The use of diagenetic signatures to distinguish marine from continental deposits in Triassic-Jurassic sandstone reservoirs from the UK Central Graben

Poor biostratigraphic control for some Triassic-Jurassic successions in the North Sea Basin and sub-basins necessitates the use of alternative correlation methods. This study examines the use of diagenetic signatures to distinguish continenetal from marine sandstone successions (Triassic-Jurassic) in the UK Central Graben. The key diagenetic alterations encountered in these successions include kaolinitization of the framework grains and the development of sphaerosiderite and pyrite. The δ ¹³C_V-PDB values of siderite (−8.1 to −8.5‰) and of ankerite (−10.8 to −9.2‰), indicate a strong contribution of dissolved carbon from the decay of plant material in soil. However, marine water likely influenced diagenesis during periods of relative sea level rise by providing the dissolved sulfate (SO₄²⁻) required for the precipitation of pyrite. The presence of diagenetic alterations such as kaolinitization of framework grains and cementation by sphaerosiderite could indicate that the sediments were deposited in an overall continental setting. However, the occurrence of pyrite and scattered grains of deep-green colored glauconite suggests occasional marine influence. Such information on the changes of the diagenetic realm provides important clues for establishing a framework for stratigraphic correlations. Caution should be exercised when interpreting petrographic data as subsequent episodes of telodiagenesis can complicate petrographic interpretations.