Correlation of Faunally Poor Clastic Successions Using Heavy Minerals: An Example From the Triassic Skagerrak Formation of the Central North Sea

Abstract

Hydrocarbons have been produced from the Triassic play of the Central North Sea since the early 1980's, yet detailed knowledge regarding the correlation and distribution of reservoirs remains relatively poor. Interest in this play has been renewed in recent years as the North Sea basin reaches its super-mature phase, with deeper HP/HT fields becoming increasingly important. Reservoirs form part of the Triassic aged Skagerrak Formation which consists of a seemingly monotonous sequence of alternating red-bed sandstones and mudstones. This represents a dryland continental succession comprising fluvial sands and floodplain-playa-lacustrine mudstones. Harsh conditions have resulted in poor palynological recovery, with palynomorphs often being non-diagnostic of specific ages and/or poorly preserved. Additional problematic factors include intermittent well coverage over what is often a seismically transparent package that has been locally deeply eroded. In contrast to the relatively poor and patchy biostratigraphic coverage throughout existing wells, provenance sensitive heavy mineral assemblages can be obtained from the vast majority of clastic borehole ditch cutting and core samples from across the area. This provides an excellent and reliable tool for aiding correlation and provenance studies, as proven with similar heavy mineral studies worldwide. The core area consists of Quads 22, 29 and 30 with the aim to extend eastwards into the Norwegian sector. With the advancement of heavy mineral techniques since existing Skagerrak heavy mineral studies were published over two decades ago, this study seeks to apply more recent techniques to both new discoveries (e.g. Culzean gas field) and key historic wells. Early results from the Quad 22 area are very encouraging where distinctive heavy mineral markers are seen to correlate between wells within the Marnock and Culzean areas despite respective operators linking these units to differing ‘J’ members. Where conventional analysis provided little in the way of diagnostic features alone and standard petrophysical interpretations infer an over-simplistic sequence, additional correlative zones have also been deduced through the use of varietal heavy mineral techniques such as apatite and zircon roundness indices. Further results from the application of heavy mineral analysis to this integrated regional study are presented as geochemical data and additional wells from across a wider area become included.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017