Influence of Oil Emplacement on Diagenetic Sequence of the Clastic Reservoir Rock and It's Relationship to the Porosity Evolution——Taking the Carboniferous quartz sandstone in southeast Tarim depression as an example

Key words: 
• oil emplacement /
• diagenetic sequence /
• porosity evolution /
• quartz sandstone

Abstract: Carboni ferous quartz sandstone distributed in the Southwest Depression of the Tarim basin is a set of widespread clastic deposit. During diagenesis, two important secondary structure units were formed in Southwest depression, i. e., Bachu uplift and Maigaiti Slope, which caused the difference in diagenetic sequence of the Carboniferous quartz sandstone in two areas. Under the stable and continuous burial dia-genetic environment, the sandstone reservior in Bachu uplift has a complete diagenetic sequence, and di-agenetic stage could reach B period of late diagenesis and even C period. Thus, the sandstone porosity there was damaged seriously and the average porosity is less than 5%. Conversely, the diagenetic stage of the sandstone in Maigaiti Slope is mainly distributed between A period and B period of the late diagen-esis due to the relatively lower burial depth compared with that in Bachu uplift. Especially, the influence caused by oil emplacement on the sandstone reservior changed the traditional diagenetic environment and made the diagenetic reaction take place in water-oil-rock three phases media which repressed the cemen-tation of some authigenic clay minerals, resulting in the discontinuity of the diagenetic sequence, but, at the same time, many effective porosity was preserved causing the average porosity exceed 7%± in Maigaiei Slope, which is clearly larger than that of the uplift area. For this reason, we believe that the Maigaiti Slope is the most favorable area for the oil exploration work in recent years.