Distribution characteristics of sedimentary organic facies in continental sequence stratigraphic framework

-Take the Middle Jurassic coal measures in Taibei sag of Tuha basin and the southern margin of Junggar basin as examples.

By using the methods of organic petrology, geochemistry and palynology, the distribution characteristics of sedimentary organic facies in Taibei sag of Turpan-Hami basin and J2x coal measures in the southern margin of Junggar basin are discussed. The research shows that, in general, all kinds of sedimentary organic facies are symmetrically distributed with lacustrine transgression system tract as the center in the longitudinal direction of sequence stratigraphic framework, and the source rocks with the strongest hydrocarbon generation capacity come from lacustrine-semi-deep lacustrine organic facies, which generally appear in the middle of lacustrine transgression system tract, and the hydrocarbon generation capacity of source rocks becomes worse from top to bottom.

Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi

I. Introduction

Sequence stratigraphy is a new discipline in the history of sedimentology. It emphasizes that the global or large-scale simultaneous deposition caused by the surface change of a certain type of large water body (ocean and inland lake) has isochronous significance. Sequence stratigraphy was originally a sedimentological model established from the study of marine sediments in passive continental margin basins in North America. In recent years, many scholars [1 ~ 4] have tried to introduce their analysis principles and methods into the study of continental basins. Turpan-Hami basin and Junggar basin are large continental depression basins in Mesozoic in China, and there are many articles about sequence stratigraphy and source rocks in this area. Ji Youliang believes that structure and climate are the main controlling factors affecting the formation of continental sequence stratigraphy [2]. TOF-SIMS and others have studied the hydrocarbon generation potential of different sedimentary facies macerals in this area [5]. Before J2x, Tuha Basin and Junggar Basin had similar geotectonic and paleoclimatic conditions, so they were comparable at least before J2x of Early and Middle Jurassic. Taking the J2x coal seam in Taibei sag of Turpan-Hami basin and the southern margin of Junggar basin as examples, the relationship between sequence stratigraphy and sedimentary organic facies in this area is discussed.

Second, the sequence stratigraphic framework in this area

See table 1 for the sequence stratigraphic framework of Taibei sag in Turpan-Hami basin. Yang Ruicai [4] divided three sequences and 10 system tracts according to the four unconformities of Jurassic in Taibei Depression. Yang Ruicai's division scheme is now adopted, which is briefly described as follows:

Sequence I: The bottom boundary is the unconformity formed by Indosinian movement between Triassic and early Jurassic Badaowan Formation (J 1b), and the top boundary is the unconformity formed by Yanshan movement I between Xishanyao Formation (J2x) and Sanjianfang Formation (J2s). I sequence was formed in warm and humid climate, and tree fern, ginkgo, pine and cypress, equisetum and cycad were very developed. When J 1b, it is a low-stage river, delta and lake swamp deposit, and a very thick coal seam is formed in the transgression stage. J 1s is the lake transgression period of sequence I, and a set of shallow lake deposits of gray-green mudstone, shale and siltstone are deposited. In the early J2x period, lacustrine and delta deposits and coal seams developed under the background of J 1s lacustrine transgression system. In the late J2x period, the climate has obviously changed to semi-dry and subtropical climate. After the I stage of Yanshan movement, the I stage tectonic sequence also ended.

Sequence II: The whole J2s Formation, the lower part is bounded by the Yanshan Movement I unconformity between J2s and J2s, and the upper part is bounded by the unconformity between J2S and J2q. During this period, the molecular content of Classopollis, which likes dry heat, increased, while the molecular content of Triceratops, Alsophila spinulosa and other wet and hot molecules decreased, which gradually changed into a semi-dry and subtropical climate. The sediments are generally a set of purple, brown, grayish green and purplish red mudstone and light gray, grayish white and grayish purple sandstone siltstone, which reflects the relatively dry shallow water environment.

Sequence III: The Turpan-Hami basin entered the structural sequence III from Qiketai period, and the unconformity formed by Yanshanian movement curtain II after Jurassic was the top boundary of this sequence, which separated it from Cretaceous strata. At the beginning of J2q, the climate changed to warm and humid in a short time, and peat and coal line deposits appeared in its low position and lake invasion period. Semi-deep lacustrine mudstone and a small amount of oil shale were formed when well Taisan-2 was invaded by J2q lake, but after the middle of J2q, the climate began to change to dry heat. By the end of Jurassic, all the lakes in this area disappeared, replaced by a long-term river environment, with the red sandy mudstone of Guqi Formation and Caraza Formation as the main body.

Table 1 Jurassic sequence division table of Taibei sag in Turpan-Hami basin

Note: According to Yang Ruicai of Xinjiang Petroleum Administration Bureau (1997). Sb 1-sb4: unconformity surface; Tk: seismic response of Jurassic-Cretaceous interface; TJ2q: Interface response between sandstone-mudstone interbedded coal seam in the lower part of Qiketai Formation and dark mudstone section in the middle and upper part of Qiketai Formation; TJ2x: Interface response between dense coal seam section of the second member of Xishanyao Formation and upper and lower surrounding rocks; TJ 1b: interface response between top coal and upper and lower surrounding rocks of Badaowan Formation; HST: advanced system domain; LHST: Late Advanced System Domain; EHST: early advanced system domain; TST: lacustrine transgression system tract; LST: low-level system domain.

Third, sequence change and sedimentary organic phase cycle

From the perspective of sequence stratigraphy, the vertical filling sequence and variation law of sedimentary organic facies in continental basins are investigated, and there are few research examples at home and abroad. In this study, the early Jurassic J 1 b and J 1s strata of Shen Ling1well in Turpan-Hami Basin and the middle and late Jurassic strata of Taisan-2 well are connected into a comprehensive profile. According to the changes of mudstone organic matter type, abundance, organic geochemistry and inorganic geochemistry parameters in the whole profile, the corresponding relationship between system tract and sedimentary organic facies is found out. As far as mudstone is concerned, TOC and S 1+S2 represent organic matter abundance and hydrocarbon generation potential; The content of SiO _ 2+Al _ 2O _ 3 can indicate the distance between the deposition area and the provenance area. Ca/Mg indicates the depth change of sedimentary medium; Sr/Ba indicates the salinity and reduction degree of the medium; Phosphorus and strontium/copper reflect climate change. Their changes in longitudinal profile can not only reflect the changes of different sedimentary system tracts, but also reflect different types of sedimentary organic phase cycles.

Fig. 1 is the variation diagram of various organic petrology, organic geochemistry and inorganic geochemistry parameters on the comprehensive profile. Mudstones with high organic matter abundance, good type and high hydrocarbon generation potential are generally distributed in lacustrine transgression system tracts, or in early high-level system tracts and late low-level system tracts. This may be because at this stage, the water tends to deepen, and the reduction type is strong and stable, so mudstone with good organic matter type and high abundance can be formed and preserved. In addition, from the change of Al2O3+SiO _ 2, except for a few points, the overall Al2O3+SiO _ 2 of lacustrine transgression system tract is lower than that of other system tracts, indicating that the sedimentary area is far from the provenance; The high value of Ca/Mg usually appears in lacustrine transgressive system tracts, but coal lines or carbonaceous mudstone in other system tracts will also affect the change of Ca/Mg if there are late carbonate veins. Sr/Ba ratio is widely used to analyze the sedimentary environment of marine and continental strata, because Ba is an element that is difficult to migrate, and the farther away from the shore, the lower the Ba content. The situation of Sr is just the opposite to that of Ba, and its migration ability is much greater than that of Ba. The farther away from the coast, the higher the salinity and the higher the strontium content. This study attempts to apply it to the analysis of inland basins. From the comprehensive profile, the changing trends of Sr and Ba are just the opposite. In J 1b low-level system tract, Ba is the highest and Sr is the lowest, while in J2q lacustrine transgression system tract, Sr is the highest and Ba is the lowest. It can be inferred indirectly from the changes of Sr and Ba that the J 1 s lake invasion of Shen Ling1well and the J2s lake invasion of Taisan 2 well may be much smaller than that of J2q well. Of course, the judgment of Sr and Ba alone is not completely reliable. As we all know, when the content of Ca is high, the content of Sr will be abnormally high, so it is necessary to combine other geological evidence to make a judgment.

P and Sr/Cu are both elements and element pairs sensitive to paleoclimate change. The enrichment of P in sedimentary rocks is related to biology. If the salinity increases sharply due to water evaporation in hot climate, some lower organisms will not adapt to this high salinity and die and participate in diagenesis, so that their horizons are relatively rich in P. Obviously, the horizons with relatively high P content represent the high salinity environment under dry and hot conditions. When the ratio of Sr/Cu in mudstone is 1.3 ~ 5.0, it indicates a warm and humid climate environment, and when it is greater than 5.0, it indicates a dry and hot climate [6]. According to the changing trend of P and Sr/Cu, during J2q lake invasion, the climate has turned to dry heat, which is basically consistent with the previous analysis. The three-bedroom period is in a dry and hot climate (P is reflected, Sr/Cu is not obvious), which is basically consistent with the previous analysis conclusions.

The separation of strontium and barium in coal seam is not obvious, and it has a good response with phosphorus and strontium/copper, reflecting the paleoclimate evolution during the development of peat swamp. Fig. 2 shows the changes of Sr, Ba, P, Sr/Cu in the upper coal seam and the lower coal seam in J2x in the southern margin of Junggar Basin. It can be seen from the figure that the Sr, Ba, P and Sr/Cu of the lower J2x coal seam are much higher than those of the middle and upper coal seams, indicating that the paleoclimate of peat swamp in the early J2x period was relatively dry. Sporopollen analysis also shows that gymnosperms are dominant in the lower coal seam and fern spores are dominant in the upper coal seam, which is consistent with coal facies analysis.

Fig. 1 Changes of fluorescence organic composition index, SiO _ 2+Al _ 2O _ 3, Ca/Mg, Sr/Cu, Sr/Ba, Ba and P of mudstone in the early and middle Jurassic schematic section of Taibei sag in Turpan-Hami basin.

Because each sedimentary organic facies also has certain organic petrology, organic geochemistry and inorganic geochemistry indexes, from the above comprehensive profile, the changes of various parameters in different system tracts show that each system tract in sequence stratigraphy can respond well to some sedimentary organisms, whether from the paleoclimate or paleogeographic environment. Under the framework of sequence stratigraphy, these sedimentary organic facies appear regularly in the longitudinal direction, showing a good mirror symmetry cycle. Table 2 is an ideal model of organic correspondence between sequence framework and sedimentation in this area, which is briefly described as follows:

Early development of J 1b low water level system tract in Taibei sag of Turpan-Hami basin. Although the climate is warm and humid, with abundant rainfall and lush plants, some early low bulges are still being flattened, so the sedimentary area is close to the provenance. Mudstones formed in this period are mostly organic phase with poor fluorescence, and the content of SiO _ 2+Al _ 2O _ 3 is high, while coal is high swamp coal developed in braided river and braided river delta, which contains inert components. In the middle and late stage of J 1b, the terrain is relatively flat, the groundwater level gradually rises, and the reducibility of the medium is enhanced. On the plane, arid forest peat swamp facies, periodic arid forest peat swamp facies and water-covered forest peat swamp facies appear in turn at the confluence of low-lying lakes. Vertically, from bottom to top. The V/I ratio of coal is getting higher and higher, and the content of homogeneous vitrinite and structural vitrinite is more and more dominant, even reaching more than 90%. Semi-deep lake-lake-bay mudstone facies rich in fluorescent organic components can be developed in local depressions, but the latter mainly developed in the full-scale lake invasion period of J1s. Of course, lake invasion is a process of gradual enhancement, and other organic phases, such as weak fluorescence and medium fluorescence, will also appear. In the late stage of lake transgression system tract development, the sediment accommodation space increased to the maximum, and then began to decrease slowly, entering the development period of high system tract. When the water level is low, the organic phases of mudstone and coal will appear in the opposite order, that is, strongly flooded forest peat swamp phase and flooded forest peat swamp phase. It can be seen that the general trend of accommodation space of each tectonic movement cycle in Taibei sag of Turpan-Hami basin is a process from small to large and then to small. This process not only constitutes the cyclicity of the development of low water level system tract, lake transgression system tract and high water level system tract in sequence stratigraphy. It also constitutes the process of fluorescent organic components from small to large and then to small, and the V/I of coal from small to large and then to small, and then constitutes a cycle series in which the sedimentary organic phase is centered on the organic phase of fluorescent mudstone in the lake transgression system, and spreads to both sides and up and down in a mirror-image symmetry in the horizontal and vertical directions. When Dai Shifeng studied the organic phase of coal in Wuda mining area in western Ordos, he also found the same change law [7, 8]. According to this ideal model, all kinds of source rocks with high organic matter abundance, good organic matter types, high fluorescence organic component index and strong gas-generating ability in this area should appear in the upper and lower layers centered on lacustrine transgression system tract vertically. The farther away from this center, the lower the EHST layer, and the worse its organic matter type, abundance and gas generating capacity. As a special organic matter-rich phase, the main coal seams of coal often appear at the turning point between LST and TST, TST and HST, and at the interval between LLST-ETST and EHST.

Fig. 2 changes of p, Sr, Ba and Sr/Cu in various coal seams of JXX formation in the southern margin of Junggar basin.

Similar changes have taken place in the fossil assemblage of plants. For example, in the upper part of J 1b formation in Junggar Basin, the number of true pteridophytes increased, accounting for about 40.8% of all plant species, Ginkgo biloba accounted for 18.4%, conifers accounted for 18.4%, and cycads were rare. Sangonghe Formation is a lacustrine transgressive system tract, with the most species of pteridophytes (about 42.2%), the corresponding increase of Ginkgo biloba and coniferous trees (20% each), and the least species of Cycas (only 9.0%). The lower part of J2x reflects the ecological environment (EHST) of the early high-water system tract. The number of true pteridophytes decreased to only 34%, while Ginkgo biloba and Cycas increased to 23.7% and 17.5% respectively. The upper part of J2x reflects the environment of the late high-water level-early low-water level system tract, and the true pteridophytes have an increasing trend, reaching 38.2%, Ginkgo biloba is 24.5%, Cycas are reduced to 14.7%, and conifers are 10.8%. It can be seen that the change period of true pteridophyte species in the vertical filling sequence of strata is very obvious. In the period of transgression, the number of true pteridophytes tends to increase, and the species of true pteridophytes reach the peak when the lake is completely invaded, but begin to decrease in the period of regression. Of course, this periodic change will never be simply repeated. Generally speaking, the Jurassic climate in Tuha Basin and Junggar Basin changed from warm and humid to dry, Cycas increased from J 1b-J2t, and true ferns decreased.

Table 2 Relationship between sedimentary organic facies, gas-generating properties and sequence stratigraphy

Note: a: organic phase of mudstone with poor fluorescence composition is1; B: organic mudstone phase 2 with poor fluorescence composition; C: Mudstone sedimentary organic phase 3, medium fluorescence composition; D: organic phase in the lake bay-semi-deep lake mudstone rich in fluorescent components; A'*: organic subfacies of high swamp coal, which is roughly equivalent to dry forest peat swamp facies; B': dry forest peat swamp facies; B ":periodically dry forest peat swamp facies; C': peat swamp facies of water-covered forest; C ":peat swamp facies of severely flooded forest; D'**: Semi-deep lake-deep lake organic phase, and no such samples have been collected in this work.

It is necessary to emphasize again that Table 2 is only an "ideal model" of sedimentary organic facies in sequence stratigraphy. The so-called ideal model refers to the ancient structure, the ancient climate, the supply of material sources, including the supply of biological precursors, and the rational allocation of ancient topography and geomorphology, so that this model can appear completely, otherwise it will not appear. For example, during the Sanjianfang period in this area, although the three system tracts of LST, TST and HST are complete, there are no various phases and organic phases rich in fluorescent mudstone, mainly because the paleoclimate has turned into dry heat at this time, which is not conducive to the full supply of organic matter. The ideal model put forward in this study is only a preliminary exploration, and few factors such as basin tectonic movement, continental source area and its change are considered, which needs to be deepened.

Acknowledgement: This article was directed by Professor Zhang Pengfei of China University of Mining and Technology, and Dr. Sun Junmin and Dr. Dai Shifeng gave specific suggestions. Dr. Zhao Fenghua and Master Xu Dewei participated in some research work, and I would like to express my heartfelt thanks!

Take the exam and contribute.

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Distribution of sedimentary organic facies in continental sequence stratigraphic framework

Framework: Taking Taibei Depression in Tuha Basin and Middle Jurassic coal-bearing measures in southern Junggar Basin as examples.

Yang Jian -ye 1 Rende -yi2 Shao Longyi 2

(1. Xi 'an Institute of Mining and Technology, Xi 'an 710054;

2. China University of Mining and Technology, Beijing 100083)

Abstract: The generation and development of sedimentary organic facies are controlled by many factors such as paleoclimate, paleostructure and paleogeography. As we all know, these factors change periodically in geological history, and this change will inevitably lead to the periodic change of sedimentary organic phase. This can be shown by the distribution of sedimentary organic facies in time stratigraphic framework or sequence stratigraphic framework. Due to the lateral change of paleoenvironment, many types of sedimentary organic facies can be developed in the whole basin in a period of time. The lateral zonation of organic phases is reflected in their vertical superposition, which conforms to Walter's law to some extent. Sedimentary system tracts in sequence stratigraphic framework have been proved to be an effective method to analyze laws and predict the organic petrology, organic geochemistry and inorganic geochemistry of potential source rocks.

On this basis, using the methods of organic petrology, geochemistry and palynology, the distribution of sedimentary organic facies in the continental stratigraphic framework of middle Jurassic coal-bearing series in Taibei sag of Turpan-Hami basin and southern margin of Junggar basin was analyzed. Turpan-Hami basin and Junggar basin are typical coal-bearing hydrocarbon-generating basins in China, and many scholars and experts have studied the continental sequence stratigraphic framework in this area. The quantity of organic matter and potential hydrocarbon generation capacity in argillaceous sandstone are indicated by the fluorescence organic component index TOC, S 1+ S2. The content of SiO _ 2+Al _ 2O _ 3 reveals the distance from provenance. The deep change of sedimentary medium is indicated by Ca/Mg. The ratio of strontium to barium reflects the salinity and reducibility of the medium. Climate change is indicated by P, Sr/Cu, and the changes of different sedimentary system tracts and cycles of different types of sedimentary organic facies can be reflected in the vertical profile. The results show that, on the whole, all kinds of organic sedimentary facies are vertically and symmetrically distributed around the transgressive system line (TST). The best source rocks are preserved in the middle of TST, and the potential of hydrocarbon generation from this position becomes poor.

Key words: continental sequence stratigraphy sedimentary organic facies Jurassic Turpan-Hami basin Junggar basin

(This paper was co-authored by Yang Jianye, Ren Deyi and Shao Longyi, originally published in Acta Sedimentary, 2000, vol. 18, No.4).