Differential characteristics of cap rocks of volcanic gas reservoirs in the Xujiaweizi fault-depression,Songliao Basin

Through analyzing the lithology,physical properties and spatial locations of the rocks overlying volcanic gas reservoirs,this paper investigated the types of cap rock and identification marks of each type in the Yingchen Formation of Xujiaweizi fault-Depression,and then discussed the spatial distribution,sealing ability and control on the formation of gas reservoirs of each cap rock type. The results showed that there were three types cap rocks of volcanic gas reservoirs in the Yingchen Formation of Xujiaweizi fault-Depression: typeⅠ is mudstone cap rock overlying just on the top of volcanic gas reservoirs; typeⅡ is compact volcanic cap rock located at the top of volcanic reservoirs; and type Ⅲ is compact volcanic cap rock interbedded with loose volcanic rocks in the upper section of volcanic reservoirs. Type Ⅱ and type Ⅲ cap rocks are mainly consisted of volcanic lava and pyroclastic rock. Both type Ⅱ and type Ⅲ cap rocks show high gamma-ray deflections and high density. The main difference between these two types of cap rocks is spatial distribution. The type Ⅱcap rock is located at the top of volcanic reservoirs,while the type Ⅲ cap rock is located at the upper part of volcanic reservoirs and usually covered by volcanic rocks with low density. The type Ⅱ cap rock had the widest distribution area and best spatial continuity than the others. The displacement pressures of type Ⅱ cap rock was up to 8. 8 MPa and single welldaily gas production in reservoirs with this type of cap rocks is the highest( up to 36. 6 × 104m3) in comparison with the other two types of cap rocks with the same thickness. The type Ⅰ cap rock has a smaller distribution area than type Ⅱand it is mainly distributed in the edge regions of fault-depression. Its displacement pressures is up to 6. 9 MPa and the highest single well daily gas production in areas with this type of cap rocks could reach 23. 2 × 104m3. The spatial continuity of type Ⅲ cap rock was poor and its displacement pressures was usually under 4. 75 MPa,resulting in the highest daily gas production of one weⅡ only of 12. 8 × 104m3 in the distribution area of type Ⅲ cap rock. Comprehensive analysis showed that type Ⅱ cap rock had the best sealing ability among all cap rock types,sealing ability of type Ⅲ cap rock was the worst,and that of type Ⅱ cap rock was moderate. This difference of sealing ability among three types of cap rocks controlled the formation and enrichment of volcanic gas reservoirs.