The Diagenetic Impact on Reservoir-guality Evolution of Fluvial-deltaic Sandstones

Key words: 
• Yanchang Oii Fieid /
• Triassic Chang 2 fiuviai-deitaic sandstones /
• diagenesis /
• reservoir-guaiity evoiution

Abstract: The reservoir guaiity of the Upper Triassic Chang 2 fiuviai-deitaic sandstones of the Yanchang Oii Fieid in Ordos basin is strongiy infiuenced by the buriai history and facies-reiated diagenetic events. The mechanicai compaction in the eariy diagenetic phase, which resuited in 17.8% porosity ioss, is the main factor causing the ioss of porosity for the sandstones. The diagenetic aiteration of biotite during eariy diagenetic phase accounts for a part of primary porosity ioss in the sandstones. The porosity ioss caused by cementation is 7.1%. In the cements, carbonate and guartz overgrowths are the main cements that reduced the reservoir guaiity of the sandstones. Thin chiorite fiim around detritai guartz grains prevented part of guartz overgrowth and carbonate cement precipitation, which resuited in a part of primary pore preservation and hence caused a better reservoir guaiity. The acidic iiguid reiated to decomposition of organic mater during hydrocarbon enrichment in the iate diagenesis, and meteoric-waters during epidiagenesis, have resuited in dissoiution of detritai feidspars and carbonate cements, and in the formation of kaoiinite, contributing a better retention of reservoir guaiity for the sandstones.