Upper Jurassic Depositional Systems and Hydrocarbon Potential of Southeast Mississippi: ABSTRACT

Abstract

ABSTRACT Upper Jurassic sedimentation in southeast Mississippi was controlled by eustatic sea level fluctuations and locally modified by salt tectonism and basement structure. The present study using conventional core data and geophysical logs indicates that a stable carbonate platform developed along the updip margin of the Mississippi Interior Salt Basin. The basin was partially barred from the main Gulf of Mexico water mass by the Wiggins Uplift, and became evaporitic during the upper Jurassic. The Smackover and Haynesville formations are separate sedimentologic packages deposited during two distinct sea level cycles. High energy carbonates extended across the northern basin margin as sedimentation kept pace with subsidence and slowing sea level rise near the close of each transgression. An effective shelf margin barrier did not develop. Lime mud generally accumulated under subtidal conditions within the interior salt basin. However, the basin was sufficiently shallow for wavebase sedimentation to occur on crests of salt-cored bathymetric highs, resulting in localized reservoir facies. To the south, eolian conditions initially prevailed on the Wiggins Uplift Later, as the Wiggins subsided, subaqueous shoals fringed and capped individual basement islands. Eustatic sea level fluctuations and salt movement controlled diagenesis and porosity formation. Limestone porosity dominates in upper Smackover platform carbonates, with a transition from secondary moldic porosity updip to preserved primary porosity downdip. Moldic, intercrystalline, and vuggy dolomite porosity is developed on the crests of intermediate and high amplitude salt highs and on the Wiggins Uplift. Jurassic source rocks are lower Smackover laminated lime mudstones. Migration into adjacent reservoirs postdated formation of porosity and the growth of salt anticlines, the most common trap type. A large potential Norphlet-Smackover gas play extends along the southern flank of the Wiggins Uplift. Salt anticlines within the interior basin remain viable targets. Small oil discoveries should continue in stratigraphic traps, subtle salt structures, and basement blocks on the platform.