Week – 8: Lithification and Diagenesis

This paper presents a mineralogical and chemical investigation of shale deposits from the U.S. Gulf coast in order to document the mechanisms and reactions involved in the diagenetic conversion of smectite into illite. Cuttings from the 1,250 to 5,500 m stratigraphic interval of a well were sorted by grain size and analzyed by x-ray diffraction to assess chemical and mineralogical trends with depth. Significant mineralogical changes are identified within the 2,000 to 3,700 m depth interval, with little change occuring outside this range. Within the depth range of major mineralogical change, illite/smectite (the most abundant mineral) undergoes a conversion from less than 20 percent illite layers to greater than 80 percent illite layers (Figure 1). In this same depth interavl several other mineralogical trends are noted within the coarser fractions of the shale: (1) Calcite undergoes a strong decrease from comprising approximately 20 percent of the shale to nearly zero. (2) Potassium feldspar decreases to zero. (3) Chlorite percentage increases. In contrast to the mineralogical trends, bulk chemical composition of the shale offers minimal evidence for substantial change with depth except for a decrease in the CaO content corresponding to the decrease in calcite abundance. In the finest grain size fraction of the shale (<0.1 m), which is essentially pure illite/smectite, more significant chemical trends are observed as a function of depth including: (1) Large increases in K2O and Al2O3. (2) A decrease in SiO2. The authors interpret all major mineralogical and chemical changes with depth to be a response of the shale to burial metamorphism. The depth-dependent mineralogical and chemical trends suggest that the primary diagenetic reaction within shales is: smectite + K-feldspar + mica = illite + chlorite + quartz, with dissolution of calcite and loss of calcium and CO2 from the metamorphosed shale also apparent. The authors suggest that diagenesis of the shale proceeded as a closed-system process for all components except H2O, CaO, Na2O, and CO2.