New evidence for the origin of quartz cements in hydrocarbon reservoirs revealed by oxygen isotope microanalyses

Quartz cements degrade hydrocarbon reservoir quality by choking off fluid flow paths, therefore, it is important to understand the sources and timing of quartz cementation. We present in situ microanalyses of oxygen isotope ratios in quartz cements from a Jurassic hydrocarbon reservoir, located offshore Norway, showing systematic variations that reveal the isotopic history of the pore fluids that precipitated the quartz. Earliest quartz cements have δ ¹⁸O values of 20-26‰. Subsequent cements drop in δ ¹⁸O value as low as 13‰ but then increase back to 23‰ with increasing cement thickness. This isotopic trend was observed in multiple overgrowths. With temperatures constrained by fluid inclusions and the burial history of the reservoir, the isotopic composition of porewaters can be estimated from the measured isotopic ratios in quartz cement. Results indicate that >97% of the quartz cement precipitated from negative δ ¹⁸O waters (originally meteoric) that were gradually modified by burial diagenetic reactions as temperatures increased. Precipitation began in the Middle to Late Cretaceous, however, the rate of quartz precipitation increased dramatically in the last 3-5 Ma as the reservoir was rapidly buried. The data show that quartz precipitation is a continuous process during burial and that silica is derived locally from silicate reactions occurring within the sandstone reservoir. The isotopic data do not record evidence of episodic cementation from convection-driven, hot basinal fluids.