AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Anatomy of an isolated Cretaceous carbonate bank in the North Atlantic Porcupine basin (Ireland Offshore). Seismic facies and internal architecture.

Abstract

The Porcupine Median Ridge (Southern Porcupine Basin) is an intrabasinal high of discussed origin (volcanic or basement) hosting deposition of shallow water carbonates at the end of the syn-rift period. Regional seismic sections display two separate build-ups above the Base-Cretaceous Unconformity, suggesting isolated carbonate banks. A well in the northern culmination drilled about 500m of Lower Cretaceous carbonates, consisting of low energy inner platform deposits, grading upward to medium-high energy platform margin deposits, with good reservoir quality and residual hydrocarbons. The internal architecture of both build-ups displays three seismic packages bounded by unconformity surfaces, however they differ in terms of reflections geometry and seismic morphology. The northern build-up is marked by significant back-stepping, suggesting a progressive shrinking of the shallow water carbonate factory up to the drowning unconformity at the top. This trend is supported by the deepening upward stacking pattern observed in the well. The southern build-up is marked by aggradation and infilling, suggesting a more persistent carbonate production and final subaerial exposure of the bank. Assuming the two banks developed in the same timeframe, the contrasting internal geometry may reflect variation of tectonic subsidence in different sectors of the Porcupine Median Ridge. Combining quantitative textural analysis and observations on reflections geometry and continuity, six seismic morphological facies were defined and mapped within each seismic unit. Integration of well data and conceptual models from outcrop and subsurface analogues, allowed interpreting depositional settings within each unit. High amplitude, variably continuous reflections are interpreted as inner platform deposits, likely alternations of grain- versus mud-rich Rudist and bioclastic carbonates. High amplitude clinoform reflections locally detectable at the bank margins are interpreted as slope to basin deposits.