Real and relict direct hydrocarbon indicators in the East Irish Sea Basin

Abstract

Seismic amplitude anomalies associated with the presence of hydrocarbon gas now play an important role in the exploration of the East Irish Sea basin. Tilted flat spots were first observed on data acquired by JEBCO Seismic Ltd. in 1986 within the productive Triassic reservoir of the Sherwood Sandstone Group located in Quadrant 110 (offshore North Wales). With hindsight Direct Hydrocarbon Indicators (DHIs) can now be identified on seismic data acquired in earlier phases of the exploration of the East Irish Sea basin, although many anomalies were not always identified as such at that time. This study determines the geophysical parameters by which amplitude anomalies associated with diagenetic variations could be separated from anomalies associated with the presence of gas in the reservoir. The predicted model results are compared with the seismic response of proven fields, discoveries and relict or palaeo-DHIs around the basin. Practical implications for effective exploration, development and production methods in the Sherwood Sandstone are suggested. Anomalous seismic reflections and amplitude variations associated with the presence of hydrocarbon fluids have been observed over the last twenty years in many localities worldwide and described by various authors, Backus and Chen (1975), White (1977) and Brown (1986). The presence of DHIs on seismic data acquired in the East Irish Sea was first reported by geoscientists working the basin in the mid and late 1970's (V. Colter pers. comm. 1991). The existence of relict or palaeo gas-water contacts associated with the distribution of diagenetic platy illite in the sub-surface, had been determined previously by mapping borehole data for the Morecambe Gas field (Bushell 1986). The hypothesis that some of the observed seismic amplitude anomalies in this basin are associated with diagenetic boundaries in the Triassic sandstones (palaeo-DHIs or direct porosity indicators) rather than the presence of present day gas was postulated. The seismic reflection character of the main lithostratigraphic units is well established in the East Irish Sea basin (Ross & Flack 1992). Seismic markers for the major formation and member boundaries of the top Ormskirk, Calder, Rottington and Manchester Marls and also the Hercynian unconformity have been recognised and used across the basin by various authors (Arter & Fagin 1993; Knipe et al. 1993). The Permo-Triassic structure and stratigraphy of the East Irish Sea basin is well known and has been described by various authors (Jackson & Mulholland 1993; Jackson et al. 1987; Colter 1978). The basin is linked to the contiguous onshore West Lancashire basin where similar stratigraphic sequences have been described (Warrington et al. 1980; Smith et al. 1974) and similar structural styles exist. The Formby Point platform is a major structural element of the study area (Fig. 1). The feature is associated with a positive Bouguer gravity anomaly which defines the platform's From Meadows, N.S., Trueblood, S.P., Hardman, M. & Cowan, G. (eds), 1997, Petroleum Geology of the Irish Sea and Adjacent Areas, Geological Society Special Publication No. 124, pp. 185-194 2 53 50'N LIVERPOOL BAY RIBBLE ESTUARY