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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 47 (1997), Pages 660-660

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

Andrew R. Thomas (1), William M. Dahl (2), C.M. Hall (3), D. York (4)

ABSTRACT

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, results in the precipitation of quartz and muscovite cement with 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 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 is the subject of well and associated porosity reducing cementation is more intense that than 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.

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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES

(1) Texaco EPTD, Houston, TX

(2) Texaco Exploration & Production Inc., New Orleans, LA

(3) University of Michigan, Ann Arbor, MI

(4) University of Toronto, Toronto, Ontario

Copyright © 1999 by The Gulf Coast Association of Geological Societies