Contact diagenesis: The effect of an intrusion on reservoir quality in the triassic sherwood sandstone group, Northern Ireland

Intrusion of a dolerite sill into clay-bearing, arkosic Triassic Sherwood Group sandstones with moderate porosity (22%) and high but anisotropic permeability (mode 500 md) led to the progressive transformation of precursor diagenetic minerals. In the host rock, locally pore-filling Mg-rich smectite (saponite), quartz, and dolomite, existing in the rock following earlier burial diagenesis, reacted to produce grain-coating flakes of talc at the lowest contact temperatures (130-180°C; based on equilibrium reaction-path and thermal-history calculations). At higher temperatures (200-230°C), talc reacted with calcite to produce an actinolitic amphibole. Close to the intrusion, at temperatures of > 250°C, bundles of acicular needles of actinolite protruding into open pores were produced by a second reaction involving saponite, hematite, and calcite. Framework feldspar minerals (mainly K-feldspars) played little or no part in the reactions because there were no K-Al rich minerals produced by contact diagenesis and because the detrital feldspar grains are essentially unaltered. Thus while equilibrium may have been approached on the intrapore scale (∼ 10 μm), it was not achieved between grains (e.g., detrital feldspars), pore water, and clays in the pores. Total porosity has been unaffected by the intrusion and consequent mineral reactions, because reactions involved recrystallization of original pore-filling minerals. Despite the contact diagenetic changes in the rock, the permeability of the sandstone remained largely unaffected by intrusion.