Master of Science (MS)
Geology and Geophysics
Predicting the effects of mudstone diagenesis on reservoir quality is an important component of successful petroleum exploration and production programs. A regional study using chemical analyses from mudstone core cuttings and SP logs from 15 wells from the western Gulf of Mexico, including the Matagorda, Brazos, Mustang offshore areas, indicates that chemical changes with depth such as overall depletion of quartz and calcite; the illite-smectite conversion; and K2O enrichment occur in the mudstone Miocene interval of the study area. Mixing of depositional sources has been suggested to be the cause of similar chemical and mineralogical changes observed in Paleocene-Eocene and Oligocene sediments in onshore Texas. A mixing line of detrital material derived from two end member sources having different TiO2/Al2O3 ratios was found in this study. However, diagenetic reactions such as, precipitation of quartz and calcite zones above the smectite-to-illite conversion interval, feldspar dissolution, and overall K2O enrichment with depth correlate throughout the study area and the onshore Texas Gulf Coast, indicating that diagenesis in the region is more likely depth controlled rather than source, age or facies dependent. In addition, mudstone diagenesis is an open chemical process throughout the study area. A second study was performed for the upper Wilcox Formation in the Righthand Creek Field in southwestern Louisiana. Produced waters have salinities as low as 9,000 to 10,000 mg/L or approximately 1/3 the salinity of seawater. The clay mineralogy of sidewall cores was studied over a depth interval from approximately 11,000 to 11,550 feet to determine whether or not there was evidence for the smectite to illite transition having occurred in these sediments. The clay minerals in the sands within this interval are typically dominated by kaolinite and chlorite. The mudstones, in contrast, are dominated by smectite, kaolinite, and illite, and the smectite-illite transition has not yet occurred in these sediments. If the low-salinity waters at Righthand Creek are the products of dehydration, it is unlikely that they have been produced by in situ reactions and are instead waters that have been expelled from underlying and/or downdip, overpressured sediments where the transition is known to have occurred.
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Pacheco, Maria Antonieta, "Diagenesis of Mudstones, Offshore Texas and Southwestern Louisiana" (2005). LSU Master's Theses. 1438.