Petrography and origin of dedolomites of the Ordovician Majiagou formation in the southeastern Ordos Basin, China: implications for reservoir quality

Lihong Liu,Zhili Du,Keke Huang,Chunlian Wang

doi:10.1007/s13146-021-00741-6

Abstract

Dedolomites, the replacement of dolomite by calcite, are widely distributed in the Ordovician Majiagou Formation of the southeastern Ordos Basin, China, which critically affects reservoir quality throughout the region. Two types of dedolomites were recognized in the upper 100 m of the Majiagou Formation, namely brecciated dedolomite and crystalline dedolomite. The petrographic and geochemical data reveal that the investigated dedolomites are often associated with clay minerals and dissolution-collapse breccia, indicating their close relationship with erosional unconformities. The relative negative δ18O and δ13C values and relative high Fe, Mn contents of dedolomite with respect to corresponding dolomite are interpreted as being result from meteoric phreatic water involvement. The dedolomitization process related to subaerial exposure is conventionally interpreted to be formed by dolomite dissolution and calcite precipitation. The rhombic shape crystals in the brecciated dedolomite was formed by an Mg2+-loss process under evaporite solution conditions, which retain the rhombic shape of the dolomite. The crystalline dedolomites, however, were formed by dissolution/precipitation and recrystallization process to form medium to coarse calcite mosaic with very low intercrystalline porosity. The resulting fabrics are mainly controlled by the paleo-topography. The precursor dolomite of the crystalline dedolomite is primarily recrystallized mudstone, which is more soluble and favorable for calcite precipitation. The crystalline dedolomite mostly occurs in the low paleotopographic locations, where the karst-saturated water with respect to CaCO3 is more concentrated, occluding the remaining porosity when excess calcium is supplied. It’s, therefore, recommended to implement drilling in tectonic highland and avoid low paleotopographic locations.

Highlights

Dedolomitization is a process by which dolomite is replaced by calcite in the presence of a calcium-rich solution, which is often associated with evaporite minerals dissolution
With respect to dedolomitization effects on the reservoir quality, a simulation performed by Escorcia et al (2013) proposed that a porosity reduction is owing to calcitization
Dolomite dissolution and calcite precipitation occur in different solutions at different time, owing to excess of calcium supplied to the system that leads to calcite cementation

Summary

Introduction

Dedolomitization is a process by which dolomite is replaced by calcite in the presence of a calcium-rich solution, which is often associated with evaporite minerals dissolution. With respect to dedolomitization effects on the reservoir quality, a simulation performed by Escorcia et al (2013) proposed that a porosity reduction is owing to calcitization. Only when the extent of dolomite and evaporite dissolution exceeds that of calcite precipitated, the overall porosity can be increased (Canaveras et al 1996; Ayora et al 1998; Nader et al 2008). Dolomite dissolution and calcite precipitation occur in different solutions at different time, owing to excess of calcium supplied to the system that leads to calcite cementation. The question of whether the reservoir quality is enhanced by dedolomitization or not remains controversial

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