Investigation of pore-type heterogeneity and its inherent genetic mechanisms in deeply buried carbonate reservoirs based on some analytical methods of rock physics

Abstract

It is recognized that pore-type diversity, in addition to porosity and mineralogy, produces a complicated porosity–velocity relationship in carbonate reservoirs. However, pore-type parameters are challenging to extract by conventional petrophysical analysis. This problem is addressed in this paper through the derivation of pore-shape parameters—the relative pore-shape factor (ΔS) and resistivity index (R)—from conventional log data. The results indicate that these parameters can quantify the effect of pore-type changes on compressional wave velocity and facilitate understanding of the complexity of the porosity–velocity relationship and the heterogeneity of the permeability. A value of ΔS > 0.1 and R < 2 are characteristics of fabric-retentive grain dolostone with dominant moldic/intragranular pores, which produces a higher velocity than dolostone containing other pore types at a given porosity. Samples with ΔS < 0.03 and R > 2 represent fabric-obliterative crystalline dolostone with dominant intercrystalline pores and have the lowest velocity. A transition zone with 0.03 < ΔS < 0.1 and 0 < R < 6 exists between the upper two zones, which contains mixed pore types and exhibits medium velocity. Additionally, ineffective reservoirs with micropores can always be distinguished from effective reservoirs with ΔS < 0.03 and R < 2. Intercrystalline pores with a larger pore throat size yield much higher permeability than other pore types at a constant porosity, and three distinctive permeability trends that represent these pore types can also be discerned using pore-shape parameters and core data. Velocity, permeability contrast and pore-type diversity all result from different depositional settings and subsequent diagenetic alterations. Fabric-retentive grain dolostones developed primarily in the core of oolitic shoal facies, and early meteoric dissolution, cementation and subsequent dolomitization caused these dolostones to have high velocity and relatively low permeability. Fabric-obliterative crystalline dolostones evolved primarily along the edge of oolitic shoal with a weak early diagenesis, but dense burial cementation, dolomitization and recrystallization, generating intercrystalline pores with low velocity but relative high permeability. Velocity scatter starts at the deposition and ends at the measured porosity–velocity values through various diagenetic trajectories; thus, velocity contrast can be used to trace particular deposition and diagenetic processes in specific reservoirs. The results of this study can be broadly applied to similar seismic-based carbonate reservoir characterization and for further stratigraphic sequence and palaeogeomorphology research.