Abstract:

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^-, N^a+, 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^-,N^a+, 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.