Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field, west of Ireland

Abstract

Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from − 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ 13C increases from about − 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ 13C then decreases to about − 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ 13C values decreasing to about − 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ 13C correlates with indicators of aridity and biological stress such that the highest δ 13C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ 13C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance.