|THE DUNQUIN PROSPECT - MATURING LOWER CRETACEOUS CARBONATE PLAY EXPLORATION THEMES IN THE SOUTH PORCUPINE BASIN, OFFSHORE IRELAND|
|Exploration Hot Spots|
1Providence Resource plc, 2University of Birmingham, 3Tonnta Energy Limited
The South Porcupine Basin (SPB) is one of a number of linked Mesozoic deepwater rift systems located on the Irish Atlantic Margin (Croker & Shannon 1987, FIG. 1). The main phase of basin extension is thought to have occurred during the Late Jurassic-Early Cretaceous period (Baxter et al. 1999) with an associated total strain (beta factor) of c. 6.0 (Conroy & Brock 1989). The SPB contains a number of significant internal ridge elements which are of unknown composition as exploration well data in this frontier basin are very limited. The most significant of these is the PMR and a number of basin development hypotheses have been proposed to explain the genesis mechanisms of this ridge. The proposed genesis mechanisms include an extrusive synrifting volcano (Tate & Dobson, 1988), a sedimentary rotated fault block (Arveschoug et al. 1999) and an extrusive serpentinitic mud mound (Reston et al. 2001, 2004). Morphological analysis from potential analogues (O’Sullivan et al. 2010) together with potential field studies (Readman et al. 2005) suggests that the PMR does not neatly conform to modern day volcanic or serpentinitic families. The composition of the PMR would provide a key insight into the history and basin development of the SPB and profoundly impact the petroleum potential of this vast and unexplored NW European Atlantic Margin frontier basin.
The majority of subsurface data coverage within the South Porcupine Basin is currently limited to potential field and conventional 2D seismic data which were primarily acquired up to the late 1990’s. Since then, just two significant long offset prospect specific seismic campaigns of any scale have been carried out in this deepwater frontier basin. Whilst these more recent data sets have dramatically improved subsurface imaging due both to improved acquisition and processing technology (FIG 2), they are spatially very restricted and afford limited imaging of the overall basin architecture. Well data are even more restricted with just three wells having been drilled on the margins of the SPB proper, however encouragingly from a hydrocarbon perspective each encountered oil and gas shows in their target Mesozoic reservoir section. Whilst an extensive potential field data base exists over the basin, the lack of significant well data makes the modeling of unique solutions for any given subsurface architecture extremely challenging.
Interpretation of available data suggests that the SPB should host a prolific Mesozoic hydrocarbon system. Basin modeling indicates that any Late Jurassic source rocks present are likely to be currently in the gas generative phase (PAD, pers. comm.) with structural highs such as the PMR acting as foci for migration and possible entrapment. The extreme extension cited for the SPB should have a profound impact on the development of petroleum systems influencing source presence/maturation, heat flow/timing, sediment input/carbonate development as well as the creational of intrabasinal structures for hydrocarbon entrapment (O’Sullivan et al. 2010). Interpretation of the recent long offset 2D seismic data has confirmed morphologies on the PMR interpreted to be associated with the development of isolated platform carbonate build-ups which comprise the Dunquin prospect. This interpretation of a Lower Cretaceous carbonate depositional system in the SPB is not novel as it supports previously published work (Newman et al. 2004). It is also worth noting that thick Lower Cretaceous platform carbonates were encountered in the Goban Spur 62/7-1 exploration well on the southern margin of the SPB. The Dunquin North and South targets as mapped comprise two discrete isochron thicks situated on the crest of the PMR. The morphology of these build-ups suggests a Lower Cretaceous NE palaeowind direction which would be consistent published global palaeoclimatic models (Markello et al. 2008). Each build-up appears to have its own unique characteristics displaying potential differing genetic links with tectonism and base level changes.
Multi-dimensional data analysis of the South Porcupine Basin has confirmed and matured a carbonate exploration play system within the Lower Cretaceous (O’Sullivan et al. 2009) and high-graded the Dunquin prospect for exploratory drilling. The development of such carbonate play systems provides a paradigm shift away from the more traditional clastic dominated play systems of the Irish Atlantic Margin which have to date enjoyed limited exploration/development success (Corrib, Spanish Point & Dooish). In contrast the Lower Cretaceous aged carbonate systems are known globally to host giant hydrocarbon accumulations and may become an important future petroleum theme in the vast and unexplored South Porcupine Basin.
The views expressed in this paper are strictly those of the authors however we would like to thank the Irish Frontier Exploration Licence 3/04 partnership of ExxonMobil Exploration and Production (Offshore) Ireland Limited, ENI Ireland BV, Providence Resources plc and Sosina Exploration Limited for providing access to some of the seismic data on which this paper is based. We are grateful to Jakub Czarcinski at Providence who drafted the various figures in this paper. The seismic data has been published courtesy of the Petroleum Affairs Division of the Irish Department of Communications, Energy and Natural Resources.