I. Division of Shuixigou Group
Shuixigou Group in ore-bearing strata can be divided into three groups: Badaowan Formation (J 1b), Sangonghe Formation (J 1s) and Xishanyao Formation (J2x), but the boundaries between these groups are still inconsistent for many years (Table 2-7- 1). According to the data of stratigraphic correlation and sporopollen analysis, I-IV cycles of Shuixigou Group in the southern margin of Yili Basin are divided into Badaowan Formation, V1cycle into Sangonghe Formation, V 2-V cycle into Xishanyao Formation, and V cycle into Upper Jurassic. The main basis for division and comparison is as follows:
1) The lake in the middle of Shuixigou Group in the southern margin of Turpan-Hami Basin is deposited as Sangonghe Formation, and the upper and lower sets of coal measures strata correspond to Xishanyao Formation and Badaowan Formation respectively. Therefore, this paper holds that the Sangonghe Formation in the southern margin of Yili Basin should be confined to the sub-cycle ⅴ 1 dominated by lacustrine facies and prodelta facies, while the coarse sub-cycle ⅴ 2 and coal-bearing sub-cycles ⅴ 3 and ⅵ should belong to the Xishanyao Formation rather than the Sangonghe Formation.
2) According to the appraisal and analysis of Petroleum Geology Experimental Research Center of China Petroleum Exploration and Development Research Institute, five gray mudstone samples were collected between 420-450m in the 2nd sub-cycle of ZK44967- 1 hole in the southern margin of Yili Basin, of which two samples (ZK44967- 1-2 and ZK44967-65438+) were The main type of fern spores is pseudolycopodium. Its age can be attributed to the early and middle Middle Jurassic. Therefore, the sporopollen identification of gray mudstone samples in ⅴ 2 subcycle shows that they belong to Xishanyao Formation rather than Sangonghe Formation.
3) The basis for dividing the eighth cycle into the Guqi Formation of the Upper Jurassic rather than the Xishanyao Formation is that there is a sedimentary discontinuity between the eighth cycle and the seventh cycle, but it is continuous with the Upper Jurassic, and the boundary is difficult to distinguish; The original environment of sandstone rocks in the eighth cycle is oxidation environment; The original environment of sandstone rocks in the seventh cycle is the reducing environment.
According to the characteristics of stratigraphic lithologic association, sedimentary rhythm, sedimentary structure, sand body scale and stability, logging curve and sediment particle size distribution curve, the Shuixigou Group, a host rock series in the southern margin of Yili Basin, is divided into four major sedimentary systems, namely, alluvial fan sedimentary system of I-IV cycle, braided river delta sedimentary system of V cycle, shallow lake swamp sedimentary system of VI cycle and meandering river delta sedimentary system of VII cycle. Among them, alluvial fan sedimentary system mainly develops alluvial fan facies and braided river facies in front of fan; Braided river delta sedimentary system mainly develops prodelta facies, delta front facies, delta plain facies and swamp facies; The meandering river delta sedimentary system mainly develops delta plain facies. Table 2-7-2 shows the main sedimentary facies and subfacies of Shuixigou Group in the southern margin of Yili Basin.
Table 2-7- 1 Stratigraphic Division and Evolution Table of Shuixigou Group in the Southern Margin of Yili Basin
Second, the sedimentary characteristics of Shuixigou Group
(A) alluvial fan sedimentary system
Alluvial fan depositional system is mainly developed in Ⅰ ~ Ⅳ cycles, in which Ⅰ ~ Ⅱ cycles are mainly fan-middle fan-end subfacies and fan-front braided fluvial facies; Ⅲ ~ Ⅳ cycles are dominated by braided fluvial facies in front of fan.
Sedimentary facies characteristics of 1. Ⅰ ~ Ⅱ period
The sedimentary characteristics of cycle ⅰ and cycle ⅱ are similar, which are characterized by conglomerate and glutenite in the lower part of cycle, medium-fine sandstone lens sometimes appearing in the upper part, and fine sandstone, siltstone and mudstone in some places. The sand-to-ground ratio of I ~ II cycle formation is generally greater than 0.6. There are less organic matter and carbonaceous debris in the formation, and the phase transition is faster. Conglomerates are mainly distributed near the southern margin of the basin, and the thickness along the basin edge is relatively stable, generally 5 ~10m; However, towards the center of the basin, it quickly becomes glutenite and gravelly sandstone (Figure 2-7- 1). The gravel size of conglomerate varies from (3× 5 cm) to (8× 10 cm), and the largest can reach more than 15×20cm. Gravel has low maturity and complex composition, which can be divided into distant siliceous rocks, metamorphic rocks, quartzite gravel (sub-circular) and near-source volcanic rocks and granite gravel (angular-sub-angular).
Table 2-7-2 Sedimentary System and Sedimentary Facies Division Table of Shuixigou Group in the Southern Margin of Yili Basin
Fig. 2-7- 1 Histogram Comparison of Ⅰ ~ Ⅱ Cyclic Strata of Shuixigou Formation in No.22 Exploration Line in Kujiertai Area, southern margin of Yili Basin
Fan terminal subfacies; B— braided fluvial facies in front of fan; T2-3xq——Xiaoquangou Group of Middle-Upper Triassic
The sedimentary structure of I ~ II cycle is dominated by massive bedding, and sometimes rough parallel bedding and large trough cross bedding can be seen. Apparent resistivity in three transverse directions is mostly box-shaped or bell-shaped (Figure 2-7- 1). The above characteristics show that I ~ II cyclic deposits are a set of alluvial fan deposits with rapid accumulation.
On the plane, the Ⅰ ~ Ⅱ cycle deposits in the southern margin of Yili Basin are generally characterized by coarse in the east and fine in the west, coarse in the south and fine in the north. According to the lithologic combination characteristics of alluvial fan system, I ~ II cyclic deposits in the southern margin of Yili Basin can be divided into alluvial fan facies and braided river facies in front of fan (Figure 2-7-2). Among them, the fan root subfacies of alluvial fan facies have been eroded, leaving only the fan tail subfacies in the fan. The subfacies at the end of the fan are mainly composed of conglomerate and glutenite with gravel sandstone, and the braided fluvial facies in front of the fan is mainly composed of glutenite and gravel sandstone with a small amount of conglomerate, siltstone and mudstone.
Figure 2-7-2 Lithofacies Paleogeographic Map of I ~ II Cycle in the Southern Margin of Yili Basin
1- denuded area; Fan end subfacies in 2 fans; 3- braided fluvial facies in front of fan; 4- Place names
2. Sedimentary facies characteristics of ⅲ ~ ⅳ cycles
The sedimentary characteristics of the third cycle and the fourth cycle are similar. The lower part is gravelly sandstone and coarse sandstone, and the local part is glutenite. The upper part is medium-fine sandstone, fine sandstone and thin mudstone, with obvious binary structure. The stratum thickness of a single cycle of ⅲ ~ ⅳ cycles is generally 10 ~ 30m, the sand thickness is generally 10 ~ 25m, and the average sand-soil ratio is 0.7 1. The apparent resistivity of three sides of ⅲ ~ ⅳ cycles is mainly box-shaped (Figure 2-7-3).
Generally speaking, the sedimentary conglomerates in ⅲ ~ ⅳ cycles are basically undeveloped, but the grain size of the sand body is coarse, with the grain size ranging from φ- 1.5 ~ 5 and the standard deviation of 0.53 ~ 0.7, so it is well sorted. The histogram is mainly unimodal (Figure 2-7-4). The probability curve of particle size distribution is two or three segments, mainly jumping population (accounting for 70% ~ 80%), with a slope of 60 ~ 70; The floating population accounts for 15% ~ 30%, and the slope is 30 ~ 40; The dividing point between jumping population and suspended population is φ 2.2 ~ 2.7, which reflects the characteristics of fluvial facies deposition. The lower part of sedimentary structure is dominated by trough and plate cross bedding, while the upper part is dominated by small cross bedding, wavy bedding and horizontal bedding. These characteristics indicate that the cyclic response of Ⅲ ~ Ⅳ belongs to braided fluvial deposits at the front of alluvial fan.
According to its lithologic combination and sand-mud ratio, the braided river facies deposits in front of the fan can be further divided into braided river subfacies and flood plain subfacies. Braided channel subfacies constitute the lower part of the binary structure, with gravelly sandstone and sandstone as the main lithology, trough and slab cross bedding as the main sedimentary structures, siltstone and mudstone as the main lithology, and small cross bedding, wave bedding and horizontal bedding as the main sedimentary structures.
(2) braided river delta sedimentary system
The braided river delta depositional system is mainly developed in the ⅴ cycle, in which ⅴ 1 sub-cycle is mainly delta front facies and prodelta facies, ⅴ 2 sub-cycle is mainly delta front facies and delta plain facies, and ⅴ 3 sub-cycle is mainly swamp facies. The characteristics of sedimentary facies are described by three sub-cycles.
Fig. 2-7-3 Partial Drilling of Exploration Line 032 in Zakistan Area, South Margin of Yili Basin
Columnar correlation map of tertiary cycle strata of Shuixigou Group
Braided river subfacies; B- flood plain subfacies
Figure 2-7-4 Grain-size Distribution Curve of Cyclic Ⅲ Sandstone in Suasugou
Sedimentary characteristics of 1. ⅴ 1 subperiod
Ⅴ1sub-cycle stratum thickness is generally 10~20m ~ 20m, and locally 20 ~ 35m. The lithology is mainly siltstone, mudstone, fine sandstone and medium sandstone, with coarse sandstone and gravel sandstone locally developed, while conglomerate and glutenite are not developed. The total thickness of the sand body in this cycle is generally less than 10m, and locally it can reach 10 ~ 15m. One of the characteristics of this sub-cycle stratum is that the thickness of mudstone is greater than that of sandstone, the sand-soil ratio is less than 0.5, and the distribution area of sandstone with sand-soil ratio greater than 0.5 is very small. Another feature of this sub-cycle stratum is that the mudstone is pure gray and dark gray mudstone, with well-developed horizontal bedding, thick single layer and stable extension, which reflects the still water sedimentary environment in deeper waters. In addition to horizontal bedding, sedimentary structures also develop massive bedding and microwave bedding. The third feature of this sub-cycle deposit is the anti-rhythmic structure with thick bottom and thick top; The lower part of the three-lateral apparent resistivity logging curve is mostly a smooth curve with low amplitude, and the upper part is mostly an inverted Christmas tree with medium and low amplitude (Figure 2-7-5). This anti-prosodic structure is a unique product of delta sedimentary environment. In addition, organic matter and carbonized plant debris are relatively developed in the sub-cycle strata, and sometimes pyrite can be seen, reflecting the relatively reduced underwater sedimentary environment.
Figure 2-7-5 Columnar Correlation Diagram of Cyclic Strata of Shuixigou Group in No.20 Exploration Line in Kujiertai Area, South Margin of Yili Basin
Prodelta facies; B- delta front sand bar and sheet sand subfacies; C- delta front distributary bay subfacies; Swamp facies; Shallow lake swamp facies
The above characteristics show that ⅴ 1 sub-cycle strata belong to delta front and pre-delta deposits, and can be subdivided into three sedimentary subfacies according to their lithologic association and sandstone development degree (Figure 2-7-6), delta front distributary channel subfacies and delta front distributary bay subfacies (mainly located in Curzi and its east area).
Figure 2-7-6 Lithofacies Paleogeographic Map of ⅴ 1 Cycle in the Southern Margin of Yili Basin
1- denuded area; 2- delta front distributary channel subfacies; 3- delta front distributary bay subfacies; 4- prodelta facies; 5- Place names
2. Sedimentary characteristics of ⅴ 2 subcycle
The thickness of ⅴ 2 sub-cycle strata in the southern margin of Yili Basin is generally 30 ~ 50m, and locally 50 ~ 60m. The lithology is mainly gravelly sandstone, coarse-grained sandstone and medium-fine-grained sandstone, and siltstone and mudstone are developed above and below the sand body. The sand-body ratio of this sub-cycle formation is generally 0.4 ~ 0.7, with an average of 0.59. The sand body of this sub-cycle formation is relatively stable, with a thickness of 15 ~ 30m in general and 35 ~ 45m locally. Regionally, from bottom to top, the complete rhythm of lithology from mudstone-siltstone-fine sandstone-coarse sandstone and gravel sandstone to coarse sandstone-fine sandstone-siltstone and mudstone is very obvious, which reflects the typical characteristics of delta sedimentary environment.
There are two main types of grain size distribution curves of delta facies sandstone in this area: one is similar to braided river facies sandstone (Figure 2-7-7A), which reflects the sedimentary characteristics of distributary channel in delta plain or subaqueous distributary channel in delta; The other has a wide granularity range (φ-2 ~ 5), with a standard deviation of 0.8 ~ 1.4, and the histogram is dominated by multiple peaks, which reflects poor sorting performance. The probability curve is mainly composed of suspended components, accounting for 80% ~ 100%, with moderate slope (40 ~ 50), little jump in general, and the graph is mostly linear (Figure 2-7-7B), which is similar to the particle size distribution curve of turbidity current deposition, reflecting the sudden carrying capacity of the river after carrying a large amount of sediment into the lake (into the delta environment).
According to lithology combination, sand body development degree and sedimentary rhythm characteristics, V2 subcycle in the southern margin of Yili Basin can be subdivided into delta plain braided river subfacies, delta plain flood plain subfacies, delta front estuary bar and sheet sand subfacies and delta front distributary bay subfacies. In addition, the sedimentary facies characteristics of V2 subcycle in different sections of the western section of the southern margin of Yili Basin are also different (Figure 2-7-8).
(1) zakistan region
ⅴ 2 sub-cycle in this area is mainly composed of braided river subfacies and flood plain subfacies in the delta plain, and its sedimentary facies characteristics are as follows: ① ⅴ 2 sub-cycle is generally characterized by anti-rhythmic deposits with thick details, but the interior of sub-cycle is composed of positive rhythmic deposits with 3-4 details (Figure 2-7-9). ② The sand body is unstable in extension, small in scale, fast in phase transition, multi-layered (generally 3-4 layers), and single-layered sand body is thin. ③ Wedge-shaped cross bedding, trough-shaped cross bedding and plate-shaped cross bedding are developed in sand bodies, and common scour surface structures are found. ④ The sand-body ratio of formation is high, generally 0.4 ~ 0.7, and 0.7 ~ 0.8 in some sections.
Figure 2-7-7 Grain size distribution curve of ⅴ 2 sub-cycle sandstone in Kujiertai deposit
Figure 2-7-8 Lithofacies Paleogeographic Map of ⅴ 2 Cycle in the Southern Margin of Yili Basin
1- denuded area; Two braided river subfacies in delta plain; 3- delta front sand bar and sheet sand subfacies; 4- delta plain flood plain subfacies; 5- delta front distributary bay subfacies; 6- Place names
(2) Kutertai-Curzi region in the west.
ⅴ 2 sub-cycle in this area is mainly composed of delta front estuary sand bar and sheet sand subfacies and delta front distributary bay subfacies, and its sedimentary facies characteristics are as follows: ① From bottom to top, ⅴ 2 sub-cycle generally shows fine-coarse-fine sedimentary characteristics, but the number of sand layers in different sections is different. For example, in Curzi area, it is often composed of 2 ~ 4 positive rhythmic deposits with thick bottom and thin top; The number of sand bodies in Sudongbulake area is reduced to 13. In Kujiertai area, several sand layers are often merged into one sand body, showing complete rhythmic sedimentary characteristics (Figure 2-7-5), reflecting that the sedimentary environment of ⅴ 2 sub-cycle in the southern margin of Yili Basin is becoming more and more stable from east to west, and the change of river channel migration and swing is small. ② Grain-ordered bedding is mainly developed in sand bodies, especially anti-grain-ordered bedding, and the scour surface structure is not common. ③ The sand-body ratio of formation is high, generally 0.4 ~ 0.7, and the sand-body development area often reaches 0.6 ~ 0.8. ④ Sand bodies are mainly distributed in the east-west direction, showing the sedimentary characteristics of (underwater) delta front sand bars and sheet sand parallel to the lakeshore direction.
Figure 2-7-9 Columnar Correlation Diagram of Cyclic Strata of Shuixigou Group on Exploration Line 024 in Zakistan Area, South Margin of Yili Basin
Delta front distributary channel subfacies; B- delta plain flood plain subfacies; Braided river subfacies in delta plain; Swamp facies
3. Sedimentary characteristics of ⅴ 3 subcycle
The secondary cycle is mainly composed of the eighth coal seam, mudstone and siltstone, and contains a lot of organic matter and carbon chips. The sand body is undeveloped, with thin thickness and fine sandstone lithology. The bedding structure is mainly horizontal bedding and massive structure. The main sedimentary feature of this cycle is that the thickness of the eighth coal seam is large, generally 10~20m ~ 20m, and its extension is very stable, which is the marker layer of regional stratigraphic correlation and reflects the long-term swamp sedimentary environment of the lake shrinking delta plain. In addition, the secondary cycle is mainly composed of coal seams, followed by mudstone and siltstone deposits. Coarse-grained sediments above the grain size of medium sandstone are very undeveloped, and the characteristics of rich organic matter and carbon chips in rocks also reflect the static swamp sedimentary environment.
(3) Shallow lake and swamp sedimentary system
The shallow lake swamp sedimentary system is mainly developed in the ⅵ cycle strata, and the shallow lake swamp facies is mainly preserved in the southern margin of Yili Basin. The main sedimentary facies features are as follows: ① Lithology is mainly composed of gray, gray-black mudstone, argillaceous siltstone, siltstone and a small amount of fine sandstone and medium sandstone, with a small amount of coarse sandstone. ② The rocks are rich in organic matter and carbon chips. ③ No.10 coal seam is at the top of the cycle, and No.9 coal seam in the middle is also well developed. ④ The sand bodies in this cycle are extremely unstable, and both the strike and the dip show the phenomenon of local expansion and sudden disappearance to both sides, but there are no sand bodies in most places. ⑤ The bedding structure is mainly horizontal bedding. ⑥ The logging curve of three lateral apparent resistivity is mainly a smooth curve with low amplitude and high amplitude teeth (ninth coal seam) (Figure 2-7- 10).
Fig. 2-7- 10 Columnar Correlation Diagram of Cyclic Strata of Shuixigou Group on Exploration Line 405 in the southern margin of Yili Basin.
(4) meandering river delta sedimentary system
The depositional system of meandering river delta is mainly developed in the seventh cycle. ⅶ cycles bounded by 1 1 coal and carbonaceous mudstone equivalent to1/coal can be clearly divided into ⅶ 1 and ⅶ 2 sub-cycles.
Fig. 2-7- 1 1 Lithofacies Paleogeographic Map of Sub-cycle in the Southern Margin of Yili Basin
1- denuded area; 2- distributary river subfacies in delta plain; 3- delta plain distributary bay subfacies; 4- delta plain flood plain subfacies; 5- Place names
Sedimentary facies characteristics of 1. VII 1 subcycle
Sub-cycle VII1in the southern margin of Yili Basin is generally a delta plain facies, which can be divided into three sedimentary subfacies: delta plain distributary channel subfacies, delta plain flood plain subfacies and delta plain distributary bay subfacies (Figure 2-7- 1 1). They have the following sedimentary facies characteristics.
(1) Lithology and Lithofacies Combination
The thickness of ⅶ 1 subcycle is relatively stable, generally 35 ~ 45m, and it can reach 45 ~ 60m locally. Lithology is mainly sandstone, siltstone and mudstone, with less conglomerate and gravelly sandstone, which generally only develop at the bottom of a single sedimentary rhythm. The sand-body ratio of ⅶ 1 subcycle formation is 0.3 ~ 0.7, and the sand-body ratio of distributary channel development area can reach 0.7 ~ 0.8, and the sand-body ratio of distributary bay development area is often less than 0.4. Carbonized plants are common in mudstone, silty mudstone and siltstone. Fine sandstone is mostly argillaceous and calcareous cementation, and the cementation is relatively tight. Coarse sandstone has loose cementation, pure composition, high siliceous content of over 70%, good sorting, good roundness and high maturity of sandstone composition and structure, which shows the sedimentary characteristics of its long-distance provenance.
Lithologic combination changes greatly in vertical and horizontal directions, and the lithologic combination of distributary channel subfacies is mostly sandstone-siltstone-mudstone-carbonaceous mudstone, generally with thick bottom and thin top. The lithologic combination of distributary bay subfacies is mostly fine-grained combination such as siltstone (or argillaceous siltstone)-mudstone coal line.
(2) sedimentary rhythm
The sedimentary rhythm of distributary channel subfacies is mostly the positive rhythm of coarse sandstone-medium sandstone-fine sandstone-siltstone-mudstone gradual change or the positive rhythm of coarse sandstone-medium sandstone-mudstone sudden change; The distributary bay has no obvious rhythm or is composed of thin fine sandstone, siltstone and thick mudstone, and the positive rhythm is very obvious.
(3) Logging curve
The three-lateral apparent resistivity curves of distributary channels are mostly serrated box or bell-shaped; The sediment in the distributary bay has fine grain size, and the amplitude of its three-lateral apparent resistivity curve is low (Figure 2-7- 12).
(4) Sedimentary structure
ⅶ 1 sub-cyclic sandstone can be seen as plate-like, trough-like, sandy cross bedding, wavy bedding and parallel bedding; Horizontal bedding is common in mudstone. Sometimes the reverse bedding of mudstone-siltstone-fine sandstone-medium coarse sandstone can be seen, which reflects the sedimentary characteristics of underwater distributary channel. The relative development of scouring and silting structures in the distributary channel development area shows that sometimes the distributary channel has strong river scouring effect and may be located in the upper delta plain environment. Therefore, the sub-cycle ⅶ 1 river is a distributary channel based on the development of delta plain environment.
(5) Shape and scale of sand body
Through the analysis of drilling data and field outcrops in the western part of the southern margin of the basin, it is found that the sand bodies in the ⅶ 1 sub-cycle are finger-shaped in the plane, and the sand bodies do not extend too far in the longitudinal and transverse sections (Figure 2-7- 13). The thickness of sand body also varies greatly, from a few meters to more than 20 meters for a single sand body. Therefore, the scale of distributary channel sand body in this area is not very large as a whole, which is much smaller than ⅴ 2 sub-cycle sand body. There is also a big difference in plane, and the distributary channel sand bodies in some sections are undeveloped or poorly developed.
(6) particle size distribution characteristics
There are two main types of sub-cycle sandstone grain size probability curves: two-stage and three-stage (Figure 2-7- 14). The curve is mainly composed of suspended and jumping populations, and sometimes there are a few traction populations. The overall slope of suspension is relatively small, mostly around 20, and φ > 2.5. The gradient of jumping population is 50 ~ 70, and the sorting is medium, with the particle size ranging from φ 0 to 2.5, and the boundary point between jumping population and suspended population is φ 2.2 ~ 2.7. The grain size of sandstone is fine, which generally reflects the sedimentary characteristics of rivers far from provenance.
Taking ⅶ 1 sub-periodic sample as the main sample, particle size analysis C? M diagram (Figure 2-7- 15). As can be seen from the figure, the sub-cycle sandstone ⅶ 1 in the southern margin of Yili Basin is mainly composed of PQ profile and QR profile, which reflects that the hydrodynamic force is strong, and the transportation mode is mainly jumping and moving, which is different from the grain size analysis of sediments in the main channel and distributary channel of the Yangtze River Delta. The characteristics of the M diagram are very similar (Figure 2-7- 16), which is equivalent to the distributary channel or main riverbed sedimentary area on the delta plain. On the distribution map of grain size parameters, sandstone samples of sub-cycle VII 1 in the southern margin of Yili Basin mainly fall in the delta plain facies area (Figure 2-7- 17).
Fig. 2-7- 12 Cross-sectional view of Curzi No.36931-kloc-0/and No.VII-1.
Delta plain flood plain subfacies; B- distributary subfacies of delta plain; C- delta plain distributary bay subfacies
Fig. 2-7- 13 vertical profile of the seventh cycle stratigraphic hole in Curzi area, Ukraine.
Figure 2-7- 14 ⅶ 1 sub-periodic sandstone granularity probability curve
Fig. 2-7- 15 C-M diagram of the seventh cycle in the west of the southern margin of Yili basin
The above grain size analysis results show that the ⅶ 1 sub-cyclic sandstone in the southern margin of Yili basin is mainly distributary channel subfacies of delta plain.
2. Characteristics of sedimentary facies of ⅶ 2 subcycle
Fig. 2-7- 16 C-M diagram of sediments in the Yangtze river delta
(According to Liu Baojun 1980)
1- river bed subfacies; 2- distributary channel subfacies; 3- front slope subfacies
A— Sedimentary area of main river bed and distributary channel; B- delta front sedimentary area; C- pre-delta sedimentary area
Fig. 2-7- 17 standard deviation of graph and scatter plot of skewness of graph
(The standard deviation and skewness are calculated by Friedman's formula for particle size parameters. )
Sub-cycle ⅶ 2 in the southern margin of Yili Basin is also a delta plain facies deposit, which can also be divided into three sedimentary subfacies: delta plain distributary channel subfacies, distributary bay subfacies and flood plain subfacies (Figure 2-7- 18). They are very similar to ⅶ 1 sub-cycle in stratigraphic lithology and lithologic combination, sedimentary rhythm, sedimentary structure and logging curve characteristics. Different from ⅶ 1 sub-cycle formation, the sand body of ⅶ-2 sub-cycle formation in the west of the southern margin (Kujiertai area) is relatively low, mostly less than 0.4, and the thickness of sand body is mostly less than 10m, which reflects that the distributary channel of ⅶ-2 sub-cycle in the west of the southern margin is not developed, and the distributary bay is developed (Figure 2-7-0
Fig. 2-7- 18 lithofacies palaeogeographic map of cycle VII-2 in the southern margin of Yili basin
1- denuded area; 2- distributary river subfacies in delta plain; 3- delta plain distributary bay subfacies; 4- delta plain flood plain subfacies; 5- Place names
3. The relationship between the sedimentary system and sedimentary facies of Shuixigou Group in the southern margin of Yili Basin and sandstone-type uranium mineralization.
Relationship between (1) sedimentary system and sandstone-type uranium mineralization
There are four major sedimentary systems in Yili Basin, namely, alluvial fan sedimentary system in the first to fourth cycles, braided river delta sedimentary system in the fifth cycle, shallow lake swamp sedimentary system in the sixth cycle and meandering river delta sedimentary system in the seventh cycle. According to the statistical analysis of the distribution ratio of known sandstone-type uranium mineralization boreholes in various sedimentary systems in the southern margin of Yili Basin (Table 2-7-3), it can be known that braided river delta sedimentary system is the most favorable sedimentary system in the southern margin of the basin, with the largest number of boreholes, accounting for 63.9% of the total boreholes in Shuixigou Formation in the southern margin of Yili Basin. Alluvial fan sedimentary system and meandering river delta sedimentary system are also favorable sedimentary systems, and their ore holes account for 22.2% and 65,438+03.9% of the total ore holes of Shuixigou Group in the southern margin of Yili Basin, respectively. However, sandstone-type uranium mineralization in shallow lake swamp sedimentary system is unfavorable, and sandstone-type industrial uranium mineralization has not been found (Table 2-7-3).
Table 2-7-3 Output Rate Table of Sandstone Uranium Deposits in Different Sedimentary Systems of Shuixigou Group in the Southern Margin of Yili Basin
The main reason why uranium mineralization in Yili basin is controlled by braided river delta sedimentary system, alluvial fan sedimentary system and meandering river delta sedimentary system is that the strata formed by these sedimentary systems have stable mudstone-sandstone-mudstone stratigraphic structure, which is beneficial to sandstone uranium deposits in interlayer oxidation zone after epigenetic transformation. However, the shallow lake swamp sedimentary system is small in scale, thin in thickness and compact in cementation, which is not conducive to the development of epigenetic interlayer oxidation and the formation of sandstone-type uranium deposits, so no sandstone-type industrial uranium mineralization has been found so far.
(2) Relationship between sedimentary facies and sandstone-type uranium mineralization.
The comprehensive statistics of sedimentary facies and ore-bearing conditions of all ore-bearing strata in the southern margin of the basin (Table 2-7-4) show that sandstone-type uranium mineralization found in the southern margin of the Yili Basin is mainly formed by delta front mouth bar and sheet sand subfacies of delta sedimentary system (accounting for 30% of the total ore-bearing holes), braided river subfacies of delta plain (accounting for 30% of the total ore-bearing holes) and fan-fan end of alluvial fan sedimentary system. Secondly, it is controlled by delta front distributary channel subfacies and delta plain flood plain subfacies, but the ore-bearing rate of these subfacies is generally less than 5%, so they are not the main ore-bearing facies of Shuixigou Group in the southern margin of Yili Basin, while the fan-front braided river subfacies, delta front distributary bay subfacies, delta plain distributary bay subfacies, pre-delta subfacies and swamp facies are not industrially uranium-enriched.
Table 2-7-4 Occurrence rate of sedimentary facies (or subfacies) sandstone-type uranium mineralization in the southern margin of Yili Basin
The sandstone-type uranium mineralization of Shuixigou Group in the southern margin of Yili Basin is controlled by delta front sand bar and sheet sand subfacies, delta plain braided river subfacies, delta plain distributary channel subfacies and fan-end subfacies of alluvial fan sedimentary system.
1) These favorable sedimentary facies have stable mudstone-sandstone-mudstone stratigraphic structure and considerable sand bodies, which are the necessary geological conditions and ore storage space for sandstone-uranium mineralization in the supergene interlayer oxidation zone. The exploration results in the southern margin of Yili basin show that the more perfect the mudstone-sandstone-mudstone stratigraphic structure and sand body development, the more favorable uranium mineralization will be. For example, delta front mouth bar and sheet sand subfacies of delta sedimentary system have the most stable stratigraphic structure, the best sand body development and the largest uranium mineralization scale.
2) These sedimentary facies are pre-enriched with uranium mineralization to some extent in the sedimentary diagenetic stage. In particular, the delta front mouth bar, sheet sand subfacies and distributary channel subfacies of delta sedimentary system are in geochemical transition under the conditions of water oxidation medium and underwater reducing medium, which is beneficial to the decomposition of uranyl complex in aqueous solution and the reduction and precipitation of uranium, thus leading to the formation of highly enriched uranium.
Meso-Cenozoic intracontinental orogeny and sandstone-type uranium mineralization in the middle subtype orogenic belt in western China.
1- distributary channel subfacies of delta plain; 2- flood plain subfacies of delta plain; 3- bare area; 4- uranium mineralization hole
Meso-Cenozoic intracontinental orogeny and sandstone-type uranium mineralization in the middle subtype orogenic belt in western China.
1- distributary channel subfacies of delta plain; 2- flood plain subfacies of delta plain; 3- bare area; 4- uranium mineralization hole
3) During the supergene transformation of the basin, these sedimentary facies were just lifted to the edge of the basin and transformed by surface water containing oxygen uranium, resulting in sandstone-type uranium mineralization enrichment in interlayer oxidation zone.
(3) The relationship between the characteristics of ancient rivers and sandstone-type uranium mineralization.
In order to accurately define the spatial position of ancient rivers in the study area in different periods and discuss the relationship between ancient rivers and uranium mineralization, the thickness of sub-cycle ⅶ 1 upper and lower layers of sand bodies in the southern margin of Ili basin was counted, and the equal thickness of the two layers of sand bodies was simulated by using Arcmap spatial analysis, and the equal thickness map of sand bodies was drawn. Then, based on the contour map of sand body, combined with the distribution characteristics of coarse-grained rocks above the glutenite which represents the river bed retention and deposition, the distribution positions of early and late ancient rivers corresponding to the ⅶ subcycle 1 upper and lower sand bodies in Curzi area of Wu area are delineated (Figure 2-7- 19, Figure 2-7-20).
As can be seen from Figure 2-7- 19 and Figure 2-7-20, the distribution of major uranium mineralization is strictly controlled in ancient rivers, and its ore-controlling effect is mainly manifested in the following two aspects: ① Large uranium mineralization zones are often distributed in larger distributary rivers, while uranium mineralization is poor in smaller ancient rivers; ② The location where the distributary channel diverges and converges is a favorable place for sandstone-type uranium mineralization.
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(Li, Han Zhongchong, Cai, Zheng Enjiu, Huang Shutao, Zhao Yongan)