Episodic dolomitization of Paleozoic limestones in the Kinta Valley, Malaysia: Implications on porosity evolution and reservoir properties

Abstract

Carbonates undergo a variety of physical, mineralogical, chemical changes during depositional and post-depositional stages that are of interest to exploration geologists and sedimentologists. World over, about 60% of hydrocarbon reserves occur in carbonates with the dominant proportion associated with dolomitic reservoirs, in particular fault-associated dolomites which appear to be proven hydrocarbon reservoirs. Dolomitization as the critical diagenetic process thus plays an important role in determining reservoir characteristics and in designing exploration strategies. The buried Paleozoic dolomites exhibit relatively higher porosity than their counterparts, Paleozoic limestones. Fault-associated dolomites are usually targets of hydrocarbon exploration due to their good reservoir quality. High heterogeneity in carbonate reservoirs due to varied porosity types and distribution makes reservoir characterization a daunting task. However, the key elements that control porosity development and evolution in these dolomites remain debatable. Studying the influence of multiphase dolomitization, associated diagenetic events, hydrothermal alteration and their causative thermo-tectonic events on porosity evolution and reservoir quality is essential to understanding the critical processes and controlling factors that result in diverse impacts on reservoir quality. In this paper, an attempt is made to characterize dolomitization in homogeneous carbonate mud and its relationship with porosity evolution and reservoir property. In the field, six facies types excluding host limestone are recognized, which are in turn characterized into seven petrographically recognizable phases, namely the host limestone, early replacement dolomite, sucrosic dolomite, metamorphosed dolomite, late replacement dolomite, brecciated limestone and dolomite and late stage calcite. Mineralogical analyses reveal the dominance of stoichiometric dolomites, followed by calcareous dolomites and magnesium calcites. Among all the field-based facies and petrographic phases, the sucrosic dolomites show appreciable porosity within a range from 4% to 8%, whereas the other phases show either nil or insignificant porosity under megascopic and petrographic observations. Our study confirms that in the process of dolomitization that substantially modifies porosity, dolomitic crystal morphologies play a significant role in enhancing or reducing the porosity.