Analysis on the Types and the Filling Process of the Anhydrite-model Pore of Ma5₄¹ in Northeast of the Palaeohigh, Ordos Basin

Key words: 
• pore types /
• gypsum-model pore /
• calcite /
• filling model /
• Majiagou Formation

Abstract: Using the data of cores and thin sections, based on the classification of Ma5₄¹ reservoir pore types in the study area and through the pore description of thin sections, the image analysis of casting thin sections and other methods, this paper gives a detailed classification of gypsum-model pore filling types and makes an elaboration of filling process. The study shows that Ma5₄¹ reservoir developed the following 4 pore types as dissolved pore (anhydrite-model pore and other solution pores), intercrystalline pore, intercrystalline solution pores and microcrack in the study area. According to the analysis to the carbon-oxygen isotopic characteristics of different fillings in the anhydrite-model pore, the fillings are mainly composed of dolomite, calcite and anhydrite, the fillings of partial filling anhydrite-model pore are mainly composed of dolomite silts, dolomite silts + quartz, dolomite silts + ankerite and the fillings of all filling anhydrite-model pore are mainly composed of dolomite silts + calcite, dolomite silts + quartz + calcite and dolomite silts + anhydrite. The comprehensive analysis reveals that the anhydrite-model pore is formed by the preservation of crystal outline after the dissolution of gypsum and other easily soluble crystals. Filling process in the anhydrite-model pore can be divided into the following several stages. Firstly, the anhydrite-model pore is filled with dolomite silts and dark-gray calcite in the naked weathering period. Then, the anhydrite-model pore is filled with authigenic quartz, white sparry calcite, ankerite and anhydrite.