Study on the sedimentary characteristics of braided fluvial fan

Abstract

As a typical tight sandstone gas reservoir in the Ordos Basin, the second member of the Shanxi Formation in the XQ gas field in the Ordos Basin has had rich gas reservoirs and good development effects in the past. On the basis of previous studies, this paper brings new understanding to the sedimentary model of this area. Nine kinds of single lithofacies and seven kinds of lithofacies combinations are determined. Braided fluvial fan deposits in distributive fluvial systems likely developed in this area. The study area is part of the middle subfacies of this sedimentary facies, and five kinds of sedimentary microfacies environments developed. Finally, establishing sedimentary model based on sedimentary characteristics. The braided fluvial fan sedimentary model based on DFS is of great significance for deepening the understanding of terrestrial sedimentary systems and providing guidance for the exploration and development of oil and gas bearing basins.

KEYWORDS:

• Ordos basin
• distributive fluvial system
• braided fluvial fan
• sedimentary facies

1. Introduction

Braided rivers are a unique sedimentary morphology; they were classified by Rust based on the river planform, river curvature index and braid index and are characterized by the lateral instability of the river, more variability and significant physical variation within a reservoir [1]. Typically, larger areas have multiple rivers, which are influenced by topography and other factors to form alluvial fans, delta deposits, etc. Fan deltas, an important category of deltas, were originally proposed by Holmes as alluvial fans or braided rivers that advance from adjacent uplands into stable bodies of water. As with alluvial fans, McGowen and Galloway classified fan deltas into arid fans and wet fans and those under arid climates and wet climates. Depending on the type of climate, fan delta plains in humid climates are usually braided river systems, and a new term, “braided deltas”, was later used by McPherson et al. to represent deltas that advance from braided floodplains into stable bodies of water [2]. Orton restricted the concept of braided river deltas to deltas formed by single braided rivers or braided systems with low curvature rivers. Braided deltas, also known as braided river deltas, are sand- and gravel-rich, as are braided rivers, with braided diversion plains fed by single or multiple bottom-loaded rivers and coarse clastic deltas. Their development is controlled by seasonal floods or by the flow of mountain rivers [3]. Their subfacies are divided into braided delta plains, braided delta fronts and pre-braided deltas. The morphologies of the sand bodies are dendritic or finger-like in plane, with a vertical or oblique intersecting distribution of fixed water bodies and a discrete broom-like shape in profile [4].

For a long time, many of the terrestrial deposits in China have been classified partly as deltas with varying morphologies, such as developing rivers and alluvial fans in mountain passes [5]. Galloway and Hobday classified alluvial fans into five types; the first two are arid fans and wet fans, where arid fans are those formed in arid environments and wet fans are those formed in humid climates. Subsequently, Galloway and Hobday generalized alluvial fans into three types, namely, alluvial fans, fan deltas and terminal fans, and divided them into mudflow fans, fluvial fans and sheet flow fans on a triangulation diagram based on the main types of alluvial fan sedimentation. On the triangulation diagram, the three terminal components of alluvial fans are divided into debris flow fans, fluvial fans and sheet flow fans [6]. Through continuous development, the basic types of alluvial fans have gradually branched out into mudflow fans, terminal fans, meandering fluvial fans, braided river fans and many others, which have expanded the concept of alluvial fans and gradually caused difficulty in classifying some specific terrestrial clastic deposits [7,8]. The original concept of alluvial fans states that they are onshore coarse clastic fan deposits formed by gravity and fluvial flows in arid and semiarid areas, usually smaller than medium size, while those formed by fluvial action on a large scale can be referred to as fluvial fans or large fluvial fans [9–11]. The concept of the fluvial fan was proposed by scholars studying alluvial fans on which sudden flows of water developed lobar bodies. These lobar bodies are usually less than 10 km, but fans with lobar bodies greater than 10 km are often observed in flat, open terrestrial environments, which cannot be largely explained by alluvial fan theory. Schumm introduced fluvial action into fan deposits with a stable fluid supply [12]. Nemec believed that fluvial action generally exists in alluvial fans; that is, fan deposits are still formed after rivers enter relatively wide and flat areas from high-lying areas.

In 2007, North et al. defined fluvial fans as a system of divergent geometries formed by frequent and continuous channel diversions on geological time scales without horizontal constraints, in which the depositional characteristics of a single channel are similar to those with ordinary fluvial deposition [13], which defines the depositional characteristics of a fluvial fan. Until 2010, the academic community still considered fluvial fans to be a special case of alluvial fans, and the idea of fluvial fans and alluvial fans came from Hartley. He proposed the distributive fluvial system (DFS), which includes fluvial and impact-dominated radiolarian sediments with lengths of 1–700 km. This range includes alluvial, fluvial and megafan systems [14], all three of which are radially spreading fluvial systems; although they all have radially spreading channels, they are not all fan-shaped. Alluvial fans are mainly generated via gravity flow and sheet flow, and river fans are mostly generated by traction and drainage. In terms of scale, the radius of an alluvial fan is usually no greater than 30 km, the radius of a fluvial fan is usually no less than 10 km, and the radius of a giant fan is greater than 100 km. These two points determine the difference between them in terms of river configuration, sediment transport type and structural maturity. Research has shown that the tectonic setting, physical supply and climatic conditions of an area can all control the development of branched river systems, which can develop into different distributive river systems under certain conditions [15,16]. Alluvial fans often develop in arid climates with high slopes, terminal fans tend to develop in arid climates with moderate to low slopes, braided fluvial fans mainly develop in semiarid-humid climates with moderate to low slopes, and meandering fluvial fans more often develop in humid climates with low to very low slopes [17–19].

Among the abovementioned river fans, braided river fans are deposited in the form of multiple braided rivers intertwined or superimposed on each other [20]. Braided fluvial fans are similar to wet fans of the alluvial fan type and, like the braided rivers in the river system, are characterized by numerous and frequently incised channels, frequent and irregular internal channel diversions, the occasional establishment and erosion of river cores, and poorly developed floodplains and outfall fans between individual channels [21,22].

The characteristics of DFS are influenced by many factors, which the tectonic background and climatic conditions during sedimentation are important factors controlling its formation and characteristics [23]. DFS is developed in all tectonic regimes [24–26]. The scale of DFS under different tectonic backgrounds. Generally, DFS developed in strike slip systems are relatively short, and the scale is usually less than 20 km. DFS developed in a compressive environment is the longest, and it developed on the edge of craton basin is the largest, extending for hundreds of kilometers [14]. In this tectonic context, the channel morphology of DFS is mainly braided channel [26]. Climate conditions have a significant impact on sediment supply. Some scholars have studied many giant fans and found that their development is influenced by significant seasonal flow fluctuations. The vast majority of giant fans are developed in low latitude areas, with large fluctuations in river flow and strong instability [14,27]. On the other hand, the development of DFS depends on the fluvial system’s ability to get rid of the restriction of valleys and the system’s lateral diffusion ability. Therefore, such as terrain slope, sediment supply and flow are secondary control factors, and the plane shape of braided channels is related to larger slope, larger sediment supply and flow [28,29]. In addition, the rapid migration and frequent breaches of fluvial channels are important factors in the formation of large-scale DFS. Sediments generated under alternating humid and arid climate conditions exceed the carrying capacity of the river, leading to channel blockage, which in turn leads to river channel breaches and migration [30,31].

As a relatively new geological concept, there is currently relatively little research on the underground under the distributive fluvial system. The Neogene glutenite is relatively developed in Chepaizi area, the Junggar Basin, China. The sand bodies in the whole study area are widely distributed in sheet shape. The sedimentary structure is a variety of cross bedding that shows the type of traction hydrodynamic force. It is believed that the braided river fan deposits developed in the Shawan Formation strata in this area [32], and areas similar to this sedimentary model include the lower member of He 8 in Sulige gas field [33]. Similar theories have been used to study the main controlling factors of reservoirs in areas such as the Abu Madi Formation in the northeastern Nile Delta of Egypt [34], and good results have been achieved. The above sedimentary examples are partially consistent with the characteristics of the distributive fluvial system. In addition, there are many DFS studies based on modern sedimentary systems, and the establishment of sedimentary models and configuration knowledge bases through modern sedimentary examples [35] can effectively provide a new theoretical basis for predicting the distribution of underground sedimentary systems, conduct comprehensive theoretical research on old oil fields, and improve the accuracy of reservoir identification [36]. In summary, the sedimentary models of distributive fluvial system can display a larger range of river sedimentary system characteristics than individual channel facies models, which is of great significance for in-depth understanding of the sedimentary systems of continental oil and gas bearing basins and guiding oil and gas exploration.

The XQ gas field is a large gas field in the Ordos Basin in China. A number of studies have been carried out in the area [37,38], most of them favoring braided deltaic deposits and agreeing that the source is in the northern part of the study area, but there are different views on the causes of the braided delta formation [39]. Some scholars believe that the Shan II section contains braided river deltaic deposits, possibly including the braided river deltaic plain phase [40]; other scholars believe that the southern part of the Yulin gas field is adjacent to a combination of river deltaic deposits and shallow marine deposits and has a marine deltaic nature [41].

A recent study of the deposition in the XQ gas field has provided new insights into the depositional pattern of the area. The author believes that the abovementioned braided fluvial fan deposits are developed in the area overall. Due to the large reservoir in the XQ gas field area, exploration results over the years have shown that the main river channel in the northern part of the field splits into multiple channels to the south, with internal bifurcation and very frequent diversions, large lateral variations in the reservoir sand body, complex and variable geological conditions, and the main river channel is basically overlain by multiple braided rivers. By comparing the characteristics of the braided fluvial fan depositional system with those of the braided river delta system, the differences between the two are illuminated, leading to the conclusion that the depositional system in this area has characteristics that are different from those of the braided river delta [5] (Table 1).

Table 1. Differences in the sedimentary characteristics of braided fluvial fans and deltas.

Type	Braided Fluvial Fan	Delta
External geometry	Radial spread, fan	Bird’s-footed or banded
Location	Open depressions on the edge of mountainous areas of the basin, adjacent to high ground	The edge of a stable water body such as a sea lake
Scale	Radius greater than 50 km	Dozens to hundreds of kilometers
Hydrodynamic conditions	Strong – medium	Medium – weak
Sediment transport mechanisms	Traction flow dominated	Presence of traction currents, modified by waves, sea/lake
Type of end	End termination	Injection into a stable water body such as a sea/lake
Lithology	Conglomerate sandstone muddy siltstone with high miscellaneous base content	Sandstone and dark mudstone with low miscellaneous base content
Typical deposition sequence	Positive rhymes dominate	Antirhyme and compound rhyme

This paper analyzes and discusses the characteristics of the braided fluvial fan depositional model by studying the new depositional perspective of the XQ area as well as to improve understanding of the characteristics of the braided fluvial fan depositional system.

2. Geological background

The XQ gas field is located in Central China and the northeastern Ordos Basin. It is approximately 80 km long from north to south and 51 km wide from east to west. The terrain of the whole study area is high in the northeast and low in the southwest, which is a typical Monocline. The structure is in the northeast southwest trend, with small slope, and the whole area is relatively flat. This relatively stable structure provides good forming conditions for large gas fields (Figure 1a).

Figure 1. (a) Location of the Ordos Basin and locations of structures in the study area. (b) Stratigraphic division of study area. (c) Cycle division of well S8.

Based on the rich research results of predecessors in this area, combined with the exploration and development of oil and gas fields in this area for many years, a mature stratigraphic division scheme is obtained. The Shanxi formation is the lower Permian formation of the Permian system (Figure 1b). The main reservoir of Shanxi formation is Shan 2 member (S2), which is divided into three sub member. The key layer in this study is Shan 23 sub member (S23). The sub member S23 is divided into three sand formations: S23-1, S23-2 and S23-3 by utilizing lithology change, logging curves, and base level cycle change. The determination of base level cycle characteristic is based on the INPEFA technique (Figure 1c).

The full name of INPEFA is Integrated Prediction Error Filter Analysis, which is a technique of maximum entropy spectrum analysis. The maximum entropy spectrum estimation value is the most suitable value obtained by inferring the previous part of the logging values that can reflect the characteristics of the formation, and subtracting the maximum entropy spectrum estimation value from the actual logging value to obtain the Prediction Error Filter Analysis. It can infer the changes of ancient water depth, and perform specific integration processing on the PEFA curve to obtain the Integrated Prediction Error Filter Analysis (INPEFA). INPEFA-GR can reflect the trend of sand and mudstone content, and determine the characteristics of base-level cycle by inferring A/S values based on the changing trends of sandstone and mudstone. It can be seen that S23 develops three sets of short-term base-level cycles, with S23-1 and S23-3 indicating a rise in the base-level cycle and S23-2 indicating a decrease in the base-level cycle. This reflects the changing characteristics of river water volume and sediment discharge during different periods.

3. Methods

In this study, we obtained multiple rock samples from multiple coring wells at different depths. Using these rock samples, a large number of detailed study of rock lithology, sandstone type, particle size and sorting characteristics. Combined with core observation, determine the sedimentary environment. On this basis, identify lithofacies associations and determine sedimentary microfacies. By analyzing the geological background and existing research results, establish the characteristics and distribution characteristics of sedimentary microfacies, and ultimately establish a sedimentary model for this type of sedimentary microfacies.

4. Results

4.1. Lithologic characteristics

The lithology of the second member of S2 in the XQ gas field is mainly sandstone, with a sandstone content of approximately 60%, which is mainly medium sandstone, coarse sandstone and fine conglomerate. The stratum thickness of the whole study layer is approximately 30 m. The stratum has unequal thickness interbedding composed of medium-coarse sandstone, conglomerate and mudstone, and the lower part is mainly dark gray black mudstone or carbonaceous mudstone, coarse sandstone and fine conglomerate, The middle and upper parts are mainly composed of light colored mudstone and medium sandstone (Figure 3a). The thickness range of a single sand body is 3∼6 m, with an average of 4 m. According to Folk’s three-category sandstone classification method, the relative contents of quartz, feldspar and rock debris show that the main types of sandstone are quartz sandstone and rock debris quartz sandstone, followed by rock debris sandstone. Combined with core analysis in other areas, the sediment source area in the study area is quartzite or quartz sandstone, which lacks feldspar. (Figure 2).

Figure 2. Lithologic characteristics of the study area: (a) statistical diagram of sandstone particle size. (b) distribution histogram of mudstone color. (c) lithologic classification of sandstone. (d) sample photos under the microscope: I-Pure quartz sandstone, MP27, 2803 m; II-Lithic quartz sandstone, AP213, 2960 m; III-Lithic sandstone, XQ7, 2808 m.

The colors of sedimentary rocks usually indicate the types and contents of iron-bearing authigenic minerals and organic matter and can accurately indicate the sedimentary environment. Core observations reveal that there are many oxidized (light gray, red-purple, and red) mudstones in the area overall, indicating that the whole study area was in an oxidizing-weakly reducing environment during the depositional period. The mudstone in the lower part of the target interval in the northern area is dark gray, suggesting relatively strong reducibility, which is basically recognized as a weakly reducing environment. The mudstone in the lower part of the target interval in the southern area is relatively light in color, most of which is brownish red, reflecting the strong oxidation of the environment. The difference in redox conditions in the sedimentary environment between the southern and northern areas reflects the environmental difference in the same period. Thus, the environment in the northern area was humid or the water body was deep, and the water body in the southern area was relatively shallow.

An analysis of the structural maturity of clastic rock samples from coring wells reveals that the sandstone is mainly coarse sandstone, the main particle size is within 0.5–2 mm, the roundness of clastic particles is mostly subrounded, the sorting is medium to good, it has porous cementation and is particle-supported. The structural maturity of sandstone in the study area is relatively good, which is consistent with the sedimentary characteristics in the middle part of the fluvial facies (Table 2).

Table 2. Statistics of the structural maturity of clastic rocks.

Wellname	Measured depth (m)	Granularity (mm)	Roundness	Sorting property
G7	2694.59	1.85	Subcircle	Medium
	2696.2	1.9	Subcircle-Subangular	Fine
	2697.2	1.7	Subangular	Medium
MP47-12	2718.67	1.3	Subcircle-Subangular	Fine
	2719.34	4	Subangular	Fine-Medium
	2759.79	3.2	Subangular	Medium-Fine
AP155	2799.06	1	Subcircle	Fine
BC139	2859.61	1.4	Subcircle-Subangular	Medium
	2865.5	1.3	Subcircle-Subangular	Medium
	2866.72	1.8	Subcircle	Medium-Fine
AP218	2808.95	1	Subcircle	Fine
AP210	2832.73	2	Subangular-Subcircle	Medium
	2835.97	1.6	Subangular-Subcircle	Medium
MP31	2806.65	0.85	Subcircle-Subangular	Medium
AP146	2659	1.75	Subangular	Fine
	2660.13	2.8	Subangular	Fine

4.2. Sedimentary structures

The core observation of the study area shows that the sedimentary structures in this area have strong hydrodynamic characteristics. Bottom scouring structures are well developed in the layered structure of the second member of the Shanxi Formation; abundant large trough cross-bedding is present, and some layers contain muddy gravel and mud debris. Various bedding structures of channel sediments with high reaction energy are relatively developed, and the grain size fines upward overall, mainly developing low-angle cross-bedding, plate cross-bedding and parallel bedding. Overlapping scour structures are found in some wells, which is obvious evidence of a sedimentary environment in a fluvial facies system (Figure 3).

Figure 3. Sedimentary structures in the study area: (a) Trough cross bedding: Well XQ7, 2833.50 m; Well MP24, 2678.52 m. (b) Bottom washout: MP47-6A, 2904.45 m; AP210, 2571.20 m. (c) Tabular cross bedding: Well XQ7, 2829.90 m; Well MP47-6A, 2903.20 m. (d) Parallel bedding: Well AP209, 2930.51 m; Well MP31, 2835.50 m.

The grain size structure of sediments is the result of sediment source rocks, hydrodynamic conditions and transport distance. In different hydrodynamic environments, due to the influence of hydrodynamic conditions and action time, the sorting of sediments is different. There are three transport modes of sediment particles in aqueous media: rolling, jumping and suspension. The grain size parameters of the sandstone in the study area are obtained by using the cast slice observation method. After classification, it is concluded that the curve types in the study area not only reflect the two-stage (Figure 4a), two-stage transition (Figure 4b) and three-stage (Figure 4c) sedimentary environments of the river but also reflect the complex hydrodynamic environment of the braided river sedimentary environment with repeated migration and diversion.

Figure 4. Cumulative probability curves of sandstone grain size (a, b, and c) and C–M diagram (d).

A C–M diagram is a diagram drawn by applying the C and m values of each sample. The C–M diagram of the core from the XQ gas field indicates that most of the samples are in the traction and drainage area, with complex hydrodynamic conditions and various debris handling modes. The C–M diagram mainly illustrates the OP section, PQ section and QR section. The QR section is the most developed and indicates an area with gradual suspension. It is characterized by a proportional increase in C to m, which is often found in river facies (Figure 4d).

4.3. Lithofacies

The grain size range of sediments is relatively small, the types of sedimentary rocks are relatively unified, and the sedimentary sequences are relatively similar. It is difficult to distinguish their microfacies from rock types alone, and it is also difficult to understand the relationship between different microfacies. Therefore, this study used lithofacies characteristics to identify sedimentary microfacies. Based on the identification of lithofacies types, single lithofacies and composite lithofacies models are established.

4.3.1. Single lithofacies

According to the rock fabric characteristics, bedding features and grain sequence in the study area, nine lithofacies types are identified in this area. The morphology and characteristics of each lithofacies are shown (Figure 5).

Figure 5. Single lithofacies types and their corresponding relations with sedimentary structures and facies.

These lithofacies include the following: 1. Glutenite facies (Gp), which is mainly plate-shaped cross-bedding and wedge-shaped cross-bedding, and the corresponding environment is longitudinal sand dam or braided channel bar. 2. The glutenite facies (Gt) is mainly trough cross-bedding, and the corresponding environment is a filled braided channel with strong hydrodynamic forces. 3. Medium sand to coarse sand (St) facies includes trough cross-bedding at the bottom, and the corresponding environment involves migration of tongue shaped dunes. 4. From medium sand to coarse sand (Sp), the geometries are plate cross-bedding and massive bedding, and the corresponding environment is usually laterally migrating braided rivers. 5. From fine sand to medium sand (Sh), parallel bedding is often developed, corresponding to an environment with a low flow regime. 6. Silty-fine sandstone facies (Fh), thin layer sandstone with fine particle size and wavy bedding reflects weak hydrodynamic intermittent sedimentation. 7. Thin interbedded sand and mud (Fl) is dominated by massive sedimentary structures, and the environment is usually a floodplain. 8. Mudstone facies (Fm), in which sedimentary structures are massive correspond to a lake swamp environment. 9. Coal or carbonaceous mudstone (C), Mainly coal seams or carbonaceous mudstones, representing the reducing environment of still water such as swamp.

4.3.2. Lithofacies combinations

There are mainly 7 types of lithofacies combinations in the braided river fan sedimentary facies of the Shan 2 member in the study area, namely, Fm-Sp-Gp-Fm, Sh-Sp-St-Fm, Fm-Sp-St-Fm, Fm-Sp-Fm-Gt-Fm, Fl-Fh-Fm, Fm-Fl-Fm, and Fm-C-Fm (Figure 6).

Figure 6. Seven combinations of lithofacies types of braided fluvial fan facies in the study area (different individual lithofacies are superimposed vertically according to certain rules).

These lithofacies assemblage types have corresponding sedimentary environments. 1. Combination 1 corresponds to a bar in the fluvial channel. At the center of the channel with a strong hydrodynamic force, the material transported by the river channel accumulates to form a channel deposit above the water surface. 2. Combination 2 corresponds to channel bar and braided channel, which is related to the vertical superposition of multistage channels. The vertical length is within 3–7 m, botton is primarily the main channel. 3. Combination 3 corresponds to a braided channel with relatively weak hydrodynamic force, its thickness is smaller than combination 2, and it is mostly branch channels. 4. Combination 4 corresponds to a braided channel and channel bar, showing a vertical flood period and drought period. 5. Combination 5 is interpreted as overbank deposition, with siltstone and fine sandstone, mainly wavy bedding and interbedded sandstone and mudstone usually exists. 6. Combination 6 is interpreted as floodplain environments, with low sandstone content, interbedded sandstone and mudstone and a large number of mudstones. 7. Combination 7 is interpreted as lacustrine and swamp environments and mainly coal seams or carbonaceous mudstones.

4.4. Distribution characteristics of sand bodies

By combining seismic attribute inversion, single well core observation, and logging interpretation in the study area, the distribution of sand bodies was analyzed, and a sand body thickness map for the S23 period in the study area was drawn (Figure 7). From the figure, it can be seen that the distribution of sand bodies in the study area gradually expands from north to south, presenting a bird foot like distribution as a whole. There are thick sand deposits in the middle of the sand bodies in various periods, with a maximum of over 12 m. Most low thickness sand bodies are mainly distributed in the southern region, with a thickness of less than 3 m. From the overall trend of changes, the thickness of sand bodies during the S23-1 period significantly decreased, reflecting a decrease in sand body sedimentation in the later stage.

Figure 7. Distribution of sand body thickness during the S23 period in the XQ gas field (Due to the need to comply with oilfield confidentiality requirements, the well numbers shown in the diagram have been encrypted and only partial well numbers are displayed).

5. Discussion

5.1. Research on sedimentary type and architecture division

Usually, the distributive fluvial system has the following characteristics: a radioactive channel feature distributed by an endpoint on a plain. As the fluvial flows downstream, the size of the fluvial gradually decreases, and the particle size of debris gradually decreases, with no restrictions on the edges of the fluvial. According to Wessinan’s theory, the scale of medium-sized DFS is usually greater than 30 km and is mainly in the progradation mode as a whole.

On the basis of core observation and description, and based on the analysis of sedimentary facies indicators such as sedimentary environment, lithological characteristics, and sedimentary structures, it is believed that the study area developed a sedimentary model of DFS. The planar morphology of the DFS is influenced by both relative unloading capacity and sediment supply. Based on research on the classification scheme of fluvial fans in the relevant distributive fluvial system (Figure 8), it is believed that the XQ gas field developed braided fluvial fans. The depositional facies was that large braided channels, under strong traction and drainage, continuously branch out on sedimentary areas with medium to low slopes, forming a branched distribution of onshore sedimentary systems dominated by sandstone. According to the characteristics of the sand body, it is believed that the research area is located in the middle of the whole braided fluvial fan, which means that the entire research area is in the middle of the braided fluvial fan subfacies area.

Figure 8. Schematic diagram of DFS plane shape types and influencing factors [10], Modified (a) Mudflow fan. (b) Laminar fan. (c) Fluvial end fan. (d) Braided fan. (e) Near shore end fan. (f) Braided fluvial fan. (g) Meandering fluvial fan.

Based on the mutual combination types and morphological characteristics of lithofacies, a series of genetic units can be divided. In the study area, the grade four architecture level is the sedimentary microfacies level, with a total of 5 types: channel bar, braided channel, overbank, flood plains and lacustrine swamp. Based on Miall’s Hierarchy of fluvial-delta depositional architecture, the sedimentary architecture levels and corresponding sedimentary facies types of the study area are established (Table 3).

Table 3. Sedimentary architecture levels and sedimentary facies.

Architecture levels	6th	5th	4th
Sedimentary facies	Middle section of river fan	Braid flow zone	Channel bar
			Braided channel
			Overbank
		Fluvial side plain	Floodplain
			Lacustrine swamp

5.2. Microfacies characteristics

Based on the lithologic characteristics, sedimentary structures, sedimentary facies markers, and sedimentary background and environment of the Ordos Basin, it is determined that the sedimentary facies is a braided fluvial fan deposit. It can be concluded that the study area was located in the middle subsection of the whole braided fluvial fan, and the subfacies of the whole area is the middle subfacies fan (Figure 9a). The sedimentary microfacies in the study area can be divided into five different parts: lacustrine swamp, floodplain, overbank, braided channel and channel bar. The logging response characteristics, sand body shape, sedimentary structures and lithology of each sedimentary microfacies sand body are very different.

Figure 9. Characteristics of the microfacies (a) Schematic diagram of middle subfacies fans sedimentary facies of in braided fluvial fans. (b) Channel bar. (c) Braided channel. (d) Overbank. (e) floodplain. (f) lacustrine swamp.

5.2.1. Channel bar

The channel bar is mostly developed in braided channels, with a sand body thickness of over 4 m. The lithology is mainly medium sandstone, coarse sandstone, and fine conglomerate, with parallel and cross bedding developed. There are multiple particle size mutations. Overall, it is dominated by a positive rhythm, and the GR curve is a serrated box type or a smooth box type (Figure 9b).

The channel bar is a type of sedimentation developed within braided channels, usually in the middle of the river. As the channel widens, the flow velocity of water in the braided channel decreases, resulting in the carrying debris gradually accumulating in the center of the channel under aggradation, ultimately forming the channel bar. Whenever the research area is in a flood period, the core beach will become larger with the increase of debris material, and a large number of medium to coarse grained debris material will overlap each other in the longitudinal direction. In the middle of large sandstone sections, there is usually a sedimentary layer dominated by mudstone.

5.2.2. Braided channel

The thickness of braided channel sand body is generally between 2 and 5 m, and the thickness of channel sand body can reach more than 5 meters in a flood period. The lithology is mainly composed of medium sandstone, coarse sandstone, and fine conglomerate. Unlike the channel bar, it develops parallel bedding, oblique bedding, and large channel like cross bedding, with the phenomenon of multiple overlapping and tangential river channels. At the bottom of the sand body, stagnant gravel deposits often occur. Overall, it is a positive rhythm, and the GR curve is a bell type (Figure 9c).

Braided channel is the main sedimentary microfacies type in the study area. During the sedimentary period of the second member of the Shanshan Formation, with strong hydrodynamic conditions, forming the braided channel. The fluvial flow carried a large amount of medium to coarse grained debris into the study area, and under the action of progradation, the content of sandstone in the channel was very considerable. Due to humid climate conditions, the fluvial flow in braided channels varies greatly. During flood periods, the water overflowed and form fluvial side plain.

From the core samples, it can be seen that the bottom of the river channel is mostly in contact with the underlying strata through erosion, with some showing abrupt contact. The lithology of the strata in contact with the bottom sandy gravel rock includes gray mudstone, carbonaceous mudstone, and coal seams. Some of the sandy gravel rocks at the bottom of river channels contain mud debris and carbon debris from the underlying strata, which indicates that these fluvial channels are developed in a fine-grained wetland environment, thereby limiting the cutting and erosion of the underlying strata by braided channel (Figure 10). Therefore, when the main braided channel develops to areas with weaker hydrodynamic strength, it will bifurcate, i.e. it will be distributed in a tree like manner on the plane, forming a planar form of distributive fluvial.

Figure 10. Contact relationship at the bottom of the braided channel: (a) Core sample. (b) Schematic diagram of ancient fluvial flow in the research area.

5.2.3. Overbank

Overbank sedimentation is formed by the overflow of fine-grained sedimentary materials carried by flood into the main river channel. Vertically, it is composed of thin layers of sandstone and mudstone interbedded with different lithofacies types alternating multiple times. The thickness of this deposit is generally 1∼2 m. It is mainly composed of silty-fine sandstone and mudstone. And sedimentary structures are mainly composed of wavy bedding, horizontal bedding, and block bedding. The GR curve presents a low amplitude finger shape (Figure 9d).

From a plane perspective, the overbank sedimentation is developed on both side of the channel, with a relatively small and limited scale. As it moves laterally away from the channel, its thickness gradually thin and its particle size becomes finer.

5.2.4. Floodplain

The thickness of the sand body is generally between 1 and 3 m, with the lithology mainly composed of fine siltstone and mudstone, and there are often sand and mud interbeds. The GR curve presents a jagged shape. Unlike overbank, their core are darker in color, more inclined to the restorative color, and have lower sand content (Figure 9e).

Floodplain is the sedimentary microfacies with the largest distribution range in the study area during the sedimentation period. It is usually formed by the accumulation of braided channels overflowed the riverbed during the flood season. Therefore, the water overflowing the riverbed carries some debris sediments to be distributed in the area around the channel. Due to the weak hydrodynamic conditions on the shore, the sandstone carried is usually fine-grained, while the rest is mainly composed of mudstone. After the flood period, the water recedes from the area, leaving behind the aforementioned substances for sedimentation.

5.2.5. Lacustrine swamp

This facies is mainly composed of mudstone and carbonaceous mudstone, with a thickness of 1∼4 m; coal seams occasionally appear in the middle, and the GR curve shows no obvious low-amplitude characteristics (Figure 9f).

This type of sedimentary microfacies is also common in modern fluvial sedimentary systems. During the sedimentation period, lacustrine swamp was developed as wich and swamps between channels, with a small scale and no fixed water source injection. The main source of water bodies is the overflow of channels during flood periods and the retention in the lower parts of the structure between the channels. The warm and humid climate during the sedimentary period allowed for its widespread development, which itself was a fine-grained deposit. Due to the aforementioned sedimentary characteristics, the sandstone content in this sedimentary microfacies is very low, and the rest is mainly composed of large sections of mudstone. In addition, it is rare for coal seams or carbonaceous mudstones to directly cover the top of large sections of sand bodies, which further indicates that the sedimentary microfacies of lacustrine swamp was in an inter channel environment and do not come into contact with channels.

It is worth mentioning that many molded fossils of pteridophyte have been found in large sections of dark mudstones of some core samples (Figure 11). This pteridophyte often exists in the terrestrial environment, generally located in warm and humid swamps, which further reflects that large sections of dark mudstones in the study area belong to the sedimentary microfacies of lakes and marshes.

Figure 11. Molded Fossils in core sample (a) Well AP364, 2836.5 m. (b) Well AP122, 2890 m.

By summarizing the logging response pattern and other characteristics of the five sedimentary microfacies, the sand body shape is predicted, and the corresponding relationship is classified in Figure 12.

Figure 12. Corresponding relationship between sedimentary microfacies and logging curves.

5.3. Single well vertical microfacies

In the whole study area, by observing the core and the above research contents, the single well facies diagram of MP47-6A in the south is drawn, which vertically shows the sedimentary microfacies corresponding to the sedimentary lithofacies combinations in this area (Figure 13).

Figure 13. Sedimentary microfacies profile of well MP47-6A.

The figure shows the single well facies diagram of wells MP47-6A and S23 in the study area. The coring depth is 2882.00∼2915.28 m. The sandstone lithology of the cored section is mainly medium and coarse sandstone and fine conglomerate. The sedimentary microfacies are mainly channel bar, floodplain, overbank, lacustrine swamp and braided channel deposits. Overall, sandstone was well developed with multiple continuous sand bodies. In the core section from 2883 to 2890.3 m, multiple stages of channel bar stacking were developed, mainly with tabular cross bedding. At the bottom of it, coarse-grained braided channel were developed, with trough cross bedding. There is a pressure suture structure at 2893 m. In the core section of 2895.25∼2897 m, there is an overlap of overbank and braided channel, which have unstable characteristics, indicating that the braided channel in this area belong to wandering braided fluvial. The core section of 2912.5∼2915 m belongs to lacustrine sediments, mainly composed of mudstone and coal seams, with almost no sand.

5.4. Distribution and model of sedimentary facies

5.5.1. Sedimentary microfacies distribution

The distribution of sand bodies is analyzed through the sandstone thickness map combined with the provenance direction and sedimentary background of the sedimentary period of this horizon and the above sedimentary microfacies characteristics. The corresponding sedimentary microfacies distribution map of the S23 member is drawn (Figure 14).

Figure 14. Distribution of sedimentary microfacies during the S23 period in the XQ gas field (Due to the need to comply with oilfield confidentiality requirements, the well numbers shown in the diagram have been encrypted and only partial well numbers are displayed).

It can be seen that the main channel in the northern source area gradually branches southward into multiple branch channels, with a larger scale and many channel bar inside the channel. Lacustrine swamps were distributed on the floodplains in the fluvial side plain. It is worth mentioning that during the S23-2 period, the number and scale of the channel bars were relatively large, and the smoothness of the channel was also better than the other two periods, which is related to the characteristics of the base level cycle during that period [42].

5.5.2. Sedimentary model

Deposition in the Ordos Basin where the XQ gas field is located began in the Ordovician. After the Taiyuan period, marine deposition ended after seawater withdrew from the region. In the Shanxi period, the deposition changed into continental deposition. The paleo-Asian Ocean in the northern part of the basin subducted under the edge of the North China plate, resulting in the formation of a mountain-forming belt in the north, which provided rich clastic materials for the basin [43].

The sedimentation of the S23 member of the Shanxi Formation in the northeast of the basin is mainly a multi-line braided fluvial fan sedimentary system, with alluvial fans developed in the northern piedmont plain and gradually turning into braided fluvial towards the south. Under warm and humid climate conditions, the flow of rivers varies greatly, belonging to a wandering braided fluvial. In addition, due to the fine sediment of wetlands limiting the cutting of rivers, the fluvial pattern is distributed in a mat shaped, forming the main fluvial pattern of braided fluvial in the study area. The abundant rainfall also leads to the extensive development of lacustrine swamp, which are distributed in the low structural areas between channels.

We can infer the total sedimentary pattern form the central region where the research area is located. According to the distance to the source area, the mode can be divided into three parts: near source, middle section, and end section (Figure 15).

Figure 15. Sedimentary model of a braided fluvial fan in member S23 of the Ordos Basin.

Near source: There are uplifts as sediment source areas in the northern part of Ordos, such as the Daqing Mountain in the northern basin. The water carries debris from the sediment source area and forms an alluvial fan in the plain area in front of the mountain. The alluvial fan has a large scale, coarse sediment particle size, and a large sedimentary thickness. At the end of this alluvial fan, many large braided channels are formed. Due to strong hydrodynamic action, the flow continues to carry debris towards the south.

Middle section: After the terrain had leveled off, the sediment discharge of the entire river had increased, but the hydrodynamic effect was still strong. In some areas, large crevasse fans may form, and the research area belonged to a large braided flow zone (channel zone) composed of multiple braided fluvial. Each large braided flow zone is composed of many braided channel, overbank and channel bar. The warm and humid climate not only replenished the water supply for fluvial, but also promoted the development of floodplains and lacustrine swamps between the channels.

End section: As the distance from the source area increases, the number of channels decreases significantly, gradually transforming into a meandering river and the hydrodynamic strength of the river gradually weakens. Usually, only fine-grained materials can be transported. As the hydrodynamic strength weakens, the river eventually disappears at the end. It should be noted that the boundary between the middle and end parts of the braided fluvial fan is not very clear, and the environmental evolution between the two belongs to a gradually changing process.

6. Conclusion

1. The lithology of the second member of the Shanxi Formation in the XQ gas field is mainly sandstone, with a sandstone content of approximately 60%. The thickness of the whole study layer is approximately 30 m. The lithology is a combination of unequal thicknesses of interbedded medium to coarse sandstone, conglomerate and mudstone. Relative contents of quartz, feldspar and rock debris are present. By the color of mudstone, indicating that the whole study area was in an oxidizing-weakly reducing environment during the depositional period. The structural maturity of sandstone in the study area is relatively good, which is consistent with the sedimentary characteristics in the middle part of the fluvial facies. The research area has a complex hydrodynamic environment with the characteristics of repeated migration and diversion.

2. Nine kinds of single lithofacies and seven kinds of lithofacies assemblages developed in the target layer in the study area. The single lithofacies are mainly Gp, Gt and St, and the main lithofacies assemblage is Sh-Sp-St-Fm, which shows the vertical superposition of sand bodies dominated by a positive rhythm, which is consistent with the microfacies characteristics of the study area.

3. The study area is determined that the sedimentary facies is a braided fluvial fan deposit. The study area was located in the middle subsection of the whole braided fluvial fan, and the subfacies of the whole area is the middle subfacies fan. The sedimentary microfacies in the study area can be divided into five different parts: lacustrine swamp, floodplain, overbank, braided channel and channel bar. During this sedimentary period, the main channel in the northern provenance area transported the debris to the whole study area, and the sand bodies were distributed in a dendritic manner. Establishing sedimentary model based on sedimentary characteristics. According to the distance to the source area, the mode can be divided into three parts: near source, middle section, and end section. It is believed that the study area is located in the middle section of the braided fluvial fan.

Acknowledgements

This work was supported by the Natural Science Foundation of Shandong Province (ZR2020MD035) and the National Natural Science Foundation of China (51504143 and 51674156). The authors would like to thank the workers of Changqing Oilfield of PetroChina for supplying research data. In addition, I would also like to express my special gratitude to Professor Zhang Jinliang, one of the co-authors of this article, who unfortunately passed away due to illness in 2022. He pointed out the research direction for this study and selflessly provides the author with a lot of guidance. The author hopes to continuously advance in this research field to improve Professor Zhang’s theory and fulfill his last wish.

Disclosure statement

No potential conflict of interest was reported by the author(s).