PETROGRAPHIC ANALYSIS OF DIAGENETIC TRENDS AND POROSITY TYPES IN THE UPPER SMACKOVER FORMATION, SOUTHWESTERN ARKANSAS

The upper sequence of the Jurassic Smackover Formation in the subsurface Gulf Coastal Plain of southern Arkansas consists primarily of ooid grainstones, which serve as substantial hydrocarbon and brine reservoirs. Petrographic analyses of these grainstones provide critical information for interpreting reservoir quality and therefore are useful for hydrocarbon and brine exploration. Thin sections of the upper Smackover Formation taken from cores in seven wells from seven oil fields in Miller, Lafayette, and Columbia counties in southwestern Arkansas were analyzed for trends in primary porosity type and diagenetic features, including cementation, dissolution, replacement, and compaction.

The seven wells were grouped into three diagenetic zones that generally correlate with Moore and Druckman’s (1981) original study. Samples from the northern zone (Paup Spur, Midway, and McNeil East fields) often exhibited secondary moldic and intraparticle porosity as well as early equant calcite mosaic cement occluding interparticle pore space. Ooids were either fully or partially dissolved or completely recrystallized. Compaction features were not prevalent. Porosity in the southern zone (Walker Creek and Atlanta fields) was predominantly primary interparticle, and ooids were either micritized or partially replaced with very fine calcite. Early calcite rim cement was common but did not completely occlude interparticle space. Medium to coarse calcite spar was the most common porosity-occluding cement. Compaction features such as pressure solution contacts and stylolites were common. The transitional zone (Mt. Vernon and Kress City SE fields) had diagenetic characteristics of both the northern and southern zones.

There were no conclusive results of the mechanisms behind these diagenetic patterns since this study did not involve geochemical data. Future work should include trace element and isotope geochemistry to determine original ooid mineralogy and diagenetic settings.