Quantitative diagenesis: Methods for studying the evolution of the physical properties of tight carbonate reservoir rocks

Conventionally, diagenesis has been studied by making qualitative morphological observations which have been organised into complex classification schemes. Petrophysics, with its many quantitative measurements, now gives us the ability to quantify the effects of the type, degree and timing of complex diagenetic process. The aim of this paper is to examine how different diagenetic processes affect the petrophysical properties of carbonate rocks and to develop quantitative methodologies to describe the results of diagenetic processes. A large number of petrophysical measurements have been made on a suite of 172 core plugs to provide a test data set. Diagenetic modification of the primary depositional fabric was observed in a wide range of measured petrophysical parameters, and that porosity and pore connectedness exert dominant control on all of the electrical and hydraulic rock parameters. This observation has been used to propose a new theoretical framework linking the effect of diagenetic process to petrophysical measureables. Cementation exponent was found to increase with permeability and pore size, especially in recrystallized rocks, and is explained by smaller porosity samples having a better connected pore network. Electrical connectedness was also found to correlate extremely well with hydraulic permeability, showing that these phenomena are linked closely in tight carbonate reservoir rocks. A method for calculating pre- and post-dolomitisation porosity and the degree of dolomitisation from the measured petrophysical and compositional data has also been developed and tested. All electrical and hydraulic properties are related to pore type, allowing cementation exponent to be obtained from optical microscopy/SEM studies or NMR measurements, providing a new approach to estimating cementation exponent in carbonate rocks. This paper also provides a powerful new approach allowing petrophysical changes associated with the type, degree and timing of different diagenetic processes to be tracked quantitatively using a method we have called ‘petrodiagenetic pathways’.