Dolomitization of carbonate rocks of Lower Ordovician in Eastern Ordos Basin can be classified into following four types: evaporative pumping-percolative reflux dolomitization, mixed water dolomitization (early and late diagenetic stages), deep-burial compaction-hypogenic thermal brine dolomitization and tectonic hydrothermal brine dolomitization. The former is genetically associated with evaporites and possesses plenty of Cl-, Na+, Sr2+ and low Mn2+, Fe2+ and high value of δ18O, δ13C, low porosity and permeability, so it is poor reservoir rock; the mixed-water dolostone is bedded-like and poor in Cl-,Na+, Sr2+ and δ18O-value is-8.59‰, PDB, δ13C-value is-1.067‰, PDB, it has wide dispersion and typical annulus fabric under luminescence, the average values of porosity and permeability are 2.64% and 7.5×10-3μm2 respectively, thus this kind of dolostone is the main reservoir rock type; the deepburial compaction-hypogenic thermal brine dolostone (nonflaser) that-filled in deep-resoluted porosity possesses low values of δ18O and δ13C, rich H2O and CO2 inclusions, lacks of hydrocarbon fluid inclusion, can not be reservoir rock; the tectonic-hydrothermal (flaser) ferredolomite developed along tectonic cracks, the distribution is controlled by the cracks and coinside with oil and gas deposits and contains the lowest δ18O-and δ13C-values, rich in Fe2+, Mn2+, abundant in H2O-and C2O-vapour-liquid inclusions and hydrocarbon vapour-liquid fluid inclusions. Therefore this kind of rock should be an important clue for hydrocarbon exploration.
