The Ordovician is composed of silty mudstone and limestone

and the Silurian is a typical flysch formation composed of four depositional cycles of glutinite in the northeastern Tarim basin. The reservoirs occur at the bottom of each cycle. All of the 4 cycles have been inverted. Diagenesis is obvious at a depth of 1 500 m where sparry calcite (Ⅰ) has been precipitated from water rich in atmospheric CO

2

with δ

13

C in the range of -4‰ and -6‰

and 818O in the range of -5‰ and -6‰. At a burial depth of 2 400-2 600 m

δ

13

C and δ

18

O in the carbonate (Ⅱ) formed in the sulfate reduction zone are in the range -7‰ to -10‰ and -8‰ to -10‰

respectively

where the dissolution of feldspar constitutes the 1st secondary pore zone. Dolomites (Ⅲ) with wavy extinction have been formed at a burial depth of 3 800-4 000 m

along with the decarboxylation in the methane generating zone

the δ

13

C is in the range of -11‰ and -16‰

while in the carbonate directly formed from CO

2

the δ

13

C is ±20‰

and the δ

18

O both in dolomite and carbonate range from -12‰ to -13‰

temperatures measured from secondary SiO

2

inclusions are in the range of 110℃ and 120℃. This interval constitutes the 2nd secondary pore zone. The depth interval of 4 500-4 700 m is a wet gas generating zone

the δ

13

C and δ

18

O in carbonate (Ⅳ) formed in this interval are in the range of -27‰ to -31‰ and -13‰ to -14‰

respectively

and the temperature measured from inclusion thermometry is 130 -140℃; and the massive dissolution of ferroan calcite has resulted in the formation of the 3rd secondary pore zone. Large amount of dry gas (Ⅴ) were generated at a burial depth of over 5 000 m

where the 8 C ranges from -18‰ to -20‰ and δ

18

O ranges from -13‰% to -14‰

being accordant with the inclusion's homogenization temperature of 150-155℃. This interval constitutes the 4 th secondary pore zone with porosity in the range of 15% to 20%. The folded strata might have been inverted during Hercynian movement

uplifted and eroded about 5 000 m and associated with'retrodiagenesis'.