1. Introduction
1.1 Project introduction
Dongyi Minefield, Wujianfang Coalfield, Xilingol League is located in the north of Inner Mongolia Plateau. Most of its surface has been semi-desertified, and some areas are covered by forest (Figure 1). The surface elevation varies from 962.26 m to 1035.42 m, the terrain is relatively flat, and there are sand dunes with a height difference of more than 30 m locally (Figure 2).
Figure 1 Surface conditions
Figure 2 Schematic diagram of the surface elevation
Prospecting was carried out in the mine field at 500m×500m grid, and the drilling revealed six groups and 10 layers of coal, among which group 2 and group 3 were bifurcated and merged. coal seam 2-3 is locally recoverable, with thickness varying from 1.1 m to 2.45 m; coal seam 3-3 has a thickness between 1.5 m and 17 m; coal seam 4 has a thickness between 0.5m and 9.5 m; coal seam 5 has a thickness between 0.5 m and 8 m. The floor elevation of coal seam 2-3 varies from 490 to 780 m; coal seam 3-3 from 430 to 920 m; coal seam 4, from 420 to 880 m; and coal seam 5 from 400 to 870 m. The structure in the mine field is a monoclinic structure, and the dip is generally less than 10°. Five faults are developed. No intrusion of magmatic rocks into coal-bearing strata was found.
The geological task is to find out the nature, occurrence and extension of faults with drop ≥ 5m in the exploration area, find out the thickness change trend of the main minable seams, and explain the bifurcation, merger and pinch-out of the main minable seams as much as possible; find out the floor fluctuation pattern and distribution range of main coal seams, find out the intrusion range of rock mass and the coal-free area.
1.2 Exploration difficulties
(1) The shallow seismic geological conditions are poor: Because the surface overburden in the exploration area is dry and loose, the absorption and attenuation of effective waves, especially high-frequency information, are strong, resulting in the attenuation of seismic wave energy.
(2) The buried depth of the target layers is shallow: Due to the large area of the exploration area, the buried depth of the target layer in the area varies greatly (100-500 m), and it is difficult for a single observation system to balance the quality of the data of the shallow, medium and deep target layers.
(3) The geological task requires high accuracy: It is difficult to identify the bifurcation and merger of coal seams on the time profile.
2. Treatment solution
In the design of observation system, the exploration area was divided by the standards of buried depth less than 200 m, buried depth 200-400 m and buried depth greater than 400 m, and different observation systems were designed to ensure times of effective coverage. In data collection, excitation was made from the vibrator vehicle and 60Hz geophones were buried to receive data so as to ensure the validity of data. The data processing focused on the comparison of static correction methods, deconvolution parameter and noise attenuation method to ensure the signal-to-noise ratio, resolution and fidelity of the data. In the data interpretation, the calibration of reflected wave horizon and the interpretation of bifurcation, merger and thickness variation trend of the coal seams (Figure 3 and Figure 4) by using wave resistance data were the key points.
Figure 3 Display of the thickness change , bifurcation and merger of the coal seam
Figure 4 Performance of coal-containing drilling and coal-free drilling on time Profile
(According to the drilling data, there is no coal in the whole hole of ZKK29-1, and there are three layers of coal in ZKKJ29-4)
3. Work situation
In this 3D seismic exploration, totally 38 swaths and 220 survey lines were completed, with a work area of 35.2872 km2 and a full coverage area of 28.9 km2. There were 20,371 production physical points and 223 test physical points, with a total of 20,594 physical points. The original records were evaluated in accordance with the Code for Seismic Exploration of Coalbed Methane, and all the test records were qualified. There were 18,153 records of grade A in the production records, representing a grade A rate of 89.112%; 2,217 records of grade B, representing a grade B rate of 10.883%; therefore, the qualified rate of the records was 99.995%.
4. Achieved Accomplishments
(1) The occurrence ranges and the floor fluctuation pattern of coal seams 3-3 and 5 in the exploration area were identified.
(2) At the southern boundary of exploration, there is a coal-free area with an area of about 1.76 km2 in coal seam 3-3; a coal-free area with an area of about 3.06 km2 in coal seam 4; a coal-free area with an area of about 3.16 km2 in coal seam 5.
(3) A total of 107 faults were combined, including 5 faults being corrected and 102 new faults as compared with those before 3D seismic exploration.
(4) The thickness variation trend of coal seams 3-3, 4 and 5 was predicted.
(5) The bifurcation area of coal group3 was delineated, covering an area of about 5.6 km2.
(6) No magmatic rocks intruded into coal seams.
5. FAQ
Q1: How to ensure the exploration effect of the target layer with different buried depths when the buried depth of the target layer varies greatly?
A: It is difficult for a single observation system to balance the data quality of shallow, medium and deep target beds. Therefore, the design scheme should be optimized according to the characteristics of exploration areas and geological tasks. The observation system should take into account the data quality of shallow, medium and deep target beds. Considering the uniform coverage times of collected data, it is generally recommended to adopt different observation systems based on the different burial depths of target beds.
Q2: How to use seismic data to interpret the change trend of coal seam thickness and the merging area of coal seam bifurcation?
A: The amplitude of reflected wave on seismic time profile is associated with the thickness of target layer. Generally, the thicker the target layer, the larger the amplitude of reflected wave, and the thinner the target layer, the smaller the amplitude of reflected wave. Due to the limitation of data processing level, there is no clear one-to-one correspondence between thickness and amplitude value, but the disappearance of reflected wave of coal seam can be used to calibrate the absence of coal seam. It is difficult to identify coal seam bifurcation and merger effectively in seismic time profile, so wave impedance profile can be used to determine the merging and bifurcating points.
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