Analysis of Seismic Response Characteristics of Coal-Bearing Sandstone Reservoirs in the Middle and Lower Jurassic in the South Turgay Basin

The Middle and Lower Jurassic coal seams in the southern margin of the South Turgay Basin are mainly formed in delta, river, lake, and coastal environments. Within the fourth order sequence framework, the formation process of coal seams can be summarized into six stages, with thin coal seams predominating and alternating with sand and mudstone. As a special lithology, the coal seam has the characteristics of low density and low P-wave impedance compared with the sand-mudstone formation. It shows strong reflection characteristics on the seismic profile, and usually forms a strong shielding effect on the surrounding reservoir, thus masking its true seismic reflection characteristics. This paper designs several forward models by changing the combination of coal seam thickness, sandstone thickness, and spacing between coal seams and sandstone, and discusses the influence of coal seams with different thickness and spacing on sandstone reservoir and seismic response characteristics. The forward modeling results show that the reflection of coal seams in this area has a large impact on the reflection interface of surrounding sandstone reservoir, with an impact radius of about 38 m, while the shielding effect of coal seams has a small change in the case of different thickness of coal seams and sandstone reservoirs. When the distance between the coal seam and the oil layer is less than 40 m, the reflection of the oil layer is submerged in the coal seam reflection without obvious seismic response characteristics; When the distance is greater than 40 m or more, the seismic reflection characteristics of the oil layer are the weak reflection characteristics of the top wave trough and the bottom wave peak.