Causes of reservoir diagenesis and pore structure differences of the Yanchang Formation in the WL area of the Ordos Basin

Abstract

Objective To clarify the causes of the vertical difference of pore structure in low permeability-tight sandstone reservoirs. Methods Taking the WL area of the Ordos Basin as an example, using physical property tests, casting thin sections, scanning electron microscopy and high-pressure mercury injection, through the division of diagenetic facies and the analysis of vertical distribution differences of different diagenetic facies the characteristics, differences and causes of pore structure in different layers of the Yanchang Formation were discussed. Results The study showed that there were five types of pore structures in the main strata of the Yanchang Formation in the study area, and there were great differences in the pore structures between the same strata and different strata. The Chang 2 and Chang 3 oil layers of the upper combination mainly developed Ⅲa- and Ⅲb-type pore structures, the Chang 4+5 and Chang 6 oil layers of the middle combination mainly developed Ⅳa- and Ⅳb-type pore structures, andthe Chang 7, Chang 8 and Chang 9 oil layers of the lower combination mainly developed Ⅳb- and Ⅴ-type pore structures. With the increasing of burial depth, the pore structure and the porosity and permeability conditions in the reservoir gradually deteriorated overall. There were four types of diagenetic facies types in the reservoir of the Yanchang Formation in the study area. The vertical difference of pore structure was mainly controlled by the difference distribution of diagenetic facies types. The dissolution dominated by atmospheric water leaching in the upper combination was strong. The diagenetic facies types dominated by unstable component dissolution facies and chlorite cementation were developed, and the pore structure formed was dominated by relatively good types. The middle combination had strong cementation, and the diagenetic facies types dominated by chlorite cementation facies and carbonate cementation facies were developed, and the pore structure formed was mainly medium type.The lower combination had strong compaction and cementation, and the diagenetic facies types dominated by carbonate cementation facies and soft-rich compaction filling facies were developed, and the pore structure formed by them was mainly relatively poor. Conclusion The research results can provide favorable guidance for oil and gas exploration and development of the Triassic Yanchang Formation in the Ordos Basin.

Keywords

• pore structure
• differential genesis
• diagenesis
• yanchang formation
• ordos basin