准噶尔盆地南缘西段白垩系深层储层特征及物性保存机制

Characteristics and physical property preservation mechanism of Cretaceous deep reservoir in western segment of southern margin of Junggar Basin

准噶尔盆地南缘西段下白垩统清水河组储层主体埋深在3600~5600 m之间,尽管埋藏深度大,但在其内部所获得的高产油气流揭示其物性条件应较为优越。但目前针对清水河组深层储层特征及物性保存机制尚缺乏系统研究,严重制约了后期油气勘探进程。据此,综合利用岩石铸体薄片、扫描电镜、物性、X-射线衍射分析及流体包裹体等多种分析测试手段,并结合钻测井等地层测试资料及区域地质资料,系统分析了南缘西段清水河组深层储层的岩石学、物性及成岩作用等特征,并在此基础上阐明其物性保存机制。研究结果表明:清水河组储层以区域连片的辫状河三角洲分流河道砂岩和砂砾岩为主,具有低成分成熟度、高凝灰岩岩屑含量及中等结构成熟度特征;储集空间以粒间孔为主,并表现为中孔—大孔及粗喉道特征,平均孔隙度为6.15%,而平均渗透率为4.25×10^(-3)μm^(2);储层成岩以压实、胶结和溶蚀作用为主,但成岩强度较弱,胶结物含量总体较低,并以方解石胶结为主,且储层整体仍处于早成岩B期—中成岩A期;富石英、长石而贫岩屑且分选较好的辫状河三角洲分流河道砂岩是形成深层优质储层的基础;成岩压实作用是物性损失的主要因素,但早期碳酸盐胶结及绿泥石包壳的抗压实效应及后期酸性溶蚀增孔延缓储层物性损失;早期长期浅埋—后期短期快速深埋的埋藏方式、超压以及不断降低的古地温梯度是在深埋条件下延缓清水河组储层成岩进程及拓宽溶蚀作用深度和时限,进而使得物性得以有效保存的关键。

The main burial depth of the Lower Cretaceous Qingshuihe Formation(K;q)in the western segment of the southern margin of the Junggar Basin is 3 600-5 600 m,although the burial depth is large,the high-yield oil and gas flow obtained inside it reveals that its physical conditions should be superior. However,there is still a lack of systematic research on deep reservoir characteristics and physical property preservation mechanism of Qingshuihe Formation,which seriously restricts the later oil and gas exploration process. Accordingly,this study comprehensively utilized various analytical and testing methods such as rock casting thin section,scanning electron microscope,physical property,X-ray diffraction analysis and fluid inclusion,and combined with formation test data such as drilling and logging and regional geological data,the petrology,physical properties and diagenesis of deep reservoirs of Qingshuihe Formation in the western segment of southern margin are systematically analyzed,and the preservation mechanism of physical properties is clarified on this basis. The results show that the Qingshuihe Formation reservoir is mainly composed of braided river delta distributary channel sandstone and glutenite with low compositional maturity,high tuff lithic content and medium structural maturity;the reservoir space is dominated by intergranular pores,and characterized by mesopore-macroporous and coarse throat.The average porosity is 6.15%,and the average permeability is 4.25×10^(-3)μm^(2);the diagenesis of the reservoir is mainly compaction,cementation and dissolution,but the diagenetic strength is weak,the cement content is generally low,and the calcite cementation is the main,and the reservoir is still in the early diagenetic stage Bmiddle diagenetic stage A;the braided river delta distributary channel sandstone with rich quartz,feldspar and poor cuttings and good sorting is the basis for the formation of deep high-quality reservoirs,and diagenetic compaction is the main factor of physical property loss,but the early carbonate cementation,the anti-compaction effect of chlorite crust and the late acid dissolution delay the physical property loss of the reservoir;the burial mode,overpressure and decreasing paleogeothermal gradient of early long-term shallow burial – late shortterm rapid deep burial are the keys to delay the diagenesis process of Qingshuihe Formation reservoir and broaden the depth and time of dissolution under deep burial conditions,thus effectively promoting the preservation of physical properties.