Introduction

Stratigraphic overlap refers to the overlaying relationship between layers, with younger layers overlaying older layers. This phenomenon is often closely related to the evolution of sedimentary facies. Sedimentary facies refer to the characteristics of sediments formed in different sedimentary environments, including the type, composition and structure of sediments1. The evolution of sedimentary facies is influenced by many factors, including tectonic activity, sea level change and sediment input2,3,4. When the sedimentary environment changes, the sedimentary facies will also change. The phenomenon of stratigraphic overlap often occurs in areas of complex tectonic activity or sedimentary environmental changes5. When fault subsidence or uplift occurs, strata may be misaligned, folded or overturned, resulting in younger strata overlaying older strata and forming stratigraphic overlap6,7. The causes of stratigraphic overlap are multifaceted, including the subsidence of basin edges caused by tectonic movements, deepening of water bodies due to changes in sedimentary environments, and alterations in sediment sources. These influencing factors interact with each other, resulting in new strata covering the basement of underlying strata or erosion zones, forming unconformable contacts8. The pattern of overlapping sedimentation at the edge of a basin is usually influenced by various factors, such as sediment sources and sedimentation rates. Moreover, overlying sedimentation is often accompanied by the downward movement of the crust, and this structural change can lead to the deepening of water bodies and the expansion of sedimentary ranges, thus forming new sedimentary environments. From the analysis of the whole tectonic evolution process, in the uniform uplift or subsidence movement of the strata, the strata overcomplex presents a relatively apparent gully filling and replenishment type of deposit, and there is a good correspondence between the sedimentary layer and the trend of the ancient terrain9,10,11. Therefore, the pinch-out lines of sedimentary sand bodies in the same period often have the same burial depth characteristics. In the target interval, the sedimentary sand body is usually distributed in a ring along the palaeo-geomorphic gullies and overlap belts, and the sedimentary facies are usually parallel to the stratigraphic boundary from the occurrence; further, the position of the formation pinch-out line is the position of the sedimentary sand body pinch-out12. Therefore, there is a close relationship between stratigraphic overlap and the evolution of sedimentary facies. The study of stratigraphic overlap can provide relevant clues for us to understand stratigraphic development and geological history, and it can help infer the evolution of sedimentary facies and environmental changes.

In the fields of geology and sedimentology, studying the relationship between stratigraphic overlap and sedimentary facies evolution is a challenging and highly relevant topic13,14,15. Previous research has mainly focused on case studies of typical regions or transitional zones between land and sea, such as the Appalachian Basin in the United States and the Surat Basin in Australia16,17. However, these studies are often limited to specific geological backgrounds and sedimentary environments, and the understanding of the relationship between stratigraphic overlap and sedimentary facies evolution in a wider range of regions is still limited18,19,20. In view of the above background and current situation, this study aims to explore the relationship between stratigraphic overlap and sedimentary facies evolution in the Hala’alat Mountain area in the northwest margin of the the Junggar Basin, China, and fill in the gaps in research in related fields. Hala’alat Mountain area is located in the northwest margin of the Junggar Basin, which is a region with complex geological structure and diverse sedimentary processes. The phenomenon of stratigraphic overlap is common in this region, and the types of sedimentary facies are abundant, making it an ideal place to study the relationship between stratigraphic overlap and sedimentary facies evolution.

Based on the above background and current situation, by comprehensively analyzing the morphology, distribution, and relationship with sedimentary facies of stratigraphic overlap, the impact mechanism of stratigraphic overlap on sedimentary facies evolution is revealed, providing a new perspective for understanding complex geological structures and sedimentary processes21. Through this study, we can further deepen our understanding of the geological structure and sedimentary history of the Hala’alat Mountain area. The phenomenon of stratigraphic overlap records the tectonic activity and sedimentary processes during geological history, and is an important clue for understanding regional geological evolution. This study will combine various research methods such as geological survey, logging interpretation, and seismic profile interpretation to reveal the complexity of geological structures and the diversity of sedimentary history in the region22,23,24. By revealing the relationship between stratigraphic overlap and sedimentary facies evolution, scientific basis and guidance can be provided for oil and gas exploration and development. Understanding the distribution and evolution patterns of sedimentary facies can help identify favorable reservoirs, while studying the phenomenon of stratigraphic overlap can help determine the distribution range and reservoir characteristics of oil and gas reservoirs.

Geological background

The Junggar Basin is located in the northern part of Xinjiang and is the second-largest inland basin in China (Fig. 1a). The Junggar Basin is surrounded by folded mountain systems, with the Zhayier and Hala’alat mountains to the northwest, the Qinggeheidi and Kelameili mountains to the east and the Heibiergen and Bogeda mountains to the south (Fig. 1b). The study area is located on the uplift of the northwest margin of the Junggar Basin and the slope belt of the southern margin of the Hala’alat Mountain structure25. The area is bordered by Heshituoluogai Basin to the north, Mahu Sag to the south, Mount Zhayier to the west and Yingshi Depression to the east (Fig. 1c).

Fig. 1
figure 1

Location map of the study area: (a) location of the Junggar Basin, (b) location of the Hala’alat Mountain in the Junggar Basin, and (c) location of the study area. (This figure was drawn using Coreldraw, version 2018. https://www.coreldraw.com/cn/).

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The southern margin of the Hala’alat Mountain is developed by the Jurassic Badaowan Formation (J1b), Sangonghe Formation (J1s), Xishanyao Formation (J2x), Tousunhe Formation (J2t), Qigu Formation (J3q), the Cretaceous Qingshuihe Formation (K1q), Hutubi Formation (K1h) and Ailikehu Formation (K2a). The target horizon of this study is the Qingshuihe Formation and Hutubi Formation of the Cretaceous, and the Ailikehu Formation is missing in the study area. The Qingshuihe Formation is divided into the second member of the Qingshuihe Formation (K1q2) and the third member of the Qingshuihe Formation (K1q3). The first member of the Qingshuihe Formation has been eroded, and no stratigraphic division has been conducted in the study area. The Hutubi Formation is divided into Hutubi Formation member 1 (K1h1), Hutubi Formation member 2 (K1h2) and Hutubi Formation member 3 (K1h3) (Fig. 2).

Since the Triassic, the the Junggar Basin has mainly undergone the transformation of Indosinian movement and Yanshan movement, and unconformities are generally developed between each sequence. At the same time, there are many local unconformities within each layer system. The Cretaceous basin sank as a whole, and the stratum thinned from south to north and from east to west, forming a unified whole with the southwest as the sedimentary center, ending the north-south division of the the Junggar Basin. The Cretaceous system in front of the Hala’alat Mountain is not fully developed, with a relatively flat geological morphology and overlying sedimentation on the old strata. The Cretaceous system is mainly composed of a set of shallow lacustrine sediments, which include gray and grayish yellow medium to coarse sandstone, as well as gray green, brownish red, and brownish red sandy mudstone and mudstone. These different types of rock layers appear in the form of interbedded layers of varying thickness and are widely distributed throughout the basin26.

The K1q of the Cretaceous System exhibits overall characteristics of obvious transgressive deposition with water progression and regression, featuring developed sand bodies in its lower part. The bottom is relatively coarse-grained, often composed of glutenite, pebbly sandstone, and coarse sandstone deposits, transitioning upwards to pebbly sandstone and medium-to-fine sandstone, with glutenite deposits also present. Overall, the grain size is finer compared to the lower part. The intercalated mudstones in the lower part are mostly gray, followed by green-gray, while the mudstone intervals at the top are predominantly gray and dark gray. The colors of the mudstones indicate that the deposition in this area is dominated by a transitional facies between water and land environments, with a subsequent expansion of the lake area in the later stage. The K1h is mainly composed of gray-green fine sandstone and brown, maroon, and gray-green mudstone interspersed with calcareous sandstone, with a gray-green breccia at the base. The overall lithology is relatively fine-grained, with some sections showing a rhythmic feature of upward coarsening. The gamma and acoustic time-difference curves present a finger-shaped pattern, indicative of deposition in a shore shallow lake beach bar facies.

Fig. 2
figure 2

Comprehensive stratigraphic column of the Hala’alat Mountain area.

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The phenomenon of formation overlying the mountain front can be seen directly by establishing the profile fence diagram of strata connecting wells in the study area. The strata in the piedmont are close to the provenance, and the southward strata gradually expand in the longitudinal range. The Cretaceous strata in the piedmont area gradually peak out to the north due to the overlap, so the Cretaceous strata appear to be thick in the south and thin in the north27 (Fig. 3).

Fig. 3
figure 3

Stratigraphic well connection profile fence map of the study area. (This figure was drawn using Coreldraw, version 2018. https://www.coreldraw.com/cn/).

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Databases and methods

Because of the limitations of the resolution of seismic data, it is difficult to predict the thickness and the formation pinch-out line directly by using seismic data. In the present study, the phase analysis technique of the 2D seismic section and the angle extrapolation method of formation are used to determine the stratigraphic overlap points, and good results are obtained. Through core observation and description, log data analysis and grain size analysis, the relationship between Cretaceous stratigraphic overlap and sedimentary facies evolution in the study area is studied. The basic data mainly include 54 well logs and approximately 210 km2 of 3D seismic data. All the data are provided by the Exploration and Development Research Institute of Shengli Oilfield, Sinopec. In addition, the cores of 21 major core wells in the Cretaceous system in the study area are observed and described in detail. The core depth is between 80.0 and 585.5 m, and the total length of the observed cores is 1,506.3 m. The colour, lithology, stratification and sedimentary structure of the rocks were analysed by taking photos and sampling more than 100 cores.

Thus, we study the division and distribution of different sedimentary facies caused by Cretaceous stratigraphic overlap in the study area and finally establish the sedimentary models of fan delta facies and beach bar facies in the study area.

Results

During the Cretaceous period, the tectonic activities of the Junggar Basin were multi-cycle strike-slip uplift and intracontinental isostatic subsidence28. The Cretaceous strata in the southern margin of Hala’alat Mountain are underdeveloped, with gentle occurrence, a monoclinal structure of southeast dip and overlying the old strata extensively. The overlap identification of a sand body based on seismic data mainly uses the contact relationship between different seismic response characteristics corresponding to different sedimentary facies and the underlying layer to determine the overlap boundary of the sand body. However, due to the accuracy of seismic data, the termination of the in-phase axis of seismic reflection near the stratigraphic overlap line does not mean the termination of the stratigraphic overlap line, and there is a certain extrapolation distance between the identified overlap points on the seismic profile and the actual overlap points29. Therefore, it is necessary to use the phase analysis technique and the extrapolation formula for stratum angle to identify and characterise the stratigraphic overlap boundary.

Phase analysis technique

This study is based on seismic stratigraphy and constrained by a sedimentary model to describe and characterise the seismic facies. The fine seismic calibration shows that the Cretaceous strata overlay the Jurassic and Carboniferous strata in a wedge-angle unconformity contact. The seismic response characteristics of the Cretaceous are mainly controlled by the sedimentary environment, palaeotectonic setting and hydrodynamic conditions30,31. The tectonic activity in the southern margin of Hala’alat Mountain is relatively weak; the slope is periodically uplifted slowly, and the seismic facies are medium–low frequency with weak amplitude oblique reflection. The backaccretion sequence can be seen in the profile, corresponding to the characteristics of water inflow deposition. The fan delta facies are developed, and apparent foreaccretion facies can be identified in the seismic facies (Fig. 4a). The instantaneous phase parameter is the optimal indicator of lateral continuity because it has little relationship with the intensity of seismic reflection and is mainly related to the continuity of seismic reflection on the in-phase axis. Therefore, the transient phase is particularly useful for describing geological phenomena such as stratigraphic overlap, pinch-outs and small faults. In the instantaneous phase profile along the source, the overlap boundary appears to be phase inversion, which is more sensitive to the lateral overlap of the sand body, and the overlap point along the up-dip direction of the sand body overlap is clearly visible (Fig. 4b).

Fig. 4
figure 4

Identification of the Cretaceous stratigraphic overlap boundary in the study area: (a) seismic profile along the source and (b) instantaneous phase profile along the source.

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Stratigraphic angle extrapolation method

The stratum angle extrapolation method establishes the relationship between the error of the stratum pinching line and the angle between the stratum and the unconformity surface through a large number of forward modelling and geological statistical methods to obtain the corresponding extrapolation distance for the target area. The main frequency of seismic data in this area is 25 Hz. The dip angle of the unconformity surface at the bottom of the Cretaceous system in the study area is approximately 4.6°, and the dip angle of each layer is between 2.3° and 3.2°. The velocity of the K1q in the research area is 2,600 m/s, and the velocity of the K1h is 2,300 m/s. The overlap boundary of the strata is calculated using the angle extrapolation formula. The larger the dip angle difference is between the strata and the unconformity, the smaller the extrapolation distance is between the overlap point and seismic reflection. The smaller the dip angle difference is between the strata and the unconformity surface, the greater the error is between the extinction point and seismic reflection. When the angle difference between the stratum and the unconformity is 1.4°, the extrapolation distance can reach 1,059.09 m. When the angle difference between the stratum and the unconformity is 2.3°, the extrapolation distance is 570.81 m (Table 1).

$$\:x=\frac{V\text{*}\text{c}\text{o}\text{s}\left({a}_{2}\right)\text{c}\text{o}\text{s}\left({a}_{1}\right)}{4f\text{*}sin({a}_{1}-{a}_{2})}$$
(1)

where V is the formation velocity (m/s), f is the earthquake frequency (Hz) and a1 and a2 are the formation dip angles (°).

Table 1 Extrapolation distance of stratum angle.
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Fitting revealed a significant power exponent relationship between the angle of the unconformity surface and the extrapolation distance of the overlap point (Fig. 5). By using the power exponent relationship between the dip angle difference and the variation of the stratigraphic overlap point, a scientific quantitative extrapolation template was established in the study area, and the relationship between the extrapolation distance y of the overlap point and the angle x of the stratigraphic overlap point in the study area was obtained as y = 1,578x− 1.242.

Fig. 5
figure 5

Error curve between stratum angle and extrapolation distance of stratum overlap point.

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Determination of the location of the overlap boundary

Based on the seismic data construction interpretation of the overlap line, we select the seismic profile perpendicular to it, measure the angle and extrapolate the overlap point using the fitted angle extrapolation template to determine the location of the overlap boundary (Fig. 6).

Fig. 6
figure 6

Location of the overlying boundary of the Cretaceous strata in the study area. (This figure was drawn using Coreldraw, version 2018. https://www.coreldraw.com/cn/).

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Lithologic characteristics

Based on the observation and description of 21 core wells in the study area (Table 2), it is believed that the Cretaceous K1q2 in the study area developed fan delta sedimentary facies. Due to the occurrence of stratigraphic overlap in the Cretaceous, representative grey gravel rocks gradually overlap forward32. According to the observation of the core well, the well with a relatively complete formation from K1q to K1h exhibits a positive rhythm feature, and the bottom of the formation is grey sandstone. Although some wells have missing strata, the bottommost strata of the Cretaceous system are characterised by the development of sandstone and conglomerate, which are generally grey in colour. Therefore, it is judged that the Cretaceous system in the study area mainly developed coarse-grained sedimentation of fan delta facies first, and as the grain size became finer, the sedimentary facies transitioned to shore shallow lake beach bar facies sedimentation.

Table 2 Data from cretaceous coring wells in the study area.
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Due to the absence of K1q1 in the Cretaceous system in the study area, the overall rock lithology and grain size of K1q2 are relatively coarse, mainly composed of grey sand conglomerate and fine conglomerate, with some grey sandstone mixed in the middle. The gravel has poor sorting ability and is mixed with sandstone for accumulation (Fig. 7a and c). The upwards grain size is relatively fine, with grey, greyish green fine sandstone and siltstone appearing. The lithology of K1q3 and K1h has changed, resulting in grey and greyish green mudstone, fine sandstone (Fig. 7d), siltstone (Fig. 7e) and muddy sandstone (Fig. 7f). The sand particle sorting and roundness are relatively good.

Fig. 7
figure 7

Lithology of Cretaceous rocks in the study area. (a) Hq21-q17 well, K1q2, 451.39 m, grey glutenite; (b) Hq21 well, K1q2, 458.08 m, grey oil-immersed fine conglomerate; (c) Hq21-q10 well, K1q2, 434.19 m, grey fine conglomerate; (d) Hq23-q1 well, K1h1, 313.44 m, grey fine sandstone; (e) Hq23-q50 well, K1h2, 237.03 m, grey siltstone and (f) Hq23-q49 well, K1h3, 168.67 m, argillaceous siltstone.

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Grain size characteristics

Due to the stratigraphic overlap phenomenon in the Cretaceous system of the study area, the coarse-grained sand and gravel sediment provided by the material source continuously overlies forward under hydrodynamic and tectonic action, resulting in the distribution of coarse-grained sediment in both K1q and K1h33 (Fig. 8a and c). Due to the continuous advancement of the sequence boundary from K1q, there is a phenomenon of grey sand conglomerate distributed in the bottom layer of the study area (Fig. 8d and f).

Fig. 8
figure 8

Grain size of Cretaceous rocks in the study area. (a) Hq23-q49 well, K1h1, 254.08 m, grey glutenite; (b) Hq23-q48 well, K1h2, 154.95 m, grey glutenite; (c) Hq23-q24 well, K1h2, 192.51 m, grey glutenite; (d) Hq23-q14 well, K1h2, 162.66 m, grey glutenite; (e) Hq23-q42 well, K1h3, 91.00 m, grey glutenite and (f) Hq23-q4 well, K1h3, 130.35 m, grey glutenite.

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In summary, the Cretaceous strata are divided into fan delta facies and beach bar facies. K1q2 only develops fan delta facies, whereas K1q3, K1h1, K1h2 and K1h3 both develop fan delta facies and beach bar facies.

Logging facies characteristics

The standard for dividing the amplitude of natural potential in this study is as follows: the amplitude is higher than the baseline by more than 1.5 cm as high amplitude; if the amplitude is 0.5–1.5 cm higher than the baseline, it is considered medium amplitude; an amplitude higher than the baseline of 0–0.5 cm is considered low. The main recognised logging curve shapes are smooth box or bell-shaped, toothed box or bell-shaped, toothed funnel-shaped, serrated and smooth tooth or finger-shaped34. By analysing the basic elements such as the combination shape, amplitude, top–bottom contact relationship and smoothness of these curves and combining them with other logging interpretation conclusions, the fan delta facies and beach bar facies in the study area were subdivided, laying the foundation for further characterising the planar distribution of sedimentary facies (Fig. 9).

Fig. 9
figure 9

Characteristics of well-logging curves for the Cretaceous fan delta and beach bar facies in the study area.

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Seismic facies characteristics

Based on the information of well-seismic calibration, the seismic facies response characteristics of fan delta facies and beach bar facies in the study area are summarized (Fig. 10). The underwater distributary channel sand body is the framework of the fan delta sand body. When the fan body is pushed into the stable water body, the channel becomes wider and spreads radially, and the seismic facies appears as a filling shape with a relatively wide and gentle concave feature vertically. The sand body at the front of the fan delta is reformed by the lake water to form shore-shallow lake beach bar deposits. The beach bar is the general term for beach sand and bar sand. It is difficult to distinguish them due to the limited accuracy of seismic data, so they are collectively referred to as beach bars here. The beach bar sand body is widely distributed. In the thicker region of the beach bar sand body, the seismic event axis will show slight bending or amplitude anomaly, showing short-axis medium-intensity reflection in a mound shape. The seismic facies of shallow lake mud is a sheet-like parallel or subparallel medium-amplitude, good continuity, and medium-frequency reflection.

Fig. 10
figure 10

Characteristics of seismic facies for the Cretaceous fan delta and beach bar facies in the study area.

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Discussion

Seismic facies analysis

Seismic facies analysis is the process of determining the type of seismic facies based on a series of seismic reflection parameters, and interpreting the sedimentary facies and sedimentary environment represented by these seismic facies. The research area has abundant drilling data in the Hq21 and Hq22 well areas. Based on the rich lithological and logging data, it is believed that there is widespread development of beach bar sediments, mainly composed of siltstone. The logging facies are box shaped and finger shaped beach bar facies integrated with hilly seismic facies, thereby delineating the sedimentary range of beach and dam facies and clarifying the distribution of beach bar facies. In areas with scarce drilling (no wells), due to the fact that the results of seismic facies analysis are mainly environmental facies and have weak ability in lithology discrimination, there are certain deficiencies and shortcomings in the conversion of sedimentary facies. At this time, it is necessary to combine sedimentary environment and classify seismic facies based on four aspects: location (shelf, shelf edge, slope), external morphology (hill, wedge, plate, etc.), internal structure (parallel, oblique, divergent, etc.), and reflection wave attributes (amplitude, frequency, continuity, etc.). The sand and gravel rocks at the boundary of the research area are relatively developed, and the terrestrial clastic sediments are mainly brought by rivers. The main water flow path can be determined by identifying erosion valleys on the seismic profile or by sandwiching high ground with low ground on the contour map. There are many concave filling or lens shaped seismic facies in the vicinity of the Hq23 well area. The southeastern strata of the research area have a large sedimentary thickness, and the seismic facies are mainly characterized by strong continuous parallel reflections of moderate amplitude, which are delineated as shallow lake sediments. Therefore, establishing an analysis method for seismic sedimentary facies can effectively improve the ability to distinguish sedimentary facies, achieve accurate reconstruction of sedimentary systems, and have important value for oil and gas exploration.

Sedimentary facies distribution

During the Cretaceous sedimentary period in the study area, K1q1 was missing, whereas K1q2 was close to the source and relatively abundant, mainly from the northwest direction, with the development of fan delta sedimentary facies. Due to the influence of the Yanshan Movement, the Cretaceous system overlapped towards the front of the Hala’alat Mountain, resulting in angular unconformity with the Jurassic system. In the early Cretaceous sedimentary period, large-scale lake intrusion occurred, and Mahu expanded towards the front of the mountain35. K1q3 evolves from fan delta facies to shore shallow lake beach bar facies. Under the influence of tectonic movements, fan delta facies are distributed at the edge of the overlying line, with isolated distribution on a small scale. Since K1q3, the beach bar facies of the shallow lake have become the main sedimentary facies in the study area, with a wide distribution range of beach bar facies parallel to the shoreline.

During the K1h sedimentation period, water influx continued, and the range of beach bar facies in the shallow lake further expanded. The fan delta facies further move northward and contract along the overlying line, and due to the influence of tectonic movements, their area is relatively small. As the river enters the water body towards the south, its energy gradually decreases and the sediment load increases, forming an environment of shallow lake beach bar facies with slow water flow speed and low sediment transport capacity. During this process, the branches of the river show a scattered trend, forming multiple branching channels, which is a clear feature of the transition from fan delta facies to shore shallow lake beach bar facies. In addition, as the water surface rises, the coastal and shallow lake environments continue to expand, and sediment starts to deposit in the direction of water depth, resulting in large-scale beach bar facies sedimentation (Fig. 11).

Fig. 11
figure 11

Plan of Cretaceous sedimentary facies in the study area. (This figure was drawn using Coreldraw, version 2018. https://www.coreldraw.com/cn/).

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Sedimentary model

The Cretaceous system in the study area developed fan delta facies and beach bar facies of shallow lakes. The Hala’alat Mountains in the north served as the sedimentary source area, and the water body carried debris from the source area to form fan delta facies in front of the mountains. Due to the overthrust movement, the fan delta uplift overthrust is closer to the mountain body, and the subfacies of the fan delta front are more developed. The sedimentary grain size of the distributary channel is coarse, and due to strong hydrodynamic forces, the water flow continues to carry debris material and spread southward. After rivers enter lakes, their hydrodynamic strength weakens and they can only transport fine-grained materials. The continuous lake intrusion causes the lake level to rapidly rise, and the effects of lake waves and coastal currents are enhanced. The sediment at the edge of the lake basin is transformed, ultimately forming a parallel distribution of shore shallow lake beach and bar facies sediment.

The Cretaceous system in the study area developed fan delta facies and shore shallow lake beach bar facies. The Hala’alat Mountain in the north served as the sedimentary source area, and the water body carried debris from the source area to form fan delta facies in front of the mountains. Due to the overthrust movement, the fan delta uplift overthrust is closer to the mountain body, and the subfacies of the fan delta front are more developed. The sedimentary grain size of the distributary channel is coarse, and due to strong hydrodynamic forces, the water flow continues to carry debris material and spread southward36. After rivers enter lakes, their hydrodynamic strength weakens and they can only transport fine-grained materials. The continuous lake intrusion causes the lake level to rapidly rise, and the effects of lake waves and coastal currents are enhanced. The sediment at the edge of the lake basin is transformed, ultimately forming a parallel distribution of shore shallow lake beach bar facies sediment37 (Fig. 12).

Fig. 12
figure 12

Sedimentary model of the Cretaceous system in the study area. (This figure was drawn using Coreldraw, version 2018. https://www.coreldraw.com/cn/).

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Conclusions

The stratigraphic overlap boundary is the main controlling factor for the pinching-out boundary of sedimentary facies sand bodies. The fan delta facies sedimentation in the study area retreats towards the front of the mountain and forms sedimentary layers that overlap towards the source area. This study uses phase analysis to preliminarily identify the stratigraphic overlap boundary and uses the stratigraphic angle extrapolation method to accurately identify the stratigraphic overlap boundary, improving the recognition accuracy of sedimentary sand body boundaries.

By clarifying the genetic mechanism and spatial distribution of stratigraphic overlap, this study is helpful to better understand the complex structures and sedimentary processes that formed these basins, and is of great significance to understand the evolution of the the Junggar Basin and other structurally complex basins. In addition, accurately identifying the overlapping boundaries of strata can provide a deeper understanding of the pinch out boundaries of sedimentary sand bodies, which is crucial for oil and gas exploration and development in such basins.

By observing the lithology, particle size and logging curve of the core well, it was found that the first section of the Qingshuihe Formation (K1q1) in the study area is missing. Starting from the second section of the Qingshuihe Formation (K1q2) at the bottom of the Cretaceous system, grey sand conglomerates were developed first, with finer particle sizes upwards, and grey, greyish green fine sandstone and siltstone appeared. Therefore, the Cretaceous system in the study area mainly developed coarse-grained sedimentation of fan delta facies first, and as the grain size became finer, the sedimentary facies transitioned to shore shallow lake beach bar facies sedimentation.

Due to the stratigraphic overlap phenomenon in the Cretaceous system of the study area, the coarse-grained sand and gravel sediment provided by the sediment source continuously overlaps forward under hydrodynamic and tectonic action, resulting in the distribution of coarse-grained sediment in both the Qingshuihe Formation (K1q) and the Hutubi Formation (K1h). The second member of the Qingshuihe Formation (K1q2) only develops fan delta facies. By contrast, the third member of the Qingshuihe Formation (K1q3), the first member of the Hutubi Formation (K1h1), the second member (K1h2) and the third member (K1h3) both develop fan delta facies and beach bar facies.