1. Introduction
The stratum structure in the exploration area is complex, and Cenozoic, Jurassic gravel layer (five-bearing stratum) and coal-bearing stratum are in unconformity with each other (Figure 1). The Jurassic gravel layer has high porosity and permeability, and is an aquifer. According to the seismic and drilling exploration, it was believed that no fault developed in the five components-bearing layer, but the roof water disaster from the layer often occurred in the mining area, and the roof water leakage accident also occurred. The main reason is that the structural development of the layer is unclear. At the same time, previous 3D seismic exploration couldn’t describe the fluctuation of the top interface of Ordovician limestone.
Figure 1: Schematic diagram of the stratigraphic structures in the exploration area
2. Treatment solution
All-digital high-density 3D seismic detection technology was adopted. The observation system consisted of 16 lines and 5 shots (Figure 2), which collected the quality original data with wide azimuth angle, high covering degree and small CDP grid, and adopted digital geophones to receive the data, so as to ensure the reception of weak reflection signals at the top interface of Ordovician limestone in the five layers. The target processing technology and the pre-stack time migration technology were used in data processing to ensure accurate imaging of complex structures and unconformity strata.
(A) Observation system 16L5S72T1R (b) Rose diagram of the observation system
Figure 2: Observation system and rose diagram
3. Work situation
This 3D seismic field exploration adopted a regular beam 3D observation system. There were 28 3D seismic wiring harnesses, 89 survey lines, 220 physical test points (86 point for tests, 6 for small refraction and 128 physical points in one test section) and 8,448 physical production points (thus a total of 8,668 physical points), with a full coverage area of 5.09k m2.
The records were graded. There were 6,334 records of the grade A, with a grade A rate of 74.98%, and 2,114 records of the grade B, with a grade B rate of 25.02%. Test and small refraction records were qualified.
4. Achieved accomplishment
(1) The time profile well reflects the Cenozoic bottom interface, the components-bearing layer, the coal seams, Ordovician limestone and their unconformity relationship (Figure 3).
(a) Conventional time profile
(b) High-density time profile
Figure 3: Comparison of the reflection of the conventional and high-density 3D seismic exploration of the five components-bearing layer, coal seams and limestone
(2) In the later curtain grouting and drilling process, the interpreted faults development in the exploration area was verified in the five components-bearing layer. 6 faults were verified, of which 4 were correct, and the other 2 had grout leakage, which need further verification.
(3) After the actual exposure of later drilling, the thickness prediction of the conglomerate by high-density 3D seismic was in good agreement with the actual situation.
5. FAQ
Q1: What is the main difference between the conventional and high-density 3D seismic explorations?
A: Compared with the conventional 3D seismic exploration, high-density 3D one is mainly characterized by the wide azimuth and the high covering degrees. Besides, the CDP grid of high-density 3D seismic exploration is smaller and the shot point density is higher.
Q2: Will the detection depth of high-density 3D seismic exploration increase?
A: The detection depth is mainly related to the well depth and dosage. However, due to the high covering degrees of high-density 3D seismic exploration, the reflection waves of deep strata are relatively clearer, so the detection depth can increase to a certain extent. However, if the dosage and well depth are unchanged, the increase of depth is very limited.
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