Simulation of AVO features and analysis of the time-frequency characteristics of the basement reservoir of the ancient central uplift zone in the northern Songliao Basin

The paleo central uplift belt is located between the Xujiaweizi and Gulong fault depressions, as a long-term inherited palaeouplift. Due to the large exploration area and well conditions for hydrocarbon accumulation in the bedrock buried-hill, the paleo central uplift belt has become one of the major areas of risk exploration at present. The lithology of the basement of the paleo central uplift belt is mainly granite and metamorphic rocks. However, because of the complexity of the reservoirs and low physical characteristics of reservoirs with fractures as the major reservoir space, research into the seismic response of fractured reservoirs is not enough as there is a low degree of exploration. In this paper, based on the formation structure of the basement weathering crust in the paleo central uplift belt, the characteristics of geophysical parameters of basement weathering crust formation are defined by the results of rock physics, logging and seismic interpretation, and the geological model of the basement is established. The AVO response and seismic wave field of strata with different gas saturation are simulated by viscoelastic wave equations, the AVO curves of caprocks of the paleo central uplift belt–weathering leaching layer interface, the AVO curves of the weathering leaching layer–fracture layer interface, and the AVO curves of the fracture layer–inner layer interface all reflect that the reflection amplitude is negatively correlated with the offset. When the water saturation is 100%, the reflection amplitude is the biggest, when water saturation changes to 50% and 0%, the amplitude obviously decreases. This indicates that the amplitude will decrease significantly when the weathering leaching layer contains gas. Based on the forward modeling of seismic records and time-frequency analysis of the basement weathering crust geological model, it is clear that when the local layer contains 100% gas, the weak time spectrum under the T₅ strong time spectrum is stronger than that with 100% water content. On the whole, the AVO characteristics and time-frequency characteristics of water-bearing and gas-bearing reservoirs in basement weathering crust are different, which can be used as possible indicators for fluid identification.