Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Intergranular Pressure Solution During the Diagenesis of Sandstone: Conditions and Controlling Factors
(1) Research and Engineering, M-I SWACO, Houston, TX.
(2) Department of Geology and Geophysics, Texas A&M University, College Station, TX.
Intergranular pressure solution (IPS) is regarded as an important mechanism of diagenesis for quartz sandstone. It is caused by stress-induced dissolution at grain contacts and diffusion down concentration gradient between grain contacts and pore fluid. But observations of natural sandstone indicate that the diagenetic role of IPS varies widely. Some sandstone shows well-developed pressure-solution structure while others have no visual evidence for IPS. To understand the conditions and controlling factors of IPS in diagenesis of sandstone, we study the dynamic processes at grain contacts and in pore fluid in compacting sandstone. The equilibrium concentration at grain contacts (ceqb) increases with stress, which depends on contact area. Therefore ceqb is independent of grain size, but is a strong function of strain. Calculations using experimental creep compaction rate and pore-fluid chemistry at 150°C and 34.5 MPa of effective pressure as input indicate that ceqb is within one order of magnitude higher than the hydrostatic equilibrium concentration (ceq) at the initial stages of creep compaction. ceqb decays rapidly with increasing compaction and becomes close to ceq after several percent strain. The evolution of ceqb is consistent to that of creep-compaction rate, both of which suggest a fast-decaying creep. The fluid-chemistry features at stressed grain contacts suggest that IPS is very sensitive to pore-fluid chemistry. To have a well-developed IPS, pore-fluid concentration is required to be close to saturated or undersaturated. Even a slight supersaturation in pore fluid could prevent the diffusion along grain contacts. Therefore, fluid flow in sedimentary basins might be an important controlling factor for IPS. Long-term descending fluid flow (undersaturated due to heating up) could result in well-developed IPS while ascending fluids (supersaturated due to cooling) would inhibit IPS. Mineral reactions in natural sandstone, such as feldspar alteration, could create a pore fluid supersaturated in silica, which would favor quartz cementation or overgrowth. Therefore, strong mineral alteration and the corresponding quartz cementation or overgrowth may suggest a poorly-developed IPS.