(A) secondary pore distribution
The distribution of secondary pores in the longitudinal direction is uneven. Primary porosity gradually decreases with the increase of buried depth, and its changing trend is basically linear. The distribution of secondary pores in Wendong area (Figure 2-4-2) shows that there are two secondary pore development zones in this area as a whole. The first development zone is located at 2500 ~ 3500 m, which is equivalent to the mature stage a; The second development zone is located at 3500 ~ 4500 m, which is equivalent to the mature B stage. The existence of two secondary pore development zones reveals multiple dissolution in this area. This multi-stage development of secondary pores can be reflected in the evolution diagram of pore structure (Figure 2-4-3). From immature to semi-mature, the primary pores have been basically reduced to an incompressible level. Secondary pores are generated in the semi-mature stage until the Ro value is 0.5 ~ 1.0, that is, the first peak is reached in the low-mature stage. When the Ro value is 1.0 ~ 1.3, it reaches the second peak of maturity. Of course, the second peak is much lower than the first peak.
Figure 2-4-2 Distribution of Secondary Pores in Wendong Area
(2) the cause of secondary porosity
The formation of secondary pores mainly depends on the dissolution and metasomatism of particles and cements. This requires constant supplementation of acidic substances.
1. Formation of acidic medium
Here mainly refers to the formation of CO2, and there are two mechanisms.
(1) organic source
The study of organic matter in Wendong area shows that in the stage of decarboxylation of organic matter and kerogen oil generation from 2000 to 4000 m, kerogen pyrolyzes to form Co2, and the reaction formula is kerogen →Co2+H2O+N2+ oil and gas+organic residue.
Figure 2-4-3 Schematic Diagram of Pore Structure Evolution in Wendong Area
(2) Inorganic genesis
Hydrolysis of 1) carbonate
Thin section identification and scanning electron microscope observation in Wendong area show that there are dispersed carbonate in clay, and its content is very rich, which can reach 20% ~ 30%. The main components are iron calcite and ankerite.
2) Reaction of clay minerals
In the deep burial stage of diagenesis, clay minerals and carbonate reflect that a large amount of inorganic CO2 can be generated, mainly the disappearance of kaolinite and the formation of chlorite, while dolomite and timely dissolution can be seen. The reaction equation is:
Development of Deep High Pressure and Low Permeability Oilfield —— Taking Es3 Reservoir of Wendong Oilfield in Dongpu Depression as an Example
The variation of clay mineral content with depth in Well Pushen 7 (Figure 2-4-4) shows that the second peak area of chlorite just corresponds to the low value area of kaolinite. The figure shows that the chlorite in the range of 3400 ~ 3600 m also shows a growth and decline relationship with kaolinite.
2. Dissolution of components
The above research shows that CO2 acid water and organic solvents in the formation are important dissolvants for forming secondary pores in sandstone, and their actions lead to different dissolution degrees of different particles. The formation of secondary pores in Wendong area is mainly due to the following dissolution:
Dissolution of (1) carbonate minerals
In acidic medium, carbonate minerals are dissolved according to the following reaction:
Development of Deep High Pressure and Low Permeability Oilfield —— Taking Es3 Reservoir of Wendong Oilfield in Dongpu Depression as an Example
The dissolution of calcite can be observed under the microscope, and iron calcite, as an early cement, has better solubility.
Figure 2-4-4 Distribution Map of Clay Mineral Content in Well Pushen 7
(2) dissolution of feldspar minerals
The chemical decomposition of feldspar, especially plagioclase, is an important reason for secondary porosity. The differentiation product of feldspar is kaolinite, and its reaction equation is:
Development of Deep High Pressure and Low Permeability Oilfield —— Taking Es3 Reservoir of Wendong Oilfield in Dongpu Depression as an Example
The selective dissolution of feldspar can be clearly observed under scanning electron microscope.
(3) Dissolution of drilling cuttings
The composition of cuttings in the third member of Shahejie Formation in Wendong area is complex, and argillaceous cuttings and metamorphic rock cuttings can also be dissolved. Flint sometimes dissolves, and the phenomenon of pressure dissolution is one of the evidences. The most soluble components are carbonate sand and oolite.
(4) Dissolution of oolite and sand.
The components of oolite and sand are calcite and dolomite, which are the most soluble components, and the formation of extra-large pores is often directly related to the dissolution of these components.
(C) Control factors of secondary porosity
The development of secondary pores is mainly controlled by diagenetic background. The diagenetic background here is a comprehensive summary of diagenetic environmental factors. These factors include structural evolution, sedimentary characteristics, burial history, thermal evolution history and fluid history, as well as abnormal high pressure peculiar to reservoirs in the study area [54-58].
1. tectonic evolution
In the late Yanshan period, after denudation from the end of Cretaceous to Neogene, Bohai Bay Basin experienced strong regional rifting and entered Cenozoic rifting cycle, and Dongpu Depression was the product of this rifting. The formation and development of Dongpu sag mainly experienced initial tension period, strengthening period, strengthening period, attenuation period and depression period. The early Oligocene was a strong rifting period, during which tension and torsion reached the strength limit and gradually formed a unified central uplift zone, which laid the foundation for the basic structural framework of Dongpu sag. At the end of Oligocene, the stress field changed from tension-torsion to compression-torsion, which led to regional uplift and the corresponding rift was in the attenuation period. Wendong Oilfield, located in the east wing of Liuwen structure, is a long and narrow anticline extending northeast, which is a structural belt formed by the reverse traction of Wendong fault. Within the range of 3km, Wen 13 and Wen 16 form two high points. The high point of Wen 13 structure is located in the reverse traction short axis anticline structure with complex faults, and the high point of Wen 16 structure is located in the nose structure inclined to the south and west. Hu-type Hebei structure in this area spans three structural units: western slope, western depression and central uplift in the range of less than 25 kilometers from east to west. Influenced by structural evolution, its sedimentation and diagenesis are obviously different.
Cracks form micro-cracks that are beneficial to the flow of water medium and form "stress active zone", which is closely related to the development law of plane secondary pores. The continuous activity of Wenxi fault, Wendong fault and Xulou fault has formed a series of favorable secondary pore development zones along the fault activity area, which is strongly evidenced by the high-yield fault-block oilfield distributed along the fault in the middle period of Shahejie Formation.
2. Sedimentary characteristics
During the main sedimentary period, Dongpu Depression experienced intense rift activity and internal structural differentiation, which led to rapid changes of sedimentary facies belts and frequent migration of sedimentary centers, providing the necessary material basis for various evolution paths of diagenesis in the later period (Figure 2-4-5).
Figure 2-4-5 Sedimentary Profile of Member 3 of Shahejie Formation in Huzhuji-Liuwen Area
Compare the mutual aid society and Liuwen area. In the early stage, the Huxing Formation was a deep area, and a set of lacustrine sediments mainly composed of fine clastic rocks and gypsum mudstone developed in the early stage. In the later stage, it evolved into terrigenous coarse clastic sedimentary rocks characterized by fan delta, with a thickness greater than10000m, and the corresponding Liuwen area developed rock salt, gypsum salt, semi-deep lake-deep lake mudstone and turbidite, and the water body became shallow, and the central uplift zone was formed. In Huzhuangji area, a set of interbedded sedimentary assemblage of sand and mudstone characterized by delta facies was developed, with a thickness of 000 m.. Liuwen area is characterized by a set of shallow lake beach deposits, mainly composed of fine sandstone and siltstone.
Controlled by the above-mentioned structure and sedimentary pattern, the sediments show different mineralogical, petrological and geochemical characteristics: ① Hu-shaped sandstone is dominated by lithic sandstone and lithic sandstone (Figure 2-4-6), with low mineral maturity, coarse grain size and high matrix content (8% ~14%); In Liuwen area, feldspar quartz sandstone and quartz quartz sandstone are dominant, with high mineral maturity, fine grain size and low matrix content (3% ~ 10%). ② The sediment concentration in Huzhuangji area is high (20% ~ 50%) and the stratification coefficient is small; The sediment concentration in Liuwen area is low (10% ~ 30%) and the stratification coefficient is large. ③ Mudstone in Huzhuangji area is rich in clastic minerals, while that in Liuwen area is relatively pure or contains a certain amount of dolomite and gypsum salt minerals. The former is rich in elements such as iron and silicon, while the latter is relatively high in elements such as calcium, manganese and strontium.
Figure 2-4-6 Reservoir Sandstone Types in Huzhuji and Liuwen Area
1 timely sandstone; 2- feldspathic sandstone; 3 timely sandstone of cuttings; 4- feldspathic lithic sandstone. 5- feldspar lithic sandstone. 6- lithic feldspathic lithic sandstone; 7- feldspathic lithic sandstone; 8— Cuttings sandstone
Sedimentary microfacies determine the structure and composition of sandstone and control the development of secondary pores. Sandstone with less shale content is often conducive to the formation of secondary pores, and shale is an important reason for low porosity and permeability of reservoirs.
3. Burial history and thermal evolution history
Burial history and thermal evolution history are comprehensive summaries of diagenetic parameters such as formation temperature, pressure and effective action time controlled by tectonic activities. The internal structural differentiation of rift basin leads to different burial and thermal evolution processes of different plots: ① rapid subsidence stage. This stage corresponds to a strong dry rift period, especially during deposition. At the end of this stage, the buried depth of the target layer in Liuwen area is slightly larger than that of the Hu-shaped sleeve, so it can exceed 2500 meters (about 90℃). ② Slow settlement stage. This stage corresponds to the decay period (Ed) of rifting, that is, the period when the stress field changes from tension-torsion to compression-torsion system. At this stage, compared with the western depression belt, Huzhuji and Liuwen area both showed relative uplift, and the basement settlement was slow and tended to recover. ③ Intense uplift stage. This stage roughly corresponds to the end of Dongying Formation (Ed) and before Shangguantao Formation (ng), and its duration is still difficult to determine. Generally speaking, the uplift range of Huzhuji area is greater than that of Liuwen area, and the buried depth and ground temperature of the target layer are obviously reduced. This structural regression leads to regional stratum denudation, especially the uplift and denudation of Hu-shaped concentrated area can reach more than 1000m, and some targets are exposed to the surface. ④ Late stable settlement stage. This stage corresponds to the Paleogene-Neogene depression period, lasting about 20Ma, with slow settlement stability and weak internal structural differentiation, which eventually led to the deep burial of the target layer and the rise of ground temperature again.
4. Fluid evolution history
The primary water in the study area is controlled by the sedimentary environment, that is, from Huxingji to Liuwen, the salinity of pore water increases, and the contents of elements such as Ca, Sr and Mn increase accordingly. In the process of burying, with the increase of temperature and pressure, the primary pore water is alternately affected by compacted water, warm pressure water and internal circulation extrusion water under the action of hydrostatic pressure or ground pressure, resulting in the increase of salinity and alkalinity. However, in the oil generation window, due to the decarboxylation of kerogen, the pore water will become mainly acidic, and with the consumption of acid in diagenetic reaction (such as dissolution), the pore water will become mainly alkaline. In addition, the uplift and erosion at the end of Paleogene made meteoric water an important factor to transform the pore fluid in the target layer, that is, the alternation of external circulation seepage water was generated under hydrostatic pressure. The research shows that atmospheric water is mainly injected along the bed, which desalinates and transforms the Hu-shaped concentrated area and weakens the alkalinity. However, this process has little influence on the destination layer of Liuwen District. The above evolution process finally leads to the target layer in Liuwen area with high salinity and strong alkalinity, while the Hu-shaped sleeve has the characteristics of relatively low salinity and weak alkalinity (Figure 2-4-7).
Figure 2-4-7 Mineralization Characteristics of Formation Water in Huzhuji-Liuwen Formation
5. Abnormal high pressure
Abnormal high pressure is an important factor to control pore fluid activity, diagenesis and oil and gas migration in petroliferous basins. The high-pressure oscillating fluid activity closely related to abnormal high pressure in sedimentary basins will quickly change the physical and chemical conditions and pressure conditions of pore media fluids inside and outside high-pressure reservoirs, and change the normal diagenesis process, thus having an important impact on diagenesis of reservoirs in abnormal high-pressure basins [59-64].
(1) Abnormal high pressure and compaction diagenesis
The basic characteristics of sedimentary diagenesis of sandstone reservoirs in Dongpu Depression are thin sand layer, fine grain size and obvious sedimentary heterogeneity. Buried more than 3000 meters, compaction, early carbonate cementation, late authigenic illite-chlorite and siliceous cementation are strong. However, the diagenesis of different structural units is also very different (Table 2-4-3). The contribution of abnormal high pressure and fluid activity to the above diagenesis difference is analyzed as follows.
Development of Deep High Pressure and Low Permeability Oilfield —— Taking Es3 Reservoir of Wendong Oilfield in Dongpu Depression as an Example
1) compaction quantity comparison
Abnormal high pressure can inhibit intergranular pressure dissolution by reducing the stress of indirect contact of sandstone particles. In this paper, samples from well PS7 and well Q24 are selected for comparison of compaction quantity, and the content of carbonate cement is less than 7%. The content of heterogroup is controlled at about 5%; The burial depth of samples is limited to 4000 ~ 4200 m, with similar burial history and the largest burial depth in geological history. The average grain size is between 0.08 and 0.10 mm, belonging to the same sedimentary facies. In this way, the statistical results of pressure solution mainly reflect the stress of interaction between particles, and the influence of abnormal high pressure on particle compaction can be reflected by calculating the percentage of pressure solution volume/particle volume. When calculating pressure solution, the main components to be considered are time-dependent overlap and feldspar-time-dependent overlap. For sandstone with poor structural maturity, the particle shape is irregular and the roundness is poor, which increases the complexity of determining the volume of overlapping particles, but it is still an effective method to estimate the volume of particles dissolved by intergranular pressure. The above method can eliminate the influence of other factors (such as grain size, early cementation and buried depth) on intergranular pressure solution, and the magnitude of intergranular pressure solution will reflect the relative magnitude of interaction stress and abnormal high pressure.
Figure 2-4-8 Pressure Solubility Comparison of Sandstone in Well PS7 and Well Q24
2) the relative relationship between compaction and cementation
Figure 2-4-9 Sandstone Compaction-Cementing Relation Diagram of Well PS7 and Well Q24
The porosity of sandstone before compaction and cementation is 36.75% of that in Wendong area.
Analyze the relative strength of profile compaction and cementation in Wendong and Qiaokou areas (Figure 2-4-9). Selected sample
Statistical results of intergranular pressure dissolution of sandstone slices in Well Q24 and Well PS7 show (Figure 2-4-9) that the intergranular pressure dissolution of Well PS7 is less than that of Well Q24, and the average pressure dissolution of Well Q24 is 1.762% between 4000 and 4200m, while that of Well Q24 is 2.1. In addition, it is found that the timely proliferation content of sandstone in the sub-member is Qiaokou, Baimiao and Wendong (Table 2-4-3), which is negatively correlated with abnormal high pressure, which may be another evidence that the abnormal high pressure inhibits compaction and leads to the weakening of timely intergranular pressure dissolution. The product granularity is carefully selected, mostly medium-fine sandstone, and the impurity content is about 5%. The effects of compaction and cementation on porosity reduction in well PS7 are basically the same; Most samples of well Q24 fall on the upper left, indicating that compaction plays a decisive role in porosity reduction, that is, the compaction experienced by well PS7 is weaker than that of well Q24, and the compaction process of well PS7 is inhibited by abnormal high pressure higher than that of well Q24.
3) Relationship between sandstone sedimentary parameters and porosity
The intergranular pore structure of sandstone was initially controlled by sedimentary parameters. With the increase of burial depth, especially the strengthening of pressure dissolution, cementation-metasomatism and particle dissolution, the controlling relationship between sedimentary parameters and porosity gradually weakens. Therefore, samples with weak cementation-metasomatism and similar burial depth can be selected to study the correlation between sedimentary parameters and porosity, and to quantitatively analyze reservoir compaction (dissolution).
The research shows that there is an obvious linear relationship between porosity and particle size parameters such as sorting coefficient and median particle size of well PS7, and the linear correlation degree is obviously stronger than that of well Q24. It shows that the intergranular pore structure of sandstone in well PS7 is still controlled by sedimentary parameters, while well Q24 is greatly affected by compaction (dissolution).
(2) Abnormal high pressure and dissolution-cementation diagenesis.
1) Abnormal high pressure and related fluid activity characteristics
The research shows that the compacted water flow is very slow in the process of thermal subsidence in the basin, and it is impossible to cause meaningful temperature field disturbance, diagenesis and mineralization. Oscillating (pulsed) fluid caused by repeated opening and closing of abnormal high pressure is a frequent geological event in the evolution of sedimentary crust and lithosphere. This self-organizing dynamic mechanism is likely to restrict the dissolution-gelation process and diagenetic belt structure on a large scale [59-65].
Abnormal high pressure developed in the process of buried diagenesis in the study area has an important influence on sandstone compaction. The homogenization temperature recorded by the fluid inclusions in the self-expanding edge of the study area is mostly between105℃ and145℃, and the frequency of homogenization temperature of the inclusions shows a multi-peak distribution, which shows the role of high-pressure episodic fluid, that is, high-pressure oscillating fluid activity. Xu Huazheng's research shows that there is a high proportion of micro-leakage space in mudstone slices in abnormal high pressure zone, and micro-cracks are the most common in micro-leakage space. At present, most micro-cracks are filled with pyrite or carbonate, which shows that oil pollution spreads along micro-cracks and a certain amount of adsorbed hydrocarbons. This phenomenon indicates that micro-fractures were once the channels of fluid migration, which may indicate that mudstone experienced episodic fracturing and fluid discharge in overpressure basins.
2) cementation-dissolution diagenesis
The existence of abnormal high pressure can increase the solubility of CO2 in porous media, enhance the acidity of porous media and enhance reservoir dissolution.
Episodic fluid discharge of high-pressure mudstone will destroy reservoir physical properties. Hydraulic fracturing and episodic fluid activities frequently occur in abnormally high pressure mudstone, which makes the fluid pressure entering sandstone decrease rapidly, and the partial pressure of CO2 in the fluid system also decreases rapidly, and leads to a sharp increase in pH value under internal buffering conditions. Finally, the reservoir (low pressure area) into which fluid is injected will precipitate and cement, and the reservoir performance will become worse. This may be an important reason for the extremely developed carbonate cementation in the later period of the study area [66].
6. Carbonate cement
The ultimate degree of secondary pore development and preservation mainly depends on the amount of cement in the later stage. The relationship curve between carbonate content and physical properties in Wendong area shows (attached figure 2-4- 10) that there is an obvious negative correlation between carbonate content and porosity. Carbonate cementation obviously reduces reservoir porosity. Only through dissolution can carbonate substances be brought out and favorable pore space for oil and gas accumulation be formed.
Development of Deep High Pressure and Low Permeability Oilfield —— Taking Es3 Reservoir of Wendong Oilfield in Dongpu Depression as an Example
Fig. 2-4- 10 Relationship curve between reservoir physical properties and carbonate content in Es3 Middle School in Wendong.
7. Pore fluid properties
Acidic medium is a necessary condition to promote the dissolution of soluble components. The change of water medium can make one place dissolve and the other place precipitate. That is, the migration of ions leads to the non-uniformity of the plane distribution of secondary pores.
When pore fluid flows through porous media, there will be mechanical retention besides chemical precipitation. When the fluid containing dissolved substances passes through the porous medium, if the molecular diameter is smaller than the inlet of the pore and larger than the outlet of the pore, the dissolved substances will mechanically stay in the porous medium, causing the pores to shrink. The mechanical retention and trap effects of low permeability reservoirs are more prominent than those of medium and high permeability reservoirs. The formation of low porosity and permeability layer in the study area can not rule out the role of mechanical retention. The main reason is that the particles are fine and mostly belong to silt grade; The fluid concentration is relatively high, and the salinity in this area is generally (20 ~ 30) ×104 ppm =10-6.
amongst
8. Authigenic minerals
Authigenic minerals not only destroy the secondary pores, but also preserve them. Authigenic minerals such as kaolinite are used as pore fillers, which make the reservoir physical properties worse. Chlorite can be deposited on the surface of sand body in the early diagenetic stage, thus preventing the secondary expansion in time and improving the preservation potential of intergranular pores. Under the thin-section microscope, it is found that the timely secondary growth with mud film is very weak. However, the argillaceous membrane sometimes plays a role in plugging pores.
9. Relationship between high porosity zone and diagenesis
There are many high porosity zones in the vertical direction of reservoirs in Wendong area, which are mainly caused by special diagenesis in typical salt lake sedimentary environment.
(1) is rich in carbonate filling.
The rich carbonate filling in the early diagenetic stage of the reservoir not only weakens the compaction, but also provides a material basis for the dissolution of high salinity media in the later diagenetic stage (roughly in the early diagenetic stage B and the late diagenetic stage A), resulting in a large number of dissolution pores. The first pore development zone is the main oil and gas reservoir distribution zone in Dongpu Depression.
(2) regional gypsum salt layer
The existence of regional gypsum salt layer is beneficial to form a "sealed box" with abnormal high pressure in the deep, slow down the compaction and preserve the original pores. It is also beneficial to the groundwater activity rich in organic acids and co: and the formation of a large number of dissolved secondary pores in the later hydrocarbon generation process (equivalent to the A and B stages of middle diagenesis). This is the main reason why the secondary pore development zone is widely distributed in the study area.
(3) clay mineral transformation
In the diagenetic change of clay minerals, the transformation from montmorillonite to illite can release more bound water. The content of illite in clay minerals of late diagenetic stage A in Dongpu Depression is relatively high, which is 67% ~ 80%, 36% ~ 69% and 33% ~ 84% in Liuwen, Weicheng and Pucheng areas, respectively, which means that montmorillonite has been transformed into illite in a large amount at the corresponding depth, which is bound to provide more carbonate cements dissolved in acidic water, which is conducive to the formation of secondary pore development zones.
The analysis of reservoir diagenesis and pore evolution history shows that the reservoir in Es3 Middle School of Wendong Oilfield has the following characteristics:
A. The buried depth is large, the structure is complex, the heterogeneity is strong, and there is a set of large gypsum and salt layers at the top and bottom.
B. compaction is the main factor of physical property attenuation of sandstone.
C carbonate has strong cementation, mainly iron-bearing calcite, iron-bearing dolomite and dolomite, and most of them exist in the form of cementation such as metasomatic clastic particles.
D. Siliceous cementation is one of the important factors that cause tight and low permeability reservoirs.
E. Dissolution has greatly improved the reservoir properties and formed a secondary pore development zone (2500 ~ 3300m); Affected by abnormal high pressure conditions, 3300 ~ 3800 m is the porosity and permeability preservation zone in high pressure area.
In the process of sedimentary burial diagenesis, the diagenetic evolution and evolution degree of clastic minerals are controlled by different factors, showing different diagenesis. Various factors can be summarized into two main aspects.
1) internal cause
Including sedimentary microfacies, sandstone type, sandstone composition and fabric characteristics, matrix content, cementation type and quantity, cementation period [67].
2) External reasons
Refers to the external environmental factors experienced by sediments during diagenesis, including the influence of temperature, pressure, pore water properties and hydrocarbons. Of course, these factors are related to burial depth, burial speed and regional tectonic conditions.
The study of sandstone in the third member of Shahejie Formation in Wendong area shows that the matrix content in internal factors has great influence on the diagenetic characteristics of sandstone. Parallel bedding silty fine sandstone and massive sandstone are mostly clean sandstone, and the impurity content is less than 10%. The main diagenesis they experienced was cementation. The cement filled between particles is usually carbonate minerals, followed by the timely cement produced by secondary expansion. Compaction is the main diagenesis of complex sandstone with matrix content greater than 10%. The obvious mechanical compaction makes the debris particles between the substrates rotate mechanically, thus forming directional diagenetic fabric, and the secondary expansion phenomenon is weak. The cementation in miscellaneous sandstone is weak, because the clay matrix restricts the circulation of pore water, making it difficult to bring out the components in sandstone and bring in the external components. Continuous and unobstructed intergranular pores in clean sandstone are very beneficial to the circulation of intergranular water carrying dissolved components. With the change of water properties, precipitation or dissolution will be promoted between particles. Wen 13 fault block area is an abnormally high pressure oilfield. Its original formation pressure is much higher than the hydrostatic pressure of the corresponding horizon, and the pressure coefficient and formation pressure gradient of each sand group are also different (Figure 1-2-2).
Among the external factors, temperature is the most important diagenetic factor. The role of temperature in buried diagenesis is as follows:
A. affect the solubility of minerals. The increase of temperature will increase the solubility of components, and the existence of temperature gradient will promote the migration of solution;
B. affect the activity of (oh)-. The results show that the reaction rate doubles with the increase of temperature at 65438 00℃.
C. promote the decomposition of organic matter. Organic matter is gradually transformed into complex hydrocarbons.
D. reduce the hydration of ions. Strongly hydrated ions such as Fe2+ and Mg2+ combine with carbonate at surface temperature to form carbonate minerals.
E. dehydrate water-bearing minerals. For example, clay minerals and zeolite minerals will dehydrate when the temperature rises, thus transforming into a more stable mineral phase.
According to the actual well temperature measurement, the geothermal gradient is 4.96℃/ 100m (Figure 1-2-3). Although this is the present ground temperature, it is different from the ancient ground temperature, but it still has important reference value. According to the principle of making the past serve the present, the ground temperature has reached 1, 10 ~ 140℃ in the mature stage of the middle diagenetic stage A(H = 2500 ~ 3500m, R0 = 0.5% ~ 1.0%). Such temperature has great influence on the chemical dissolution of particles, the formation of various authigenic minerals, the diagenetic evolution of clay minerals and the pyrolysis of organic matter.