The role of the Mid-Cimmerian Unconformity on the quality of the underlying Skagerrak sandstone reservoir in the Kittiwake Field,central North Sea

The central aim of this paper is to address the role of unconformities in affecting reservoir quality.Do they facilitate diagenesis that leads to either enhanced or reduced porosity through dissolution or cementation?Or,do they have little effect?We have investigated the Late Triassic Skagerrak sandstone reservoir underlying the Mid-Cimmerian Unconformity in the Kittiwake Field,central North Sea.There is strong evidence for the development of secondary porosity through the dissolution of unstable silicate minerals,primarily feldspars.This includes the presence of oversized pores,partial dissolution of framework grains,and complete dissolution of grains leaving remnant grain margins and partially filled cores.This dissolution as a late-stage event is demonstrated by the complete lack of compaction effects on the secondary pores and diagenetic products despite present burial depths in excess of 3000 m.These observations,coupled with an absence of systematic trends linked to the unconformity surface in respects of reservoir porosity,feldspar amount and dissolution,and kaolinization,lead to the conclusion that there has been no effect of the Mid-Cimmerian Unconformity on reservoir quality in the Kittiwake Field.There is no evidence for leaching or cementation linked to meteoric water influx either shortly after deposition or following the uplift and exposure,which led to development of the Mid-Cimmerian Unconformity.Instead,we propose that the late-stage dissolution of feldspar and generation of secondary porosity are most likely related to the influx of organic acids and carbon dioxide generated either from thermogenic maturation of the source rock or from biodegradation of oil within the reservoir near the oil-water contact(OWC)transition zone.

Tracing the Methane Events by Stable Carbon Isotopes of Benthic Foraminifera at Glacial Periods in the Andaman Sea

Stable isotopes of carbon and oxygen variations in foraminiferal shells have been widely used in paleo-environment studies.However,studies about the shells of benthic foraminifera in methanehydrate-bearing sediments as reliable geochemical proxies to reconstruct the potential methane release events in the geologic past are rare.In this study,we present the stable carbon and oxygen isotopes of fossil benthic foraminifera including one epifaunal species(Cibicidoides wuellerstorfi)and two infaunal species(Bulimina mexicana and Uvigerina peregrina)from the Site U1447 of IODP 353 Expedition to trace methane events in the Andaman Sea,where one of the thickest and deepest gas hydrate stability zones was discovered.Theδ^(13)C values of benthic foraminifera show that there are eight distinct intervals with negative values in the last~10 Myr,interpreted as a record of long-term fluctuations in methane emission.Six of these methane events occurred during the glacial sea-level lowstands in the last~1.1 Myr.We,therefore,infer that the trigger mechanism for these events might be the hydrate destabilization caused by sea level fall.The methane events that occurred at~2.11 and~5.93 Ma are more likely related to the sudden changes in sedimentation,either slide events or marked variations in sedimentation rate.