Robert A. Horton1
(1) California State University, Bakersfield, CA
ABSTRACT: Depth-porosity relationships in Eocene through Miocene arkosic sandstones, San Joaquin basin, California
During burial, primary porosity is rapidly reduced through grain rotation and labile-grain alteration/deformation. Porosity is maintained by grain dissolution. Dissolution is dependent on an open hydrologic network. The best development of secondary porosity occurs in clean arkosic sands at shallow burial depths that have maintained open intergranular pore networks. In sands containing abundant labile grains intergranular porosity is quickly lost as pseudomatrix forms and clogs the pore network. This restricts development of secondary porosity. With increasing burial porosity is maintained in clean sands by creation of secondary pores as older pores (primary and secondary) collapse. Eventually the intergranular pore network becomes dominated by secondary porosity. As the burial depth increases the interconnectedness of the pore network decreases. This restricts the flow of diagentic fluids into the sands and grain dissolution decreases. However, tectonic activity has caused extensive fracturing. As the degree of induration increases with burial the nature of the fracturing changes. Whereas fracturing in loosely consolidated sands occurs mainly on the scale of individual grains, in highly indurated sands the fractures are through-going. This alters the hydrologic regime concentrating fluid movement and dissolution along the fractures. The result is a change with depth from an open, evenly distributed porosity network dominated by primary intergranular pore spaces, through a stage of less evenly distributed intergranular (primary and secondary) porosity, to an interconnected network of secondary porosity occurring mainly adjacent to fractures.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado