Subsurface correlation in the Upper Carboniferous (Westphalian) of the Anglo-Dutch Basin using the climate stratigraphic approach

The Upper Carboniferous play in the Anglo-Dutch offshore continues to be a challenging exploration target, as well as providing a significant portion of the region’s existing gas production. Despite the application of various techniques, a secure genetic stratigraphic framework, applicable at field as well as regional level, has continued to be elusive. This paper presents such a framework, using only routine wireline log data supported by the limited amount of published stratigraphic information. In addition to their conventional properties, wireline logs conceal information encoded in their waveform properties. Treated as complex waveforms, logs are amenable to a range of ‘time-series’ analytical methods. Applying the concepts of global cyclostratigraphy (Perlmutter et al., 1990, 1998), so-called spectral trend (or INPEFA) curves, which show uphole changes in the waveform properties of the data, are used to generate a framework of near-synchronous well-to-well correlations. Using this approach, we have subdivided the top part of the Carboniferous succession – essentially the Westphalian – into nine first-order stratigraphic packages, W1000 to W9000 from bottom to top. Most of these packages can be further subdivided into second-order packages, and some of these into third-order packages, taking the resolution of this scheme down to a few tens of metres, or even to a few metres. These packages have been identified in a total of over 50 wells in the offshore UK and offshore Netherlands sectors.Comparison with the limited information publicly available on previous stratigraphic classifications indicates that our scheme is far more widely applicable, and probably considerably more reliable than any other previously attempted at the regional scale. Also, the scheme has the potential for further subdivision, to the limit of resolution of the log data, at the local (field/reservoir) scale. As our subdivisions are inherently time-related, they will now serve as the most appropriate framework within which to understand basin paleogeographic development, and the distribution of reservoir and seal facies within the Upper Carboniferous. The purpose of the paper is two-fold. First, we present our stratigraphic scheme and the method of climate stratigraphy upon which it is based. Second, we show how systematic application of this method in well-to-well correlations leads to the identification of important intra-formational unconformities.