ABSTRACT

The Smackover at Boyd Hill Field, although located in the Northern Zone and exhibiting oomoldic porosity diagnositic of exposure to a regional meteoric system, has features more commonly associated with the Transitional and Southern Zones. These include overpacking of allochems, some mechanical and solution compaction, bladed rather than equant rim cements, and coarse mosaic and poikilotopic calcspar cements. Twenty-two samples provided 50 homogenization temperatures fro two-phase fluid inclusions. These data have not been pressure corrected, but can be compared directly with similarly uncorrected data for the Southern Zone from earlier workers.

Bladed scaleonohedral rim cements (64-114°C) are non-luminescent; these may have stabilized in highly oxidizing pore waters. Fine-grained calcite cements (79-129°C), which commonly overlie the bladed cements, show bright luminescence and may have formed from mildly reducing pore waters. Post-compaction calcite cements occure as two modes: intergranular and oomoldic mosaid (92-110°C) and poikilotopic (83°C). Both cements show luminescent zoning; a dull center is surrounded by a non-luminescent zone and then alternating bright and dull zones. Echinoderm grains (75-102°C) show dull luminescence; their overgrowths show a bright to non-luminescent zoning. Some post-compaction dolomites (77-89°C) have dull luminescence and are ooid-replacive. Others, slightly coarser and oomoldic, are non-luminescent or zoned bright to non-luminescent and are slightly ferroan. Replacive anhydrite occurs as two generations--a fabric-selective phase (76-99°C) and a cross-cutting phase (84-100°C).

There is enough overall match between homogenization temperatures at Boyde Hill Field and those previously reported from the Southern Zone to conclude that the thermal histories are very similar. Based on a published Jurassic geothermal model, our oldest cements, the bladed rim cements, were in place no later than 130 mybp at a depth of 800 m or less, and probably much earlier. Most of the various cements apparently have re-equilibrated to the point that it is impossible to meet one of the original goals of this study -- to establish a stratigraphy of cementation/diagenesis based on temperature.

Zoning developed in these local Northern Zone cements shows that the pore waters had a complex oxidation-reduction history that cannot be attributed to a simple progression of oxidizing through mildly reducing to anoxic conditions due to increasing burial depth. The alternating bright and dull bands of much of the zoning must represent episodic variations in the redox potential of pore waters and perhaps in the critical Fe/Mn ratio as well. Global sea-level fluctuations during the Late Jurassic-Ealry Cretaceous provide a plausible mechanism for episodic incursions of mixed waters of varying redox states into part of the Northern Zone. Perhaps this may explain why Boyd Hill Field has a diagenetic pattern more common to the Transitional and Southern Zones.

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