Loading…
Petroleum Potential of Cretaceous and Tertiary sediments of the Mackenzie Delta (Canada): Influence of organic facies variations and gas system
Saison, Anne
Um die Zusammensetzung und Generierung von Erdöl in einem regionalen Maßstab vorherzusagen, ist eine genaue Definition der organischen Fazies erforderlich. Einerseits sind die sedimentären Fazies eng mit der Qualität des organischen Materials und somit mit seinem Potential zur Erdölbildung verknüpft, da lateralen und vertikalen Änderungen im Muttergestein entsprechende Änderungen in der Erdölzusammensetzung erwarten lassen. Andererseits ermöglichen die zunehmend genaueren analytischen Möglichkeiten hochauflösende Studien und regen dazu an, die Auflösung bei der Simulierung geologischer Strukturen und Prozesse, die auf die Bildung von Erdöl Einfluss nehmen, voranzutreiben. Im Mackenzie Delta haben die Unterschiede der organischen Fazies, hervorgerufen durch unterschiedliche Ablagerungsmilieus, eine starke Auswirkung auf die Zusammensetzung des Erdöls. Meeresspiegelschwankungen während der Ablagerung der Kugmallit Sequenz führten zu einer Verschiebung des Erdölpotentials von „low-wax“ zu „high-wax“ Ölen innerhalb der der Delta plain und Delta front Sedimente, sowie zu einer Anhebung des Ölfensters um bis zu 1500 m. Die ästuarinen/lagunalen Abschnitte der Taglu Sequenz hingegen neigen zur Gaserzeugung in den terrigenen Bereichen, während die Bereiche mit einem starkem limnisch/marinen Charakter der organischen Fazies, Potential für die Bildung von Öl und eine enge Verteilung der Aktivierungsenergien zeigen. Die direkte Konsequenz ist eine Absenkung des Ölfensters um bis zu 2500 m zwischen den ästuarinen und den terrerigenen Faziesbereichen der Taglu Sequenz. Dieser Ansatz hat nicht nur die Aufstellung eines regionalen Modells der Erdölbildung ermöglicht, sondern auch die Identifizierung möglicher Quellen für die Gase der Gashydrate.
A crucial part of petroleum exploration strategy is to evaluate the quality of kerogens because this is the key to determining both the quantity and type of petroleum generated, and the extent of this generation within the basin. Surprisingly, despite the availability of more and more analytical tools, the inferred source rock generative profile is never used considering the depositional history of the region, albeit the source rock quality is genetically tied to the geological context at the time of deposition. By contrast with conventional methods, which consider wholly homogeneous source rock to which an average petroleum potential is attributed, what is proposed here is to point to the occurrence of lateral and vertical variations of organic facies and then petroleum potential within a given source rock. For this, an original multi parameter approach is used, which involves the assessment of the genetic potential, petroleum potential, petrographic facies and thermal stability of the sediments. This is applied to five source rock units of Canada’s Mackenzie Delta. Lateral and vertical sedimentary organic matter heterogeneity is shown to impact on source rock petroleum potential assessment. Within the Upper Cretaceous Smoking Hills sequence, organic facies variations are explained by the control of current speed and redox conditions in a context of low sedimentation rate, which have favoured selective transport and deposition of oil precursors. For the Late Palaeocene-Early Eocene Aklak sequence, the shifting from marine-influenced deltaic environment to coastal plain not only coincides with a shifting from oil to gas potential, but it results in the incongruous situation where gas-prone sediments of the upper Aklak sequence are less stable than oil-prone sediments of the lower Aklak sequence. Oil prone estuarine episode within the Early to Middle Eocene Taglu sequence contrasts with gas-prone continental episodes by its macerals assemblage and thermal stability, resulting in a deepening of the oil window up to 2500 m. The Middle to Late Eocene Richards sequence cannot be considered as a source rock despite the strong facies variation observed. Fluctuating sea level during the deposition of the Kugmallit sequence leads to changes in the geomorphology of the delta from river- and wave-dominated delta to tidal estuary. Consequently, specific organic facies are developed according the depositional history and a shallowing of the oil window up to 1500 m between both phases is observed. Source rock heterogeneity is also crucial when considering the fate of organic matter with maturation. This is shown by performing artificial maturation experiments on two judiciously selected samples: a high-wax, oil-prone, delta-plain of the Kugmallit sequence and a low-wax oil prone, delta-slope sample from the Smoking Hills sequence. In open system, reaction pathways comprise mainly cracking reactions for the Smoking Hills sequence, while aromatization/condensation reactions within the residual kerogen predominate for the sample of the Kugmallit sequence. Residual potential of both samples increases in early maturation stages (300-340°C), which may be ascribed to bitumen formation. Although the Smoking Hills sample generates in open-system around 50% gas more than the Kugmallit sequence, this latter shows patently higher gas generative capacity in closed-system and notably methane generation does not reach an end by the highest maturity level tested (by 600°C). Such late methane generation from the secondary cracking of a neoformed residue issued from the aromatization/condensation of the residual kerogen with newly generated products at low maturity. These observations, altogether with Mackenzie Delta’s play elements configuration, indicate that the Kugmallit sequence is likely the source of gas hydrates.
