Geotechnical properties of Mississippi River delta sediments utilizing in situ pressure sampling techniques

Abstract

Rapid accumulation of organic-rich sediments and the subsequent anaerobic, biogenic decomposition of the organic material on the Mississippi River delta have led to sediment regions with high methane concentrations. The presence of the very soft, underconsolidated, gassy sediments creates a variety of problems for the acquisition of seismic data and samples for geotechnical analyses. A pressure core barrel has been developed by Texas A&M University, in conjunction with the U.S. Geological Survey Mississippi Delta Project, to investigate these shallow gas-charged sediments. Substantial quantities of biogenic methane gas are found in the surficial sediments (0-50 m subsurface) of the Mississippi Delta. The presence of methane gas in the pro-delta sediments is believed to play a major role in sediment instability phenomena. In addition, the acoustic attenuation of high resolution seismic signals that causes an acoustic wipeout commonly observed over large areas of pro-delta sediments, as well as other regions worldwide, is also believed to be caused by the presence of biogenic and/or thermogenic gas in bubble form. New techniques developed for geotechnical sampling with a pressure core barrel and pressurized Shelby tubes have provided insight into the effects of methane solubility and the resultant physical characteristics of samples collected and tested at in situ and ambient pressures. Sample testing procedures require the use of a manned hyperbaric chamber, pressurized to in situ sample pressures. Test results indicate samples that contain high methane concentrations show marked reduction in strength characteristics resulting from evolution of methane gas in bubble form upon release of in situ pressures. Gas bubble formation is directly evidenced as increased sample void ratio and porosity. Consolidation test results indicate that this process can be correlated to increased coefficients of consolidation and compressibility.