Geochemical Investigation of Vertical Migration of Petroleum through Mudstones I: Origin of the Hydrocarbon Shows

Abstract

Hydrocarbon shows and seeps are frequently associated with petroleum migration through caprocks. However, leakage of petroleum from reservoirs does not necessarily result in surface seeps, nor do all occurrences of surface petroleum result from trap leakage. Cretaceous Shetland Group mudrocks which serves as caprock (seal) for the petroleum reservoir of the Heidrun Field was observed to contain liquid hydrocarbon as evidenced from UV florescence. Is this oil indigenous of the caprock or from the underlying reservoir thereby indicating possible failure of the seal? Samples from the caprock were analysed with the aim of establishing the origin of the oil stains. The investigation comprised bulk geochemical analyses in order to estimate various leakage indicators (S1 & PI). High Extractable Organic Matter (EOM) values of up to 6 mg/g rock were obtained in most of the samples. The high S1 and PI values obtained from the samples suggest oil staining. Furthermore, Iatroscan SARA analysis was used to identify compositional variability between the oil present in the caprock and the reservoir oil. The reservoir oil predominantly consisted of saturated hydrocarbons, which, in contrast, represent only a fraction of the caprock EOM. The caprock EOM is dominated by polar compounds, and the saturate fraction increases with increasing proximity to the caprock/reservoir boundary. High gas wetness of more than 80% was recorded at caprock/reservoir boundary, suggesting a good relationship between the distribution of gaseous hydrocarbons in the caprock and the gas content of the reservoir oil. The oil stains in the caprock are therefore established to be sourced from the reservoir.

Keywords: Leakage; Mudrock; Extractable organic matter; Production index; Gas wetness