Application of Geological Modeling Technique Based on Seismic Frequency Division Processing and Fluids Substitution Forward Modeling in Lithologic Gas Reservoir Risk Assessment

Abstract

Abstract JZ Gasfield is an offshore sublacustrine fan lithologic gas field located in Bohai Bay Basin, in China, with a buried depth of about 1750m. It is currently in the stage of overall development plan preparation. Two exploration wells have been drilled. They show that the gas field has characteristics of rapid lateral change in reservoir, complex distribution range and connectivity of sand bodies. In addition, the few drilling data, large well spacing and low resolution of seismic data in the evaluation stage make it extremely challenging to establish an accurate geological model of gas reservoir. In order to solve the above problems, in this paper, an integrated geological modeling method based on seismic frequency division processing and fluid substitution forward modeling is proposed. There are four key aspects to this method. (1) Using seismic sedimentology, the lateral distribution ranges of sublacustrine fan sand bodies are described by seismic frequency division processing and interpretation technique and seismic variance attribute. (2) Based on markers of single-well, logging and seismic faceis, the average instantaneous frequency attribute is selected by optimizing the seismic attributes in the dominant frequency band, and then the distribution ranges of three subfacies including inner fan, middle fan and outer fan of the sublacustrine fan are characterized, and the fine sedimentary pattern of the sublacustrine fan is established. (3) Based on an understanding of the sedimentary pattern, fluid substitution forward modeling is carried out, and the superposition and connectivity of the sublacustrine fan sand body are described. (4) A 3D geological model of the sublacustrine fan sand body is established by using a new four-step modeling method including the concept of facies-in-facies modeling: 1) A model of the distribution of the sand body is established through a deterministic modeling method; 2) Within the sand body, according to the division results of subfacies, the distribution model of sub-facies is established; 3) A fine lithofacies model is established by combining stochastic simulation with deterministic modeling and hierarchical modeling from rough to fine under the constraint of a seismic attribute; 4) Petrophysical models are established by lithofacies control, which provide a more accurate geological model for preparation of the gas field development plan. Based on this comprehensive study, it is concluded that there is a great possibility of reservoir disconnection between the inner fan and the middle fan, where the gas reservoir is located, and the inner fan in the updip position of the structure has the risk of no gas accumulation, and the drilling risk in the development stage is greater than the potential. Based on this geological understanding, the available reserves of the gas field are reduced by nearly 40%.