Seismic Wave Propagation Characteristics Of Fractured Marl Reservoirs

As one of the super-low porosity and permeability reservoirs,marl reservoirs have gradually aroused much more concern while the exploration of conventional oil and gas couldn't meet the needs of economic development.Taking Shulu Depression for example,although the matrix porosity of marl formation is poorly developed,the conglomerate which deposited with marl simultaneously is important storage space in such formation.In addition,cracks near the fault are extensively developed.The above features result in marl fractured reservoir both heterogeneity and anisotropy,which makes seismic wave propagation mechanism is unclear in such reservoir.Therefore,this paper carried out the research of seismic wave propagation characteristics of fractured marl reservoir.Marl refers to the transitional rocks between carbonates and clay rocks,and its clay content is in the range of 25%~50%.The rock framework of marl reservoir is composed of calcite,followed by dolomite,quartz,feldspar,while clay minerals are mainly kaolinite.In this paper,based on equivalent differential medium theory,calcite was regarded as the main phase of marl.Other mineral components were added in the order of mineral contents from more to less.Isotropic background of marl matrix was obtained by calculating bulk modulus and shear modulus of marl matrix.We used Linear Slip fractured theory to describe the anisotropic characteristics of marl reservoir induced by the paralleled arrangement fractures under the condition of long wavelength approximation.Hudson models were used to build the connection between the stiffness matrix of fractured marl reservoir and the microscopic parameters of cracks.And on this basis we established the anisotropic effective medium model which is applicable to describe the fractured marl reservoir in this paper.Based on the anisotropic effective medium model,controlling variable method was used to get a series of equivalent medium models with different parameters.To analyze the seismic wave propagation features,rotated staggered grid finite-difference algorithm was utilized to perform the forward modeling of the equivalent medium models,and we applied the complex frequency shift recursive integral NPML boundary condition to absorb the boundary reflection.Results show that the clay content increasing will lead to weaker reflection characteristics,while limestone mineral content increasing lead to stronger ones.Earthquake response caused by cracks can obtained only when the fractures developed to a certain density,which makes it generally impossible to recognize fracture development area in a seismic section.Plane wave analysis was performed by solving Christoffel equation to calculate the seismic wave velocity varying with different marl mineral content and different fracture parameters.We studied the AVO responses characteristics in equivalent medium models of fractured marl reservoirs with different marl mineral content and different fracture parameters,and summarized the conclusions.Based on the description of fractured marl reservoirs,we built the equivalent medium models,and summarized the wave propagation principles with different parameters.The studies in this paper may provide theoretical instructions for the exploration and development of fractured marl reservoirs.