One-step versus two-step dolomite calcitization (dedolomitization): differences and inferences

Abstract

Polycrystalline calcite pseudomorphs after dolomite can exhibit mosaic or drusy fabric caused by replacement of the dolomite by calcite (one-step dedolomitization) or dolomite dissolution followed by calcite cementation (two-step dedolomitization), respectively. This study discusses the differences between the two types and interprets the characteristics of the fluid causing them. Conclusions drawn here are founded on a microscopic and petrophysical examination of an outcrop of Tuwaiq Mountain Formation, central Arabian Peninsula. Petrographic examination shows that dedolomite has distinct microfabrics in each part of the section. The lower part of the section is not affected by dolomite calcitization, the middle part is dominated by replacement fabrics, and the upper part is dominated by dissolution and/or cementation fabrics. Petrophysical examination shows a wider range of porosity and permeability values for the upper part of the section, relative to the middle and lower parts. One-step dedolomitization is characterized by preservation of the external morphology of the parent dolomite, confinement to the volume previously occupied by the dolomite, development of intracrystalline porosity and permeability that allows the fluid to maintain contact with the reaction front, and no significant effect on porosity of the original rock. Two-step dedolomitization is characterized by corrosion of the external morphology of the parent dolomite, association with other forms of calcite cementation, development without intracrystalline porosity and permeability, and positive or negative effects on the porosity. While one-step dedolomitization appears to occur within a stagnant zone of fluid circulation, the two-step dedolomitization is considered to reflect an active fluid-flow system.