Subsurface correlation of the Triassic of the UK southern Central Graben: new look at an old problem

Mat De Jong, David Smith, S. Djin Nio, and Nick Hardy discuss climate change as a primary driver of vertical lithofacies change allowing a time-significant stratigraphic classification to be derived from a standard facies-sensitive wireline log such as the GR. Knowledge of Triassic stratigraphy in the Central Graben is impeded by lack of regional seismic markers, poor recovery of microfossils, and regional structural complexity. Of the various sources of data available (seismic, logs, cuttings, core/sidewall core), the most reliable and continuous must surely be the wireline log. With the exception of occasional short intervals of bad data, logs are more or less universally available for all North Sea wells, providing an unparalleled source of objective and closely spaced samples of various physical quantities. An ideal stratigraphic method would extract time-significant information from the logs, allowing correlation at a resolution approaching that of the logs themselves. We here describe the experimental application of just such an approach to 16 wells in the southern part of the UK Central Graben, quads 22 and 30 (Figure 1). Our method relies on (a) a new method of extracting trends in spectral (wavelength, frequency and phase) content of a wireline log, and (b) the interpretation of this information in terms of orbital-forcing of climate change in the 104 to 105 year waveband. We first outline the regional background to the need for a unifying scheme for Triassic correlation that is readily applicable to all significant Triassic well-penetrations in the area. We next describe the key principles underlying the method. Finally, we describe the stratigraphic scheme that emerges from this initial study, and we discuss its implications.