Analysis of the Diagenetic Changes in Complex Carbonates Using Rock Physics Algorithms

Abstract Carbonate reservoirs are prone to significant diagenetic changes. Diagenetic processes can interact with each other and either increase or reduce porosity and permeability. The shear modulus of the rock is not affected by the saturating fluid and hence provides an excellent tool for analyzing diagenetic changes such as dolomitization, cementation and karstification. The proposed workflow combines inclusion theories with the multimineral analysis to assess mineralogical complexity. The accuracy of this analysis is crucial to characterize the effects of various diagenetic processes on elastic moduli of the effective media. The cementation of the grains is evaluated using contact theories based on the constant cement model. The inclusion theory allows to describe isolated vuggy pores and fractures as the spherical and elongated ellipsoid inclusions, respectively. The porosity of fractures is assessed using the total porosity versus shear modulus cross-plot and is identified as the points falling below certain threshold. An analysis was carried out on the carbonate reservoir of complex mineralogy, which was affected by diagenetic changes. The workflow produced variable cementation curve over the reservoir interval. The analysis identifies intervals with the presence of dissolution pores and fractured intervals. In many cases, the vuggy pores are not well connected, and the presence of such pores leads to a significant permeability reduction. On the other hand, the presence of open or dissolution-enhanced fractures leads to permeability increase. The mineralogy should be taken into account in the diagenesis analysis because of the mineralogy-related variations in the matrix shear modulus. Thus, ignoring the minerology can lead to erroneous results. The combination of the contact and inclusion models with the multimineral analysis allows to provide qualitative and quantitative estimation of various diagenetic processes, such as dolomitization, cementation and karstification. The combination of different models, based on the inclusions and contact theories, provides a novel approach to the analysis of the aforementioned processes.