Diagenetic Interaction of Sandstones and Shales in Gulf Coast Clastic Sediments: Predicting Reservoir Porosity and Permeability Evolution in Deep Reservoirs

Anthony Park¹ and Kitty Milliken^2
1 Sienna Geodynamics & Consulting, Inc, Bloomington, IN
² University of Texas, Austin, TX

It has been long suspected that mudrocks are significant sources and sinks of various cations that affect diagenesis of nearby sands. Cations are mobilized from within mudrocks as chemically incompatible clays and feldspars react toward establishing a more stable mineralogical assemblage. Because reaction rates of both clays and feldspars are dramatically temperature-dependent, understanding this disequilibrium-driven network of reactions has particular significance for predicting porosity and permeability characteristics of deeper reservoirs.

Sym.8 is a diagenesis simulator that addresses compaction, diffusive and advective mass-transfer, conservation of mass through kinetic and equilibrium reactions, and associated sediment compositional and textural changes.

In this study Sym.8 is used to assess the role of coupled reactions, diffusion-driven mass-transfer and compaction, on porosity and permeability modification of Gulf Coast reservoir sand-shale pairs. Initial results verify that dissolution of detrital smectite and feldspars in shales contribute to early quartz cementation and progressive albitization of the feldspar grains in adjacent sandstones. Relative immobility of aluminum is clearly demonstrated, with significant precipitation of authigenic clay within the shale. The results show a range of mobility for cations, with calcium being one of the most mobile. It is estimated that the range of mobility is in the scale of meters, however sediment heterogeneity can strongly affect the mass redistribution process. Therefore, a significant component of late-stage reservoir permeability modification can be attributed to detrital sediment heterogeneity as well as the mineralogy and burial history.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005