四川盆地上三叠统香溪组二段和四段砂岩沉积模式

四川盆地上三叠统香溪组二、四段砂岩是盆地内主要天然气储层之一,近年来勘探形势喜人。香二和香四段砂岩分布面积约为10×10⁴km²,厚度稳定,矿物和结构成熟度高,较厚砂岩层大多呈逆粒序构造,常发育冲洗层理和互成180°交角的楔形或板状斜层理,风暴岩频繁发育,伽马曲线成箱形,夹5%～10%的炭质泥岩或含薄煤层炭质泥岩。结合全川的资料,对川中4×10⁴km²内数十条地面和岩心剖面研究结果表明,四川盆地香二和香四时周边各山系为构造活动期,经冲积扇、河流搬运入湖的碎屑物质经波浪和湖流反复簸洗、改造和再分配,形成了沿岸和近岸的滩坝相砂体,滩坝相砂体后发育为滨岸含煤沼泽。随湖盆脉动式的升降、岸线的迁移,滩坝砂体也将垂直或沿岸线迁移,最终形成遍布盆地内的砂岩体。提出了香二、香四段滩坝砂体及所夹炭质泥岩的发育、演化模式。

Sedimentary model of sandstone in second and fourth members of Xiangxi Formation in the Upper Triassic of Sichuan Basin

The sandstones in second and fourth members of Xiangxi Formation of the Upper Triassic are the main natural gas reservoirs in Sichuan Basin. There has got good exploration and exploitation results recently. The sandstone of second and fourth members of Xiangxi Formation with stable thickness and high mineral maturity are distributed in the area of about100thousands of square kilometers in Sichuan Basin. The thicker bed always shows reverses graded bedding and develops tempestite，washing stratification and wedge-shaped cross-bedding or planar stratification with 180° angles with each other. GR curves usually show box girder. The whole bed has alternating layers with five percent to ten percent of carbonaceous mudstone and thin coal layer. In the depositional period of second and fourth members of Xiangxi Formation，the mountains all around Sichuan Basin were in the tectonic movement.The sediments brought into the lake by alluvial fans and rivers were washed and removed by wave and lake current and then formed bank-bar sandy body along or near the lake shore. With the migration of lake-coast line，the bank-bar sandy body was migrated perpendicularly to or along the lake-coast line and finally formed sandstone distributed in all over the inner basin. A sedimentary model in the second and forth members of Xiangxi Formation of the Upper Triassic of Sichuan Basin is presented.