--> ABSTRACT: Massive, High Temperature Recrystallization of Low-Mg Calcite in Hydrocarbon Source Rocks, Upper Jurassic Smackover Formation, Mississippi, by Ezat Heydari and William J. Wade; #90906(2001)
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Ezat Heydari1, William J. Wade2

(1) Mississippi Office of Geology, Jackson, MS
(2) LSS International, Houston, TX

ABSTRACT: Massive, High Temperature Recrystallization of Low-Mg Calcite in Hydrocarbon Source Rocks, Upper Jurassic Smackover Previous HitFormationNext Hit, Mississippi

We document massive recrystallization of low-Mg calcite lime mudstone source rocks of the Smackover Previous HitFormationNext Hit, Mississippi, at moderately high temperatures driven by organic acids produced during kerogen maturation. This conclusion is based on the comparison of the following petrographic and bulk rock geochemical characteristics of organic-poor lime mudstones versus those of organic-rich lime mudstones: (1) TOC (total organic carbon): 0.01 vs. 0.5%; (2) d13C: +4.5o/oo PDB vs. +5.5o/oo PDB; (3) d18O: +1o/oo PDB vs. -3.5o/oo PDB; (4) Sr content: 900 ppm vs. 400 ppm; (4) 87Sr/86Sr ratio: 0.7069 vs. 0.7070; and (5) small calcite crystals (12µm) with smooth crystal surface and edges vs. large calcite crystals (20 µm) with pitted surface and irregular edges.

Evaluation of data suggests that Smackover lime mud formed inorganically as low-Mg calcite from a near normal salinity Late Jurassic seawater and was deposited in deep waters (>100 m) unaffected by meteoric diagenesis. High d18O and Sr compositions (+1o/oo PDB, 900 ppm, respectively) of organic-poor lime mudstone layers are nearly identical to the composition of modern low-Mg calcite, suggesting preservation of original mineralogy and geochemical signatures. Low d18O and Sr values (-3.5o/oo PDB, 400 ppm) of organic-rich lime mudstones are best explained by recrystallization at moderately high temperatures (50-75°C). This interpretation is supported by radiogenic 87Sr/86Sr compositions of organic-rich layers indicative of Previous HitrockNext Hit-Previous HitwaterNext Hit interactions in the presence of Previous HitformationTop waters. The driving mechanism for high temperature recrystallization of low-Mg calcite is inferred to have been organic acids produced during kerogen maturation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado