Jurassic non-marine source rocks and oils of the Porcupine Basin and other North Atlantic margin basins

Abstract

A detailed geochemical study has shown that oils from the Porcupine and Jeanne d’Arc basins were not sourced from a typical, fully marine Upper Jurassic Kimmeridge Clay Formation equivalent. Rather, these oils are thought to be a mixture of hydrocarbons derived from an atypical Upper Jurassic shale and from a Middle Jurassic non-marine source interval. Standard geochemical techniques were used to analyse a suite of oil samples from the Atlantic margin, along with new proprietary geochemical parameters which can discriminate age and depositional environment. The results indicate that oils from the Porcupine and Jeanne d’Arc basins are very similar, and originated from a mixture of an unusual marine source rock and a lacustrine algal source rock. This conclusion is strengthened by considering the regional geology, as several marine and non-marine potential source rock intervals are present in both the Porcupine and Jeanne d’Arc basins. The chemistry of the oils from the Jeanne d’Arc Basin suggests that the Egret Member of the Rankin Formation is the unusual Upper Jurassic marine source rock. The Egret Member is also an excellent analogue for the Upper Jurassic source contributor in the Porcupine Basin, and palaeogeographical reconstructions for this time suggest that similar source facies were deposited in both basins. The lacustrine source contribution recognized in the Porcupine Basin oils is thought to originate from Middle Jurassic algal shales, which are moderate to very good oil-prone source rocks (total organic carbon contents: 1.3–3.9%; hydrogen indices: 143–573). This newly recognized non-marine oil component in North Atlantic margin basins reduces the charge risk of Atlantic margin plays, particularly where the Kimmeridge Clay Formation is known to be absent or immature.