The main functions and processes of the formation and evolution of modern deepwater chemistry

The difference in chemical balance between modern chemistry and initial chemistry only determines the basis and scale of the formation and evolution of initial chemistry in the process of geological history, but does not answer how the deep-water modern chemistry of each research layer evolved in the process of geological history. Therefore, the main functions and processes of studying the formation and evolution of strata should be discussed according to three hydrogeological periods: sedimentation, subsidence and burial and hydrothermal tectonism.

The main function and process of 1 hydrochemical metamorphism. T3x2

(1) Hydrogeological period of sedimentation: this period refers to the duration from the beginning to the end of T3x2 sedimentation, which is the shortest period that T3x2 has experienced in geological history. After T3x2 deposition, it was covered by seawater, and the water in the layer kept alternating with seawater. The cumulative pressure caused by the superposition of the static rock pressure of the sedimentary layer itself and the weight of the overlying seawater is very low, so the compaction strength of the sedimentary layer is very small, and the water circulation in the layer is alternately sluggish; The sedimentary layer is in the initial stage of diagenesis and remains loose.

Table 8-8 Comparison between Initial Chemistry and Modern Chemistry of Upper Triassic

The main functions of changing the chemical composition of water are biochemical reduction and leaching of sulfate. Biochemical reduction of sulfate removal is common. Because of the large binding energy of oxygen and sulfur in SO _ 42- in water, at low temperature (

Under the superposition of these two effects, the concentrations of Na+, Ca2+, Mg2+, Cl- and HCO3-in water all increase and produce H2S, while the content of so42- decreases. Because of the low solubility product of carbonate and sulfate of calcium and magnesium, it is easy to form precipitate and enter the solid phase, resulting in the decrease of the concentration of Ca2+ and Mg2+. This period lasted for a short time and was in a low temperature and low pressure redox environment. Sediment deposition is mainly in loose state, and the effect of changing the increase or decrease of hydrochemical composition is weak. The increase of TDS concentration is very small, which basically maintains the main characteristics of syngenetic sedimentary water chemistry.

(2) Hydrogeological period of subsidence and burial: this period refers to the duration from the end of T3x2 deposition to the end of Cretaceous deposition. T3x2 is covered and buried by sedimentary layers with a cumulative thickness of about 6000m, which is the longest period in its geological history. In a strong reducing environment with high temperature, high pressure and closed confining pressure; Loose sand and mud deposits entered the late diagenetic stage and all solidified into rocks; The oil and gas in the source rocks of Xujiahe Formation have matured, with primary and secondary migration and natural gas accumulation. In the process of rock consolidation, the sediment migrates and accumulates in the reservoir with the migration of oil and gas and the release of compacted water, and it is accompanied by the alternation of internal circulation squeezing water, which leads to the renewal of water in the reservoir and the migration in the direction of pressure reduction.

Under the above background, the initial chemistry of T3x2 sedimentary water has undergone a profound and essential change, and changing the chemical composition of water has six main functions.

1. Leaching: In the long geological period, the leaching (including other effects) in the closed hydrogeological structural system with high temperature, high pressure and strong reduction environment is far greater than that in the previous period and lasts longer. The chloride leaching stage of sediments has been completed, which leads to a significant increase in the concentration of Cl- and Na+ stable components. Leaching of sulfate and carbonate increases the concentrations of SO _ 42-,CO _ 32-,Ca ~ (2+) and Mg ~ (2+) in water. Because the concentration product of Ca ~ (2+), Mg ~ (2+), CO _ 32- and SO _ 42- is very small, it is easy to combine to generate sulfate and carbonate secondary minerals, which enter the solid phase and then decrease. In the presence of CO2, CO32-is transformed into HCO3-,which increases the concentration and makes the aqueous solution weakly alkaline. However, the solubility of new minerals formed by incomplete dissolution of aluminosilicate minerals is lower than that of original minerals, and the dissolution of original minerals by water is unsaturated. In the geological period when water and original minerals get along, incomplete dissolution can continue. For example, the reaction formula of incomplete dissolution of albite, sodium montmorillonite, potash feldspar and anorthite is:

Simulation and tracing of deep water formation and evolution and oil and gas migration and accumulation in the basin

And incomplete dissolution of calcium montmorillonite, illite and biotite. It is the reason for the increase of the concentration of cations K+, Na+, Ca2+ and Mg2+ in water.

Second, organic matter metamorphism: the Upper Triassic source rocks are thermally degraded to generate a large number of gaseous hydrocarbons, CO2 and liquid hydrocarbons, mainly in Jurassic, and the Cretaceous is the peak of gas generation. Organic matter is produced by different decomposition stages and ways of biological debris. After conversion, organic matter is decomposed into simple compounds, such as hydrocarbons, carbon dioxide, H2, sulfides, salts and water. The organic matter metamorphism not only strengthens the biochemical process of sulfate removal, but also the interaction between methane, hydrogen and sulfate is extremely active under high temperature and high pressure, resulting in the depletion of so42- in water and the aggregation of reaction products co32- and H2S. Because of the existence of CO2, the carbonate equilibrium system moves, HCO3-generates CO32-, and CO32- combines with Ca2+ and Mg2+ to generate carbonate, which enters the solid phase. Carbonate cements are common in reservoirs. Therefore, the stable components Na+ and Cl- in water are highly concentrated, and the unstable components SO4 2-, Ca2+, Mg2+ and HCO3-are exhausted. The above actions lead to the differentiation and purification of the main salts in water, but the concentrations of trace, inorganic and organic components, as well as the concentrations of biological and metamorphic gases have increased to varying degrees.

The third is hydrolysis: from the formation of aluminosilicate in Xujiahe Formation to the transformation after diagenetic stage, a certain amount of water is needed to participate in the transformation of clay minerals, and hydrolysis has taken place for water. The uneven dissolution of aluminosilicate minerals is actually hydrolysis, because water molecules have also undergone chemical reactions. Hydrolysis begins with the decomposition of water molecules into H+ and OH-. Part of OH- generated by dissociation is consolidated in clay minerals generated by aluminosilicate hydrolysis reaction, and the other part reacts with CO2 to generate HCO3-, while H+ enters products H4SiO4 and HCO3-. When carbonic acid is saturated, CO 32 dissociates into H+ and CO 32-, so that OH- originally combined with CO2 is separated into hydrogen and oxygen, and oxygen is fixed by carbonate, and H+ can combine with silicate anion to form minerals such as clay. Therefore, water dissociates into H+ and OH-, both of which enter into new substances generated by the reaction. As a result, the dissociation equilibrium of water was destroyed and water molecules were dissociated again. The dissociation equilibrium is constantly destroyed, and water molecules can be continuously dissociated. The dissociation of water molecules is huge, and its scale depends on the intensity and duration of hydrolysis. The transformation of clay minerals caused by the transformation and destruction of aluminosilicate leads to a large amount of water loss, which is one of the reasons for the salinization of water bodies. In the lower part of the supergene zone, with the increase of buried depth, montmorillonite obviously decreases or even disappears, and the transformation to illite and chlorite is a common phenomenon. For example, tertiary sandstone and mudstone sedimentary systems such as Xihu sag in the East China Sea Shelf Basin, Jizhong in Bohai Bay and Huanghua Rift Basin are buried below 2 100m, and mixed layers of Mongolia and Iraq appear, and illite occurs below 3 100m ... The buried depth of Upper Cretaceous in Douala Basin in Cameroon is 3658m, and the mixed layers of montmorillonite and Mongolia and Iraq are reduced.

Fourth, thermal geochemistry: the continuous increase of ground temperature accelerates the leaching process, promotes the transfer of soluble salts in surrounding rocks to water, and also promotes the thermal degradation of organic matter in hydrocarbon-generating rocks in surrounding rocks to generate a large number of different forms of organic matter and chemical components into water, leading to changes in chemical components in water; In addition, underground evaporation is also one of the important reasons for directly improving the degree of salinization of water bodies.

The fifth is interfacial chemical action: this action includes two independent actions: one is adsorption, that is, the solid adsorbs an ion (molecule) from the aqueous solution; The second ion exchange is that the ions adsorbed on the solid surface are exchanged with the same number of ions with the same molar concentration in water. Both of these effects will change the chemical composition of water. Xujiahe Formation T3x2, its underlying and overlying T3x 1 and T3x3 are marine sandstone and mudstone, which form the adsorption complex of cation Na+ in seawater environment. Because the sedimentary layer is rich in organic matter, the adsorption capacity is very large. Other cations in water exchange with Na+ adsorbed on solid phase. The main factors affecting ion exchange are the concentration of aqueous solution, pH, chemical composition and its concentration, ion radius and valence state. The adsorption affinity of divalent cations is usually stronger than that of monovalent cations, and the selection order is CS+>. r b+ & gt； k+& gt； na+& gt； Li+，Ba2+& gt； sr2+& gt； Ca2+>； Mg2+. Because the concentration of Ca2+ in divalent cations in water is the highest, followed by Mg2+, Ca2+(Mg2+) exchanges with Na+ first, which leads to the increase of Na+ and the decrease of Ca2+(Mg2+) in water.

6. Differentiation and purification of chemical components in water: In the process of increasing TDS in aqueous solution system, the differentiation of component concentration with aggregation or dilution is another important role in changing chemical components in water. In order to explore the correlation between them, 22 chemical constituents (including TDS) were selected for R-cluster analysis (Figure 8- 15). The results show that when the correlation coefficient is 0.6980, TDS, Na+, Cl-, Ca2+ and SO42-in the 22 component variables form a class, namely, Sr2+ and Ba2+. When the correlation coefficient is 0.7839, TDS and Na+, Cl-, Sr2+ and Ba2+, Li+ and K+ each form a class, and other 15 variables form an independent class. When the correlation coefficient is 0.988 1, TDS, Na+ and Cl- form a class, and other 19 variables form an independent class.

This shows that:

The increase of TDS in water mainly depends on macro components Na+ and Cl-, followed by Ca2+, so42- and Mg2+, followed by trace and trace components. There are significant differences in the contribution of various components to the degree of salinization.

Major elements Na+, Cl-, Ca2+, so42-, Mg2+, trace elements Li+, K+, I-, Sr2+, Ba2+, trace elements Fe, Mn, etc. Among them, Na+ and Cl-, Li+ and K+, Sr2+ and Ba2+, Mn and Fe are symbiotic. The increase of TDS in water has a low correlation with Rb+, Br-, HCO 3-, Si, Al, Cu, Zn and Pb, and there is no obvious symbiotic aggregation relationship among the components.

Because Cl- in water is neither oxidized nor adsorbed, when the concentration of water reaches 275g/kg, Cl- can combine with Na+ and precipitate into salt rock, which is a stable component in water. When carbon dioxide is saturated, HCO 3- is converted into carbon dioxide. Because the solubility product of carbonate is small, it is easy to produce Ca and Mg carbonate. So42- combines with Ca and Mg to generate sulfate, which enters the solid phase. Ba2+ in water can easily generate BaSO4 and enter the solid phase. The biochemical action of sulfate in reducing environment leads to the consumption of so42- in water. Ca2+, Mg2+, so42- and HCO3-are unstable components in water. Finally, it leads to the differentiation and purification of macro components in water, forming Cl-Na type or Cl-Na Ca type water.

Fig. 8- 15 pedigree diagram of r-type cluster analysis of deep water chemistry

According to the above six aspects, it shows that during the hydrogeological period of subsidence and burial, T3x2 sedimentary brine absorbs a lot of soluble salts from the solid phase, which is a main salt accumulation period, but it is a salt washing period for the solid phase. The main concentration of water increased to about 100g/kg, which was the first salt-forming period, and evolved into Cl-Na or Cl-Na Ca type epigenetic sedimentary brine with differentiated purification of major components, high concentration of trace components and rich organic components.

(3) Hydrogeological period of tectonic hydrothermal process: this period refers to the duration between the two tectonic episodes of Yanshan movement at the end of Cretaceous and Himalayan movement at the end of Tertiary. Between these two tectonic episodes, T3x2 experienced intense thermal metamorphism.

Evidence of tectonic hydrothermal process includes the following five aspects:

First, the large-scale Yanshan movement caused the Mesozoic large-scale subsidence basin to rise unevenly, and the sedimentary history ended in the central and eastern regions. The strata around the basin are folded and uplifted into mountains, and the hot magma upwelling from the mantle erupts at the western margin of the basin, or forms plutonic rocks along the fault zone and fragile parts of the crust, baking its surrounding rocks. According to the data at home and abroad, the temperature of basalt lava measured under normal pressure can be used as the upper limit temperature of mineral crystallization of basic eruptive rocks, and its range is 980 ~ 1800℃. Circulating water with hydrothermal temperature above 200℃ can be distributed around the magma for hundreds of meters, and water with temperature of 90 ~ 100℃ can reach several kilometers to 15 kilometers. The fault between Huaying Mountain and Qiyaoshan Mountain in the east of the basin formed the fold belt in the East Sichuan Depression, and the Triassic was exposed to the surface. The fault between Longquan Mountain and Huaying Mountain in the middle part forms the uplift belt in the middle of Sichuan, and the Triassic is buried underground. The western Sichuan depression belt is formed between Longmenshan and Longquanshan faults in the west, and the Triassic is buried deep underground. Yanshan movement shaped the abnormal development of faults and folds in the basin, forming three stepped platforms rising from west to east. The modern outline of Sichuan basin has been formed, which was formed under the strong extrusion and folding of Himalayan movement. Huge ground pressure and upwelling hydrothermal solution will inevitably destroy the original distribution pattern of static pressure field, underground brine hydrodynamic field, chemical field and temperature field in the Great Sichuan Basin.

Secondly, according to the homogeneous temperature data of timely and calcite crystal gas-liquid inclusions in 68 samples of T3x sandstone fracture fillings from drilling and ground geological profiles in West Sichuan Depression, it is shown that the homogeneous temperature of T3x 1 is102 ~108℃, T3x138 ~ 3576 ~ 3724m4491m 168.2 ~ 186℃, 5303m197.2 ~ 201℃. T3x4 1797m is 79.3℃, 2 12 1 ~ 2736m is 90 ~ 100.3℃, and 2900~3455m is1/5.7.

Thirdly, the phenomenon of timely dissolution and recrystallization of sandstone is common in the upper Triassic T3x rock slices under the microscope, such as timely edge-expanding cone, timely harbor dissolution, creeping timely and microcline silicification. Cracks formed after cutting and traction of minerals, such as cracks and twisted cracks formed after tensile fracture of feldspar, cracks formed after timely traction, and later cracks passing through earlier cracks, etc.

Fourthly, there is a thin lens lapis lazuli interlayer near the top of the Middle Triassic (T2l) stratum, and there are gray-black argillaceous lapis lazuli bands and lapis lazuli in the anhydrite layer at the bottom. The homogenization temperature of inclusions (2 ~ 20μ m) in the Lower Triassic strontium ore in Hechuan is 150 ~ 155℃.

Fifthly, the theory of organic genesis in the late stage of oil and gas holds that the organic matter in sediments needs to reach a certain temperature before it can be converted into oil. When the temperature is higher than 130 ~ 148.9℃, liquid hydrocarbons are destroyed and cracked into natural gas. The Triassic in Sichuan basin is almost all natural gas; The generation, migration and trap of T3x oil and gas occurred before Jurassic, and the subsequent Yanshan-Himalayan movement will surely make fruitful contributions to the transformation of residual liquid hydrocarbons into gaseous hydrocarbons.

The main geological effects of thermal metamorphism are as follows:

First, the Sichuan Basin and its periphery experienced strong orogeny caused by faults, magmatic eruption and intrusion, and hot magma in the deep crust and liquid water in the mantle upwelling along faults or fragile parts. Because the western Sichuan depression is closest to the magmatic eruption zone, the Upper Triassic is the deepest buried and the hydrothermal roasting is the strongest, and the substances carried by the rising of mantle liquid water have the highest concentration on the metal components of brine, which strongly changes the chemistry of T3x2(T3x4, T3x6) brine by thermal metamorphism.

Secondly, the upper Triassic brine water is in the hot boiling thermal geochemical environment above 150℃, which has a strong leaching effect on the surrounding rocks again, making more material components accumulate in the water, and the high temperature accelerates the evaporation of the water itself. This is the period when the brine concentration and salinization reach the highest level, and it becomes the secondary salinization period of T3x2 brine.

Thirdly, thermal metamorphism releases active compounds, hydrocarbons, H2S, etc. It is transmitted to the surrounding areas in the form of chemistry and energy, accompanied by a series of chemical actions. Judging from the sedimentary fractures, there is no evaporite, only insoluble calcium carbonate and magnesium carbonate are filled in the fractures. According to this, it can be logically judged that the highest concentration of T3x2 brine reaches 160g/kg, the macro components Na+ and Cl- in water are highly enriched, and Ca2+ is relatively aggregated, while the concentrations of HCO3-and Mg2+ are low, SO42-is very low or disappears, and the concentrations of micro and trace components increase. Biogenic gases are mainly hydrocarbons, CO2, H2S and N2.

Fourthly, Triassic ancient brine migrated from the high pressure zone in the western Sichuan depression to the low pressure zone in the eastern Sichuan depression. Copper-bearing sandstone and siderite are formed in sandstone of Shahejie Formation (T3x) flowing through Luzhou. Thin-layer lenticular celestite interlayer is commonly found in Leikoupo Formation (T2l), and the anhydrite layer is mixed with gray-black argillaceous celestite bands and celestite metasomatism anhydrite phenomenon. Strontium deposit in Hechuan was formed in Jialingjiang Formation (T 1j). With the extinction of Yanshan Movement and Himalayan Movement, geothermal energy dissipated, ground pressure readjusted, and a new fluid dynamic equilibrium system was gradually established.

2.2. The main function and process of formation and evolution. T3x4 hydrochemistry.

The types and sequences of the three hydrogeological periods that T3x4 experienced in geological history are the same as T3x2, and the main functions and processes of hydrochemical formation and evolution are similar, so the details of its formation are not detailed, and the conclusions of its hydrochemical formation and evolution are mainly expounded.

T3x4 water comes from TDS.

During the hydrogeological period of sedimentation, the concentration of various components in water increased and decreased, and the concentration of TDS increased and decreased slightly, which basically maintained the characteristics of initial chemistry.

In the hydrogeological period of subsidence and burial, T3x4 has the same five functions, but the intensity of the functions is different, except that the interfacial chemical action is opposite to that of T3x2. In this long geological period, fresh water of T3x4 continuously absorbs a large amount of soluble salts from surrounding rocks. Due to the compaction caused by the continuous subsidence of sedimentary layers, the underlying T3x2 high salinity water enters T3x4 along the rising channels of cracks, cracks and joints with good water permeability to mix, which leads to the increase of salinity of T3x4 water and the first salinization period, forming the concentrations of macro components Na+ and Cl-, and increasing the concentrations of micro and trace components. Rich in organic components, TDS increased to 20 ~ 80g/kg, and evolved into highly concentrated salinized deep metamorphic Cl HCO 3-Na and Cl-Na-type pore brine after deposition.

In the hydrogeological stage of tectonic hydrothermal process, T3x4 brine experienced intense thermal metamorphism, and then salt formation stage occurred again. The concentration of each component is obviously higher than that of the previous stage, the macro components are differentiated and purified, and Na+ and Cl- are highly enriched. Finally, it evolved into Cl-Na Ca and Cl-Na type metamorphism, and the TDS increased to 30 ~ 100 g/kg, which has the main characteristics of marine sedimentary hydrochemistry.

The main functions and processes of 3.3. Formation and evolution of T3x6 hydrochemistry.

The hydrogeological period of T3x6 is the same as that of T3x2, and its formation and evolution conclusions are as follows: the source of T3x6 water is the same as that of T3x4. During the hydrogeological period of sedimentation, the basic characteristics of initial chemistry were maintained. During the hydrogeological period of subsidence and burial, the same effect as T3x4 occurred, and the concentrations of various components increased or decreased similarly. After the first salt-forming period, it evolved into deep metamorphic sedimentary pore brine with the highest TDS concentration of 60g/kg. In the hydrogeological stage of tectonic hydrothermal process, the salt-forming stage occurred again, and finally evolved into Cl-Na Ca and Cl-Na post-metamorphic pore fissure brine with marine sedimentary hydrochemical characteristics and the highest TDS concentration of 84g/kg.