About This Item
Share This Item
The exploration fairway of the Upper Jurassic Smackover, from the Rio Grande to the Panhandle of Florida, consists of a rather simple carbonate ramp depositional system characterized by thick, widespread blanket ooid sands. The ooid sand belt gives way landward to quartzose
clastics, and locally to shales and evaporites. The high-energy ooid sand belt changes sharply into dark, fine-grained limestones and shales in a seaward direction. Early salt movement, buried basement structures, and growth faulting locally affected sedimentation patterns within the upper Smackover. Salt anticlines active during Smackover deposition leading to localization of favorable facies are particularly important along the east Texas and south Arkansas parts of the trend. Buried basement structures as well as salt structures controlled detailed sedimentation patterns in the Alabama-Florida parts of the trend, while growth faults controlled sedimentation along a narrow belt straddling the Arkansas-Louisiana border. Initial porosities and permeabilities were, however, generally h gh across the entire Smackover fairway prior to burial. Presently observed porosity-permeability trends are controlled generally by postdepositional processes including compaction, dissolution, cementation, and dolomitization. Regional differences in burial history along the Gulf rim, related in part to proximity to the isolated interior salt basins (Mississippi, north Louisiana, and east Texas salt basins), have resulted in striking differences in reservoir characteristics across the trend, reflecting significant regional differences in pathways of porosity evolution.
East Texas is a province dominated by dolomitized reservoirs, with production controlled by proximity to major fault trends, such as the Mexia-Talco, or to well-defined salt anticlines. Reservoir quality is tied inextricably to dolomitization. Dolomitization occurred early, associated with a regional fresh meteoric water system, hence reservoir characteristics were generally established prior to significant burial. Fracturing is a general feature of east Texas sequences, and there is evidence that fractures have acted as conduits for hydrocarbon migration. East Texas is generally a gas province, with oil migration occurring relatively early in the burial history of the sequence. Late burial diagenetic events in east Texas seem to have had little influence over ultimate porosity evolut on.
Because of the depths involved, the south Texas Jurassic has been explored only superficially and no production has been established. Recent works by Loucks and Budd on the available material indicates strong similarity to east Texas relative to porosity evolution and general diagenetic framework. However, Loucks and Budd do note significant, late, subsurface, secondary porosity development.
South Arkansas, Louisiana, and Mississippi stand out in stark contrast to the Texas part of the trend. Reservoirs are generally limestones, with porosity either early fresh meteoric, secondary moldic, or preserved primary porosity. The early meteoric moldic porosity occurs in a predictable trend across the updip portion of the fairway. Reservoirs with preserved primary porosity occur in a band seaward of the secondary trend, and show no evidence of freshwater influence, or early diagenetic processes other than minor marine cements. Porosity preservation in this zone is a function of grain type (ooids versus pellets) and original sediment texture, and hence it is controlled ultimately by depositional processes. In the primary porosity zone, production is almost always associated with s lt-related structures, while in the early secondary zone, updip permeability barriers (diagenetic and stratigraphic?) as well as salt-related structures are important. Late diagenetic events associated with migration of basinal derived fluids across the shelf during moderate burial include cementation dedolomitization, and calcite dissolution. All reservoirs in this part of the trend show ample evidence of significant porosity enhancement during this late solution phase. The limestone trend of Arkansas, Louisiana, and Mississippi is primarily an oil province with hydrocarbon migration taking place much later than to the west in Texas.
In Alabama and Florida, the trend is again toward dolomite reservoirs, with most dolomite and hence porosity permeability establishment early, and associated with meteoric water processes. Most large reservoirs such as Jay are associated with salt anticlines; minor production comes from updip basement structures. This part of the trend is mixed gas and oil. Hydrocarbon migration into reservoirs seems to have been a relatively late event.
The Jurassic upper Smackover of the Gulf rim is a simple sedimentologic system that has seen a complex and variable burial history along the trend which is distinctly reflected in major changes in diagenetic history, reservoir-porosity type, trap characteristics, and hydrocarbon migration timing.
End_of_Article - Last_Page 1469------------