Secondary porosity development in incised valley sandstones from two wells from the Flemish Pass area, offshore Newfoundland

Abstract

In this study we discuss the diagenetic processes that have contributed to the preservation of anomalously high secondary porosity (20–30%) and associated high permeability in relative deeply buried (2–3 km) Tithonian-aged stacked incised valley reservoir sandstone intervals (informally named Ti-2 and Ti-3) in the Flemish Pass area, offshore Newfoundland. These incised valley sandstones are bounded by thick organic-rich and/or organic-lean siltstone and mudstone deltaic deposits. Here we use a multi disciplinary approach including thin section petrography, cathodoluminescence (CL) petrography, programmed pyrolysis, and organic petrology to examine the controls on organic matter and secondary porosity development in the studied sandstones. The incised valley sandstones are dominated by litharenites, sublitharenites, and feldspathic litharenites. The most significant early diagenetic pore-occluding event is pervasive poikilotopic calcite cementation that arrested the affects of mechanical compaction preserving framework grains in point contact. Secondary porosity formed by dissolution of the calcite cement is commonly speculated to be related to thermal maturation of organic matter (OM) and associated generation of short-chained carboxylic acid (SCCA). %VRo data shows the OM to be immature (0.5%) therefore the estimated burial temperature of the sediment likely never got above approximately 60 °C in the late diagenesis to early catagenesis stage. This thermal maturity is consistent with the pre-oil generation window and the onset of SCCA generation allowing early calcite dissolution to take place. It is likely a second phase of meteoric water flushing took place, however, the scale of investigation of this study was not adequate to verify meteoric water flushing as a mechanism for calcite cement dissolution.