鄂尔多斯盆地上古生界致密砂岩气成藏特征

Accumulation Character of the Upper Paleozoic Tight Sandstone Gas in Ordos Basin

鄂尔多斯盆地上古生界气藏为致密砂岩气藏。通过对盆地上古生界烃源岩、储集层、储层致密和成藏时间以及天然气运聚的分析,详细研究盆地上古生界致密砂岩气藏的成藏特征。本溪组-山西组煤层是主要的烃源岩,烃源岩有机质丰度高,干烙根为Ⅲ型干烙根,有机质成熟度高,以高-过成熟为主,为盆地天然气成藏提供充足的物质基础。储层岩性以岩屑石英砂岩、石英砂岩和岩屑砂岩为主,孔隙度主要分布在4%~8%,渗透率在0.1×10^(-3)~0.5×10^(-3)μm^2,为低孔低渗致密砂岩储层。储层具有"小孔隙、细喉道、微裂缝不发育、孔喉连通性差"的特点。致密气藏是先致密后成藏,储层在早-中侏罗世储层演化为致密砂岩储层;而后在晚侏罗世-早白垩世天然气大量充注形成气藏。天然气在气体膨胀力的作用下沿着微裂缝近距离运移聚集形成大面积连片分布的致密砂岩气藏。

The Upper Paleozoic gas reservoir in Ordos Basin is tight sandstone gas reservoir. Through the analysis of the source rock,the reservoir,the timing of gas accumulation,reservoir tightening and the gas migration,the accumulation conditions of the Upper Paleozoic tight sandstone gas in the basin is studied. The coal bed in the Benxi Formation-Shanxi Formation is the main source rock. The kerogen is the type Ⅲ kerogen,the organic matter abundance and maturity are high,which supplies enough material. The reservoir lithology is mainly lithic quartz sandstone,the quartz sandstones and lithic sandstone. The porosity is mainly 4% 8%,the permeability is mainly0. 1 × 10-3 0. 5 × 10-3μm2. The reservoir is low porosity and low permeability,with small pore,fine throat,little microfracture,poor connectivity of the pore and throat. The gas reservoir is accumulating after the tighting,the reservoir becomes the tight sandstone reservoir in early-middle Jurassic,the gas reservoir accumulates in late Jurassic-early Cretaceous. Under the force of gas expansion,the gas migrates in short distant along the microfracture,then forms the widespread tight sandstone gas reservoir.