Experimental study of chlorite authigenesis and influence on porosity maintenance in sandstones

Abstract

Chlorite is recognized as a key mineral for preserving reservoir quality in deeply buried sandstones, as chlorite coatings inhibit the nucleation of quartz overgrowths. A limited understanding of the mechanisms and conditions under which these authigenic chlorite coatings form prevents the accurate forward modeling of diagenesis and limits reservoir quality models critical to a wide range of geoscience applications. We present experimental data that show how authigenic chlorite grain coatings preserve porosity in deeply buried sandstone reservoirs, using a series of hydrothermal reactor experiments to simulate quartz cementation and capture the evolving porosity. To simulate reservoir evolution, berthierine-bearing sandstone samples (Lower Jurassic Cook Formation, Oseberg Field, 30/6-17R, Norway) were exposed to a silica-supersaturated Na2CO3 (0.1 M) solution for 72 hours at temperatures of between 100 and 250 °C. Quantification of the temperature-dependent changes in the volume of authigenic chlorite, the thickness and coverage of the clay coatings, and the sample porosity shows increases in chlorite volume (from ∼ 2% to ∼ 14%). This occurs by the transformation, of patchy amorphous berthierine into grain-coating Fe-chlorite cements through a mixture of the solid-state transformation and dissolution–precipitation mechanisms, siderite replacement, and direct precipitation on clay-free surfaces. With increasing temperature, the chlorite coatings increase from ∼ 3.8 μm to ∼ 5.4 μm thick and expand their grain surface coverage from ∼ 28% to ∼ 50%. The face-to-edge and face-to-face foliaceous structure of the clay coatings produced are morphologically similar to those observed in deeply buried sandstones. Only above temperatures of 175 °C is porosity preserved as a consequence of inhibition of quartz overgrowths and the generation of secondary porosity.