Distribution law of hydrochemical anomalies in the basin

(a) There are differences in hydrochemical anomalies in different hydrogeological units.

From the point of view that oil-bearing basin is a part of artesian basin, hydrochemical anomalies related to oil and gas can be distributed in different hydrogeological units. In the groundwater recharge area near the source depression (except the exposed surface of water-bearing rock series), faults, folds and other structures are developed, which provides a certain space for oil and gas accumulation. If there are hydrochemical anomalies in this area, most of them are low-salinity fresh water, and most of the indicators are not high, which belongs to low-value anomalies, except for high-grade units matching fresh water. In the groundwater runoff area, with the increase of regional hydrogeochemical background value, the anomaly value also increases, and the background and anomaly are relatively easy to separate and determine, which improves the reliability of anomaly. However, when it enters the drainage area of groundwater, the abnormal index content reaches the highest value in the basin. According to the statistical results of artesian basins containing oil and gas in China, the probability of hydrochemical anomalies in different hydrogeological units is the highest in groundwater runoff area, followed by drainage area and the lowest in recharge area. Under the premise of the same oilfield scale or trap area, the range and area of hydrochemical anomalies in water system area are the largest; The abnormal range of runoff area is closest to the closed area of oil and gas trap (but mostly larger than the closed area of trap); The hydrochemical anomalies near the groundwater recharge area are often scattered along the direction of groundwater flow. When evaluating or predicting favorable zones, the spatial range should be extended upstream, and the most favorable parts for oil and gas are mostly confined near the relative highest point of exponential intensity. In the anomaly, the composition of hydrochemical indexes in different hydrogeological units is different, which is related to the activity, stability and chemical properties of each index, as well as the great changes in the natural geographical environment, topography and geomorphology of the supergene zone, as well as the vertical and horizontal heterogeneity of deep oil-bearing reservoirs (including the oil-rich degree of reservoirs, the difference of oil and gas properties, the change of lithology-lithofacies, intermittent sedimentation, the compaction and consolidation of sediments and the degree of tectonic activity, etc.). ) About groundwater. Even above the same oil and gas reservoir, the hydrochemical anomaly components of different observation points will be different, especially in environmental indicators. From this point of view, the shallow hydrochemical effect itself is not determined by the type and quantity of hydrochemical indexes, but by selecting an effective index combination in combination with geological and hydrogeological conditions and according to the change range of content. Examples are as follows.

1) Ordos basin is a large gravity basin with well-developed groundwater in Mesozoic and Paleozoic. The upper phreatic water or shallow confined water in the Loess Plateau is buried deeply and the water quantity is poor. The denuded area in the north changes greatly with the nature and topography of the aquifer, with poor drainage and poor water quality. The western margin of the basin is close to the groundwater recharge area, and a series of north-south and northwest-west faults have developed. These fault systems extend far (hundreds of kilometers) and have a large fault distance (up to 3000 meters), which are the main channels for vertical micro-migration of oil, gas and groundwater, and form hydrochemical anomalies in Quaternary-Tertiary aquifer groundwater (Figure 5-94). As can be seen from Table 5-59, the salinity of the recharge groundwater in this area is low, less than 1g/L, and it is NaH-CO3 type water, and its main component is anions. The salinity of groundwater in hydrochemical anomaly points increased obviously. Because the water-bearing rock series is rich in gypsum, the water content is high, mainly Na2SO4 water, and the order of anions is NH4, I- and Br-. In the groundwater runoff area inside the basin, not only the hydrochemical anomalies increase, but also the hydrochemical components develop towards metamorphism. Jingbian, Dingbian, Zhidan, Wuqi, Maling and other areas are generally CaCl2 _ 2 type water with high salinity (above 5g/L), and the content of soluble gaseous hydrocarbons in the water increases sharply (Table 5-60). The range of carbon isotope changes mostly belongs to the category of pyrolysis gas (Table 5-665438). In Yongping, Yanchang and Yanchuan areas of groundwater discharge area, the salinity rises sharply, the chemical composition of water is concentrated and deteriorated, and it evolves in the direction of enriching Cl- and Na+, which has some basic characteristics of oilfield water (Table 5-62).

Fig. 5-94 Hydrocarbon Anomalies in Groundwater in Majiatan Oilfield

1- sampling point; 2- structural isoline; 3- failure; 4- heavy hydrocarbon anomaly zone

Table 5-59 Comparison Table of Chemical Compositions of Different Types of Water in Near-recharge Area Unit: mg/L

Table 5-60 Characteristic unit of soluble hydrocarbons in hydrochemical anomalies in runoff area: μL/L

Table 5- δ 13C 1 value of hydrochemical anomalies in runoff area

Table 5-62 Comparison Table of Chemical Composition between Abnormal Water in Discharge Area and Oilfield Water

2) The hydrochemical anomalies of different hydrogeological units in Songliao Basin are similar. From the NW-SE hydrogeological profile (Figure 5-95), it can be seen that the groundwater in the recharge area of Daxinganling mainly flows to the southeast of the basin, and the drainage area is located in Sanzhao area in the southeast of Daqing placanticline. The groundwater replenished from the southeast (Zhangguangcailing) and the east (Xiaoxing 'anling) flows to the northwest and is mainly discharged into the Songhua River Basin. Under the control of topography, neotectonic movement and sediments (aquifers), the chemical composition of water changes regularly on the plane (Figure 5-96). Generally speaking, from the recharge area (especially in the west or northwest of the basin) to the discharge area (southeast), the chemical composition of water changes from light to salty, and from low salinity (less than 0.5g/L) to bicarbonate. Under the background of this change, the hydrochemical anomalies related to oil and gas also show orderly changes (Table 5-63).

Figure 5-95 Schematic Diagram of Quaternary Hydrogeological Section in Songliao Basin

Table 5-63 Characteristics of hydrochemical anomalies in different hydrogeological units

Figure 5-96 Plane Distribution of Quaternary Hydrochemical Compositions in Songliao Basin

1-bicarbonate; 2- sulfate water; 3- chlorine water

The above two examples show that the hydrochemical anomalies are clear, and their spatial distribution, anomaly intensity and index combination are consistent, whether in a complex oil-bearing basin with complex landforms and multiple source rocks or in an oil-bearing basin with vast plains and single source rocks.

(2) The probability of hydrochemical anomalies in the oil-controlling fault zone is high.

The reasons why hydrochemical anomalies are distributed in fault zones are as follows: firstly, the widely developed fault activities in oil-bearing basins (especially in the eastern platform area of China) destroy the structure and continuity of strata, and the formation of fracture zones around faults (including fault intersection and faults in different directions). ) provides channel conditions for vertical micro-migration of oil and gas, especially those inherited faults with early development, long activity time and large fault distance, which communicate the oil and gas relations between oil and gas reservoirs and overlying strata or near the surface. Second, faults form various traps, which are the main places for oil and gas enrichment, and oil and gas micro-leakage forms hydrochemical anomalies near the surface. Many petroliferous basins in China have shallow hydrochemical effects related to fault activities.

Pan Lin fault zone is located in Huimin sag in the southwest of Jiyang depression, which is divided into northern uplift area and southern relative subsidence area. The fault zone is 76 kilometers long and its strike is close to NEE direction. Its profile is inclined to the south with an inclination angle of 45 ~ 60, and the fault distance is1000m ... This fault existed in the sedimentary period of Shahejie Formation, and moved into Guantao Formation and Minghuazhen Formation for a long time, especially at the end of Dongying, and its activity intensified, forming two groups of faults parallel to the neE direction of the fault zone and inclined on both sides. Many fault blocks and fault nose structures formed along the fault zone control the distribution and accumulation of oil and gas in this area. The main oil-forming assemblages are Paleogene Es4-Es3, Es2-Es 1 and Dongying Formation. The reservoir-cap assemblage consists of Neogene Guantao Formation sandstone and Minghuazhen Formation mudstone. * * * It is divided into eight sets of oil-bearing series (Es4, Es3, Es3, Es2, Es 1, Dongying, Guantao and Minghuazhen). The preservation degree of reservoir-cap assemblage on both sides of the fault is very different, and the south wall is well developed, and the north side is mainly below the third member of Shahejie Formation. The oil-water relationship in this area is complex, and the properties of crude oil and formation pressure coefficient change with the steps formed by fault blocks. It has the characteristics of multi-faults (80% of drilling encounters faults, and there are 132 faults within 300 square kilometers), small fault blocks (there are more than 140 fault blocks in this area) and layered oil and gas reservoirs. There is no uniform oil-water interface between fault blocks, but there is a multi-oil-water system, and the groundwater salinity is high (Table 5-64). During the sedimentary period from Guantao Formation to Minghuazhen Formation, faults continued to move, resulting in upward migration of oil and gas and groundwater, which made the properties of crude oil in the upper layer quite different from those in the lower layer (Table 5-65). In addition, the groundwater pressure in the main water-bearing rock series in this area is high or overflows the surface in most boreholes, with high salt concentration. The above oil and gas geological and hydrogeological conditions laid a geological foundation for the formation of chemical anomalies in near-surface water.

Table 5-64 Chemical Composition Characteristics of Oilfield Water in Pan Lin Fault Zone

(According to Shengli Oilfield Research Institute 1979)

Table 5-65 Vertical Variation of Crude Oil Properties

(According to Shengli Oilfield Research Institute 1978)

Figure 5-97 summarizes the hydrochemical anomaly characteristics of the fault zone, in which salinity reflects the comprehensive characteristics and variation degree of hydrochemical components, and the high-value area of quartic trend surface on the plane is surrounded by oil and gas reservoirs (see Figure 4- 19). According to 134 points with positive residual (349 samples in total), the abnormal components of each point are calculated, and the points with high residual value are mainly distributed near the fault zone. In the fracture zone, with the increase of salinity, Cl- and Na+ always occupy the first place, maintaining the ion sequence of Na+> Ca2+> Mg2+ and Cl- >, which is consistent with the ion combination of oilfield water and obviously different from the background area. The results of stepwise regression and factor analysis showed that the main components were salinity, Cl- and Na+. All water samples are divided into oil-bearing structures and non-oil-bearing structures for discriminant analysis. It can be seen from the results that the distribution of abnormal points has certain laws to follow with the distribution of geological structures and oil and gas reservoirs, and the normal distribution of abnormal points shows that its forming factors are basically consistent from one side. In addition, the high value area of quartic trend surface analysis of phenol and its homologues is consistent with that of oil fields, and most of the remaining abnormal points are distributed along fault zones. On the fault zone, the soluble gaseous hydrocarbons basically maintain the characteristics of all components, with high C 1 > C2 > C3 > C4 and heavy hydrocarbons.

Figure 5-98 is another example of hydrochemical anomaly located on the fault zone. There are four hydrochemical anomalies in the figure, with different types and modes and different control areas, but they are all related to NWW-trending main faults and associated faults in spatial distribution. See Table 5-66 for the parameter characteristics of each anomaly. The abnormal index combination is relatively stable, with good superposition degree and relatively high intensity. These anomalies have the characteristics of multi-methods (water medium and soil medium), and the multi-indexes show anomalies at the same time with great contrast, which shows that the formation of anomalies is on the one hand due to the migration of deep source rocks along certain channels and the long-term influence on shallow geochemical effects; On the other hand, it also shows that there are traps for oil and gas accumulation near the fault zone.

Figure 5-97 Characteristics of hydrochemical anomalies in Pan Lin fault zone

1- fracture; 2- known oil-bearing area; 3— Isogram of quartic trend surface of phenol; 4— Residual abnormal points on the quartic trend surface of phenol; 5— The discriminant values of salinity, Cl and Na are greater than 0.2; 6— The residual value of the fourth trend surface of salinity is greater than 50.

Table 5-66 Characteristics of hydrochemical anomalies in fault structural zones

Hetao depression belongs to a part of Yinshan-Ordos Plateau hydrological region in hydrogeology. Low degree of oil and gas exploration. The author and his colleagues (1969) investigated the hydrogeological conditions of the depression and its surrounding mountainous areas from this area, and studied the variation law of hydrochemical composition. By taking groundwater samples in different seasons, simply pumping water and establishing experimental observation stations, according to the statistical results of 392 water samples, three water system chemical abnormal areas are delineated (Figure 5-99). The salinity of hydrochemical components in these abnormal zones is high, and sulfate ions are generally low, mainly in NaHCO3 and CaCl2 _ 2 water, containing trace components such as Br and I, and the soluble gaseous hydrocarbons and redox potential in water are abnormal (Table 5-67). It is worth pointing out that methane is the main soluble gaseous hydrocarbon in this area, but the anomaly is not only complete in composition, but also obviously increases in heavy hydrocarbon content, exceeding methane in absolute value, and the anomaly value is as much as 10 times higher than that of recharge source or local lake (marsh) water, which is obviously not the result of modern biogeochemistry. Compared with the chemical composition of deep groundwater, it has similar or consistent characteristics, which is the result of deep groundwater flooding along the fracture zone and infecting shallow groundwater. As can be seen from Figure 5-99, three shallow hydrochemical anomaly zones are all distributed near the fault zone.

Figure 5-98 Relationship between hydrochemical anomalies and fault zones

Table 5-67 Hydrochemical Anomalies and Background Values in He Lin Depression

Figure 5-99 Relationship between hydrochemical anomaly of phreatic water and fault zone in He Lin sag.

1- fracture; 2- hydrochemical anomaly

To sum up, the hydrochemical components in the oil-controlling fault zone include both indigenous components and immigrants from far away. Hydrochemical anomalies are distributed in a beaded shape along the fault zone, which is an objective geological and geochemical phenomenon.

(3) Hydrochemical anomalies are distributed around the center of oil source in the depression.

The main oil fields in sedimentary basins are mostly distributed in favorable traps and reservoirs near oil sources, and it is a basic law of continental oil and gas fields that oil-generating centers restrict oil and gas distribution. The spatial distribution of hydrochemical anomalies is mainly controlled by three factors: first, oil and gas fields; The second is the channel conditions of vertical micro-migration; The third is the peak surface of groundwater. These factors are often organically combined in sedimentary depressions (oil-generating depressions) to promote the formation of hydrochemical anomalies. The author (198 1) pointed out that sedimentary depression is the basic geological unit that controls the evolution of hydrochemical composition in oil fields. On the basis of this basic understanding, the distribution law of hydrochemical anomalies is discussed.

The high hydrocarbon generation intensity of Paleogene oil source center in Dongying Depression increased from 3.6 1× 106t/km2 in Kenli-Binzhou-Boxing to 32.4× 10-6t/km2 in the center, showing a regular centripetal increase. There are many hydrochemical anomalies of different sizes and shapes around the center of oil source. From one aspect, it shows that oil and gas resources are very rich. These abnormal index parameters vary greatly in different parts of the depression. Generally speaking, the salinity in the center of the depression is the highest (3 ~ 5g/L), which is mainly composed of Cl-Na+ ions. Salinity (2 ~ 4g/L) is the second hydrochemical anomaly in the northern part of the depression, and Cl-Na+ ion combination is the main one, but the rising speed is faster. The hydrochemical abnormal salinity in the south is generally low (1g/L), and Cl- binding is the main factor. Soluble gaseous hydrocarbons, benzene-phenol and its homologues, fluorescence spectra, etc. All anomalies have high contrast values. The total hydrocarbon content of hydrocarbon gas in soil medium is generally high, but methane accounts for a large proportion. Modern transgression has a great influence on the hydrocarbon content in modern sediments.

Hydrocarbon hydrogeochemical investigation was carried out in Biyang sag 1984, and 2 12 water samples were collected. After that, a grid of 500m×500m was used for detailed investigation, and two methods (hydrochemistry and soil medium) were used for comprehensive determination of many indicators. See Table 5-68 for the concentration distribution characteristics of each parameter. The discovery of 12 hydrochemical anomaly, together with the shallow geochemical effect of known oil fields, forms an ordered annular anomaly zone with the source rock as the core (Figure 5- 100). Because of the abundant oil and gas resources, high hydrocarbon generation intensity, moderate reservoir burial, good conditions of element diffusion convection and oil and gas micro-leakage, this area shows strong hydrochemical anomalies above the known oil fields, with the characteristics of good multi-parameter consistency (Table 5-69). The differences in geological conditions and supergene geochemistry make each anomaly different in primary and secondary relationship, combination and content, morphological type and scale. Table 5-70 lists the characteristics of hydrochemical anomalies, which were confirmed by drilling in later oil and gas exploration and in oil fields. The oil and gas information reflected by anomalies are mainly ordinary (light) oil in the south of the sag and heavy oil and natural gas in the north of the sag.

Table 5-68 Distribution Characteristics of Hydrochemical Indexes in Biyang Depression

Figure 5- 100 Relationship between Oil Source Center and Hydrochemical Anomaly in Biyang Depression

The phenomenon that the oil source center controls the formation and distribution of hydrochemical anomalies is also a common phenomenon in sedimentary depressions in other basins.

(d) Hydrochemical anomalies related to oil and natural gas have high stability.

The vertical micro-migration of oil and gas mainly takes water as the carrier. Driven by vertical pressure gradient and buoyancy, water moves upward along the pore-fracture system of rock in the form of convection-diffusion in the changing temperature and pressure environment. In the development of geological history, vertical micro-migration is intermittent and phased, and it is a continuous progressive process.

Table 5-69 Shallow Geochemical Effect Parameters of Known Oilfields in Biyang Depression

Table 5-70 Characteristics of hydrochemical anomalies confirmed by drilling holes

Most chemical components of oil and natural gas are soluble in water. The vertical micro-migration of oil and gas makes the groundwater above the oilfield have a long-term recharge source, which is affected by deep oil, gas and water for a long time. The accumulation effect makes the chemical composition of water tend to the nature of oilfield water and is relatively stable. The observation data of the above hydrogeochemical test site for many years prove that the hydrochemical anomalies related to oil and gas have good reproducibility, and the parameters and characteristics of repeated inspections remain unchanged. The data in Table 5-7 1 further shows that not only inorganic components but also volatile organic components are highly comparable in different years, and it is stable and reliable to judge anomalies by them. After 20 years of repeated sampling, the shallow hydrochemical anomalies in Biyang Depression were analyzed. The average value of C 1 only decreased by 0.09 μL/kg, and the standard deviation changed from 0.92 to 0.94. Due to the long-term waterflooding development of the oilfield, the average water-soluble hydrocarbon has dropped significantly, but it is only 2.3 1μL/L, and the standard deviation has changed from 0.94 to 0.96, indicating that it is extremely stable. The simulation experiment of water soaking oil-bearing rocks proves the migration process of oil-water chemical components and the stable state of water chemical components from another angle. It can be seen from Figure 5- 10 1 that the hydrochemical composition of oil-bearing rocks increases rapidly in the initial stage of surface water immersion (crushing, heating, ultrasonic vibration, etc.) with salinity of 0.15g/L. ), and gradually slow down with the passage of time, and basically remain unchanged after reaching a certain value. In addition, the groundwater is buried at a certain depth, which is less affected by climate change and other factors, and there are relatively few interference factors, which is also a factor that the chemical composition of groundwater is relatively stable. Although the groundwater is flowing, its flowing speed is very slow, and it moves regularly in the region, and the anomalies will not drift or disappear greatly. Even if it deviates, it can be traced back to the source area through certain technical means and research.

Table 5-7 1 Comparison Table of Organic Components in Hydrochemical Components in Different Years

Fig. 5- Variation curve of hydrochemical composition of oil-bearing rocks after soaking in water +00 1

(5) There are many hydrochemical anomalies confirmed by oil and gas exploration.

The formation of chemical anomalies near surface water is the result of the influence and action of deep oil, gas and water. According to the distribution law of hydrochemical anomalies, the oil and gas prospect of the basin can be predicted and the favorable areas for oil and gas enrichment can be pointed out. The practice of oil and gas exploration proves that the comprehensive application of hydrogeochemical exploration technology is an aspect of accelerating the pace of oil and gas exploration and breaking through the oil export barrier. The effectiveness of some hydrochemical anomalies in providing oil and gas information has been described before, and now several successful exploration examples are listed.

1. Liangshui Town is not normal.

This anomaly is a comprehensive anomaly found in regional hydrogeochemical survey (area 1300km2). According to the statistics of 1306 water quality analysis data, the abundance characteristics of main hydrochemical parameters in this area are shown in Table 5-72.

Through mathematical statistics, combined with the results of hydrocarbon adsorption in soil medium, three geochemical zones are determined: high background, medium high background and low background. The oil-gas bearing property of each anomaly is evaluated by zoning, and the most favorable oil-gas anomaly is delineated. The comprehensive anomaly in Liangshuizhen is a first-class anomaly, and the main hydrochemical indexes are as follows. The comprehensive evaluation index is as high as 12.0, with phenol and spectrum in water as the center, the block-ring combination of hydrocarbon adsorbed on the periphery is abnormal, and other indicators are distributed in sporadic points (Figure 5- 102). The anomaly is controlled by six water samples, among which the phenol content is as high as (9.85 ~120.96 )×10-9, the fluorescence at 320nm is 17.4 ~ 30.4 int, and the ultraviolet derivative spectrum at 222nm is 9.0 ~15.0. In addition, the synchronous fluorescence is mainly in light band, but the high band peak is missing. The wavelength pair of the three-dimensional fluorescence main peak is 228nm/340nm, and the T4 peak is missing. The light-to-weight ratio of the ultraviolet absorption spectrum (222nm/232nm) is greater than 1. These spectral characteristics reflect the oil and gas properties in the anomaly, mainly ordinary crude oil. δ 13C 1 is about -55‰, which proves that the formation of anomalies is related to deep oil and gas. According to the above characteristics, it is considered that this anomaly is the key target area for oil and gas exploration, and industrial oil flow has been seen in Jia 2 well.

Table 5-72 Abundance Characteristics of Abnormal Hydrochemical Parameters in Liangshui Town

Note: Variation amplitude value = maximum-minimum/average.

Figure 5- 102 Comprehensive hydrochemical anomaly map of Liangshui Town

1- comprehensive anomaly; 2- abnormal phenol; 3- Spectral abnormality; 4-c1abnormal; 5- abnormal water-soluble hydrocarbon

2. Post-Wuzhuang anomaly

It is located on the nose structure of the southern boundary fault zone of Nanyang sag in Nanxiang basin. It is determined by geochemical exploration (densifying soil medium samples) in the favorable area of hydrochemical survey. The anomaly is characterized by the abnormal combination of block and ring in the fluorescence spectrum around the comprehensive hydrocarbon gas (water-soluble hydrocarbon and adsorbed hydrocarbon). See Table 5-73 for the combination characteristics of abnormal indicators.

Table 5-73 Characteristics of hydrochemical anomalies in Houwuzhuang

The comprehensive evaluation index of this anomaly is as high as 3 1.6. The synchronous fluorescence spectra of 320nm, 380nm and 405nm all show certain intensities, but the intensity of 320nm, which reflects the characteristics of low-ring aromatic hydrocarbons, is the highest, indicating that the oil attribute of this anomaly is mainly light oil. At present, Well N65 and Well N67 have obtained high-yield oil flow (Figure 5- 103), in which the third member of Well N65 is 2678.5 ~ 268 1.9m, the first layer is tested at 3.4m, and the daily crude oil output of 8mm choke is 23.1m3; . 2,704.9-2,707.5 m, with a daily crude oil production of 4.5m3；; The third member of Well N67 is 2,623.3 ~ 2,626.8 meters, with a layer thickness of 3.5 meters. The tested daily crude oil production is 4.32 cubic meters, the abnormal control area is 3.2 square meters, and the newly-increased petroleum geological reserve is1160,000 tons, which has created certain economic benefits.

3. Nanlin Slope Belt

This area is a part of the secondary structural unit of Huimin sag in Jiyang depression. According to the geochemical exploration procedures of general survey, detailed investigation and precise investigation, 1244 water samples and 5 175 soil samples were collected in the range of 2000km2. The investigation results show that the northern zone near Xiakou fault is a favorable zone for oil and gas enrichment. Then, using the method of combining hydrochemistry with soil medium, the northern belt and key areas were investigated in detail. Among the discovered hydrochemical anomalies, Well Qugu 1 is located at1515 ~1520m (the second member of Shahejie Formation) and Well Qu 10, Well Quxie 8 and Quxie. The hydrochemical anomaly of Duoshiqiao is one of the anomalies found through in-depth investigation. See Table 5-74 for the main index characteristics. It is worth mentioning that the parameters such as abnormal adsorption of filaments, three-dimensional fluorescence and stable carbon isotope of methane in water all show the characteristics of coal-type gas.

Table 5-74 Characteristics of hydrochemical anomalies of Duoshi Bridge

Fig. 5- 103 houwuzhuang anomaly map of nanxiang basin

According to the statistics of nearly 3000 data from 60 measuring points, the laws are as follows:

1) is similar to the standard spectrum of natural gas in neighboring areas (Figure 5- 104). Abnormal peripheral points have poor correlation; Taking heptane (normal isomer), cycloheptane and toluene as the three vertices of the triangle, all the abnormal points fall on the end near toluene, indicating that the content of toluene is much higher than that of heptane and cycloheptane. The concentrations of benzene and toluene vary with the types of oil and gas accumulation, and their high values reflect the characteristics of coal-formed gas. The ratio of normal isoparaffins reflects the evolution characteristics of oil and gas. Taking isobutane/n-butane as an example, if it is more than 60, it is considered to be related to strong biodegradation, and if it is less than 60, it is considered to be related to weak biodegradation. The water chemistry of Duoshiqiao is abnormal, and the abnormal point of isobutane/n-butane is more than 60 when it is more than 2/3. The ratio of isomeric hexane can also reflect the biodegradation degree of oil and gas. Usually, the normal order of organic matter evolution is 2-mc5/3mc5 > 1, and when there is biodegradation, the ratio is < 1. The above ratio of most sampling points in Duoshiqiao area is less than 1. According to the above characteristics, it is judged that the oil source in this area may have suffered biodegradation.

2) The plane graphic features of three-dimensional fluorescence are between "P" type and "O" type, showing the sign of coal-formed gas (Figure 5- 105). The wavelength pair of the main peak position is 227nm/34 1nm, and a peak appears, which has the peak matching characteristics of coal-formed oil (gas) -T 1, T2, T2. The ratio (R) of main peak to secondary peak is 3.88, and the steepness (K) of main peak is 0.6.

3) The dispersion range of methane stable carbon isotope in water is between-44% and-32%, and the average value of δ 13C 1 is obviously heavier than the regional background value, which is close to the carbon isotope value of coal measures strata in this area (-32.64‰).

4) The ultraviolet derivative spectrum has the highest intensity among light components, such as 2 14nm reflected monocyclic aromatic hydrocarbons and its previous bands, and its high intensity is a reflection of rich gas.

Fig. 5- 104 Total ion flow chromatogram of hydrochemical anomalies in Duoshiqiao

Fig. 5- 105 Three-dimensional fluorescence spectrum of hydrochemical anomalies in Duoshiqiao

The hydrochemical anomaly of Duoshiqiao reflects the understanding of coal-formed gas, which is confirmed by drilling. The daily output of coal-formed gas is greater than 5× 104m3.

The above examples and practices prove that hydrochemical method is a mature method in oil and gas geochemical exploration. It is an effective oil exploration technology and method in eastern and central China. Under the condition of insufficient groundwater outcrop, combining with hydrocarbon gas in soil medium and following the method of determining hydrocarbon gas boundary in abnormal soil samples with water, good geological effects can be achieved. In the case of insufficient groundwater samples in arid and semi-arid areas, the water-soluble rock (soil) method can be applied, which has certain application value.