(a) hydrocarbon microleakage theory
1. Two manifestations of hydrocarbon leakage
(1) Visible oil and gas seedlings: oil and gas seedlings that can be observed by naked eyes. Its migration mechanism is the migration of oil and gas reservoirs under the influence of geological activities (such as stratigraphic dip, buried depth change, fractures, faults, etc.). Early oil and gas exploration mainly focused on finding oil and gas seedlings.
(2) Microscopic oil and gas seedlings: oil and gas seedlings that cannot be observed by naked eyes. The migration mechanism of microscopic oil and gas seedlings is "vertical migration theory", that is, oil and gas leak upward through overlying strata. Vertical leakage theory is the basis of direct oil exploration by remote sensing technology and geochemical exploration technology.
2. The vertical migration of hydrocarbons mainly depends on the upward penetration of hydrocarbon molecules.
After oil accumulates in the trap, hydrocarbons are carried by water, gas and other media, supplemented by buoyancy, evaporation and other effects, resulting in natural upward penetration. The results show that the molecular diameter of complex cyclic hydrocarbons with larger diameter is 15 ~ 20, while the pore size of shale caprock with a depth of 4000m is 20. Therefore, all kinds of hydrocarbon molecules can pass through the caprock and reach the surface.
(2) The theoretical mechanism and method of detecting trace hydrocarbons by remote sensing.
1. Ground effect of hydrocarbon microleakage
The vertical migration of hydrocarbons to the surface may form a chemical field covering, thus changing the redox conditions of the surface, causing a series of physical and chemical reactions among soil, rocks and vegetation, and forming a series of corresponding surface characteristics. These phenomena are called "alteration effect" or "alteration phenomenon" of hydrocarbon microleakage, which are collectively called ground * * * generation effect. Specifically including:
The increase of (1) hydrocarbon content and its associated elements, such as △C and Hg, leads to top anomaly or edge effect.
(2) The transformation from Fe3+ to Fe2+ in soil leads to the fading of the red layer.
(3) Carbonated alteration (△C) is formed in the soil.
(4) Vegetation is directly affected by soil changes or hydrocarbon substances, resulting in plant variation.
(5) Due to the effect of hydrocarbon substances, radioactive substances are redistributed, resulting in radioactive anomalies.
(6) Causing changes in the geophysical and geochemical fields on the ground.
(7) Due to the change of Eh/Ph value, feldspar minerals are altered to form clay minerals.
2. Remote sensing detection mechanism
The physical basis of remote sensing is the different electromagnetic wave characteristics of different ground objects. Due to the different composition and structure, all kinds of ground objects have their own specific characteristics of reflecting, scattering, absorbing and radiating electromagnetic waves. The change of color and shadow structure of remote sensing image reflects the difference of spectral characteristics of ground objects.
Typical spectral characteristics of (1) hydrocarbons
Through the laboratory transmission spectrum measurement of crude oil samples, it is found that hydrocarbons are at the wavelengths of 1.725μm, 2.270μm, 2.348μm, 3.30 ~ 3.53 μ m, 6.23μm, 7.246μm, 6438+0 1.363 μ m and 6438+02. 6.68 ~ 7.38 micron absorption band (overlapping with atmospheric absorption band, inconvenient to use). The first two bands are practical bands for detecting hydrocarbon microleakage by remote sensing. Obviously, Landsat TM5, TM6 and TM7 all contain hydrocarbon absorption bands.
Figure 1 transmission spectrum characteristics of hydrocarbons
(1)-crude oil; ②-oil seedling; ③—Asphalt
(2) Typical spectral characteristics of surface altered minerals formed by hydrocarbon microleakage. The main types of surface material alteration caused by hydrocarbon microleakage are "red bed fading" (Fe3+→ Fe2+), mineral clay alteration and carbonation alteration. The typical spectral characteristics of these altered minerals show that Fe3+ has an absorption peak near 0.9μm, and Fe2+ has an absorption peak at 1. 1 μ m. The absorption peak of clay minerals is about 2.2 ~ 2.3 μ m, and carbonate minerals have multiple absorption peaks, of which 2.35μm and 2.5μm are the most obvious.
(3) Abnormal Vegetation Spectrum The health status and abnormal distribution of vegetation above the oil and gas fields have aroused great concern of petroleum remote sensing geologists. It has been found that the soil changes caused by the slight leakage of oil and natural gas will make plants wither or get sick. The spectral curves of diseased plants and normal plants are obviously different. The main feature is that the near-infrared (0.7 ~ 0.75 μ m) reflection curve of the diseased plants is "blue shifted", that is, it moves from the position of 0.70μm to the short wave direction, and the near-infrared reflectance is obviously reduced (Figure 2).
R λ′ = R λ-RO/RS-RO standardized formula
A = a r λ′+b red edge regression curve
(4) The hue and structure, abnormal hue and shadow structure of remote sensing images reflect the spectral characteristics of ground objects. Rock, soil alteration and plant growth variation caused by hydrocarbon microleakage are abnormal in color tone and schlieren structure in remote sensing images, and the main feature is light color tone. The shape of these color anomalies is limited by underground reservoirs or structures.
Fig. 2 Schematic diagram of spectral changes of plant diseases
λ ρ-blue shift index RS- infrared shoulder height RO- chlorophyll absorption peak
(5) The geochemical information and remote sensing information of the ground effect of micro-leakage of oil and gas are two manifestations of the same content. Geochemical exploration is an early direct oil exploration method, and its theoretical basis is also based on the vertical migration theory of oil and gas micro-leakage, which has many internal relations with remote sensing detection.
Geochemical data are sampled according to a certain layout of the survey network, and the geochemical indicators of ground objects are expressed in the form of data, which is the evidence for directly monitoring the existence of oil and gas micro-leakage.
Remote sensing data (airborne and airborne) are spectral values recorded by each object in a certain form, which directly reflect the ground * * * effect caused by oil and gas micro-leakage. Therefore, there is an inevitable connection between remote sensing and geochemical exploration, and the combination of the two will certainly improve the effectiveness of oil and gas exploration. Based on remote sensing information, supplemented by other oil and gas related information, the identification model of ground * * * effect of oil and gas micro-leakage is established.
Second, the geological and geographical background of the exploration area
The experimental area is located in the vast Quaternary coverage area west of Korla, east of Shaya, south of Wuka Highway and north of Tarim River, covering an area of about 20,000 square kilometers. Geomorphologically, this area is a slightly inclined Gobi with high northwest and low southeast. Because the Tarim Basin is deeply located in the inland center, its north is separated by Tianshan Mountain, with scarce rainfall, dry and hot climate and sparse vegetation.
Tabei area is located in the east of Kuqa depression, the northern slope of Tabei uplift and northern depression. Judging from the macro features of 9 MSS combined processing maps, the regional structural features in Tazhong area have obvious three-part characteristics. That is, the north of Xinhe-Luntai line (equivalent to the east of Kuqa depression) is the distribution area of ne-oriented linear structure or long-axis structure; The south of Alar-Tarim Township-Hadadun Line (equivalent to most of Tazhong Uplift and Northern Depression) is a large-scale long-axis structure distribution area in NNE or near NNE. The long-axis structures all have the characteristics of expanding at the southern end, and the arrangement (left oblique row) is orderly. Between them (equivalent to a part of Tabei uplift and northern depression) is a short-axis or massive structural distribution area. Its axis direction is northeast in the east and northwest in the west.
Further research on remote sensing data shows that the shallow geological structure of Tabei uplift is the product of superposition of alluvial fans or alluvial fans of different periods, scales and shapes. From the analysis of fan distribution characteristics, with the development of local history, there is a tendency to retreat to the southern Tianshan Mountains. According to geological data, there was a rudiment of uplift in Tabei area since the end of Proterozoic, and the uplift amplitude suddenly increased in the early Paleozoic. In Mesozoic, although the area and the northern depression subsided as a whole, Tabei was still a relatively uplifted area. Although the northern depression rose sharply in Cenozoic, the Tabei uplift gradually disappeared. However, as far as Tabei uplift itself is concerned, its internal structure still has relatively stable layout characteristics except the influence of regional background. Therefore, even though the obvious alluvial fan can compensate the ups and downs caused by the tectonic movement since Cenozoic, it has not completely cut off the connection between the deep geological structure and the surface, which is an important geological background condition for the application of remote sensing data in this area.
3. Different remote sensing data are consistent in reflecting geological information.
There are six different types of remote sensing data used in northern Tarim, namely MSS, TM, SPOT, airborne multispectral scanning, color infrared photography, thermal infrared (night flight) and other data (see plates ⅸ and ⅹ). Although these six kinds of data are not completely coincident in space, the corresponding relationship between them can be found through different connection surfaces of different data. Its main features are as follows:
(1) The underground forward structure is generally characterized by light color in various pictures.
From the analysis of the composite image of 145-3 1Tm453 (see plate VII- 1), 33 annular images are determined, of which 17 is light (Figure 3). On the image processed by MSS combination method, 23 ring images are identified, of which 18 is a light-colored or light-colored mottled block ring image (Figure 4). According to the analysis, the light-colored images in all kinds of pictures are mostly micro-geomorphic protrusions reflected by water system and ground humidity, which are closely related to the underground normal structure.
(1) The second dam abutment structure (No.168, Figure 3-RT 12) has obtained oil flow. This structure is a normal fault block controlled by two NE-trending faults, with positive gravity and magnetic anomalies, and it is an inherited uplift at the end of Paleozoic. Its image feature is the abnormal tone of bright ellipse under dark background.
(2) Langalungdong structure is located in the southwest of Lunnan buried hill belt. The structure (or buried hill head) is composed of Ordovician, and obvious light-colored image anomalies are shown in TM (Figure 3-RT14), MSS (Figure 4-RR51) and SPOT (Figure 5-RS-2). The northeast end of this light-colored image anomaly is Lunnan buried hill structural belt. Lunnan buried hill structural belt is in the abnormal area of light color image on a large scale. Although the buried hill belt is complicated by the east-west faults, it is generally a massive structure, which is basically consistent with the abnormal shape of light-colored images. Lunnan buried hill structural belt is one of the famous oil-generating structures.
(3) Donghetang fault block is a NW-trending secondary fault block on the background of NE-trending giant fault uplift. Its northwest section is located at the high point of Mesozoic basement structure map and the northern edge of abnormal geological body in II- 10 earthquake sequence (equivalent to Carboniferous-Permian). At present, Donghe 1 well has produced oil. Its oil producing layer is Carboniferous sandstone body. The fault block is characterized by light-colored images in MSS combination diagram (Figure 4-RR45) and aerial multispectral scanning diagram.
(4) Recently, industrial oil flow was obtained in the Triassic structure of girac, and girac structure is a light-colored annular image on MSS composite map (RR55), SPOT (Figure 5Rs 14) and TM (Figure 3RT 14-2), and the light-colored image is characterized by micro-landform uplift surrounded by water system. Therefore, the above examples show that the underground normal structure in this area is closely related to the present micro-landform uplift.
Fig. 3 TM geological interpretation map
Fig. 4 Geological Interpretation Map of MSS Synthetic Image
Fig. 5 Field Geological Interpretation Map
(2) There is a certain correlation between known oil fields and geochemical exploration.
A geochemical exploration area of 8000km2 was explored in northern Tarim area. 12 effective indicators were selected from 30 indicators in the experimental area. Through the overall analysis, it is considered that CH4, C2H6, C3H8, iC4 and nC4 are related to oil and gas accumulation, and often show the edge effect or cap effect of the reservoir. And Hg, △C, Uv, etc. Show the edge effect of the oil field (Figure 6).
(1) Yakela structure, extending to the northeast, obtains oil flow (—O), which is the high anomaly area of CH4 and C3H8, and the low value area of Hg and △ C (edge effect).
(2) The nearly east-west Lunnan buried hill structural belt produces oil from many wells on the Ordovician buried hill, showing high anomaly areas of CH4 and C3H8.
(3) The Upper Paleozoic fault block (RR45, Rt 1 1) in Donghetang structure extends to the northwest, and its northern edge Donghe 1 well has produced oil (Carboniferous sandstone), which is also a high anomaly area of hydrocarbons.
(4) Langalungdong structure (Rt 14, RS-2) 1990+00 was an Ordovician low-yield oilfield after the oil test in June. The characteristics of light color image are not only consistent with the structural trap at the top of Ordovician, but also an abnormally high value area of oil and gas.
Therefore, according to the analysis of the existing data, there is a good correlation between the oilfield area and geochemical anomalies, which means that there is a geochemical field coverage that can be monitored above the oilfield.
(3) As we all know, the oil field is characterized by the positive and abnormal micro-magnetic field.
The practice of oil and gas exploration in recent years shows that there are weak secondary magnetic anomalies above most oil and gas reservoirs. This micro-magnetic anomaly is due to the leakage and escape of carbon and oxygen compounds, which creates a reducing environment above the oil and gas reservoir, and makes the physical and chemical changes of rocks gradually lead to magnetite, and then forms a positive micro-magnetic anomaly (Figure 7). Usually, this positive micro-magnetic anomaly is submerged in the regional magnetic field and is ignored by people. The current apparent depth filtering method can be extracted from the regional magnetic field as one of the evidences of oil and gas information. By filtering the high-precision aeromagnetic data in Kuqa-Luntai area, the results show that the positive micro-magnetic anomalies are in good agreement with the interpretation and remote sensing oil and gas information of known oil fields, especially in Donghetang area, such as RT30, RT 1 1 and RT 14 in the north of Tali Township, which are in good agreement with the first-level evaluation area of micro-magnetic anomalies in this area (Figure 8).
(4) As we all know, the oil field is a highly reflective area of the ground spectrum.
In northern Tarim area, two surface spectral profiles (about 150 spectral points) were made simultaneously by geochemical exploration line I (Lunnan 2 well) and line II (Husha 14 well). It can be seen from the spectral curves of four adjacent measuring points that the reflectivity of each band in the measured spectral range has obvious correlation (Figure 9). The profile changes are basically the same, and the relative relationship is tm7 > tm5 > tm4 > tm2 > tm 1. Because of the direct influence of the surface characteristics, they are unstable in known oil areas, but the general trend is relatively high value areas. This has a certain correspondence with various remote sensing data.
(5) It is known that there are infrared normal thermal anomalies in the oilfield.
The known oil fields in the range of TM6 (145 ~ 3 1) map all have normal thermal anomalies in different degrees. Ba 'er Station (168) is characterized by weak positive thermal anomaly disturbed by ground vegetation and water system. Lunnan and Santamu buried hill belts are obvious normal thermal anomaly areas; Donghetang structure is a normal thermal anomaly divided by northwest water system; At the end of last year, the structure of Langalv, which produced oil, was also characterized by weak normal thermal anomalies. Therefore, normal thermal anomaly is one of the characteristics of oilfield surface.
Fig. 6 Surface geochemical profile of Yakela structure
Fig. 7 schematic diagram of supergene magnetite formation process
Fig. 8 Evaluation Map of Micromagnetic Anomaly in Tabei Area
1- oil well; 2- Dry well; 3- earthquake fracture; 4- Inferring intrusive rocks; 5- Inferring Permian basalt; 6— Inferring the first-class favorable oil-bearing zone; 7— Inferring favorable zones for secondary oil bearing; 8- Inferring the magnetic layer of sedimentary rocks; 9— Abnormal; 10-Silurian pinchout line; 1 1- ground anticline
Fig. 9 Spectral reflection characteristic curves of oily sand in Lunnan area (four measuring points: 96, 97, 98 and 100).
Iv. Basic laws and preliminary conclusions
From the above examples and overall analysis, the known oil fields in northern Tarim are closely related to remote sensing information, geochemical anomalies and ground spectra. This connection can be summarized into the following modes, and is represented by a block diagram:
Selected papers on remote sensing of oil and gas geology in Yehefei
Preliminary conclusion: The above relationship shows that with the support of geological and geophysical exploration, the oil and gas fields in Tabei can be predicted directly by using space remote sensing information.
Verb (abbreviation of verb) Prediction of several favorable oil and gas areas
(1) Akkumu area east of Tarim Township: There are a series of annular (massive) structures reflected by obviously light-colored annular images in this area. The northern end of Rk 17(Rs 13, Rs8RR54) is the produced Santamu buried hill structural belt. There are ring structures such as Rk 18 and Rk 19 at the southern end, which are also favorable oil-bearing structures with obvious ring characteristics.
(2) Halahatang area to the south of Donghetang: There are three rows of light-colored annular images reflecting medium-sized annular (massive) structures in this area. Among them, the northern edge of Rk 12 structure (Donghe 1 well) has obtained industrial oil flow. South Rk8, Rk9, Rk 10, Rk 1 1, Rk20 all have very similar characteristics. Therefore, it is considered that these are all oil-bearing structures with similar characteristics.
refer to
Zhu. Remote sensing geology. Beijing: Geological Publishing House, 199 1.
Geological characteristics of petroleum basins in western China. Selected papers on petroleum geology in Li Desheng. Beijing: Petroleum Industry Press, 1992.
Kang Yuzhu. Characteristics of tectonic movement in tarim basin. Geology of Xinjiang,1986,6 (1).
Wang Jialin, etc. Gravity and magnetic interpretation of oil. Beijing: Petroleum Industry Press, 199 1.
Teng jiwen Geophysical field and oil and gas in Tarim basin, Beijing: Science Press, 199 1.