Diagenesis Of The Eighth Reservoir Unit Of Triassic Yanchang Formation In Pingzhuang-Shangliyuan Area, Ordos Basin

The eighth reservoir unit of Triassic Yanchang Formation (C8) is one of the most important oil-bearing seris of strate in Pingzhuang-Shangliyuan area, which lies in the southwest of the Ordos basin. On the basis of essential and subsidiary index beds for stragraphic correlation, the eighth reservoir unit of Triassic Yanchang Formation were divised into C81 and C82. C8 deposited in the deltaic front environments. Distributary channels are advantage belts for development of reservoir sand bodies. Based on the theory and method of sedimentology and the reservoir geology this paper deals with lithologic features, physical characteristics and pore structure, and diagenesis and its influence on reservoir through thin section analysis, SEM, X-ray and cathodeluminescence methods.The reservoir rocks mainly consit of lithic arkose and feldspathic litharenite, which are characterized with higher textural maturity and lower compositional maturity. The diagenesis reservoir sandstone mainly underwent comprise compaction, pressolution, cementation, metasomatism, dissolution and replacement. Compaction and cementation reduced porosity. Dissolution is beneficial to improvement of the reservior. According to comprehensive analysis, the division of diagenetic stage and the diagenetic evolutionary series are studied. Most reservoir rocks experienced stage A in the middle diagenesis, and a small part of reservoir rocks experienced stage B in the middle diagenesis. The reservoir rocks are characterized with low porosity and ultra-low permeability, because the physical property of reservoir is influenced by microfacies, diagenesis and tectonism. Reservoir rocks are classified into four types. Through analysis of diagenesis, distribution of reservoir sand, reservoir property, oil productive capacity of reservoir, prospective areas for high quality of reservoir are indicated.