Mark Wilkinson¹, R. Stuart Haszeldine¹, Kitty L. Milliken²
(1) Edinburgh University, Edinburgh, United Kingdom
(2) University of Texas, Austin, TX

Abstract: Secondary porosity in deep prospects: Implications of alkali feldspar dissolution in sandstones

Alkali feldspar dissolution continuously generates new secondary porosity over the interval of 0 - 4,500m. Consequently, the ratio of secondary porosity: primary porosity increases with depth, and secondary porosity may dominate in deep prospects. However, reliably estimating the proportion of secondary porosity is difficult, as conventional petrographic techniques are subjective. Also, the petrographic evidence (e.g. skeletal grains) is continually destroyed by compaction during burial. By studying the dissolution of the reactive mineral grains, so porosity evolution can be assessed realistically.

A compilation of data from sedimentary basins in the North Sea (Europe) and the Gulf coast (USA) reveals similar trends of decreasing alkali feldspar abundance with depth (c. 1500 to 4500m). This is interpreted to indicate that alkali feldspar dissolution is an important process over this depth interval, such that up to 30% feldspar may be lost. This loss produces economic porespace which is not balanced by the precipitation of authigenic clay. An alternative interpretation of the data, that the decline in alkali feldspar with present-day burial depth is due to proximal-to-distal depositional variation, can be dismissed. Firstly, it is unlikely that all the sedimentary basins studied have the same primary pattern of alkali feldspar distribution as they represent a number of sedimentary settings. Secondly, in the Texas Gulf Coast, distal Plio-Pleistocene sediments have the same alkali feldspar content as the modern Mississippi River which supplied them.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana