CONSIDERATIONS AND APPLICATIONS OF THE ILLITE-SMECTITE GEOTHERMOMETERIN HYDROCARBON-BEARING ROCKS OF MIOCENE TO MISSISSIPPIAN AGE

Empirical relationships between clay mineral transformations and tempe rature provide a basis for the use of clay minerals as geothermometers . Clay-mineral geothermometry has been applied mainly to diagenetic, h ydrothermal, and contact- and burial-metamorphic settings to better un derstand the thermal histories of migrating fluids, hydrocarbon source beds, and ore and mineral formation. Quantitatively, the most importa nt diagenetic clay mineral reaction in sedimentary rocks is the progre ssive transformation of smectite to illite via mixed-layer illite/smec tite (I/S). Changes in both the illite/smectite ratio and ordering of I/S, as determined from X-ray powder diffraction profiles, correlate w ith changes in temperature due to burial depth. Although the smectite- to-illite reaction may be influenced by several factors, reaction prog ress appears to be strongly controlled by temperature. Studies show th at the model proposed by Hoffman and Hower in 1979 is applicable in bu rial diagenetic settings from about 5 to 330 Ma, and includes most roc ks about Miocene to Mississippian in age. Reliability of the I/S geoth ermometer is, however, dependent upon a good understanding of the rock 's original clay-mineral composition. Changes in the ordering of I/S a re particularly useful in the exploration for hydrocarbons because of the common coincidence between the temperatures for the conversion fro m random-to-ordered I/S and those for the onset of peak, or main phase , oil generation. Here, the utility of the I/S geothermometer is revie wed in hydrocarbon-bearing rocks of Miocene to Mississippian age. Usin g three common applications, the I/S geothermometer is compared to oth er mineral geothermometers, organic maturation indices, and grades of indigenous hydrocarbons. Good agreement between changes in ordering of I/S and calculated maximum burial temperatures or hydrocarbon maturit y suggests that I/S is a reliable semiquantitative geothermometer and an excellent measures of thermal maturity.