The Secondary Porosity Trick: Arkose to Quartzite in the North Sea
Stuart Haszeldine, M. Wilkinson, D. Darby, O. McLaughlin,
R. Stewart, G. McAulay, G. Couples, and A. E. Fallick
Observations of mineralogy, texture, poroperm, stratigraphy and overpressure compiled from major reservoirs of the North Sea at different burial, structural and pressure settings, indicate to us that secondary porosity has continued to form at depths of 10-18,000ft (3-6km). This has resulted in strata-bound economic improvements to reservoir quality by as much as 8% and 2,000mD.
The Brent Sandstone Group shows a decrease of feldspar with depth from 20% to 5%; clay content does not increase; identifiable secondary porosity does not increase. The HPHT Fulmar Sandstone shows a decrease of feldspar from 40% to 10%; aluminium (clay) content decreases; identifiable secondary porosity does not increase. Differences within these two sandstone Groups can not be explained by depositional variations. The Brae Formation shows loss of feldspar and calcite from 10% to 5%; only minimal clay is present; secondary porosity is often present, and is strata-bound. In this last case, an adjacent oilfield contains oil leaked up-dip from the deeper sandstones. This West Brae oilfield contains up to 13% kaolin clay, a big excess over the usual 2% expected for its present depth. The deep sand has lost clay, the shallow sand has imported clay.
We conclude that feldspars continue to dissolve below 10,000ft. The liberated aluminium is not precipitated locally, but can be transported for distances of at least 5 miles (7km) to form clay. Poroperm in the deeper sandstones is improved selectively along routes of overpressured fluid expulsion; these can become quartzites. Secondary porosity must also compact during burial to trick petrographers into textural mis-identification.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California