Post-Stack Seismic Characterization of Pore Structure Variations for Predicting Permeability Heterogeneity in Deeply-Buried Carbonate Reservoirs, Puguang Gas Field, China

Abstract

Alteration of depositional environment and diagenesis of carbonate rocks create various pore structures that cause strong heterogeneity in permeability. In this research, the petrophysical and elastic characteristics of diverse carbonate reservoir pore types in deeply-buried Puguang Gas Field, China are analyzed by integrating core, well log and seismic data. Core and well log measurements were first investigated using a frame flexibility factor (γ) derived from a rock physics model of poroelasticity to classify different pore types in Feixianguan Formation of Puguang Gas Field and build the relationship between porosity and permeability for different pore type groups. The frame flexibility factor (γ) has a good correlation with pore shape instead of porosity and can be used as the pore structure indicator to classify moldic (γ < 4.5), intercrystalline (5.5 < γ < 15), and microcrystalline pores (γ > 15) in the studied reservoir. When 4.5 < γ < 5.5, the reservoir rocks have mixed pore types, including both moldic and intercrystalline pores. Two distinct permeability trends were observed within two main pore types. At a similar porosity value, permeability is high in well-connected intercrystalline pores and low in isolated moldic pores. The effect of pore structure variations on acoustic velocity and impedance was then quantified using the pore structure indicator (γ). A more linear correlation of acoustic impedance (AI) and the product of porosity and γ was established. Results show that moldic pores have higher AI, whereas intercrystalline pores have lower AI at a given porosity. These relationships were used to interpret seismic AI inversion results and estimate the spatial variation of permeability using the post-stack seismic data. Moldic pores generated in platform margin ooid shoals and restricted platform after exposure and selectively dissolution as well as refluxion have lower permeability appearing as high AI; whereas dolostone with intercrystalline pores deposited in platform margin experienced reflux and burial dolomitization has relatively higher permeability, manifested in low AI values. The result shows great influence of varied carbonate pore structures on permeability heterogeneity and can be useful for further reservoir properties prediction.