Depositional Setting and Reservoir Quality of the Frome Clay Limestone Reservoir (Middle Jurassic), Witch Farm Field Southern England
Peter Gutteridge and Peter Robinson
The middle Jurassic of southern England was a southerly-dipping carbonate ramp system with shallow oolitic carbonates passing down-dip into mudstone with isolated bioclastic carbonate banks. Several of these banks are present at outcrop and in the subsurface and one, the Frome Clay Limestone, forms a reservoir in the Witch Farm Oilfield.
These banks formed as a result of prolific growth of the oyster Liostrea below wave base on the mid- to distal part of the carbonate ramp. The banks are of the order of 100sm across and up to 45m thick. The Frome Clay Limestone reservoir was initiated as a sheet which was followed by upward growth over a more restricted area, probably influenced by in sea level rise.
Off-bank facies include: Trigonia wackestone and bioturbated argillaceous wackestone, the latter facies was also deposited as the bank was established.
Bank facies include: Liostrea clay packstone with Liostrea valves in a clay matrix with nodular carbonate cement, and Liostrea grainstone. These facies form the bulk of the reservoir and contain the best reservoir quality. Minor facies include lenses of intraclast clay packstone formed by storm reworking and whole Liostrea clay packstone which represent areas of Liostrea growth.
Clay-rich limestones contain the best reservoir quality which is controlled by a balance between compaction and cementation. Precipitation of carbonate cement reduces compaction, keeping intergranular pores open. Occlusion of porosity is prevented by the clay matrix which inhibits cementation.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California