四川盆地西北部上三叠统须家河组纵向上发育多套致密砂砾岩气藏,具备多层系立体勘探的广阔前景。目前针对该区域须家河组沉积古环境及物源体系的认识还不够深入,导致勘探上有点无面,严重制约了下一步攻关方向,需要加强该区域沉积古环境的研究。通过显微薄片鉴定、全岩矿物X衍射、稀土微量元素分析,结合井、震资料,对主要层段须三段—须二段含气砂体岩石类型、矿物含量、物源条件、古气候、古盐度及古氧化还原条件进行系统分析和对比研究。研究结果表明：①须二上亚段以长石岩屑砂岩为主,石英及长石矿物发育,石英含量超过50%;②受龙门山碳酸盐物源供给影响,须三下亚段普遍含钙质,岩屑成分以碳酸盐岩为主,方解石与白云石矿物发育,石英含量低于50%;③须三下亚段砂体稀土元素特征较须二上亚段更为复杂,指示多物源特征,两者沉积时该区整体处于潮湿性干热古气候,沉积环境偏淡水氧化环境,但须三下沉积期比须二上沉积期气候更为潮湿、古盐度略高、氧化性也更强。研究成果对川西北部须家河组重点砂岩层段微观特征、沉积古环境及地层划分新方案具有一定参考意义。
Multiple suites of tight glutenite gas reservoirs are extended vertically into the Upper Triassic Xujiahe Formation, Zitong-Qiulin area, indicating a broad prospect in multi-layered stereoscopic exploration. However, there was a lack of intensive awareness on both sedimentary paleoenvironment and provenance, resulting that it seems difficult to make the breakthrough in regional exploration to constrain the next research badly. It is necessary to investigate thoroughly this paleoenvironment. Thus, the third to second members of Xujiahe Formation (Xujiahe 3 Member to Xujiahe 2 Member), as the principal gas sands in Zitong-Qiulin area, were taken as examples. And based on drilling and seismic data, the rock types, mineral content, provenance, paleoclimate, paleosalinity and paleoredox conditions were systematically discussed through microscopic thin-section identification, X-ray diffraction (XRD) of whole rock minerals, and rare earth element (REE) analysis. Results show that (i) the upper Xujiahe 2 Member is dominated by feldspathic lithic sandstone well developed with quartz (˃50%) and feldspar; (ii) due to carbonate stemmed from Longmenshan Mountain, calcium is always found in the lower Xujiahe 3 Member which is dominated by carbonate debris developed with calcite and dolomite, and quartz content below 50%; and (iii) the lower member represents more complex features in REEs than those in the upper member, indicating an impression of multiple source. The study area as a whole is in a humid dry and hot paleoclimate when both members are deposited in the relatively freshwater-prone oxidizing setting. By contrast, with higher paleosalinity and stronger oxidation, the lower member is deposited in a more humid circumstance. The above results are referential for understanding the microscopic characteristics and sedimentary paleoenvironment together with updating the stratigraphic division on the principal sands of Xujiahe Formation, northwestern Sichuan Basin.
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