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

Andrew R. Thomas, 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 #1 well. Stylolitization of quartz and k-spar, the two most common framework grain types, resulted in the precipitation of quartz and muscovite cement within the Norphlet Fm. 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 by far 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. Younger muscovite pore fillings are found in close proximity to the stylolites, and have not yet been conveyed back to the insoluble residue seam. Large 1M muscovite growth is correlated to late, intense stylolitization.

Burial history reconstructions constrained with vitrinite reflectance show that the muscovite grew at burial depths ranging from 13,000^prime to 18,000^prime. Large 1M muscovite occurrence coincides with the top of the dry gas window. Compaction and microbrecciation concomitant with final expulsion from the Lower Smackover source/seal rock is speculated to have played a part in the timing of intense stylolitization. Such a compromised seal could have prompted fluctuations in reservoir fluid (gas) pressure during leakage. Such fluctuations would have temporarily increased effective overburden, catalyzed the pressure solution process, and caused extensive late quartz cementation after hydrocarbon cracking. Other high pressure Norphlet wells contain very little stylolitization and quartz cement, and in those wells fluid pressure is interpreted to have minimized effective overburden and retarded stylolitization.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California