The discovery of the huge conglomerate oilfield in the Mahu sag, the Junggar Basin, has once again aroused the interests to conglomerate reservoirs in the industrial and academic circles ^[1,2,3,4,5], and “supported conglomerate” is considered as the major reservoir rock in this oilfield^[6,7]. The term “supported conglomerate” is a special concept proposed by Zhang Jiyi through investigation of modern sediments of coarse clastic alluvial fans in Xinjiang Oilfield and observation of coarse clastic cores of Karamay Oilfield^[8,9]. This term is widely accepted in exploration and development in Xinjiang Oilfield and has made important contribution to the exploration and development of conglomerate reservoirs. However, it hasn’t been frequently cited by the academic community, possibly for four reasons: (1) the formation mechanism of sup-ported conglomerate has stayed at its original definition, and needs to be further investigated theoretically; (2) supported conglomerate shows similarities with grain-supported conglomerate and matrix-supported conglomerate, but its particularity has not been paid enough attention to; (3) flood deposition has been taken as the only genesis of supported conglomerate, restricting the discussion on its genetic diversity; and (4) the supported conglomerate is similar with openwork conglomerate commonly used worldwide, which also hinders the promotion of the concept.

In this study, similar concepts of supported conglomerate, grain-supported conglomerate and openwork conglomerate are compared, the modern sediments in Wulungu Lake, Baiyanghe Fan, and Huangyangquan Fan in Xinjiang are surveyed, and the Triassic and Permian drilling cores from the Mahu sag, Xinjiang are described to find out the similarities and differences between them, the formation environment and deposition mechanism of supported conglomerate, and the sedimentary characteristics of supported conglomerates developing in different sedimentary microfacies. On this basis, the formation mechanisms of supported conglomerate and their influence on the reservoir porosity are investigated to provide references for the study on sedimentary genesis and heterogeneity of conglomerate reservoirs and the economical and effective development of the huge conglomerate oilfield in the Mahu sag.

Supported conglomerate, grain-supported conglomerate, and openwork conglomerate are petrological terms proposed by different researchers in different periods. They have similarities and differences in connotation.

Zhang Jiyi (1980) stated in his paper published in Xinjiang Petroleum Geology that “There is a special type of rock in the clastic sediments of alluvial fan - supported conglomerate. The supported conglomerate has a low shale content and contains angular gravels which support each other, leaving large void spaces in the rock and having large pores and high permeability. And its sedimentary thickness is only a few to a dozen centimeters, and it is unstable laterally”^[8]. In another paper “Some deposition characteristics and microfancies subdivision of coarse clastic alluvial fans” published in 1985 in Acta Sedimentologica Sinica, Zhang Jiyi pointed out that “Supported conglomerate refers to the loose gravel layer in alluvial fan, where well-sorted gravels support each other, and there are no or little mud filling in the pores”. He believed that supported conglomerate is a special rock texture developed in alluvial bedding, and particularly noted that “supported gravel and sieve deposits are not of the same origin”^[9].

Grain-support refers to a rock texture in which rock grains support each other, and fine components fill between the grains ^[10,11,12]. The sandstone or conglomerate with this texture is called grain-supported sandstone/conglomerate, or multi-scale grain- supported sandstone/conglomerate when there are multiple sizes of grains. Grain-support is defined relative to matrix-support. Grain-supported conglomerate may also contain matrix, but the matrix always fills the pores. In contrast, matrix-support features grains floating in the matrix.

The concept of openwork gravel was proposed earlier, but it is not widely used in China. In 1862, Erdmann made the first description of openwork gravel in a tillite formation north of Falköping in Sweden^[13]. A translation from the Swedish work reads: One could observe 12-15 m long and 3.0-3.6 m thick lens or stock-shaped deposit composed entirely of roll-stones with very little fine gravels between the grains which are surrounded on all sides by gravelly or sandy layers^[13]. Davis introduced in 1892 the term openwork gravel for such layers of gravel in which the space between the pebbles are empty^[13]. Since then, the term openwork gravel has been frequently adopted in literatures on river engineering related to riverbed evolution^{[14,15,16,17,18]} and sedimentology research^{[13, 19-21]}.

Fig. 1 shows the variation of sedimentary texture with the decrease of gravel content and the increase of fine-grained sediment content without considering the cement filling formed by diagenesis^[22]. Texture ① exhibits no sediment filling between gravels, indicative of openwork gravel, and also satisfying the characteristics of grain-supported conglomerate and supported conglomerate. Texture ② represents a certain content of fine-grained components and no occurrence of matrix, showing the characteristics of supported gravel and grain-supported structures, but rigidly it is no longer an openwork conglomerate. Texture ③ shows two groups of gravels with different grain sizes, with finer grains filling between gravels and relatively a small amount of matrix, indicative of multi-scale grain-support, which is still a supported gravel structure, but not an openwork structure. Texture ④ still shows a grain-support, which is called matrix-grain-support because of the high content of matrix, and greatly derivative from the definition of supported gravel texture which requires an extremely small amount of matrix. Texture ⑤ shows that grains are floating in the matrix, indicative of matrix-supported conglomerate. Texture ⑥ is not a conglomerate structure. It is thus inferred that openwork gravel is an extreme characteristic of the gravel texture Although the matrix-support is one of the support patterns of gravelly sediments, it is neither a grain-support, nor meets the characteristics of supported gravel texture The supported gravel texture includes openwork, grain-supported, and multi-scale grain-supported textures as shown by the left three textures in Fig. 1.

Since the term openwork gravel was proposed earlier, it should be used according to the principle of time preference; however, it is rarely used in China. In contrast, grain-support and multi-scale grain-support are widely used in China, but involve two types of rock textures. Moreover, there is a matrix-grain-support texture relevant with them, so the three concepts cannot be collectively referred to as grain-support. On the other hand, the term supported conglomerate has been used for a long time in the exploration and development of conglomerate oilfields in Xinjiang, and has played an important role, hence this term can be used continuously. However, the definition of supported conglomerate as a special conglomerate formed by flood deposition impedes the research on the genetic diversity of supported conglomerate. People may misunderstand that all rocks with supported conglomerate texture are originated from flood events, thereby resulting in the errors in sedimentary facies analysis. Surveys of some modern sediments reveal that supported gravels with the characteristics of “well-sorted gravels in good supporting and no or little mud fillings in the pores” might be formed in a variety of sedimentary environments, and supported conglomerates can be found in multiple types of sedimentary facies Therefore, identifying the characteristics and formation process of supported gravels in modern sedimentary environment can provide important references for analyzing the formation and distribution of supported conglomerate.

Many scholars have conducted in-depth studies on the genesis of supported gravel and openwork gravel. In 1951, Cary studied the openwork gravel in rivers flowing into northwestern Pacific Ocean, and indicated that openwork gravel was caused by eddies between the gravels^[19]. In 1955, Braden proposed that the openwork gravel in rivers in northern Minnesota was formed due to the steep riverbed gradient which was unfavorable for the preservation of fine-grained sediments^[20]. Yu Kuanhong et al. called the supported conglomerates in alluvial fans as weakly cemented conglomerates, and considered the weakly cemented conglomerates as sieve-like deposits on alluvial fan plane or braided bars in trough of upper fan and braided channels in mid-fan^[23]. By investigating the modern fluvial and lacustrine deposits in the Baiyanghe alluvial fan, Huangyangquan alluvial fan, and Wulungu Lake in Xinjiang, it is found that the supported gravels may be formed in braid channels in inter-mountain creeks, foothill colluvial fans, Gobi desert, and alluvial fans, and other sedimentary environments, and the supported gravels might be deposited by sieving, colluvium, gravity deposition, river deposition, wind and wave effects, and the likes. Due to the limitations of the survey conditions and the limited scale of supported gravels, the trenches were not excavated to identify the sedimentary characteristics of supported gravels. Instead, only the characteristics of sediments exposed from the surface and river banks were used to investigate the depositional process and forming mechanism of supported gravels.

Sieve deposits show typical supported gravel texture^[23,24,25]. According to the Oxford Dictionary of Earth Sciences, “sieve deposits are a kind of conglomerate that is well sorted and has a low content of matrix, and are formed in a sedimentary place where the deposited sediments contain only gravels”^[26]. In 1967, Hooke found through experiments that the sieve deposits are developed in the lower fan and the middle fan. The sieve deposits in the middle fan were formed due to the infiltration of the water in the form of groundwater flow, and the sieve deposits in the lower fan were formed due to the rolling down of particles in the slope break, both showing a typical openwork gravel texture^[27]. This is similar to the mechanism proposed by Zhang Jiyi, namely, supported conglomerate was formed by the gravel skeleton left after the sediments in the pre-existing alluvial deposits in the fine trenches on the top of fan body were taken away by subsequent floods. In Jiangjungou at the southern margin of the Junggar Basin, the maximum diameter of boulders in sieve deposits exceeds 1 m. Extra-large pores appear between the gravels. Water flow and fine-grained sediments pass through the pores of the boulder to form a boulder-like sieve (Fig. 2a), with evident supported gravel texture.

Foothill colluvial fan is also one of the survival places of supported gravel. It is generally distributed near the mountain at the margin of the basin, and is a fan-shaped accumulation which is formed by coarse clasts derived from the weathered mountain after rolling down the slope break under gravity^[28]. The foothill colluvial fan at the edge of the old mountain in the upper part of Baiyanghe fan in Karamay, where the sediments are mainly sourced from the mountain adjacent to the fan body, is composed of rock clasts formed by weathering and fragmentation of the mountain rock. It includes mainly gravels, and minor sandy and muddy sediments. Sediment particles are angular, with a gravel diameter of 10-15 cm at most, and generally 3-8 cm, forming supported gravel. The largest particles exist in the lowermost part of the fining upward foothill colluvial fan deposits There are usually some radial trenches on the surface of foothill colluvial fan, and coarser-grained supported gravels are distributed along the trenches (Fig. 2b). This phenomenon is common in sand piles in sand yards (Fig. 2c).

Supported gravel is widespread in modern gravelly riverbed sediments ^[29,30,31]. In the flood season, seasonal rivers have high velocity and high discharge of water, and thus strong hydrodynamic power, sandy/muddy materials are carried downstream by the water, while the coarse-grained gravels remain on the riverbed to form supported gravels. In the middle part of the modern Baiyanhe alluvial fan, the gravels in the main channel are 5-30 cm in diameter and in an imbricate pattern (Fig. 2d). It can be seen on the section that the supported gravel layers interbedded with multi-scale grain-supported gravel layers along the bank of the stream. The supported gravels are generally 5-10 cm in diameter, have no fine-grain fillings in between, and show distinctive orientation and imbricate texture (Fig. 3a).

On the surface of alluvial fan and Gobi desert, the effect of continuous wind takes away the fine-grained components in the sediments, leaving well-sorted gravels, forming gravelly ripples and contributing the development of supported gravels. On the dry riverbed in the western outskirt of Karamay, the wind reworked the sediments to form gravelly ripples which are 10-20 cm wide and spaced at 0.5 to 1.5 m, with the height equivalent to the diameters of about 3 to 5 gravels. The gravels are angular, with the diameter ranging from 0.5 to 3.0 cm (Fig. 3b); they are well sorted and have the characteristics of supported gravels. Similar gravelly ripples are developed on the dry riverbed of the lower reach of the Jinghe River, Xinjiang, with the gravel diameter of generally 2 to 5 mm the ripple height, length and spacing of 1-5 cm, 10-20 cm and 50-70 cm, respectively (Fig. 3c). In the Huangyangquan alluvial fan, Wuerhe, Xinjiang, the temporary channel sediments were reworked by wind to form well-sorted gravel beaches, where the gravels are sub-rounded and well sorted, with the diameter of 2-5 mm, without fine sediments, and with supported gravel characteristics. Each gravel beach has an area of less than 100 m² and a thickness not more than the diameters of 5 gravels (Fig. 3d). The Gobi desert developed in the inter-distributary channel in the middle Baiyanghe alluvial fan is tens to hundreds of meters long (Fig. 3e). The gravels are angular and sub-angular (Fig. 3f), with the development of supported gravels.

Bedrock lake shoreline is developed on the west bank of modern Wulungu Lake. The bedrock is eroded by lake waves to form curved cliffs, under which the gravelly beaches are developed (Fig. 4a). The beaches are 2 to 10 m wide. Lakeshore gravels are mainly 5 to 15 cm in diameter and poorly sorted. They are sub-angular to sub-rounded and grain-supported, with a scarce of sandy and muddy sediments between the gravels, and the existence of supported gravels.

On the west bank of Wulungu Lake, episodic floods carried gravels from nearby mountains to the lakeshore area where the gravels accumulate to form gravelly braided river delta. The lakeshore waves continued to rework the delta front to form gravelly beach bars. Gravelly beach bars can extend up to hundreds of meters along the lakeshore, and the facies is 3-20 m wide (Fig. 4b). The beach bar sediments range from 5 to 20 cm in diameter and are round to sub-rounded, well sorted and medium in sphericity, forming supported gravels (Fig. 4c). Similar phenomena were reported in the study of Qinghai Lake^[32].

Debris flow is a surged high-mud content gravity flow. A debris flow event passes through several stages from start to ebb^[33,34]. Debris flow gives rise to coarse conglomerate in the early stage, argillaceous conglomerate supported by the matrix in the flowing stage, and the debris flow changes into a traction flow since the water turns clear in the late stage. The current carries the fine-grained muddy and sandy components in the surface of the debris flow downstream, forming supported gravel deposits at the top of the debris flow deposit sequence. The phenomenon is evident in small debris flow aggregates (Fig. 4d).

During the diagenesis, some of the pores between the supported gravels might not be filled with cements, and some might be partially or completely filled with cements, showing the characteristics of weak to strong cementation. All of them have supported gravel texture, so they can be called supported conglomerates, and the supported conglomerates formed in the same environment may also differ in cementation degree. The cores of the Triassic Baikouquan Formation and the Permian upper Wuerhe Formation in the Mahu sag demonstrate that gravelly fluvial channel deposits, channel deposits reworked by wind, gravelly beach bar deposits, delta front deposits reworked by waves, mouth bar deposits, grain flow deposits, and debris flow deposits all may have supported conglomerate layers of different thicknesses and characteristics.

The 13-16^th cors (2250-2391 m) of Well Xia 10 in Mahu Oilfield drilled red-brown conglomerate of the Baikouquan Formation with gravel-bearing sandstone. The gravels are sub-rounded to rounded, and in distinctive orientation, revealing a number of distinct upward-thinning cycles. Particularly, the 15^th core (2341.2-2344.0 m) can be divided into five upward-fining cycles (Fig. 5), which are ten centimeters to tens of centimeters in thickness, and exhibit relatively coarse grains at the bottom, generally with the matrix-grain-supported conglomerate, and slightly finer grains in the middle-upper part, with supported conglomerate.

Well Ma 18 in the Mahu sag encountered gray conglomerate and purple mudstone at 3850.3-3866.4 m (Fig. 6a). It is inferred that the gray conglomerate is the braided channel deposit in arid climate, and the purple mudstone is interchannel deposit. The channel deposits are composed of largely gray pebble conglomerate, showing massive bedding and rough cross bedding, granule conglomerate with parallel and massive bedded granule conglomerate at the top of the sequence. Gravels in the granule conglomerate are 2 to 4 mm or about 5 mm at maximum in size, sub-angular to sub-rounded and well sorted. The granule conglomerate with no fine-grained matrix shows supported gravel texture but has muddy cements filling between the gravels (Fig. 6b-6d). It is inferred that the supported gravels formed by the wind reworking to the sediments during the river drying period were cemented during the diagenesis to form the supported conglomerate.

Sandy beach bars have been frequently reported^{[35,36,37,38,39]}, but gravelly beach bars have been rarely studied^[32]. In Karamay Oilfield, Well Jin 206 encountered a set of thick conglomerate intercalated with thin siltstone and sandstone in the Permian Upper Wuerhe Formation (the 8^th core, 4071.16- 4078.18 m). The existence of low-angle cross bedding and wave ripple in the sandstone suggests that the formation is gravelly beach bar deposit (Fig. 7). The core shows that the gravels are well sorted, rounded, and spherical, there is little matrix filling between the gravels, corresponding to the characteristics of supported conglomerate (Fig. 7c, 7d). Similar characteristics were also found in the 2^nd core in the Baikouquan Formation of Well Mazhong 2 in the Mahu sag.

The Baikouquan Formation (2527.10-2540.70 m) in Well Fengnan 401, the Mahu sag, is composed of gray and dark gray conglomerate with gray and purple mudstone, and argillaceous siltstone interbeds, and massive bedding and rough cross bedding (Fig. 8a), representing the deposits of gravelly braided river delta front. When the lake water level was high, subaqueous distributary channels developed, the deposits are gray and dark gray, and multi-scale grain-supported structure appears in the coarse-grained section (Fig. 8d); occasionally, grain-supported conglomerates exist due to the wave washing. When the lake water level was low, the delta front was exposed out of the water. The fine-grained section consists of purple and maroon mudstone and siltstone, while the coarse-grained section maroon granule conglomerate (Fig. 8c). Some deposits have little matrix filling between grains due to wave washing, and show supported conglomerate textures (Fig. 8b).

River mouth bar is an important sedimentary body in delta front. In Karamay Oilfield, Well Jin 211 revealed delta front river mouth bar deposits in the upper Wuerhe Formation (the 4^th core, 3752.06-3755.66 m), which are composed of conglomerate and silty fine sandstone interbeds. The silty fine sandstone includes extremely fine sandstone, siltstone, and argillaceous siltstone in gray and dark gray, with parallel bedding, massive bedding and small ripple laminae. The conglomerate is mainly gray granule conglomerate, showing two types of cycles (upward-fining and upward-coarsening) (Fig. 9). The top of the mouth bar cycle has dense gravels but low content of matrix, showing supported conglomerate texture, and well-developed zeolite cements (red box in Fig. 9a, and Fig. 9b, 9c). At the position with high content of matrix, the conglomerate shows matrix-supported texture, with little zeolite cement (Fig. 9d).

Well Jin 205 of Karamay Oilfield revealed possible supported conglomerate of grain flow deposits. In the Upper Wuerhe Formation (the 4^th and 5^th cores, 3838.85- 3922.31 m), the well encountered a set of gray and gray-green sandstone, siltstone and mudstone with thin layers of granule conglomerate. The granule conglomerate contacts sandstone and siltstone in gradual or abrupt manner (Fig. 10). The sandstone presents massive bedding, graded bedding and soft sediment deformation and may be deposited by turbidite gravity flows (Fig.10). The conglomerate section shows grain-support texture and multi-scale grain-support texture, and massive bedding, horizontal bedding, and low-angle cross bedding. The cores show good sorting, low mud content, loose cementation, and are possibly grain flow deposits (Fig. 10). The grain flow deposits exhibit low content of matrix in both the cross and vertical sections of the cores (Fig. 10a-10c), a typical characteristic of supported conglomerate.

The Baikouquan Formation (the 5^th-6^th cores 2805.02- 2817.7 m) in Well Fengnan 16 in the Mahu sag is mainly a set of debris flow deposits (Fig. 11), consisting of reddish brown and gray pebble conglomerate and granule conglomerate intercalated with reddish brown mudstone and siltstone. The mudstone is massive and contains scattered fine gravels. The conglomerate shows of massive bedding and rough parallel bedding; the gravels are angular and sub-angular, or partly sub-rounded, in non-directional or weakly-directional arrangement. The debris flow conglomerate generally presents matrix-support texture and matrix-grain-supported texture (Fig. 11a, 11b), and the characteristics of supported conglomerate in local parts with little matrix (Fig. 11c).

Absence of fine-grained sediment filling between gravels is the most typical feature of supported conglomerate. The gravel “cavity” is very complex in mechanism and possibly formed in one depositional phase, or after multiple depositional phases or even in the diagenetic stage. The supported gravel texture formed may be preserved for a long time, or may be reworked by subsequent deposition or diagenesis to form multi-scale grain-support texture, grain-matrix-support texture, or even matrix-support texture. Deposition is mainly responsible for these changes. According to the sequence of sedimentary diagenesis, the formation mechanism of supported conglomerate is divided into deposition stage, sedimentary reworking stage and diagenetic stage.

The supported gravels of sieve deposits, colluvium deposits and gravelly riverbed deposits are formed in the deposition stage. The colluvium deposits, with short distance of transport, lack fine gravels and sand/muddy matrix, and thus can form supported gravel texture itself. Inter-mountains creeks and water-rock flows may also carry gravels of different sizes to form supported gravels. Supported gravels on the gravelly riverbed are formed by the gravels rolling and accumulating along the riverbed driven by the current, and there is no or little fine-grained matrix during its movement ^[40] (Fig. 12a).

Pores between the supported gravels formed in the early stage might be filled with fine gravels or sandy/muddy sediments to form multi-scale grain-support texture or matrix-grain-support texture (Fig. 12b, 12c), or be reworked into matrix-support texture in case of liquidation or slump of sediments; the unfilled supported gravels might be filled with cements during diagenesis to form a basement cement texture. Therefore, the original deposits might have more supported gravel textures than what can be observed on the core and outcrop. This conclusion is very important for recovering the sedimentary hydrodynamic process of the conglomerate.

The matrix-grain-supported and matrix-supported gravels formed at the earlier deposition may also be reworked into supported gravels after deposition (Fig. 12d, 12e). The post-deposition reworking has positive and negative effects on the formation and preservation of the pores of supported conglomerate. After the formation of supported gravels, fine- grained sediments might infiltrate between the gravels due to the reworking by wind, water current, debris flow, and the likes, which resulted in the pores to be completely or partially filled, thereby evolving into a matrix- grain-supported texture or grain-matrix-supported texture^[14,15]. The differences in the grain sizes of supported gravels and the sizes of original pores between the gravels lead to the differences in the grain size of sediments infiltrated and the quantity of grains infiltrated, which has a great impact on the original porosity of the channel sediments^{[22, 41]}. Through flume simulation experiment, numerical analysis, empirical models, and modern sediment surveys, the degree of infiltration of gravelly riverbeds can be estimated, and the original texture of supported gravels and the pore texture characteristics of the riverbed sediments after infiltration can be reconstructed^{[40, 42-47]}.

Post-deposition reworking may also be beneficial to the formation of supported conglomerate. During the recession of the debris flow, the water current carried away the finer sediment downstream, leaving supported conglomerate on top of the debris sequence. Wind took fine sediments away from the Gobi desert and temporary riverbeds, giving rise to supported gravel texture. The flood water falling in level might wash away fine sediments on both flanks of riverbed and in the flood plain to form supported conglomerate. The sandy/gravelly deposits in the shore and river mouth areas were reworked by waves, resulting in the formation of supported gravels in in coastal areas. Clearly, the genesis of supported conglomerates is diverse, and it is difficult to explain adequately by using only one sedimentary mechanism.

Diagenesis generally has negative effect on the porosity of supported conglomerate. Although high temperature and high pressure may cause certain deformation of the gravel particles and thereby change the geometry of pores between the gravels, the compaction generally has little effect on the shape of gravel particles. Cementation has a greater impact on the porosity of supported conglomerate. During the diagenetic process, the underground fluid precipitated between the gravels to form cements to reduce the intergranular pores of supported conglomerate. Moreover, fine solid particles may enter between the gravels; the dissolution has a certain but weak effect on the morphological changes of gravel particles.

Conglomerate reservoirs are widely distributed in petroliferous basins in China, such as Xinjiang, Northeast China, Bohai Bay, and Pearl River Mouth ^{[48,49,50,51,52]}. Most of supported conglomerates show ample pore spaces, thereby creating superior conditions for oil and gas storage. However, the pore space also provides preferential pathways for the migration of other underground fluids, making the supported gravels be earlier and more frequently filled with muddy and other types of cements. In view of the possibility of pore preservation, the supported gravels formed by colluvium fans and sieve deposits are more likely to be reworked later; the supported gravels formed by debris flow, grain flow, beach bars and aeolian gravel beaches after deposition and reworking may be preserved to have higher porosity. In view of rock texture, the openwork and grain-supported gravels are easily cemented to form a dense texture, and the multi-scale grain-supported conglomerate is more likely to preserve a large number of micro pores. Statistics on the physical properties of reservoirs in the Baikouquan Formation of Mahu Oilfield, Xinjiang, show that coarse sandstone has the highest porosity, pebbly conglomerate, granule conglomerate and medium-fine sandstone have good porosity, and coarse pebble conglomerate and cobble conglomerate have relatively low porosity (Fig. 13). According to the permeability of reservoirs with different lithologies, granule conglomerate and pebble conglomerate have the highest permeability, and cobble conglomerate and coarse pebble conglomerate have lower permeability. This indicates that the supported gravel texture is susceptible to reworking by post-deposition filtration and diagenesis, resulting in a decrease in porosity and permeability.

In terms of concepts, supported conglomerate has both similarities to and differences from openwork conglomerate, grain-supported conglomerate, and matrix-grain-supported conglomerate. According to the original definition, supported conglomerate is mainly characterized by well-sorted grains in supported contact, with no or little muddy filling in the pores. Clearly, openwork conglomerate, grain-supported conglomerate and multi-scale grain-supported conglomerate are all assigned to the category of supported conglomerate. It is recommended to keep adopting the term supported conglomerate since it has been followed for years and has played an important role in the exploration and development of conglomerate oilfields. Initially, however, supported conglomerate was defined as a special type of conglomerate formed by flood deposition, which impeded the understanding of the genetic diversity and thus might lead to errors in the analysis of sedimentary facies. Supported conglomerate was formed by supported gravels after burial and diagenesis. Identifying the characteristics and formation process of supported gravels in modern sedimentary environment is of great reference for analyzing the formation and distribution of supported conglomerate. Modern depositional investigations of modern rivers and lakes such as Baiyanghe alluvial fan, Huangyangquan alluvial fan, and Wulungu Lake in Xinjiang reveal a variety of modern depositional environments for supported gravels. For example, supported gravels can be formed in inter-mountain creeks, foothill colluvium fans, alluvial fan gravelly riverbeds, Gobi desert on alluvial fan surfaces, gravelly beaches, gravelly delta fronts, debris flow, and other depositional environments, following the mechanisms of sieve deposition, colluvium deposition, gravity flow deposition, river deposition, wind and wave reworking.

Core observations reflect seven types of supported conglomerates in the Triassic Baikouquan Formation and Permian Upper Wuerhe Formation in the Mahu sag, the Junggar Basin, namely, supported conglomerate of gravelly riverbed deposits, supported conglomerate formed by wind reworking, supported conglomerate of gravelly beach bar deposits, supported conglomerate formed by wave reworking in delta front, supported conglomerate of river mouth bar deposits, supported conglomerate of grain flow deposits, and supported conglomerate of debris flow deposits. Supported conglomerate might be formed by one depositional phase, or after multiple depositional phases or even in the diagenetic stage. The post-deposition reworking has positive and negative effects on the formation and preservation of the pores of supported conglomerate. Diagenesis generally negatively affects the porosity changes of supported conglomerates. The genesis of supported conglomerates is diverse, and it is difficult to explain adequately by using only one sedimentary dynamic mechanism.

Conglomerate reservoirs are widely distributed in petroliferous basins in China, such as Xinjiang, Northeast China, Bohai Bay, and Pearl River Mouth. The pore spaces are well developed in the original supported gravel texture, thereby creating superior conditions for oil and gas storage. Statistics on the properties of reservoirs in the Baikouquan Formation of Mahu Oilfield, Xinjiang, show that coarse sandstone has the highest porosity, pebble conglomerate, granule conglomerate and medium-fine sandstone have good porosity, and coarse pebble conglomerate and cobble conglomerate have relatively low porosity. Granule conglomerate and pebble conglomerate have the highest permeability, and cobble conglomerate and coarse pebble conglomerate have lower permeability. This indicates that the supported gravel texture is susceptible to reworking by post-deposition filtration and diagenesis, resulting in a decrease in porosity and permeability.