Research on Diagenesis System of Yanchang Formation Reservoirs, Southeast Ordos Basin

Key words: 
• diagenesis /
• chlorite rims /
• calcite /
• Ordos basin /
• autogenetic quartz /
• tight sandstones

Abstract: Yanchang Formation sited in the southerneastern Ordos Basin has been proven a prolific reservoir unit for hydrocarbon resources. Featured by relatively low porosity and effective permeability, the quality of the reservoir has a significant impact on the migration and accumulation of oil and gas. By analyzing the petrophysic data from thinsection samples, BSE, Energy Spectrum Analysis and core plugs, we reached the following conclusions: 1. The rimming of chlorite has an important control on the reservoir quality  such diagenetic process is characterized as a product of delta front depositional environment where is mineralogically rich in black mica and feldspar. 2. It effectively reduced the compaction effect from the overburden through burial history, therefore contributed to the preservation of primary porosity, simultaneously, ceased the diagenetic overgrowth of quartz grains. 3. The associate presence of calcite reorientated the grains into porphyroblastic structure, but arbitrarily distributed due to the small quantity.4.Authquartz has a prominent vertical change, which is often observed as small crystal forming with chlorite rimming in a close space. However, in semiclosed spaces, quartz is prone to migrate upwards, stop and develop as overgrowth where the intergranular space is not fully taken by chlorite. Through the study, we sorted out the controls and developing mechanisms of qualified reservoirs in Yanchang Formation, and therefore guide the future exploration.