Deep to Shallow Kaolinite Relocation Generates Porosity

Orla M. McLaughlin, Gavin E. Mc Aulay, R. Stuart Haszeldine, and Rogers Graeme

Eocene and Paleocene sands make up the reservoir in the West Brae Field, which is located on the Fladen Ground Spur, Quadrant 16/7a, UK North Sea. The Tertiary sandstones were derived from a mixed source area of sediments and volcanics. Early carbonate cementation and late dissolution of carbonate cement and feldspars has resulted in sandstones possessing excellent poroperm characteristics, similar to the high quality upper reservoir sandstones described in the South, Central and North Brae fields.

The diagenetic feature which most adversely affects these sandstones is the presence of large amounts of authigenic kaolinite. Quantities of up to 13% kaolinite have been reported for the West Brae reservoir, an unusually high volume considering its current depth of 7,700 ft. In contrast, the South, Central and North Brae fields exhibit only 1-3% diagenetic kaolinite, even though up to 8% feldspar dissolution porosity has been recorded.

This study tests the hypothesis that feldspar dissolution at 12,000 ft in the Upper Jurassic reservoirs of South, Central and North Brae, has enabled aluminium to be exported from the deep Jurassic sandstones, to form kaolinite in the shallow Tertiary sandstones of West Brae. As Jurassic derived fluids have a distinct radiogenic strontium signature, in contrast with Paleocene fluids which have an unradiogenic Sr signature, Sr isotope studies provide a test for this hypothesis. ⁸⁷Sr/⁸⁶Sr analyses of the clays have been carried out to ascertain if their signature has been derived from dissolution of Jurassic minerals at depth, or if the signature is locally derived from Palaeocene sediments.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California