Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Clyde H. Moore, Jr

Abstract

The sequence stratigraphic framework of the Smackover Formation and other Jurassic strata in southwest Alabama is delineated, with the objective of eventual correlation of equivalent sequences across the northern Gulf. Additionally, geologic controls on the diagenesis of organic matter in the Smackover Formation are examined, in order to better understand organic-inorganic interaction and hydrocarbon evolution during burial. Jurassic systems tracts of southwest Alabama differ significantly from those of the north central Gulf in that they show profound influence of antecedent topography, consequent large continental clastic influx, and a subsidence history less influenced by thermal cooling. Norphlet siliciclastics constitute a continental lowstand systems tract. Marine-reworked uppermost Norphlet sandstone and lower Smackover laminated mudstone form a subsequent transgressive systems tract. The subsequent highstand systems tract is characterized by formation of upper Smackover rimmed shelves. Three type-2 sequence boundaries occur within the Buckner Formation and are marked by interfingering Cotton Valley continental sandstones. Lack of biostratigraphic control precludes assignment of definitive ages to Alabama Jurassic sequences. Jurassic sequences of southwest Alabama are regionally atypical and should not be used as models for Gulf-wide sequences. Analyses of Smackover crude oils and condensates reveal that these crude oils evolve along two distinct pathways. Normal hydrocarbon maturation is dominated by thermal cracking; hydrocarbons in high H$\sb2$S reservoirs are altered by reaction with reduced sulfur compounds. Because the latter causes preferential cracking of saturates and higher weight fractions, the criteria of: (1) low C$\sb{15+}$ saturate content, and (2) low saturate/aromatic ratio serve to differentiate altered and unaltered hydrocarbons. Hydrogen sulfide concentrations in Alabama Smackover reservoirs correlate with temperature above 115$\sp\circ$C and proximity of siliciclastic strata. Impermeable carbonate intervals surrounding Smackover reservoirs promote closed system behavior with respect to H$\sb2$S, thereby favoring thermochemical sulfate reduction. Local thicknesses of impermeable intervals therefore correlate closely with reservoir H$\sb2$S contents. Although low H$\sb2$S concentrations and absence of hydrocarbon alteration in most Arkansas Smackover reservoirs indicate that thermochemical sulfate reduction is not presently occurring there, crude oils in three Arkansas reservoirs were altered during an earlier local thermal episode.

Pages

287

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