TEMPERATURE CONTROLLED POROSITY/PERMEABILITY REDUCTION, FLUID MIGRATION, AND PETROLEUM EXPLORATION IN SEDIMENTARY BASINS

Abstract

Findings mainly from the Norwegian Continental Shelf indicate that at temperatures greater than approximately 60°C, internally sourced quartz cementation and diagenetic clay become important porosity and permeability reduction factors, respectively. Porosity loss due to quartz cementation in sandstones and siltstones proceeds mainly independent of effective stress or fluid pressure. Porosity loss rates approach those required to generate high fluid overpressures at approximately 120°C, which can result in hydrofracturing of the overlying low permeability shales. In thick low permeable sediment sequences, the probability of km scale vertical fluid migration increases. These thermally driven processes are capable of generating and sustaining fluid overpressure and facilitate fluid migration for up to tens of millions of years. Unlike mechanical compaction models, this model predicts that porosity loss and therefore fluid migration will continue despite the buildup of overpressure, even during periods of no sedimentation. In sealed compartments, fluid flow from more deeply buried high permeable lithologies will occur by hydrofracturing of overlying low permeable lithologies, preferentially along near vertical faults if present. When hydrofractures are induced from high permeable sediments, they will propagate vertically through overlying low permeable sediments, unless they enter lithologies with sufficient permeability and volume to bleed off the fracture fluid propagation pressure. Quantitative analysis of the mineral reactions causing porosity loss cannot only identify sediments in thermal zones which are expelling fluids, but also lithologies in thermal zones which are likely to receive those fluids, in addition to the timing and rates of fluid migration. The model has important applications for evaluating petroleum exploration risks and the potential for remigrated hydrocarbon plays both at basin scale and prospect level.