1. Introduction
The magmatic activity in the exploration area is strong, not only invading a wide range, but also invading many positions (Figure 1), which affects coal mining. Drilling technology can identify the magmatic rocks, but it is difficult to divide the distribution scope and lateral variation trend of the magmatic rocks in detail by drilling.
Figure 1: Scope of magmatic intrusion
2. Treatment solution
The 3D seismic exploration studies the intrusion mode and the scope of magmatic rocks mainly by the following two methods: study the seismic time profile characteristics of magmatic rocks invading coal seams in different ways according to the drilling data and the geological profiles of connecting wells; study the erosion scope of magmatic rocks by multi-attribute analysis.
(1) Time profile interpretation of magmatic rocks
① When thick magmatic rocks intruded between two coal seams, the time difference of reflected waves in coal seams would gradually increase;
② When magmatic rock passed through a coal seam, it would cause obvious dislocation of the event of coal seam with weak traction, but there is no dislocation of the events in the upper and the lower coal seams (Figure 2).
(a) Magmatic rock penetration in time profile CDP1800
(b) Magmatic rock penetration in time profile CDP1812
Figure 2: Reflection of the magmatic intrusion in time profiles
(2) Attribute interpretation of magmatic rocks
Taking seismic attributes as sample data, magmatic rocks were identified by multi-attribute clustering analysis. On the basis of fine position interpretation, various interlayer attributes were extracted, and the attributes which are sensitive to magmatic rocks were selected after normalization of seismic attributes, and the magma distribution was determined through the cluster analysis (Figure 3).
Figure 3: Distribution of the magmatic rocks in coal seam 82 (red area)
3. Work situation
There were 183 3D seismic wiring harnesses, 246 survey lines, 309 physical test points (149 points for tests, 10 for small refraction and 150 physical points in one test section) and 36,902 physical production points (thus a total of 37,211 physical points), with a full coverage area of 22.33km2.
The records were graded. There were 23,842 records of the grade A, with a grade A rate of 64.61%, 12,716 records of grade B, with a grade B rate of 34.46%, and 344 scrap records with a rejection rate of 0.93%. Test and small refraction records were all qualified.
4. Achieved accomplishment
(1) The outcrops of the main target strata in the exploration area are controlled, and the outcrop boundary of the main coal seam is basically consistent with the original exploration results, but the outcrop boundary of the Ordovician limestone top interface expands westward greatly.
(2) It is found that the faults and folds are developed in the main coal seam and the Ordovician limestone top interface has a drop of over 3m. A total of 2,319 faults have been interpreted.
(3) The influence range of magmatic rocks on the main coal seam in the exploration area was analyzed and interpreted, and it is found that magmatic rock intrusion is characterized by longitudinal penetration and transverse bedding erosion. A total of 38 longitudinal penetration phenomena were found in the main coal seam (Figure 4).
Figure 4: Schematic diagram of the magmatic rock penetrating the boundary line of the roof and the floor of the main mined coal seam
5. FAQ
Q1: Can magmatic rocks form reflected waves in seismic data volume?
A: Because of lithology, the magmatic intrusion has the high large wave velocity, which can form a big wave resistance contrast with the surrounding rocks, so it can theoretically form reflected waves, but the strength of reflected waves is also related to the thickness of magmatic intrusion.
Q2: Is the boundary of magmatic rock penetrating coal seam a fault?
A: The magmatic rock penetrating coal seam will cause the elevation change of the coal seams on both sides, which has the nature of fault in shape, but it is not a diastrophism caused by the tectonic stress, but a geological phenomenon formed by the magmatic rock penetrating through strata, so it cannot be called a fault, but it can be treated as a fault in actual production for the convenience for production.
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