徐家围子断陷火山岩天然气盖层差异特征

通过分析松辽盆地徐家围子断陷营城组火山岩气藏上覆岩层的岩性、物性、空间位置及厚度等差异分布规律，确定了火山岩天然气盖层的类型及划分标准，探讨了各类型盖层的空间分布规律、天然气封闭性及其对气藏发育控制作用的差异性。分析结果表明，徐家围子断陷营城组火山岩天然气藏盖层具有3种类型：Ⅰ型为顶部泥岩盖层，Ⅱ型为顶部致密火山岩盖层，Ⅲ型为上部致密火山岩夹层盖层。其中，Ⅱ型和Ⅲ型盖层岩性均含有火山熔岩及火山碎屑岩两类岩性，且伽马及密度测井曲线都显示高值特征；二者测井响应主要区别在于空间分布位置上的差异，Ⅱ型盖层位于火山岩顶部，而Ⅲ型盖层则位于火山岩段上部，其上还覆盖低密度火山岩段。Ⅱ型盖层覆盖面积大，空间连续性好，排替压力最大可达8.8MPa，相同盖层厚度条件下所对应单井日产气量最高，可达36.6×10⁴ m³；Ⅰ型盖层覆盖面积次之，且主要发育在断陷边部地区，排替压力最大为6.9 MPa，覆盖范围内单井日产气量最高为23.2×10⁴ m³；而Ⅲ型盖层分布区域内致密封盖层之间空间连续性较差，盖层排替压力大部分在4.75 MPa以内，单井日产气量最高仅为12.8×10⁴ m³。Ⅱ型盖层对天然气具有更强的封盖作用，Ⅰ型盖层次之，Ⅲ型盖层封盖能力最差。各类型盖层封盖能力的差异性控制着火山岩气藏的形成与富集。

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:typeⅠis mudstone cap rock overlying just on the top of volcanic gas reservoirs;type Ⅱ 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 Ⅱ cap 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 well daily gas production in reservoirs with this type of cap rocks is the highest (up to 36.6×10⁴ m³) 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 Ⅱ and 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 23.2×10⁴ m³.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 we Ⅱ only of 12.8×10⁴ m³ 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.