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+

Sr

2+

and low Mn

2+

Fe

2+

and high value of δ

18

O

δ

13

C

low porosity and permeability

so it is poor reservoir rock; the mixed-water dolostone is bedded-like and poor in Cl

-

N

a+

Sr

2+

and δ

18

O-value is-8.59‰

PDB

δ

13

C-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

-3

μm

2

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 δ

18

O and δ

13

C

rich H

2

O and CO

2

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 δ

18

O-and δ

13

C-values

rich in Fe

2+

Mn

2+

abundant in H

2

O-and C

2

O-vapour-liquid inclusions and hydrocarbon vapour-liquid fluid inclusions. Therefore this kind of rock should be an important clue for hydrocarbon exploration.