ABSTRACT: Authigenic Muscovite and Stylolitization Timing, Jurassic Norphlet Formation, Offshore Alabama

THOMAS, ANDREW R., WILLIAM M. DAHL, C. M. HALL, and D. YORK

Stylolitization is common in some Norphlet Fm. areas, and is observed to vary in abundance from 0.4 to 1.1 stylolites/foot in the Mobile Area 872 No.1 well. Stylolitization of quartz and k-spar, the two most common framework grain types, results in the precipitation of quartz and muscovite cement within the Norphlet Formation. Three authigenic muscovite morphologies are associated with Norphlet Fm. stylolitization; 1) large crystals of 1M muscovite found in vertical stylolite offsets; 2) fine-grained platy muscovite pore-fillings which only occur near stylolites; and 3) fine-grained platy pods of muscovite found in stylolite insoluble residue.

Thirty one Ar⁴⁰/Ar³⁹ analyses of groups of these mica morphologies indicate that the large 1M muscovite grew at 51 +/- 9 ma, pore fillings at 77 +/- 22 ma and pods at 86 +/- 16 ma. The large 1M muscovite is the most abundant of the three muscovite types.

Petrography illustrates that the muscovite paragenesis is related to Norphlet Fm. stylolitization. Early muscovite pore fillings grew near incipient stylolites due to k-spar dissolution. As stylolitization continued, the earliest pore fillings were conveyed back to the stylolite and currently are found within the seam of insoluble residue as mica pods. Large 1M muscovite growth is correlated to late, intense stylolitization.

Stylolitization in the subject well and associated porosity reducing cementation is more intense than that observed in penetrations from higher pressured areas. In higher pressure regions of the Norphlet we interpret that high fluid pressure can minimize effective overburden pressure, retard stylolitization, and maintain high reservoir quality.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana