Wayne A. Dunlap

Initial results and progress of the Mississippi delta sediment pore water pressure experiment

Abstract This report describes the instrumentation, initial results, and progress of an experiment designed to measure and monitor submarine sediment pore water and hydrostatic pressures in a selected area of the Mississippi Delta. The experiment also is intended to monitor significant pressure perturbations during active storm periods. Initial analysis of the data revealed excess pore water pressures in the silty clay sediment at selected depths below the mudline. Continuous monitoring of the pore water and hydrostatic pressures was expected to reveal important information regarding sediment pore water pressure variations as a function of the geological processes active in the Mississippi Delta.

Consolidation Characteristics And Excess Pore Water Pressures Of Mississippi Delta Sediments

Examination of the geotechnical properties of the Quaternary sediments from the middle and lower portions of the Mississippi Fan shows that the sediments are normally consolidated to overconsolidated to a depth of 40 to 50 below the seafloor. Beyond that depth, the sediments become highly underconsolidated and exhibit high excess pore water pressures. The state of underconsolidation is attributed to the rapid rates of accumulation and the low permeability of smectite clays.

Abstract: Massive Bed-Forms, Mega-Furrows, on the Continental Rise at the Base of the Sigsbee Escarpment, Northwest Gulf of Mexico

Abstract Some of the largest deep-water sedimentary bed-forms ever found in the world oceans were recently discovered along the base of the Sigsbee Escarpment in the northwest Gulf of Mexico. Deep-Tow high-resolution geophysical surveys on the continental rise south of the Sigsbee Escarpment have revealed the existence of mega-furrows, ~10 m deep and ~30 m wide, spaced ~100 m apart that occupy an area on the Continental Rise 20 to 40 km wide. A single furrow within a field of furrows in the Green Knoll Diapir area can be traced over a distance of 150 km. The furrows are located at the base of the Sigsbee Escarpment from 90° to 93° West Longitude, from north of Green Knoll to the area just west of Bryant Canyon. The presence of furrows, longitudinal bed-forms, suggests that unidirectional bottom currents up to 100 cm/sec may exist at the base of the Escarpment. The furrows are inferred to be formed by helical secondary circulation. Allen (1968) produced sedimentary furrows in the lab and found a hierarchy of bed forms ranging from uniform furrows at low flow conditions to meandering furrows and flute casts at the highest flow conditions. All the conditions described by Allen are present in the area due south of Bryant Canyon on the continental rise. In terms of hazards relating to deepwater hydrocarbon exploration and production, the formation of mega-furrows demonstrates the existence of previously undocumented high-velocity currents at the base of the escarpment. End_of_Record - Last_Page 410-------

Comparison of Geological and Engineering Parameters of Marine Sediments

Abstract : Various tests are conducted on sediment cores taken from the Gulf of Mexico in order to determine possible correlations between fast survey techniques and time-consuming laboratory tests.

Offshore Geologic Hazards: A Short Course Presented at Rice University, May 2-3, 1981 for the Offshore Technology Conference

Practically all parts of the United States continental shelves and some segments of the adjacent upper continental slopes are presently subject, or will be in the near future, to exploration and development. The same is true for many continental margins all over the world. Unless the potential influence of hazards is taken into account in the design, installation, and operation of any offshore structure, such structures can pose a threat that could result in pollution, damage, or loss of lives and equipment. This publication, written to accompany an AAPG Short Course, provides some kind of summary of current [at the time of writing] knowledge. Higher categories of geologic hazards as well as individual potentially hazardous geologic phenomena are described and discussed.

Methane Concentration and Distribution in Pressurized Core Samples from Mississippian Delta Sediments: ABSTRACT

Elevated concentrations of shallow biogenic methane are common in regions of rapid sediment deposition. A pressure core barrel was designed and implemented by Texas A&M University, in conjunction with the U.S. Geological Survey Mississippi Delta Project, to study these gas-charged shallow sediments of the continental shelf. Methane measurements were made on 10 pressurized cores taken in the South Pass region of the Mississippi delta. The results of these measurements indicated methane was present in concentrations ranging from 3,450 to 137,140 ppm (µl CH4(STP)/l wet sediment). Methane values were generally higher than found in comparative samples taken with conventional wire-line equipment. However, these values were lower than predicted by theoretical calculations and also lower than pressure corrected saturation values for methane solubility in seawater. Stepwise decompression experiments were performed on selected samples to study the rate of methane release into the surrounding inert atmosphere of the pressurized sampling container. Results showed that at least 98% of the methane was released from the sediment matrix within 3 to 5 hours after opening the pressurized core. Samples containing higher total methane concentrations demonstrated no change in concentration with pressure during stepwise decompression while those containing lower total methane concentration demonstrated a marked change in concentration with pressure. These experimental results, successfully obtained with pressurized core barrel techniques, provide further in ight into the physical and chemical properties of gas in sediments. End_of_Article - Last_Page 1007------------