准噶尔盆地南缘中段超深层相对优质砂岩储层发育特征及主控因素

The characteristics and main controlling factors of ultra-deep sandstone reservoir in the middle part of the southern margin of Junggar Basin

准噶尔盆地南缘中段HT1井在白垩系清水河组底部砂岩储层中获得了高产气流,证实该套储层是一套相对优质储层。利用薄片观察、孔渗分析、压汞分析、场发射扫描电镜观察、激光共聚焦显微成像等多种分析测试手段,对清水河组砂岩样品开展了岩石矿物、物性、孔隙结构等分析,并结合埋藏史开展成岩演化研究。结果表明:(1)南缘中段清水河组储层岩石类型主要为长石岩屑砂岩,结构成熟度较高。(2)储层中胶结物含量中等,残余粒间孔和裂缝发育,现今处于中成岩B阶段。(3)相对优质储层的形成受控于四大因素:远源辫状河三角洲沉积体系使砂岩的结构成熟度较高;较低的地温梯度减弱了砂岩热压实效应;深层流体膨胀作用导致的异常高压有利于粒间孔隙的保存;裂缝提高了砂岩储层的渗流能力。

In the southern margin of Junggar Basin, multiple rows of thrust faults and anticlines are widely developed under the tectonic stress during the Himalayan period, and three reservoir-seal assemblages including the upper, middle and lower are developed vertically, among which the lower assemblage with multiple sets of Jurassic-Cretaceous reservoir is the most important successor for oil and gas exploration. The Lower Cretaceous Qingshuihe Formation has two provenance systems from south and north respectively, and the braid-delta sandbodies in the north provenance system is widely distributed. High-production gas flow was obtained in Qingshuihe Formation of Well HT1, which not only indicates the great exploration potential, but also confirms a set of high-quality ultra-deep reservoir(the depth is more than 7 000m). Based on the regional geological data, this paper analyzes the rock minerals, physical property and pore structure of the sandstone in Qingshuihe Formation in detail by means of thin section observation, pore permeability analysis, mercury injection analysis, field emission scanning electron microscope observation, laser confocal microscopy,and other analysis and testing methods. The diagenetic evolution sequence of the reservoir is established by the former result combined with burial history. The studies show that the reservoir rock type of Qingshuihe Formation in the middle part of the southern margin is mainly feldspathic litharenite sandstone with high texture maturity, which has moderate cement content. The reservoir space is dominated by primary pores and fractures. The reservoir is in middle diagenetic stage B. Based on the comprehensive analysis, the formation of relatively high-quality reservoir of Qingshuihe Formation is controlled by four factors:(1)sandstone with high texture maturity from distal provenance is the foundation to form high-quality reservoir;(2)thermal compaction effect in sandstone is relieved by low geothermal gradient;(3)overpressure controlled by fluid expansion reduces the loss of pore-space during compaction;(4)fractures improve the seepage capability of the sandstone reservoir. This research provides an important geological example for the exploration of ultradeep clastic reservoirs in continental basins.