Grid density overlapping hierarchical algorithm for clustering of carbonate reservoir rock types: A case from Mishrif Formation of West Qurna-1 oilfield, Iraq

Abstract

The primary goal of this study is to solve a major challenge of reservoir rock typing in quantitatively understanding the relationship between geological and petrophysical properties of carbonate reservoir. This paper first presents an improved overlapping index based on the grid density to define the overlapping relationship between different clusters and a grid density overlapping hierarchical algorithm (GDOH) for clustering of carbonate reservoir rock types. GDOH clustering algorithm is a hybrid type which integrates the gird, density, overlapping index and the hierarchical clustering method using the number of grid divisions and the threshold value of the overlapping index as the key parameters. Taking the Mishrif Formation of West Qurna Oilfield-1 as a case, the GDOH clustering algorithm is employed for clustering of carbonate reservoir rock types. As a result, ten geological rock types are identified according to the lithological type and diagenetic type using the thin section data. We finally got four reservoir rock types using the GDOH clustering algorithm with the overlapping index of core porosity and the permeability data as the metric. These reservoir rock types are different in reservoir qualities and porosity-permeability relationships. Each reservoir rock type also shows a similar set of capillary pressure curves and a typical and unique set of pore throat size distribution. The clustering result gives us some clues to understand the control of depositional and diagenesis features on the variation of reservoir rock types in reservoir quality and porosity-permeability relationship. Compared with the model-based petrophysical rock-typing techniques such as Winland r35, FZI and FZI-Star (FZI*), GDOH algorithm shows some apparent advantages in the evaluation of reservoir quality and prediction of permeability in three dimensions based on geologic principles in addition to understanding the relationship between reservoir rock type and petrophysical static and dynamic rock type.