Spatial and Temporal Evolution of Burial Diagenesis Driven by Geothermal Convection in Pre-salt Lacustrine Carbonate Reservoirs

Prolific deepwater hydrocarbon discoveries in the pre-salt section of the Santos Basin have created a business imperative to predict lacustrine carbonate reservoir quality. Dissolution related to exposure would likely have occurred in lacustrine carbonates subject to climatically driven changes in lake level as evidenced by seismic scale unconformities. In contrast, the potential to enhance reservoir quality by dissolution in the burial environment is poorly understood. Geothermal convection is a style of groundwater flow, driven by temperature induced variations in fluid density, capable of diagenetically modifying reservoir quality. Based on a Santos Basin half-graben conceptual model, we used Reactive Transport Models to evaluate the potential for diagenesis driven by convection. Specifically we tested the effect of variations in basal heat flux, lake temperature, permeability, faults, stratigraphic architecture (thickness and geometry), salt thickness, salt rugosity, salt minerals and pore fluid composition.

Diagenetic rates are critically controlled by temperature gradient and fluid flux following the principles of retrograde solubility. Simulations predict that convection operates in lacustrine carbonates, prior to salt deposition but rates of dissolution in the reservoir interval are generally low being on the order of 10-2 to 10-4 volume % / M.y., and thus insignificant with respect to net diagenesis. The exception is around vertical faults where dissolution rates are amplified to 1 - 10 volume % / M.y to locally enhance vertical connectivity. Post-burial simulations demonstrate the critical role of salt rugosity and the presence of salt withdrawal basins. The greatest potential for dissolution at rates of 0.1 - 1 volume % / M.y occurs where salt welds thin to less than 300 m. Dissolution is greatest beneath the edge of the withdrawal basin in the top of the reservoir. With 10’s of M.y. available residence time, when the above conditions are satisfied, convection could locally result in porosity changes of 1-10 % and potentially an order of magnitude or more in reservoir permeability. By integrating our model driven predictive diagenetic concepts for geothermal convection and other diagenetic processes with traditional subsurface datasets we are able to further refine our exploration to production scale understanding and hypotheses of lacustrine reservoir presence and quality in the Santos Basin and elsewhere.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California