Indicators for an Isolated Thermal Event Affecting the Late Carboniferous Sediments, Saudi Arabia

The diagenetic history of Late Carboniferous sandstones reflects the physical and chemical changes occurring through geologic time. Given their Late Paleozoic age, quartz-rich composition, and long thermal exposure, it is not surprising that diagenesis in these sandstones is dominated by compaction and quartz cementation.

A “typical” diagenetic mineral, petrographic and paragenetic analyses revealed a suite of minerals, occurring within limited stratigraphic intervals, that indicate or strongly suggest localized thermal alteration. These minerals include galena, barite, coarsely-crystalline pyrite, and dickite, all of which may be associated with thermal alteration. Dickite and barite occur together in discontinuous, sub-millimeter-width, bedding-perpendicular microfractures, which may themselves have been created by thermal fluids exceeding local lithostatic gradient. Coarsely-crystalline pyrite, which is crystallographically distinct from the framboidal pyrite that is common in early diagenesis, is associated with galena, barite, and dickite.

The co-occurrence of galena, barite, dickite, and late pyrite is highly suggestive of stratiform or stratabound thermal mineralization occurring in a localized, evolving fluid system. Although, thermal events can theoretically occur at almost any time during burial diagenesis, all indications are of late stage, but episodic, emplacement.

Studies to date do not fully constrain the spatial, temporal, and physical extent of thermal modification, which complicates prediction of rock properties. Paragenetic relationships suggest that thermal fluids invaded these sandstones at different times and in the sandstones’ diagenetic histories. Thermal events have the potential to significantly alter rock properties by elevating thermal gradients and precipitating pore-filling minerals, and on compaction and quartz cementation.