Microscopic characteristics and sedimentary paleoenvironment of tight sandstone:Examples from Xujiahe 3 and Xujiahe 2 members,Zitong-Qiulin area,northwestern Sichuan Basin
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 sedi-mentary 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.
Northwestern Sichuan BasinUpper TriassicTrace elementSedimentary paleoenvironmentProvenance system
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