STONECIPHER, SHARON A.¹, HAYDN C. TANNER², and D. ANNE GRAU^1
1Marathon Oil Company, Petroleum Technology Center, Littleton CO
²Marathon Oil (UK), London, UK

Abstract: Depositional and Diagenetic Controls on Reservoir Quality, Lower Triassic Sherwood Sandstone, St. George's Channel, Irish Sea

The Lower Triassic Sherwood Sandstone was deposited in an arid, low relief continental basin similar to the present interior of Australia. The Sherwood interval in the 106/18-1 well, St. George's Channel, is interpreted to have been deposited along the margin of a shallow saline lake that formed in a depression in the alluvial/playa plain during a relatively wet climatic interval. The lake appears to have had no point of outflow, making the lake level and salinity subject to rapid fluctuation as a function of rainfall and evaporation. Lacustrine/playa deposits are closely intermingled with braid plain, eolian, and sabkha intervals, all of which were reworked in the lacustrine environment. In addition to neighboring clastic environments, a localized carbonate terrane provided detrital material to the lakes. Deposits that formed in close proximity to the carbonate terrane and contain abundant carbonate detritus will be termed 'coincident' shorelines, whereas lacustrine sediments deposited distally and discretely from the carbonates are termed "non-coincident".

Reservoir quality in these sandstones is controlled by facies and carbonate cementation. Porosities in the 106/18-1 well are significantly lower than expected, averaging only 6% compared to 10-15% in neighboring wells. Early carbonate cementation is a primary cause for porosity occlusion. Sandstones developed along the lake margin coincident with the carbonate terrane contain abundant carbonate detritus and carbonate cement and exhibit poor reservoir quality (4-6% f, ~0.05 md K). Braided stream/braid plain deposits are typically very poorly sorted, contain abundant clay matrix, and, with the exception of a few cleaner braided stream intervals, are generally non-reservoir. The best reservoir quality (7-10% f, up to 6 md K) is seen in sands that appear to have formed either along non-coincident shorelines or in a more offshore lacustrine setting. These sands contain less carbonate detritus and less carbonate cement. In some of these sandstones, however, welldeveloped fibrous illite fills most pores. In these illite-cemented sands, although porosity is good (up to 11%), most of ft is microporosity and permeability is poor (0.04 md). Thicker aggradational shoreline sands tend to have better reservoir quality than ones thinly Interstratified with lacustrine muds or sabkha deposits.