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Improving Prediction of Porosity Preservation in Thermally-Stressed Deep Marine Sandstones: A Synthesis of Grain-Coating Chlorite Observations


Many offshore basins produce from Neogene reservoirs, where depth-porosity trends are successful predictive tools; however, the results of recent exploration in pre-Neogene rocks are mixed. Due to their often deeper burial depths and prolonged residence time above the temperature threshold for quartz overgrowth formation, these older sandstones risk suffering advanced quartz cementation. One common mitigator to this high thermal stress is the overgrowth-inhibiting morphology of grain-coating chlorite (GCC). While GCC is commonly observed in terrestrial and shallow marine sandstones, its presence and effectiveness in preserving porosity in sandstones deposited in deep marine environments is not well documented. Deep marine turbidites are a significant reservoir type in many untested plays, therefore the prediction of GCC in these environments is critical to future exploration success. Despite the volume of data concerning deep marine reservoirs, GCC is claimed to inhibit quartz cementation in only six cases globally. We review these examples and their geological controls, and propose a simple framework for the formation of GCC in deep marine sandstones. The shared compositional and depositional characteristics of the reported examples show that the probability of GCC in deep marine sandstones is increased by satisfying three primary requirements. First, sands develop precursor grain coats in shelf environments. Secondly, rejuvenation of precursor coats, partially abraded during transport to deepwater, occurs via redistribution of pore-blocking clay to a grain-coating morphology by the dewatering of high-density flows. Finally, the rejuvenated precursor coats are chloritized by drawing on an in situ source of iron and magnesium. None of the six examples reviewed were deposited further than 60 kilometers from their shelf edge. We posit that the key limiting factor in this process is the retention of high density flow at large transport distances by confined bathymetric topography, whereas low density flow separates the clay and sand fractions. While the claims of GCC in deep marine sandstones effective at inhibiting quartz cementation are generally validated, the phenomena is limited in distribution. Deep marine sandstones containing abundant GCC similar to shallow marine or terrestrial examples remain unknown; however, this synthesis forms a robust starting point for further investigation of the controls on reservoir quality in thermally-stressed environments.