Hydrocarbon filling history and reservoir continuity of oil fields evaluated using Sr-87/Sr-86 isotope ratio variations in formation water, with examples from the North Sea

This paper describes how Sr-87/Sr-86 ratios in formation waters are used to evaluate compartmentalization of hydrocarbon reservoirs. Strontium Isotope Residual Salt Analysis (SrRSA) of core samples provides a means of measuri ng Sr-87/Sr-86 ratios in formation water from hydrocarbon columns and aquif ers. Smooth SrRSA profiles suggest progressive, uninterrupted, filling and the absence of sealed barriers, while a step change in a profile normally s uggests a barrier sealed up-dip from the well penetration. Inferences about lateral connectivity are made by comparing SrRSA profiles from neighbourin g wells at TVD. Profiles that are superimposed when plotted at TVD suggest the well sections share a common filling history and lie in the same flow u nit. Neighbouring SrRSA profiles that are not superimposed normally suggest segmented compartmentalization of the reservoir. Post-fill structural tilt ing of reservoirs and hydrodynamism synchronous with filling are processes which complicate data interpretation. Drilling mud contamination of core wa ter is the most serious technical limitation of the SrRSA technique.