Hydrocarbons and Evaporites

Abstract

Even though evaporites constitute less than 2 % of the world’s sedimentary rocks, one-half of the world’s largest oilfields are sealed by evaporites, the other half are sealed by shales (Fig. 10.1; Grunau 1987). Kirkland and Evans (1981) argued that evaporites overlie or seal carbonates containing an estimated 50 % of the world’s known total petroleum reserve. Of the world’s 25 largest gas fields, nine are sealed by evaporites and sixteen by shales and hydrates. Sixteen are capped by Mesozoic seals, 7 by Palaeozoic seals, and only two by Tertiary seals. Fourteen are in the 1,000–2,000 m seal-depth interval, nine in the 2,000–3,000 m interval; and two in the 0–1,000 m interval. As one would expect, more gas fields than oilfields are sealed by Palaeozoic caprocks, and more oilfields than gas fields are sealed by Tertiary caprocks. Surprisingly, the seal depth intervals for the 25 largest oil and gas fields do not differ significantly. However, Grunau argues many “supergiant” gas accumulations below depths of 3,000 m and have either not yet been discovered, or have not yet been put in production (e.g. much of the Khuff-hosted gas in North Dome in the Middle East).