Depositional age definition of the Açu Formation (Potiguar Basin, 40 39 northeastern Brazil) through Ar- Ar dating of early-authigenic K-feldspar overgrowths

Abstract

Early-authigenic K-feldspar overgrowths are widespread in the Cretaceous fluvial Açu Formation sandstones from the Potiguar Basin, one of the most important onshore oil reservoirs of Brazil. These overgrowths were formed at near-surface conditions (early eodiagenesis), directly on detrital Kfeldspar grains. The physical continuity between overgrowths and K-feldspar grains rendered impossi ble their separation by conventional means for ⁴⁰K- ⁴⁰Ar isotope analysis. An in situ ⁴⁰Ar- ³⁹Ar step-heating method was, therefore, applied and the minimum age of 120 Ma obtained for the authigenic overgrowths agrees with the depositional biostratigraphic age previously estimated for the Açu Formation. The method applied to the Açu Formation sandstones shows excellent potential for depositional age determination of other reservoir-sandstones. We suggest that future developments of the method will, however, greatly benefit from the use of ultra-violet (UV) laser for reducing chances of contamination of authigenic K-feldspar overgrowths analyses by detrital grains.