Rock-physics-based carbonate pore type characterization and reservoir permeability heterogeneity evaluation, Upper San Andres reservoir, Permian Basin, west Texas

摘要

In addition to mineral composition and pore fluid, pore type variations play an important role in affecting the complexity of velocity-porosity relationship and permeability heterogeneity of carbonate reservoirs. Without consideration of pore type diversity, most rock physics models applicable to clastic rocks for explaining the rock acoustic properties and reservoir parameters relationship may not work well for carbonate reservoirs. A frame flexibility factor γ defined in a new carbonate rock physics model can quantify the effect of pore structure changes on seismic wave velocity and permeability heterogeneity in carbonate reservoirs. Our study of an Upper San Andres carbonate reservoir, Permian Basin, shows that for core samples of given porosity, the lower the frame flexibility factor γ, the higher the sonic wave velocity. For the studied reservoir, samples with frame flexibility factor γ < 3.85 represent either visible vuggy pore space in a dolopackstone or intercrystalline pore space in dolowackstone. On the other hand, samples with frame flexibility factor γ > 3.85 indicate either dominant interparticle pore space in dolopackstone or microcrack pore space in dolowackstone or dolomudstone. Using the frame flexibility factor γ different porosity-impedance and porosity-permeability trends can be classified with clear geologic interpretation such as pore type and rock texture variations to improve porosity and permeability prediction accuracy. New porosity-permeability relations with γ classification help delineate permeability heterogeneity in the Upper San Andres reservoir, and could be useful for other similar carbonate reservoir studies. In addition, results from analysis of amplitude variation with offset (AVO) and impedance modeling indicate that by combining rock physics model and pre-stack seismic inversion, simultaneous estimation of porosity and frame flexibility factor γ is quite feasible because of the strong influence of carbonate pore types on AVO especially when offset is large.