Abstract: A Comparative Sedimentological and Sequence Stratigraphic Study of the Middle Jurassic Beryl Formation within Quad 9, UKCS

Gregor Maxwell

Thirty two wells (nineteen cored) from the Middle Jurassic Beryl Formation have been studied from two separate areas approximately thirty kilometres apart. The Beryl Formation (Bajocian-Bathonian) varies in thickness (from 410^prime to 1100^prime) across the Beryl Embayment within Quad 9.

In the northern Bruce and Keith areas the thickness variations are caused by local hiatuses and intraformational erosion events. The lithofacies present are highly variable with initial coals, fluvial sandstones and floodplain to tidally-influenced sandstones, bioturbated offshore sandstones and mudstones with storm deposits. These are stacked in an overall transgressive sequence capped by the offshore mudstone of the Heather Formation (Callovian aged).

In the southern Buckland and "U" area the thickness variations are caused by post depositional erosion. Sedimentologically the section is different to that found in the north with a basal coal and shoreface deposits immediately above. The rest of the section is dominated by tidally influenced sandstones, lagoonal mudstones and a prograding shore face deposit which is topped with a coal. This sequence has a regressive trend with the erosion occurring after

the coal deposition and before the sharp transition into the basinal mudstone of the Heather Formation (Callovian). This sequence has only minor syn-sedimentary fault activity with good correlation between lithostratigraphic units whereas in the northern area the sequence is far more complex due to the fault activity.

1. small scale (3-4m) units often not correlatable between wells and attributed to auto cyclic processes.

2. medium scale (10-15m) stacked parasequences correlatable within each area with a part tectonic part eustatic control.

3. large scale (30m-) sequence tracts correlated throughout the study areas with a solely eustatic control.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994