Origin of diagenetic fluids in the Zechstein Main Dolomite reservoir rocks, West Pomerania, Poland

摘要

Fluid inclusion analyses and petrographic investigations were carried out on Zechstein Main Dolomite oolitic dolostones from wells Benice-3, Ciechnowo-5, Czame-I and Gardomino-1 located in the area of the Kamien Pomorski carbonate platform, West Pomerania, NW Poland. The dolostones within the oolite shoal facies were primarily deposited during a relative sea level highstand and display the following microfacies: oolitic dolograinstones and dolopackstones, in-traclast dolofloatstones and biogenic boundstone types (bindstone/microbial laminites and framestone/stromatolites). Two stages of diagenesis related to reservoir properties have been identified: eodiagenesis which slightly reduced the high pri-mary porosity and mcsodiagenesis, associated with gradual burial, which played a critical role in decreasing the reservoir potential. Pore-filling overgrowths, chemical compaction and diverse cementation processes also acted at this stage. Fractur-ing and stylolitisation occurred in some places resulting in the formation of potential migration routes for hydrocarbons and mineralising solutions. Microthermometric studies of fluid inclusions, found only in Benice-3 anhydrite cements, revealed the composition of the mineralising solutions to be H_2O-CO_2-NaCl-CaCl_2 brines. Calculated temperatures and pressures of crystallisation vary from 94-110 °C and 270-330 bars. The average composition of the brine is 3.9 wt% NaCI equivalent. It is suggested that the formation brines represent an anhydritisation process of the Main Dolomite during shallow to intermediate burial. This occurred after dolomitisation and migration of hydrocarbon-bearing solutions during Early Jurassic times.