Abstract: Sandstone Diagenesis-The Hole Truth

John B. Hayes

Most sand when deposited has high primary porosity and permeability. Soon after deposition and continuing through deep burial, many physical and chemical diagenetic processes modify primary porosity so much that, in most hydrocarbon reservoir sandstones, primary intergranular porosity is scarcely recognizable. Compaction, cementation, recrystallization, and replacement are widely recognized porosity-reducing mechanisms. Diagenetic dissolution of detrital and authigenic minerals can restore or enhance porosity at depth; many pores in most reservoir sandstones actually are solution or secondary pores, a fact not well documented in the published record. Because secondary porosity differs from primary porosity in terms of origin, subsurface distribution, pore-size distributio , pore shape, surface area, and attendant permeability, secondary porosity must be recognized and evaluated as such by explorationists and reservoir managers. Except for compaction, diagenetic processes which modify sandstone porosity and permeability are essentially chemical reactions between minerals and migrating pore fluids. Diagenetic modifications, therefore, are controlled in part by sand composition and by texture. Composition and texture, in turn, are determined by: (1) tectonic setting of the sedimentary basin and sediment source areas; (2) composition of rocks and weathering regime in sediment source areas (the provenance); and (3) environment of deposition. These pre-diagenetic factors are interrelated, and together they program the course of diagenesis. A novel or unique element in these factors can cause pronounced adverse or beneficial effects on reservoir properties.

Our greatest ignorance about diagenesis has to do with the tens and hundreds of thousands of pore volumes of water which had to pass through a sandstone to produce the observed mineral reactions. Where and when do subsurface waters originate, how do they evolve chemically as they migrate and react, and how can we decipher their character and genesis from the precipitated or dissolved solids in their wake? Progress on these problems will require multidisciplinary approaches. Rewards will be more accurate porosity prediction in exploration and more efficient management of rock-fluid interactions in producing reservoirs.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado