WORDEN, RICHARD H.
Queen's University, Belfast, BT7, UK.

Abstract: Understanding the origin and production geochemistry of radioactive lead (Pb²¹⁰) in NORM-contaminated formation waters

Radioactive produced formation waters in a Triassic fluvial reservoir, onshore southern UK, are unusual in that they are enriched in Pb²¹⁰, instead of radioactive radium which is more typical of Naturally Occurring Radioactive Material (NORM) contamination in petroleum reservoirs. Like Ra²²⁶, Ph²¹⁰ belongs to the U²³⁸ decay chain. Key issues include the origin and distribution of radiogenic lead in subsurface waters and the absence of the more common type of NORM.

NORM was sourced internally within the reservoir as the sandstones are unusually enriched in uranium having local concentrations up to 70 ppm. Stratigraphic and aerial distribution of high levels of subsurface uranium correlates well with the highest level of NORM-contamination in produced waters. Triassic formation waters have very high sulphate and very low barium concentrations and are at saturation with respect to barite. Radium is not a NORM problem presumably because its concentration is limited by barite solubility. There is so much sulphate in the formation water that daughter Ra²²⁶ precipitates as RaSO₄ within the reservoir. While the sulphate concentration is very high, the sulphide concentration is below conventional detection limits. Geochemical modelling indicates that Triassic formation waters are undersaturated with respect to PbS (galena) thus allowing radiogenic lead to remain in aqueous solution.

One of the main sites for NORM precipitation in the facilities, is where Triassic waters are co-mingled with waters produced from local H₂S-bearing, Mid Jurassic oil field formation waters. The addition of sulphide leads to supersaturation with respect to PbS.

Downhole Pb²¹⁰ geochemical-amelioration procedures have been suggested, based upon the saturation state of galena.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas