Based on the analysis of source rocks

reservoirs and cap rocks in the Upper Paleozoic

Longdong eara

Ordos Basin

studies were performed on the controlling factors of natural gas accumulation.It's shown that the formation and distribution of gas reservoirs in the Upper Paleozoic are mainly controlled by the source rocks

reservoirs and cap rocks and the coupling relationship among them.Source rocks and cap rocks of the Shanxi Formation jointly control the formation and distribution of the gas reservior in the region.The gas play fairway of the first member of Shanxi Formation(P

2

s

1

) are distributed in area with high gas generation intensity and thick P

2

s

1

cap rock over 35 meters.In contrast

gas play fairway of the eighth member of Shihezi Formation(P

2

h

8

) occurs in area where the thickness of P

2

s

1

cap rock is less than 35 meters.Local enrichment of gas is dermermined by the reservior conditions.When the source rocks and cap rocks of the first member of the Shanxi Formation are similiar

"sweet spot" controls gas distribution

that is to say the better the quality of the reservoir is

the more enrichment the natural gas is.When both the source rocks and cap rocks of the first member of Shanxi Formation are favorable

gas reservoir can also be formed in the relatively poor formations.It is worth noting that best gas enrichment area is not the one where the gas source

seal and reservoir conditions are all favorable

rather the one where the gas source

seal and reservoir conditions match and compensate best.