Correlation, facies distribution and sequence stratigraphic analysis of the Westphalian B coal measures in quadrant 44 of the southern North Sea

Abstract

In the early 1980's a number of significant gas discoveries were made in reservoirs of Westphalian B age in Quadrant 44, Southern North Sea. This study develops an integrated scheme to enhance the correlation of existing wells and to assist in future predictability. It allows for an improved understanding of the succession thickness and character, utilising the concepts of high-resolution non-marine sequence stratigraphy. This is based on a chronostratigraphic framework built on palynostratigraphy and the identification of the marine bands which subdivide the Silesian. Conventional palynological analysis provides the basis for the chronostratigraphic framework; the palynological assemblages described from Quadrant 44 wells compare favourably with similar studies on equivalent sections onshore. This allows the upper Westphalian A to lower Westphalian C interval to be subdivided into 5 broad zones. However, there is a considerable margin of error in these deep Carboniferous exploration wells due to a number of sedimentological and drilling related factors. To achieve the chronostratigraphic resolution required it is necessary to identify the 9 well-known marine bands which subdivide the Westphalian B to lower Westphalian C interval. In the subsurface marine band log recognition relies on spectral gamma ray logs, where the individual marine bands can be recognised from their uranium enrichment. This process required the definition of a marine band characterisation scheme, whereby 4 types of marine band are recognised from their uranium response, out of a total of 8 marine bands showing varing degrees of uranium enrichment. A further marine band relies on the location of marine macrofauna. The marine bands are then incorporated into the established chronostratigraphic framework. An additional stratigraphic marker, the Sub-Clowne tonstein, is identified from the spectral gamma ray logs, and provides an independently determined time line. A method whereby the common Westphalian B facies could be identified in Quadrant 44 wells was developed in this study. It was based on the work of Fielding (1984, 1986), Giuon and Fielding (1989) and Haszeldine (1981, 1983a,b) combined with Westphalian B outcrops on the Northumberland coast and core material from Westphalian B intervals in Quadrant 44 wells. This facies identification scheme defined 13 lithofacies, which were grouped on typical wireline response into 8 wireline log facies associations (Leeder et al. 1990). Facies distribution, determined from the facies analysis within the chronostratigraphic framework was used to develop a high- resolution sequence stratigraphic scheme. This enabled a 3rd order sequence in the terminology of Galloway (1989a,b), from maximum flooding surface to maximum flooding surface to be defined with an important 3rd order sequence boundary and overlying alluvial transgressive systems tract. The remainder of the succession was interpreted as a transgress I've sequence set, subdivided by a number of 4th order sub- regional flooding surfaces. Both the 3rd and 4th order surfaces were used to provide a high resolution correlation for Westphalian B sediments in Quadrant 44.