Abstract
Illite polytype distribution is compared to pyrolysis T max values in the Smackover Formation in the Manila Embayment, southwest Alabama to determine its potential as an indicator of thermal maturity. The percent 2M illite polytype (2M/1M + 2M) increases from 20 to 80% over the formation temperature interval 100–165°C and as T max values increase from 432 to 461°C. Illite polytype distribution is considered a good qualitative indicator of thermal maturity in the Smackover Formation of the study area. The correlation between the percentage of 2M illite and temperature and thermal maturity in the Smackover Formation is believed to be primarily the result of the diagenetic transition from the 1M to the 2M polytype. Depositional effects are manifested in four samples as anomalously high 2M polytype percentages. This variation is interpreted to be the result of locally high concentrations of detrital 2M muscovite. Because original sediment composition and illite chemistry may vary significantly both locally and regionally, illite polytype distribution is a useful thermal maturity indicator only within a particular basin, and when calibrated with other thermal maturity indicators.
Published Version
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