1. Coal and Rock Characteristics of Leping Fm Coal Seam in Pingle Depression
Leping coal is a special kind of coal, which contains a lot of bark tissue. Typical Leping coal-bark residual coal (containing more than 50% bark tissue) mainly occurs in the B coal groups of Mingshan, Huiyuanling and Qiaotou A near Leping, and the B coal groups of Zhongjiashan in Leping and Li Zhushan are mostly dark bark coal and bright dark bark coal. However, no residual bark coal was found in Fengcheng in the west of Pingle depression and B coal group in Yichun.
There are great differences in coal-bearing property and coal seam characteristics among the eastern, central and western parts of the depression.
There are three minable coal seams B 1, B2 and B3 in the east of coal group B, and the B 1 coal seam in the middle and west is often missing, and the B3 coal seam is often replaced by shallow sea facies and lagoon facies. The features of B2 coal seam developed in the whole region are also different. The eastern coal seam is not very stable, and the thickness can range from zero to three or four meters, and sometimes it can be changed from two meters to thirty or fifty meters. The structure of coal seam is complex, the lateral change is great, and the number of gangue layers changes greatly. The coal seam is layered and convex, mainly composed of semi-dark coal balls, the lower part is dark and the upper part is bright. Due to the frequent alternation of various coal and rock components, an uneven and complex linear structure has been formed. Pyrite inclusions in coal seams are mostly convex mirror-shaped, with a maximum of 10 cm× 55 cm. The floor of coal seam is mostly clay rock and siltstone, with oolitic structure (siderite oolite) and a large number of plant root fossils-trace wood. Its top is often sandwiched with thin calcareous sandstone or marl, and contains marine animal fossils. In the middle and west of the depression, take Fengcheng as an example. Although there are two or three layers of gangue in B2 coal seam, the structure is also complex, but it is stable in the lateral direction, and generally does not change much within a few kilometers. It belongs to a relatively stable layered coal seam, with relatively uniform coal and rock composition, mainly semi-bright briquette, interbedded with semi-dark coal, showing a wide strip structure. Its floor is also clay rock and siltstone, but there is no oolitic structure. The direct roof is mainly lacustrine siltstone and clay rock.
The microscopic characteristics of group B coal in the eastern and western regions are also different. Typical bark residual coal is developed in B2 coal seam in Mingshan area of Leping in the east. The content of bark tissue in maceral is more than 50% ~ 60%, and the bark tissue is orange. Due to the different expansion degree, the original structure is not obvious, the surface is gelatinous, and only band-like scale structure can be seen locally. Bark tissue varies in size, with larger pieces (up to 5 mm) distributed along the layer, and smaller bark fragments (mostly 0. 1 ~ 0.0 1 mm) is irregularly distributed in the dark brown weak filament carbonization matrix. The gel matrix is quite uniform, interbedded with bark tissue, showing horizontal microwave bedding, and the matrix can also be seen with corrugated structure. There are few xylem tissues in coal, mainly composed of massive lignite vitrinite and vitrinite semi-silk charcoal, and few typical silk carbonization components. Among the mineral impurities, pyrite is abundant and distributed around the bark tissue in the form of fine crystals, while others include clay minerals and timely debris dispersed in the carbonized matrix of weak filaments. The bedding and microscopic characteristics of bark tissue enrichment in coal seams in other eastern regions are similar. The microscopic characteristics of B2 coal seam in Fengcheng area in central China and vast areas in western China are that there are many cementitious components, the content of which is above 70%. Except for some bark tissue found in the lower part of Meixianling and Wusheli coal seams, there is generally no bark tissue. There is more matrix in coal and less plant tissue. Clay minerals are the main mineral impurities, which are dispersed in the cementitious matrix in the form of worms or small convex mirrors, and their contents vary greatly in each coal seam.
Second, the coal-forming environment and coal-forming prerequisites
According to the genetic characteristics of coal seam floor, coal seam structure, coal petrographic composition and coal microstructure, we think that the residual bark coal was not generated in different places, and the original coal-forming material was not transported from other places by rivers and floods, but was formed in the flooded swamp with strong water flow. The enrichment of bark tissue is related to local transportation in swamp-micro-transportation in different places.
юемчуников) and иив (иив ... We think that the formation environment of bark residual coal in Leping area is similar to this.
In the coal with high content of bark tissue in Leping area in the east of Pingle Depression, the content of gelation component is always much higher than that of silk carbonization component, and the bark tissue itself expands due to gelation. The existence of pyrite particles shows that the accumulation environment at that time was not a strong oxidation environment, not a "dry" swamp, but a deep-water swamp. But the environment in the swamp is not very calm, and the water flow is more active. Coal has obvious microwave bedding, and the structure in the cementitious matrix is corrugated. Part of bark tissue is broken into thin strips and fragments, which form a convex mirror with silk charcoal fragments, weakly carbonized briquettes, clay minerals and timely, indicating that the water flow was active at that time, and the floor of coal seam with residual bark plants had wooden traces, sometimes perpendicular to the floor. Generally, the structure of coal seam is obvious, but the thickness is unstable, and the bark content of the same coal seam in the same mining area changes greatly. In the coal seam profile, bark tissue is often enriched at the bottom of the coal seam and near the gangue layer, which reflects that the original materials of coal grow and accumulate in situ, and the bottom of coal-forming peat swamp is uneven, which is beneficial to the enrichment of bark tissue under the condition of water flow.
In the late Permian, the climate was humid, and the cortex of coal-forming plants such as Lycopodium and Pteridophyte was very thick. There are a lot of bark tissues in the coal-bearing formations of many coalfields in late Permian in China. Therefore, there is no doubt that paleobotany is a necessary condition for the formation of residual coal with bark.
According to the available data, there is little difference in fossil plant composition in Pingle Depression. Why is the residual coal with bark only developed in the east? We think this is related to the paleogeography and tectonic conditions during the coal-forming period.
The Wu Dong movement caused the Pingle sag to rise as a whole, and many secondary sags were formed in the sag due to the wavy swing of the fault block. At that time, the terrain height difference was large and the river was washed strongly. Many rivers flow to sedimentary basins with a large amount of coarse clastic materials. The eastern part of the depression is influenced by the Jiangnan ancient land and the Wugong uplift in Huai Yu, and the uplift is more obvious than that in the central and western parts. In the areas of Mingshan and Zhongjiashan in Leping, delta facies deposits at the bottom of coal-bearing formations are widely developed, with a thickness of more than 100 meters, and the grain size changes greatly, with multiple cycles, reflecting the frequent crustal activity at that time. The crustal activities in this area are quite different, and the lacustrine facies and closed-flow basin facies before coal formation are unstable (Figure 1), in which there are many minerals such as siderite nodules, pyrite fine crystals, hydromica and dolomite, which have oolitic structure, reflecting the weak reducing environment of water flow activities. The peat swamp developed on this basis has a small scale and uneven base, which is beneficial to the bark tissue left after plant decomposition to be transported from one place to another with the water flow in the swamp and enriched in a deeper and calmer place. After peat accumulation, it is quickly covered by lagoon facies and shallow sea facies deposits.
However, in Fengcheng area in the middle of Pingle Depression, the tectonic movement is relatively stable for a long time. After accepting delta facies deposition, lacustrine facies and closed-flow basin facies are generally developed and thick (Figure 1). After a long period of erosion and deposition, the area has gradually leveled off, and plants are overgrown, forming a vast coastal peat swamp. At this time, the swamp is deeply covered by water, and the water flow activity is small. The lignocellulose tissue in plants is fully decomposed, completely gelled and preserved.
Figure 1 Lithofacies Histogram of Coal Group B in Fengcheng Area, Leping
Third, the metamorphism of coal.
In order to understand the problem of coal metamorphism, we must first accurately determine the stage and law of coal metamorphism. According to the existing data, the reflection of unstructured gel in vitrinite is a good sign to reflect the metamorphic stage. We have determined the metamorphic stages of coal in Pingle Depression by comparing the reflectivity, which are divided into I, II, III, V, V, VI and VII metamorphic stages according to the metamorphic degree, and these metamorphic stages are roughly equivalent to long-flame coal, gas coal, fat coal, coking coal, lean coal, lean coal and anthracite. Each stage can be divided into three small stages: Ⅱ1,Ⅱ 2 and Ⅱ 3. The vitrinite content of B2, the main minable coal seam in the central and western part of the depression, is above 80%, and the chemical analysis data of coal is less affected by the composition of coal and rock, mainly by the metamorphic stage, so the brand and ⅴ γ value of coal are also referred to when discussing the metamorphic law.
(1) metamorphic bands of coal in Pingle sag are obviously distributed. In the horizontal direction, taking B2 coal seam as an example, the general trend is that the metamorphic degree is getting higher and higher from east to west. For example, Leping area is in the second metamorphic stage, Fengcheng is in the fourth metamorphic stage, Yinggangling is in the sixth-seventh metamorphic stage, and Wujinjing is in the seventh metamorphic stage. At the same time, we can also see the phenomenon that metamorphism gradually strengthens from north to south and from Jiangnan ancient land to the center of depression. Generally, the metamorphic band is low-high, and the intermediate metamorphic band is narrow.
On the stratigraphic section, the metamorphic degree of the upper coal seam is low, such as the B2 coal seam in Leping Mountain is Ⅱ1period, and the C6 coal seam about 140m above it is Ⅰ 3, with little difference. B2 of Xianguling in Fengcheng is Ⅳ 2, while C 18C28 coal seams, which are 280m apart, are all Ⅱ 3, with a difference of 5 small steps.
This zonal distribution law is mainly caused by regional metamorphism. Pingle sag is one of many belt-shaped active depressions in the south, and the inheritance and difference of crustal movement in this area are quite obvious. The original subsidence amplitude of each part in the depression is different, and the northern part is shallowly controlled by Jiangnan ancient land, while the southern part is deeper. Affected by the Wugong Uplift in Huai Yu, the subsidence in the eastern region is shallow. The total thickness of the caprock from the formation of B2 coal seam to the Indosinian fold is only 500-600 meters, while the subsidence of Yichun and Pingxiang in the west is deep, and the caprock can reach more than 2000 meters. It can be seen that in the same tectonic unit, under similar geothermal conditions, in the process of continuous subsidence after the formation of coal seam, due to the different subsidence depth and the influence of different temperatures and pressures, the coal with different metamorphic degrees has a greater impact. Due to the difference and inheritance of crustal movement, it is possible that the syncline basin has a high degree of metamorphism and a slightly lower edge.
(2) Magmatic activities were very frequent in different periods of Mesozoic in Pingle Depression, which had an important influence on the degree of coal metamorphism.
Around Mengshan, where the Indosinian granite body was formed, the volatile isoline and metamorphic belt were distributed concentrically and annularly (Figure 2). Even in the highly metamorphic VII metamorphic belt, several sub-zones (VII1,VII 2, VII 3) can be divided. The closer to the complex rock mass, the higher the degree of metamorphism and the obvious influence of thermal metamorphism. In addition, Yanshanian diabase intrusion and local contact metamorphism can also be seen in Jiansha, Qiaotou A and Li Zhushan.
Fig. 2 Lithofacies histogram of coal group B in Fengcheng area, Leping.
(3) The structure in the west of Pingle Depression is complex, the linear composite fold is closed and the dip angle is steep. Faults are mainly reverse faults and overthrust faults with frequent inversion and high degree of coal metamorphism. In the central area, however, folds are gentle, faults are mainly normal faults and flat inference layers, and the metamorphic degree is moderate. Therefore, the influence of tectonic force (power) on coal metamorphism is also worthy of attention.
In a word, different types of metamorphism are the concrete manifestations of heat, pressure and other factors in various stages of geological development, which are closely related to each other and have a comprehensive impact on coal metamorphism.
[Note] Coal seam naming: The number of coal-bearing coal seams in Leping Fm coal-bearing strata in Pingle Depression varies greatly, and the development degree varies from place to place. At present, the bottom coal seam is used as the first coal seam in production, which is arranged in turn, and the coal group code (such as A 1B3C6, etc.). ) The front is the serial number, so the coal seams of the same horizon have different names in different regions. According to the sedimentary cycle characteristics of coal-bearing formations and the comparison results of coal seam lithofacies marks, we try to name the coal seam of coal group B uniformly according to the sedimentary cycle (table 1).
Table 1 Cyclic Naming of Pingle Depression
(This article is co-authored by Gao, Liu Xiangsheng and originally published in Selected Papers of the 32nd Annual Academic Conference of geological society of china, 1963).