The Culzean Field, located in UK CNS Block 22/25a between the Lomond Deep and the Heron Cluster, was discovered in 2008 by the 22/25a-9Z well and subsequently appraised by wells 22/25a-10, 22/25a-10Z, 22/25a-10Y and 22/25a-11. The primary reservoir is the Triassic Skagerrak Formation, with a secondary reservoir in the Middle Jurassic Pentland Formation. Total recoverable reserves are estimated to be 250 – 300 MMBOE. One principal challenge concerning the Skagerrak reservoir in Culzean is the stratigraphy, and especially how it relates to adjacent wells in Quadrant 22. Historically, biostratigraphic age determinations of the Skagerrak Formation in Quadrant 22 have been unsuccessful, unlike in Quadrant 30 where biostratigraphic recovery is good and ages are well established. However, good recovery in the Culzean discovery well 22/25a-9Z enabled identification of the Jonathan Mudstone Member, the Joanne Sandstone Member, the Julius Mudstone Member, the Judy Sandstone Member and Marnock Mudstone. The Joanne Sandstone Member and the Jonathan Mudstone Member are therefore more widely distributed in Quadrant 22 than has been hitherto recognized. The Skagerrak Formation in Culzean also has a well-defined heavy mineral stratigraphy, enabling correlation into the adjacent Marnock, Skua, Egret and Heron fields. This correlation shows that the reservoir succession in Egret and Heron is largely equivalent to the Joanne Sandstone Member, whereas Marnock and Skua contain the Judy Sandstone Member overlain by truncated Joanne Sandstone Member and Julius Mudstone Member intervals.
The Triassic of the Central North Sea is a continental succession that contains prolific hydrocarbon-bearing fluvial sandstone reservoirs stratigraphically partitioned by mudstones. Within the Skagerrak Formation of the UK sector, hydrocarbon accumulations in the Judy, Joanne and Josephine Sandstone members are top sealed by the Julius, Jonathan and Joshua Mudstone members, respectively. However, UK and Norwegian stratigraphic correlations have been problematical for decades, largely due to biostratigraphic challenges but also due to the non-uniqueness of the lithotypes and because the cross-border stratigraphic nomenclature differs and has yet to be rationalized. This study focuses on mudstones rather than sandstones to unify cross-border correlation efforts at a regional scale. The mudstone members have been characterized by integrating sedimentological, petrophysical and geophysical data. The facies are indicative of playa lakes that frequently desiccated and preserved minor anhydrite. These conditions alternated with periods of marshy, palustrine conditions favourable for the formation of dolostones. Regional correlations have detected lateral facies changes in the mudstones which are important for their seismically mappable extents, resulting palaeogeographies and, ultimately, their competency as intraformational top seals. Significant diachroneity is associated with the lithological transitions at sandstone–mudstone member boundaries and although lithostratigraphic surfaces can be used as timelines over short distances (e.g. within a field), they should not be assumed to represent timelines over longer correlation lengths. Palaeoclimatic trends are interpreted and compared to those of adjacent regions to test the extent and impact of climate change as a predictive allogenic forcing factor on sedimentation. Mudstone member deposition occurred as a result of the retreat of large-scale terminal fluvial systems during a return to more arid ‘background’ climatic conditions. The cause of the member-scale climatic cyclicity observed within the Skagerrak Formation may be related to volcanic activity in large igneous provinces which triggered the episodic progradation of fluvial systems.
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