Richard W. Lahann

Occurrence and prediction of high-pressure sediment along the West African margin

High-pressure sediments in which oil and gas are generated and accumulate in traps are proven in many operating areas along the entire West African margin. Although classic areas for high pressure are found in Tertiary deltas, such as the Niger Delta, other types of basins in which young clastic sediments have accumulated also create the environment for high pressure and drilling challenges. The BP Macondo oil spill in the Gulf of Mexico has highlighted the technical challenge of drilling for oil in deep water, and the high pressures there added complexity to the control incident and the high volumes of fluids which blew out to the seabed.

Gulf of Mexico overpressure and clay diagenesis without unloading: An anomaly?

ABSTRACT Compaction disequilibrium is a widely accepted cause of overpressure, especially in clay-rich, rapidly deposited sediments. Clay diagenesis has been associated with the occurrence of overpressure greater than the compaction disequilibrium overpressure. These observations have led to the expectation that overpressure will be greater than the compaction disequilibrium contribution when clay diagenesis occurs within an overpressured mudstone. Clay diagenesis have been reported in a Pliocene section of a well from the Gulf of Mexico, offshore Louisiana. Pressure and log data from that well indicate that despite clay diagenesis, the overpressure can be attributed solely to compaction disequilibrium. This paper examines the whole mudstone and clay mineralogy composition and petrophysical characteristics of the offshore Louisiana well with clay diagenesis, but without a diagenesis contribution to overpressure and contrasts that data with results from other clay diagenesis and petrophysical studies. The comparison suggests that the offshore Louisiana well was relatively smectite poor compared with wells from regions associated with a clay diagenesis contribution to overpressure. The lower smectite content resulted in a lower percentage of reacted volume that was insufficient to allow the load transfer often associated with clay diagenesis. Petrophysical features of the offshore Louisiana well and nearby wells differ from the features associated with clay diagenesis in other Gulf of Mexico wells and a limited number of international wells. Comparison of location, age, depositional package, clay mineralogy, and petrophysical features suggests that provenance may control the occurrence of Gulf of Mexico mudstones that do not experience increased overpressure as a result of clay diagenesis.

Mechanism for Framework Grain Dissolution (Secondary Porosity in Sandstones): ABSTRACT

We propose that organic and clay maturation in concert are responsible for much framework grain dissolution (secondary porosity). Petrographic observations indicate a pulse of porosity formation near the top of the oil-generation window and that there is often not enough authigenic clay to account for the aluminum removed from the dissolved grains. Geochemical considerations indicate that H+ ions are required for aluminosilicate dissolution and that the aluminum must be complexed to concentrations greater than 100 ppm in order to transport aluminum out of the sandstone using water volumes available in most basins. The smectite to illite conversion, which is coincident with early organic maturation, produces additional pore water and can desorb organic molecules from the s ectite interlayers. The early stages of organic-matter maturation generates H+ (as carbon dioxide), volume-change pressures to move fluids, and water-soluble organic matter. The soluble organic matter can contain ligand compounds (e.g., short-chain fatty acids) which complex aluminum. The organic ligands in the shale complex aluminum at relatively low concentrations because aqueous aluminum activity is depressed by the formation of illite from smectite. The H+ and organic ligand-bearing solution is expelled into sandstones where the aluminum activity is buffered at higher levels by feldspar, thus allowing higher levels of complexed aluminum. The solution dissolves the feldspars and other aluminosilicate components and complexes much of the resulting aluminum for transport out of the sand tone. End_of_Article - Last_Page 630------------

Diagenesis of Sandstone/Shale Package, GCO/DOE No. 1 Well, Brazoria County, Texas: ABSTRACT

Petrographic analysis of closely spaced sandstone samples (GCO/DOE No. 1 well) of a sandstone/shale package from the Frio Formation (Oligocene) indicates that sandstone reservoir quality was influenced by shale diagenesis. Three alteration zones at increasing distance from the sandstone/shale contact are observed. This zonation may be explained as follows. Organic and inorganic maturation processes modified shale fluids which, upon expulsion into the sand, resulted in the precipitation of thin, isopachous chlorite grain coatings in the contact sand. Late in the chloritization process and thereafter, unstable framework grain silicates began to dissolve within the sand. We believe that aluminum from framework grains was removed from the contact zone by mobile organic complexes. Silica released from grain dissolution reprecipitated as quartz cement. This contact zone is about 1-ft (0.3 m) thick. As fluids passed into the second zone (about 1-ft (0.3 m) thick) the sandstone framework grain leaching continued but to a lesser degree. Kaolinite was produced from internal mass sources and from aluminum imported from the first zone. The net addition of alumina from the contact zone prevented development of quartz overgrowths in this zone and the third zone. In the third zone, dissolution of framework grain silicates was least thorough because of greater distance from the shale and alteration appears to have been aluminum conservative. End_of_Article - Last_Page 609------------

Silica Dissolution from Montmorillonite; Effect of Solution Chemistry: ABSTRACT

The rate of silica removal from two montmorillonites (Chambers and Polkville) has been measured as a function End_Page 483------------------------------ of time, temperature, solution composition, and exchange ion on the clay. Solution compositions ranged from 400 to 4,000 ppm potassium in all samples. Sodium concentration ranged from 0 to 9,400 ppm, calcium from 0 to 380 and magnesium from 0 to 10 ppm. Silica removal rate increased as the temperature increased from 200 to 350°C, decreased with time, and could be approximated initially by a parabolic rate law. Within the time range (from 1 to 10 days) approximated by the parabolic-rate law, comparison of rate constants allows quantitative evaluation of the effects of solution chemistry and exchange ion. Calcium-saturation of the clay reduced the value of the rate constant, relative to sodium-saturation, by about 50%. In all analyses, increasing solution concentration of an ion de reased the rate of silica removal. On an equimolar basis, magnesium was most effective at inhibiting dissolution, followed by calcium, sodium, and potassium. Reductions of the rate constant by 50 to 75% were observed for a Na-clay with 9,400 ppm sodium and for Ca-clay with 380 ppm calcium, relative to the sodium and calcium-free solutions. Activation energies for silica removal range from 3 to 12 kcal/mole. The highest values are associated with the largest concentrations of ions in solution, thus suggesting dissolution-inhibition by an ion adsorption mechanism. These results demonstrate that silica dissolution rate depends dramatically on solution composition. This relation should be incorporated into models constructed to describe sandstone cementation or porosity enhancement by dissol tion and transport of dissolved silica from clays in sandstones or interbedded shales and sandstone sequences. End_of_Article - Last_Page 484------------