High H

2

S concentrations found in gas reservoirs within the Upper Permian Changxing Fm(P

3

ch)and Lower Triassic Feixianguan Fm(T

1

f)are generally thought to be formed via a process known as thermochemical sulfate reduction(TSR).However

unlike the T

1

f Fm

rare gypsum/anhydrite beds or anhydrite nodular were found in the P

3

ch Fm.This cast doubts over the accepted model for the souring process of the P

3

ch reservoirs.This study provides an analysis of palaegeopressure and carrier system within this system using carbonate-associated sulfate(CAS)

isotopes

trace and rare earth element data.Results shows that

the H

2

S of high concentrations within the P

3

ch reservoirs were not derived from downward migration of sour gas from the T

1

f Fm

but were generated in situ through TSR;reactive sulfates were mainly derived from cross-formational brines migration from the T

1

f Fm

but with insignificant contributions from the Late Permian P

3

ch seawater and sulfate-rich brines within the Lower Triassic Jialingjiang Fm and Middle Carboniferous Honglong Fm formations

and the down-migration of T

1

f brines may have resulted in the P

3

ch TSR calcites having positive Eu anomaly

high Sr contents(up to 7 767 ppm)and Ba contents(up to 1 279 ppm)close to the T

1

f TSR calcite

and

87

Sr/

86

Sr ratios(0.707 24~0.707 55)more radiogenic than those of the late Permian seawater;the cross-formational brine flow mainly occurred during sedimentary stage or the early stages of diagenesis

and seepage reflux during contemporaneous-penecontemporaneous stage and differential compaction flux during early-mid burial stage might serve as the main migration mechanisms;④CAS released during dolomization was another possible sulfate source

which brought the δ

34

S values of H

2

S and bitumen a little lower than those of the T

1

f Fm.