Characteristics and Development Mechanism of Dolomite Reservoirs in North Truva of Eastern Pre-Caspian Basin

Key words: 
• reservoir characteristics /
• dolomitization /
• dissolution /
• carboniferous /
• Pre-Caspian Basin

Abstract: Dolostone is a significant reservoir type of pre-salt Carboniferous carbonate rocks in Pre-Caspian Basin. Based on the analysis of core, thin section, petrophysical properties, geochemical data and other related data, reservoir classification and performance of dolostones in KT-Ⅰ Formation were analysed, and their controlling factors and development models were also investigated. The results show that types of dolostones have a significant influence on their petrophysical properties. Among all the types, reservoir property and oil richness of residual bioclast dolostone mainly with moldic pore and dissolution enlarged pores is the best, that of silt to fine crystalline dolostone mainly with intercrystal pore and its dissolution pore is medium, as well as mud to silt crystalline dolostone mainly with moldic pore and intercrystal pore, but mud crystalline dolostone mainly with intercrystal micropore is poor on reservoir property. Bioclast shoal with more primary porosity formed in the relatively high hydrodynamic environment is favorable for dolomite reservoir formation of high quality. Penecontemporaneous dissolution of fresh water and early dolomitization mainly via seepage-reflux dolomitization (dolomite order degree falls in between 0.32 and 0.67, δ¹³C ranges from -0.22‰ to 5.94‰ and δ¹⁸O ranges from -1.09‰ to 2.45‰) before massive compaction have a dominant effct on early porosity formation and preservation for dolomite reservoir in the Late Carboniferous. At the burial diagenesis stage the formation of fractures with different causes and dissolution of acidic fluids are the vital factors for further porosity increasement of dolomite reservoir since the Early Permian.