Panarama of channel and sheet turbidite sandstones at Kilhaha Bay this and other photos in the gallery below are by Christopher Kendall, and Peter Haughton.
Kilbaha Bay-Architecture of Deepwater Sheet Lobes and Channels in Mid-Ross Sandstone Formation
Outcrops at the Kilbaha Bay on north west shore of Shannon Estuary east of Loop Head expose the nature of the deepwater sediments of the Ross Formation where they are characterized by channeled deepwater fan lobes, megaflutes and channel fill. As at the Bridges of Ross there are a series of incised and stacked channels with channel fill that extends beyond the channels as wings and are associated with megaflutes (Elliot , et al, 2000; & Elliott, 2000) which can be identified in the image galleries.
As explained in the 2008 SEPM Field Guide (Martinsen et al., 2008) Kllbaha Bay is flanked by low cliffs that provide a continuous 1.6 km well-exposed outcrop above a wave-cut intertidal platform oblique to depositional strike and in which the channelized-sheets can be seen. Morgan Sullivan in the guide book makes the following key points linking these to the cross sectional diagram above:
The outcrops at Kilbaha Bay are characterized by compensationally stacked, very broad channelized sheets which represent the transition from channels to sheets.
These elements are up to a thousand meters (3270 feet) wide and B to 13 meters (26 to 42 feet) thick. Their bases tend to be non-erosional, suggesting that they are primarily aggradational in origin. In general, these channel complexes do not infill erosional scours; rather they are compensationally stacked due preexisting highs related to underlying channel complexes.
Individual elements can be further subdivided into distinct axis and margin facies associations. Highly amalgamated, massive sandstones characterize axial deposits. Away from the axis, beds become distinctly less amalgamated and extremely continuous to produce laterally extensive, layered wings at the margins.
Net-to-gross ratios for these channelized sheets range from 70 to 90% with an average of 80%.
Detailed bed-by-bed correlations show that these sandstones have lengths much greater than the dimensions of the Kilbaha Bay outcrop, while
mudstones have much shorter lengths (Fig. 9). Therefore, the key to understanding reservoir continuity and internal heterogeneities that affect reservoir performance is knowing the thickness and lengths of mudstone barriers and not the distribution of sandstones.
80% of these mudstones are less than 0.3 meters (1 foot) thick and have bed lengths less than 200 meters (655 feet).
Although this is a relatively high net-te-gross reservoir type with excellent lateral continuity, the vertical continuity would be moderate to low. This is due to the preserved interbedded mudstones, even though only 5 to 10% of these mudstones have lengths approaching or greater
than 700 meters (2295 feet). It is these continuous mudstones that likely would produce significant vertical barriers to fluid flow.
Wednesday, September 25, 2013