History of hydrocarbon charge on the Atlantic margin: Evidence from fluid-inclusion studies, West of Shetland

Abstract

Fluid inclusions in Jurassic reservoir sandstones help us to understand the history of hydrocarbon charge in the West of Shetland region. The sandstones contain both hydrocarbon and aqueous fluid inclusions. Hydrocarbons are present in trails of secondary inclusions through detrital grains, and as primary inclusions in quartz overgrowths. This is direct evidence for at least two episodes of hydrocarbon charge before the present oil entered the rock, consistent with indirect evidence from burial history modeling and organic geochemistry. Fluorescence colors indicate that the two oils are of distinct composition. The first oil was degraded, reflecting an episode of uplift and exposure to near-surface fluids before reburial and migration of a second live oil. Oils comparable to these two paleocharges were mixed to produce the oils that are encountered in the current reservoir pores, showing that they were preserved despite repeated tectonic events at the Atlantic margin.