 m, the lithofacies are mostly muddy siltstone and siltstone (F4), frequently interbedded with mudstone (F5). High natural gamma (GR) values, finger-like or peaked curve shapes, and low resistivity are characteristics of well-log responses. Coarsening-upward sequences are also seen in the logging curves (Figure 6).
Mixed siliciclastic–carbonate channels represent a distinctive type of channel formed under unique sedimentary conditions. Although their spatial distribution is similar to that of conventional distributary channels, their depositional environment differs. They develop in transitional settings characterized by both terrigenous clastic input and endogenous carbonate chemical or biological deposition. These channels receive both carbonate-rich waters and terrigenous clastics, leading to the mixed deposition of the two elements. They are usually found in distal regions with high water salinity. Mixed sedimentary sandstone (F7) and fine sandstone (F3) with a low clay content predominate in the lithofacies. Due to the high carbonate content, well-log responses exhibit “box-shaped” or “bell-shaped” SP and GR curves, high resistivity on RT curves, and exceptionally low AC values-with lower AC values correlating with higher calcareous content (Figure 7).
Mixed siliciclastic–carbonate mouth bar: At the outlets of mixed channels, mixed mouth bars formed. The lithofacies, which exhibit a typical reverse grading sequence, are composed of mixed sandstone (F7) and siltstone (F4). Funnel-shaped SP and GR curves, high resistivity on RT curves, and incredibly low AC values are examples of well-log characteristics (Figure 7).
Mixed siliciclastic–carbonate Overbank Sands: Usually found close to the main channel, mixed overflow sands are lateral sedimentary bodies created by the terminal overflow or lateral extension of mixed channels. The two main lithofacies are siltstone/muddy siltstone (F4) and mixed sandstone (F7), which are distinguished by weak hydrodynamics and fine grain size. In contrast to the medium to low resistivity typical of ordinary overflow sands, well-log responses exhibit “finger-shaped” GR curves, very low AC values, and high resistivity on RT curves (Figure 7).
Mixed siliciclastic–carbonate beach bar: Developed in a littoral-shallow lacustrine environment near a saline lacustrine basin, with the lithofacies mainly consisting of inter-bedded mixed sandstone (F7) and marl (F8). The natural gamma curve (GR) shows a “finger-like” or “peak-like” response, the resistivity curve (RT) shows high resistivity, and the acoustic curve (AC) shows extremely low values (Figure 7).
Algal Mounds: The algal limestone facies (F6) make up the majority of algal mounds, with trace amounts of oolitic grains and bioclastic debris. The upper and lower lithologies are mostly composed of marl facies, though they are sometimes interbedded with deformed marl (F8). Two common locations for algal limestone are the elevated central regions of marl flats, where it is created by algal growth, and the delta front sand bodies, where it forms above mouth bars and distributary channels. This distribution suggests that autocyclic processes can cause algal mounds to form in both open aquatic environments and temporarily abandoned deltaic areas. Algal limestone deposits are generally small, with widths of tens to hundreds of meters and thicknesses of 0.25 to 5 m. The spontaneous potential (SP) curve is bell-shaped, the natural gamma curve (GR) shows extremely high values and a “spiky” response, the resistivity curve (RT) shows extremely high resistivity, and the acoustic curve (AC) shows extremely low values (Figure 7).
Marl Flats: Dark gray-green marl facies (F8), which usually form in deeper water environments and have a low mud content, predominate in marl flats. The deposits frequently have multiple dolomite patches and primarily show horizontal bedding. Marl flat sediments have a wide lateral distribution and are frequently found in open-water environments like distal delta fronts and prodelta regions. Although the thickness of individual marl layers is typically less than 1 m, their planar distribution can cover a large portion of the Huatugou Oilfield. The alternating deposition of chemical and suspended sediments in shallow to semi-deep lacustrine environments is reflected in the vertical interbedding of marl layers with mudstone.

4.5. Mixed Sedimentary Characteristics

This study distinguishes between component-type and stratigraphic-type mixed sedimentary characteristics based on lithofacies analysis. Identifying these types demonstrates that their vertical distribution patterns are important markers for reconstructing the sedimentary evolution sequence and aids in elucidating the mechanisms governing mixed sedimentation [19].
(1)
Component type mixed rocks
Mixed Sedimentary Rocks by Component Type: The microscale mixing of terrigenous clastic and carbonate components, which are scattered throughout the rock strata, is what distinguishes component-type sedimentary rocks. This type is mostly found in the VIII–X oil formations and corresponds to mixed sandstone facies (F7) and marl facies (F8). Under a microscope, ooids, bioclastic fragments, and a micritic calcite matrix coexist with terrigenous grains like quartz, feldspar, and clay minerals (Figure 8).
The terrigenous clastic grains are mainly composed of silt-sized rock fragments and quartz, mostly in a mixed accumulation state. The contact relationships are mainly point-line or floating contact, with a sub-angular to sub-rounded shape and moderate sorting. The carbonate component is mainly micritic calcite, with oolitic grains or intra-clasts scattered throughout.
(2)
Sequence-Type Mixed Sedimentary Rock
Clastic–carbonate mixed sediments are characterized by the vertical alternation of terrigenous clastic and carbonate rocks, forming a clear rhythmic stratification [40,41]. They are mainly characterized by inter-bedded mudstone, sandstone, and algal limestone (Figure 9a–c). They widely developed in the VI-VII oil formations of the Upper Ganchaigou Formation. Common types in the study area include inter-bedded mudstone-algal limestone, fine sandstone–algal limestone, and medium sandstone–algal limestone-mudstone. The recurring rise and fall of the lacustrine level is reflected in these mixed sedimentary layers. Clastic layers are formed during the comparatively lower lacustrine level phase due to an abundance of terrigenous clastic material; during the higher or stable lacustrine level phase, carbonate layers form, lacustrine water clarity increases, and clastic input decreases. This kind of stratigraphic mixed rocks represents the dynamic equilibrium between chemical deposition and terrigenous supply in shallow water and is a typical sedimentary response of the lacustrine regression system tract.
The mixed sedimentary type is characterized by vertically interbedded mixed sedimentary rocks composed of calcareous siltstone, calcareous mudstone, calcareous sandstone, and muddy limestone (Figure 9d–f). They mainly developed in the VIII–X oil formations. This type of strata formed in shallow to semi-deep-water environments with variable hydrodynamics: under strong hydrodynamic conditions, mixed sedimentary sections dominated by clastic components were formed. When hydrodynamics weakened and lacustrine water chemical conditions were suitable, carbonate components precipitated or algal activity occurred, forming mixed sedimentary layers or thin carbonate rocks.
The distribution of the two aforementioned mixing patterns within the study area exhibits significant regularity and demonstrates a clear genetic linkage with the sequence stratigraphic framework and sedimentary microfacies. Component mixing dominates the transgressive systems tract (Oil Groups VIII–X) and is concentrated in microfacies such as mixed channels, mixed mouth bars, mixed overbank sands, and mixed beach bars. The persistent low-energy hydrodynamic conditions during this period facilitated the thorough intermixing of fine-grained terrigenous clastics and intrabasinal carbonate components near the sediment–water interface, thereby forming component mixing at the microscopic scale. Sequence-scale mixing, however, exhibits two typical patterns. The clastic–carbonate type predominantly develops in the delta-front environment of the regressive systems tract (Oil Groups VI–VII). It is characterized by rhythmic interbeds of algal mounds and clastic microfacies such as distributary channels and mouth bars, reflecting the periodic environmental shifts induced by high-frequency lake-level fluctuations. The mixed rock–mixed rock type is primarily distributed in the VIII–X Oil Groups. It manifests as the vertical alternation and stacking of microfacies that are inherently mixed in nature, such as mixed channels, mixed mouth bars, and mixed beach bars. This pattern records the frequent oscillations in water chemistry and depositional energy during the transgressive phase.

4.6. Vertical Evolution Sequence

The Upper Ganchaigou Formation (N1) in the study area shows clear progradational features in its vertical evolution. In the lower VIII–X oil formations, the sediments change from a littoral-shallow lacustrine beach bar–mixed sedimentary system to a braided-river delta front system in the upper VI–VII oil formations. As a result, distributary channels, mouth bars, and overflow sands replace mixed sedimentary beach bars, channels, and mouth bars as the predominant sedimentary microfacies. This vertical evolution documents gradual shifts in the supply of materials and sedimentary dynamics over time.
Analysis of the well profiles from the IX oil formation (Figure 10) indicates that during the VIII-X oil formation period, the study area had frequent interbeds of thinly interbedded mixed beach-bar sands and lacustrine mudstones. The mixed channels and mouth-bar sand bodies were limited in scale and exhibited poor lateral continuity. These characteristics suggest that the lacustrine level was high during this period, with high accommodation space, weak sediment supply intensity, and enhanced lacustrine chemical deposition and wave action.
The sedimentary environment entered a transitional phase as lacustrine water levels changed from rising to falling. The VI–X oil formations developed algal mounds at various intervals during this time because the water depths were ideal for algal blooms and carbonate production. Whereas algal mounds appeared in conjunction with delta front sand bodies in the VI–VII formations, frequently directly overlying distributary channels or mouth bars, marl flats were frequently found above and below the algal mounds in the VIII–X formations. This variation reflects a transitional response to decreasing lacustrine levels and an unstable depositional environment, showing increasingly shallower water, salinity conditions conducive to algal growth, and weakened hydrodynamics.
During the VII-VI oil formations period, the lacustrine level continued to decline, reducing the available space for sediment deposition while ensuring abundant sediment supply. The sedimentary fill during this period is characterized by thick, vertically stacked, and laterally extensive distributary channel and mouth bar sand bodies, as evidenced by well profile correlations of the VII oil formation (Figure 11). These sand bodies indicate that river action became the dominant sedimentary force, chemical sedimentation weakened, and the braided-river delta front system greatly prograded towards the center of the lacustrine basin.
Based on the analysis of the combined well profiles, the Upper Ganchaigou Formation (N1) constitutes a complete lacustrine transgression–regression cycle from bottom to top (X → VI oil formation). Its sedimentary evolution can be summarized as: transgressive mixing → high-level transition → lacustrine regression. This sequence records the systematic transformation of the sedimentary environment from “weak hydrodynamics, weak sediment supply, relatively enhanced chemical deposition, and predominantly mixed sandstone” to “strong traction currents, strong sediment supply, weakened chemical deposition, and clastic-dominated”. It is also a direct sedimentary response to the paleo-lacustrine level decrease.

4.7. Planar Sedimentary Distribution

The planar distribution of the sedimentary microfacies of the Upper Ganchaigou Formation (N1) in the study area exhibits obvious regularity. It is mainly controlled by the combined intensity of sediment supply from the west and southwest, and lake-level changes. Different oil formations exhibit obvious sedimentary differentiation characteristics due to variations in sedimentary settings (Figure 12).
Braided-river delta fronts, which were near the primary western sediment source and had a plentiful supply of clastics, dominated the VI–VII oil formations in the study area. The most developed microfacies were mouth bars and subaqueous distributary channels, which formed laterally lobe-shaped sand bodies that prograded eastward into the lacustrine basin (Figure 12a). Delta construction was constrained as the source supply weakened gradually farther into the basin. Subaqueous distributary channels shifted northeastward into thinner, more widely dispersed sheet-sand bodies in these distal regions. This spatial variation demonstrates how the distribution and architecture of the depositional system are controlled by the supply of sediment.
Algal mound microfacies developed in some intervals of the VI-VII oil formations in the study area. Algal mounds were widely distributed in the study area as patches or in continuous patterns (Figure 12b). Only sporadic distributary channels and mouth bars developed in the marginal areas near the western source area, reflecting a sedimentary environment with relatively weak water energy that was suitable for algal growth.
Accommodation space increased as lacustrine water levels reached a highstand during the VIII–X oil formations period. Fine-grained clastic sediments predominated in the deposition as the western source’s sediment supply declined. Only minor, small-scale mixed delta front subfacies developed close to the western source in the sedimentary system, which was mainly a shallow-lacustrine bar–mixed beach bar environment. Shallow-lacustrine bars and mixed beach bars, divided by lacustrine mudstone, covered large portions of the study area (Figure 12c), indicating a depositional setting with decreased sediment supply and increased wave reworking.
Near the source area, a small-scale mixed sedimentary delta front subfacies developed. It extended in a narrow band along the source direction and exhibited clear differentiation of the source-directed facies zones. Near the source area, there were mixed sedimentary channels and mixed sedimentary mouth bars. With increasing distance from the lake basin center, the water depth increased and the clastic supply weakened. It transitioned into mixed sedimentary beach bars, eventually pinching out into lacustrine mudstone (Figure 12d).

4.8. Mixed Sedimentary Models

Based on a comprehensive analysis of mixed sedimentary characteristics, vertical evolution, and planar distribution, this study established two mixed sedimentary models for the Upper Ganchaigou Formation (N1) during the lacustrine regression period (VI–VII oil formations) and the lacustrine transgression period (VIII–X oil formations) in the Huatugou oilfield (Figure 13). These two models revealed the evolutionary patterns of the sedimentary system and mixed sedimentary characteristics under the control of lacustrine level changes. The study area mainly developed delta front, pre-delta, shallow lacustrine bar, and algal mound sediments. Based on the planar distribution of sedimentary microfacies, it can be divided into four sedimentary units: the near-end delta front, middle delta front, far-end delta front, and the shallow lacustrine bar [33].

4.8.1. Sedimentary Model of the VI–VII Oil Formations

Along the source direction, large-scale distributary channels and mouth bars developed near the delta front. It had thick, vertically stacked, and laterally continuous sand bodies. In the middle delta front, a symbiotic combination of delta sand bodies and algal mounds appeared, indicating a transitional response of moderately shallow water and a more unstable environment. Further to the distal delta front and into the littoral-shallow lacustrine environment, sheet sands and thin-layered beach bars were widely deposited, eventually pinching out towards the center of the basin in pre-delta mudstone (Figure 13a). This model fully records the progradation process of the sediments towards the basin during lacustrine regression.

4.8.2. Sedimentary Model of the VIII–X Oil Formations

This period was characterized by mixed sediments within a lacustrine transgressive system (Figure 13b). The study area exhibited a broad system of mixed beach bars and deltas, showing a systematic facies sequence from the western source area towards the center of the basin: mixed deltas (mixed channels, mixed mouth bars), mixed beach bars, beach bars, algal mounds, and marl flats. This model is dominated by component mixing (including limestone and calcareous sandstone) and interbedded mixed sedimentary rocks. It reflects a spatiotemporal mixed sedimentation of continuous terrigenous clastic supply and enhanced carbonate precipitation during lacustrine-level rises.

5. Discussion

5.1. Coupling Mechanism of “Sediment Supply–Lake Level” and Its Control on Mixed Sedimentary Characteristics

The development of mixed sedimentation in the Upper Ganchaigou Formation (N1) of the study area is controlled by the synergistic interaction between lake-level fluctuations and sediment supply rates. Their coupling governs the spatiotemporal differentiation of mixing patterns and sedimentary architectures [21,30].
During the transgressive period (Oil Groups VIII–X), contrary to the interruption of terrigenous clastic supply due to increased accommodation space predicted by classic models [1], a unique “active transgressive mixing” model was formed instead. This pattern is characterized by the high-degree spatiotemporal overlap of continuous terrigenous clastic input and enhanced carbonate production, leading to the widespread development of microscale component mixing. This phenomenon differs from the traditional understanding that pure chemical sedimentation dominates during transgressive periods, revealing the critical control exerted by the persistence of short-axis sediment supply on mixing patterns in an arid climatic setting [27,28,42].
During the regressive period (Oil Groups VI–VII), high-frequency lake-level fluctuations replaced continuous mixing as the dominant control, and the sedimentary record correspondingly shifted to rhythmic interbeds of clastic and carbonate rocks—that is, stratigraphic mixing. Previous studies have noted that carbonate rocks in saline lacustrine basins mainly form during regressive periods and are controlled by lake water chemistry [43]. This study proposes that the formation of stratigraphic mixing was controlled not only by lacustrine chemical conditions but, critically, by the high-frequency lake-level-driven switching of the depositional environment between “clastic-dominated” and “chemically/biologically dominated” states. Grain-size analysis data provide dynamic evidence supporting this understanding, showing a marked increase in the variability of hydrodynamic energy from the VIII–X to the VI–VII oil groups, reflecting the intensifying control of base-level cycles.

5.2. Mixed Siliciclastic–Carbonate Depositional Model of a Braided River Delta–Littoral-Shallow Lacustrine System

During the deposition of the VIII–X oil groups in the Upper Ganchaigou Formation, the study area was predominantly within a transgressive systems tract, characterized by the widespread development of component mixing. In the medial part of the delta front, microfacies such as mixed channels, mixed mouth bars, and mixed overbank sands developed and coexisted with algal mounds. In the prodelta and littoral-shallow lacustrine beach-bar environments, mixed beach bars and algal mounds developed, collectively forming a mixed beach bar–mixed delta sedimentary system. During the transgression, as lake level rose and water deepened, terrigenous clastic supply was significantly inhibited, and this inhibitory effect intensified with increasing water depth. Under this setting, sedimentation manifested as thorough mixing at the component level (component mixing).
The VI–VII oil groups are characterized by a braided river delta front system developed under a regressive systems tract background, with sedimentary microfacies exhibiting clear zonation. The proximal delta front features vertically stacked and laterally continuous distributary channel and mouth bar sand bodies, reflecting conditions of ample sediment supply and relatively shallow lake levels. The medial delta front, in addition to the aforementioned microfacies, also includes algal mounds and sheet sands. The distal delta front is dominated by beach bar deposits. The clastic–carbonate sequence-scale mixing developed during this period is the result of the combined effects of high-frequency lake-level fluctuations and sustained, strong sediment supply [44,45,46]. High-frequency lake-level changes caused the depositional environment to switch frequently and rapidly between “river-dominated clastic depositional phases” and “chemically/biologically dominated carbonate depositional phases,” thereby forming clear rhythmic interbeds (Figure 9a–c) and directly controlling the relative thickness ratio of siliciclastic layers to carbonate layers [30].

6. Conclusions

(1)
The Upper Ganchaigou Formation (N1) in the Huatugou Oilfield of the Qaidam Basin is characterized by a braided river delta–shallow lacustrine bar mixed sedimentary system. Macroscopic features in outcrops, such as the distribution of algal limestone and the coarsening-upward sequences of the mouth bar, provided critical empirical evidence for lithofacies and microfacies classification. Based on this, core, well logging, and grain size analysis identified 8 lithofacies, 10 sedimentary microfacies, and 2 types of mixed sedimentary patterns. The lithofacies included 5 clastic lithofacies (including conglomeratic sandstone, medium- to coarse-grained sandstone, fine sandstone, siltstone/muddy siltstone, and mudstone) and 3 mixed sedimentary lithofacies (algal limestone, mixed sandstone, and marl).
(2)
The Upper Ganchaigou Formation (N1) formed a complete transgression–regression cycle from bottom to top. The sedimentation gradually transitioned from a littoral-shallow lacustrine beach bar mixed sediments to a braided-river delta front. The planar distribution was controlled by the combined influence of sediment supply intensity from the western short-axis source and fluctuations in lake level. During the transgression, it was dominated by a laterally extensive mixed beach bar. While, during the regression, it formed laterally braided-river delta front sand bodies.
(3)
A mixed sedimentary evolution model for the short-axis source in arid saline terrestrial lacustrine basins was established. Based on the “source–lacustrine level” model, two evolution models were established for the lacustrine transgression and regression periods. The unique evolution paths of “weak source, high lacustrine level, strong mixing” and “strong source, low lacustrine level, weak mixing” under the background of arid salinization were clarified. This model provides an important reference for mixed sedimentary research and oil and gas exploration in similar arid saline terrestrial lacustrine basins.

Author Contributions

Conceptualization, Y.L. and X.S.; methodology, Y.W.; software, Y.L.; validation, Y.L. and Y.W.; investigation, Y.L.; resources, W.F.; data curation, Y.L.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L. and X.S.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Science and technology project of PetroChina Company Limited (2023YQX10201, 2023ZG18).

Data Availability Statement

Data are contained within the article.

Acknowledgments

We sincerely thank the Research Institute of Exploration and Development, PetroChina Xinjiang Oilfield Company, for their invaluable support and guidance during the research and manuscript preparation process.

Conflicts of Interest

The authors declare that this study received funding from the Science and Technology Project of PetroChina Company Limited (Grant Numbers: 2023YQX10201, 2023ZG18). The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Figure 1. Location (a), structural distribution (b), and composite stratigraphic column (c) of the Huatugou Oilfield in the Qaidam Basin, China.
Figure 1. Location (a), structural distribution (b), and composite stratigraphic column (c) of the Huatugou Oilfield in the Qaidam Basin, China.
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Figure 2. Typical field outcrop features of the Upper Ganchaigou Formation. (a) Thick-bedded algal limestone with laterally extensive distribution; (b) Prominent geomorphology and weathered surface features of the algal limestone; (c) Vertical association of algal limestone overlying thinly bedded marl; (d) Direct contact between medium-grained sandstone and marl; (e) Inverse-grading (coarsening-upward) succession of a mouth bar; (f) Pebbly siltstone; (g) Vertical association of algal limestone overlying thinly bedded marl.
Figure 2. Typical field outcrop features of the Upper Ganchaigou Formation. (a) Thick-bedded algal limestone with laterally extensive distribution; (b) Prominent geomorphology and weathered surface features of the algal limestone; (c) Vertical association of algal limestone overlying thinly bedded marl; (d) Direct contact between medium-grained sandstone and marl; (e) Inverse-grading (coarsening-upward) succession of a mouth bar; (f) Pebbly siltstone; (g) Vertical association of algal limestone overlying thinly bedded marl.
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Figure 3. Typical lithofacies photographs. (a) Core from Well C5, 962.6 m, conglomeratic sandstone (F1), massive structure; (b) Core from Well C1, 1379.8 m, medium- to coarse-grained sandstone (F2); (c) Core from Well C1, 1510.19 m, fine-grained sandstone (F3); (d) Core from Well C1, 1517.69 m, siltstone (F4); (e) Core from Well C1, 1517.69 m, siltstone (F4); (f) Core from Well C1, 1542.54 m, mudstone (F5), deformed structure; (g) Core from Well C1, 1517.5 m, algal limestone (F6), deformed bedding; (h) Core from Well C1, 1538 m, mixed sandstone (F7); (i) Core from Well C1, 1545 m, marl (F8). Core locations are shown in Figure 1b.
Figure 3. Typical lithofacies photographs. (a) Core from Well C5, 962.6 m, conglomeratic sandstone (F1), massive structure; (b) Core from Well C1, 1379.8 m, medium- to coarse-grained sandstone (F2); (c) Core from Well C1, 1510.19 m, fine-grained sandstone (F3); (d) Core from Well C1, 1517.69 m, siltstone (F4); (e) Core from Well C1, 1517.69 m, siltstone (F4); (f) Core from Well C1, 1542.54 m, mudstone (F5), deformed structure; (g) Core from Well C1, 1517.5 m, algal limestone (F6), deformed bedding; (h) Core from Well C1, 1538 m, mixed sandstone (F7); (i) Core from Well C1, 1545 m, marl (F8). Core locations are shown in Figure 1b.
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Figure 4. Grain size diagram of the Upper Ganchaigou Formation core from Huatugou Oilfield (CM diagram). (a) Grain-size C-M diagram of Oil Groups VI–VII; (b) Grain-size C-M diagram of Oil Groups VIII–X. NO: Coarse material transported by rolling; OP: Dominantly rolling with minor suspension; PQ: Dominantly suspension with minor rolling; QR: Graded suspension segment; RS: Uniform suspension segment (Modified from [39]).
Figure 4. Grain size diagram of the Upper Ganchaigou Formation core from Huatugou Oilfield (CM diagram). (a) Grain-size C-M diagram of Oil Groups VI–VII; (b) Grain-size C-M diagram of Oil Groups VIII–X. NO: Coarse material transported by rolling; OP: Dominantly rolling with minor suspension; PQ: Dominantly suspension with minor rolling; QR: Graded suspension segment; RS: Uniform suspension segment (Modified from [39]).
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Figure 5. Mean grain size, standard deviation, and kurtosis characteristics of core samples from the Huatugou Upper Ganchaigou Formation. (a) Mean grain size of Oil Groups VI–VII; (b) Mean grain size of Oil Groups VIII–X; (c) Standard deviation of Oil Groups VI–VII; (d) Standard deviation of Oil Groups VIII–X; (e) Kurtosis of Oil Groups VI–VII; (f) Kurtosis of Oil Groups VIII–X.
Figure 5. Mean grain size, standard deviation, and kurtosis characteristics of core samples from the Huatugou Upper Ganchaigou Formation. (a) Mean grain size of Oil Groups VI–VII; (b) Mean grain size of Oil Groups VIII–X; (c) Standard deviation of Oil Groups VI–VII; (d) Standard deviation of Oil Groups VIII–X; (e) Kurtosis of Oil Groups VI–VII; (f) Kurtosis of Oil Groups VIII–X.
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Figure 6. Well-log interpretation template for conventional clastic sedimentary microfacies.
Figure 6. Well-log interpretation template for conventional clastic sedimentary microfacies.
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Figure 7. Well-log interpretation template for mixed sedimentary microfacies.
Figure 7. Well-log interpretation template for mixed sedimentary microfacies.
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Figure 8. Mixed sedimentary Microscopic Characteristics of the Upper Ganchaigou Formation. (a) Core from Well C1, 1531.01 m, crossed-polarized light (XPL), calcareous sandstone containing ooids. (b) Core from Well C1, 1530.6 m, XPL, calcareous sandstone with minor ooids. (c) Core from Well C1, 1505.50 m, XPL, calcitic sandstone containing minor intraclasts. (d) Core from Well C5, 976.64 m, XPL, calcareous siltstone. (e) Core from Well C3, 1597.55 m, XPL, packstone containing minor terrigenous clastics. (f) Core from Well C3, 1643.30 m, XPL, calcareous siltstone. (g) Core from Well C3, 945.55 m, XPL, sandy micrite. (h) Core from Well C3, 1650.45 m, XPL, calcareous mudstone containing 25% micritic calcite. (i) Core from Well C3, 1655.30 m, XPL, argillaceous micrite with approximately 30% clay minerals and 15% silt to very fine sand.
Figure 8. Mixed sedimentary Microscopic Characteristics of the Upper Ganchaigou Formation. (a) Core from Well C1, 1531.01 m, crossed-polarized light (XPL), calcareous sandstone containing ooids. (b) Core from Well C1, 1530.6 m, XPL, calcareous sandstone with minor ooids. (c) Core from Well C1, 1505.50 m, XPL, calcitic sandstone containing minor intraclasts. (d) Core from Well C5, 976.64 m, XPL, calcareous siltstone. (e) Core from Well C3, 1597.55 m, XPL, packstone containing minor terrigenous clastics. (f) Core from Well C3, 1643.30 m, XPL, calcareous siltstone. (g) Core from Well C3, 945.55 m, XPL, sandy micrite. (h) Core from Well C3, 1650.45 m, XPL, calcareous mudstone containing 25% micritic calcite. (i) Core from Well C3, 1655.30 m, XPL, argillaceous micrite with approximately 30% clay minerals and 15% silt to very fine sand.
Minerals 16 00092 g008
Minerals 16 00092 g009
Figure 9. Types of mixed sedimentary strata of the Upper Ganchaigou Formation in the study area. (a) Core from Well C1, 1374.0 m; (b) Core from Well C5, 990.7 m; (c) Core from Well C1, 1389.6 m; (d) Core from Well C1, 1546.5 m; (e) Core from Well C1, 1526.3 m; (f) Core from Well C1, 1516.4 m.
Figure 9. Types of mixed sedimentary strata of the Upper Ganchaigou Formation in the study area. (a) Core from Well C1, 1374.0 m; (b) Core from Well C5, 990.7 m; (c) Core from Well C1, 1389.6 m; (d) Core from Well C1, 1546.5 m; (e) Core from Well C1, 1526.3 m; (f) Core from Well C1, 1516.4 m.
Minerals 16 00092 g009
Minerals 16 00092 g010
Figure 10. Profile of sedimentary microfacies of connected wells in the NW-SE direction of the IX oil formation (profile location is shown in Figure 1b, B-B’).
Figure 10. Profile of sedimentary microfacies of connected wells in the NW-SE direction of the IX oil formation (profile location is shown in Figure 1b, B-B’).
Minerals 16 00092 g010
Minerals 16 00092 g011
Figure 11. Profile of sedimentary microfacies of connected wells in the NW-SE direction of VII oil formation (profile location is shown in Figure 1b, B-B’).
Figure 11. Profile of sedimentary microfacies of connected wells in the NW-SE direction of VII oil formation (profile location is shown in Figure 1b, B-B’).
Minerals 16 00092 g011
Minerals 16 00092 g012
Figure 12. Planar distribution of sedimentary microfacies. (a) Oil Group VI, Sub-layer 17; (b) Oil Group VI, Sub-layer 5; (c) Oil Group VIII, Sub-layer 5; (d) Oil Group VIII, Sub-layer 9.
Figure 12. Planar distribution of sedimentary microfacies. (a) Oil Group VI, Sub-layer 17; (b) Oil Group VI, Sub-layer 5; (c) Oil Group VIII, Sub-layer 5; (d) Oil Group VIII, Sub-layer 9.
Minerals 16 00092 g012
Minerals 16 00092 g013
Figure 13. Sedimentary model diagram of the Upper Ganchaigou Formation (N1) in Huatugou Oilfield. (a) Depositional model of Oil Groups VI–VII; (b) Depositional model of Oil Groups VIII–X.
Figure 13. Sedimentary model diagram of the Upper Ganchaigou Formation (N1) in Huatugou Oilfield. (a) Depositional model of Oil Groups VI–VII; (b) Depositional model of Oil Groups VIII–X.
Minerals 16 00092 g013
Table 1. Lithofacies classification and characteristics of the Upper Ganchaigou Formation in the Huatugou Oilfield.
Table 1. Lithofacies classification and characteristics of the Upper Ganchaigou Formation in the Huatugou Oilfield.
Lithofacies TypeLithofacies (Code)CompositionGenetic InterpretationSedimentary Microfacies
Clastic LithofaciesGravelly sandstone facies (F1)Gravel (5%–30%) and sandHigh-energy traction
currents; channel lag or basal mouth bar		Distributary
channels, mouth bars
medium- to coarse-grained sandstone facies (F2)Dominantly medium and coarse sandStrong traction currentsDistributary
channels, mouth bars
Fine sandstone facies (F3)Dominantly fine sandModerate to weak traction currentsTerminal
distributary
channels, mouth bars
Siltstone
facies/muddy
siltstone facies (F4)		Dominantly siltLow-energy suspension
settling with minor
reworking		Overbank sands, sheet sands, beach bars
Mudstone (F5)Dominantly clay
minerals with minor silt		Suspension deposition in quiet waterInterdistributary bays, prodelta
Mixed LithofaciesAlgal Limestone
Facies (F6)		Abundant algal
laminae/stromatolites, ooids, bioclasts		Dominantly biological/chemical depositionAlgal mounds
Mixed sandstone
facies (F7)		Terrigenous sand/silt (matrix) and carbonate components (>10%)Syn-depositional mixing of terrigenous input and
carbonate precipitation/growth		Mixed channels, mixed mouth bars, mixed beach bars
Marl facies (F8):Micritic calcite matrix and terrigenous clay/silt (>10%)Mixing of chemical precipitation and fine-grained
suspension deposition		Marl flats, prodelta mixed zone
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MDPI and ACS Style

Liang, Y.; Song, X.; Wang, Y.; Feng, W. Development of Braided River Delta–Shallow Lacustrine Siliciclastic–Carbonate Mixed Sedimentation in the Upper Ganchaigou Formation, Huatugou Oilfield, Qaidam Basin, China. Minerals 2026, 16, 92. https://doi.org/10.3390/min16010092

AMA Style

Liang Y, Song X, Wang Y, Feng W. Development of Braided River Delta–Shallow Lacustrine Siliciclastic–Carbonate Mixed Sedimentation in the Upper Ganchaigou Formation, Huatugou Oilfield, Qaidam Basin, China. Minerals. 2026; 16(1):92. https://doi.org/10.3390/min16010092

Chicago/Turabian Style

Liang, Yuxin, Xinmin Song, Youjing Wang, and Wenjie Feng. 2026. "Development of Braided River Delta–Shallow Lacustrine Siliciclastic–Carbonate Mixed Sedimentation in the Upper Ganchaigou Formation, Huatugou Oilfield, Qaidam Basin, China" Minerals 16, no. 1: 92. https://doi.org/10.3390/min16010092

APA Style

Liang, Y., Song, X., Wang, Y., & Feng, W. (2026). Development of Braided River Delta–Shallow Lacustrine Siliciclastic–Carbonate Mixed Sedimentation in the Upper Ganchaigou Formation, Huatugou Oilfield, Qaidam Basin, China. Minerals, 16(1), 92. https://doi.org/10.3390/min16010092

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