ABSTRACT

The Jay Field Smackover Formation (Oxfordian Age) produces oil from a complex network of dolomite horizons. Petrography and geochemistry indicate multiple generations of diagenetic alteration, including dolomitization, within the Smackover.

Calcium, magnesium, and five trace elements (iron, manganese, strontium, sodium, and potassium) were analyzed in duplicate by atomic absorption spectrometry. Mineralogy exerts the strongest control on the distribution of these trace elements; i.e., percent dolomite, calcite, and, to a lesser extent, insoluble residue most strongly affect trace element distributions. To negate these effects, elemental ratios were used. It is apparent that diagenesis results in observable trace element trends; extensive diagenesis masks any trace element trends which may have been due to original environmental differences. Indeed, certain trace elements may be useful as tracers for particular diagenetic processes but not for particular depositional environments in altered carbonates.

Evidence indicates early dolomitization of the upper Smackover grainstones in an evaporitic-sabkha environment. Geochemistry indicates that diagenesis of the lowermost Smackover may have been influenced by fluids migrating upward from the underlying Norphlet Sandstone.

At least one generation of dolomite in the lower and middle Smackover may be late diagenetic (deep burial?) and is associated with stylocumulate. Petrography shows dolomite precipitation in dilated, horizontal stylolites. Geochemistry suggests meteoric or mixed meteoric-marine waters for the diagenetic fluids. The dilated stylolites suggest overpressured zones (fluid pressure greater than overburden pressure) which would have promoted upward migration of fluids. Significantly, this provides a mechanism for diagenesis of portions of the lower to middle Smackover by meteoric or mixed waters long after burial.