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

Abstract Dedolomites, the replacement of dolomite by calcite, are widely distributed in Ordovician Majiagou Formation southeastern Ordos Basin, China, which critically affects reservoir quality throughout region. Two types dedolomites were recognized upper 100 m Formation, namely brecciated dedolomite and crystalline dedolomite. The petrographic geochemical data reveal that investigated often associated with clay minerals dissolution-collapse breccia, indicating their close relationship erosional unconformities. relative negative δ 18 O 13 C values high Fe, Mn contents respect to corresponding interpreted as being result from meteoric phreatic water involvement. dedolomitization process related subaerial exposure is conventionally be formed dissolution calcite precipitation. rhombic shape crystals was an Mg 2+ -loss under evaporite solution conditions, retain dolomite. dedolomites, however, dissolution/precipitation recrystallization form medium coarse mosaic very low intercrystalline porosity. resulting fabrics mainly controlled paleo-topography. precursor primarily recrystallized mudstone, more soluble favorable for mostly occurs paleotopographic locations, where karst-saturated CaCO 3 concentrated, occluding remaining porosity when excess calcium supplied. It’s, therefore, recommended implement drilling tectonic highland avoid locations.