Comparative Sedimentology and Diagenesis of Upper Jurassic Ooid Grainstone Sequences, East Texas Basin
Ooid grainstones in Upper Jurassic carbonates in East Texas have been hydrocarbon exploration targets for some thirty years. Shoaling-upward sequences through mudstones/wackestones and packstones culminate in well-sorted ooid grainstones in many cores. A succession of several shoaling-upward sequences, each averaging 12-18 m (40-60 ft) thick, is commonly developed in upper Smackover carbonates around the basin margin where it produces large cumulative thicknesses of grainstones. In contrast, basinal carbonates contain fewer and thinner grainstone sequences.
Porosity and permeability within the grainstones are controlled by their diagenetic history. Along the northern flank of the East Texas Salt Basin it is possible to trace a lateral continuation of the northern, transitional and southern diagenetic zones recognized in Louisiana and Arkansas. Oomoldic porosity is well developed in the northern zone and patchily present in the transitional zone. Early complete dolomitization is prevalent at the top of the Upper Smackover in the northern and transitional zones; late dolomitization is minimal. Little or no dolomitization occurs in the southern zone.
On the western flank of the basin early dolomitization of upper Smackover carbonates is less common but some percentage of later, post-compaction dolomite is present in both updip and downdip carbonates. In updip areas fracturing is common, with fracture fill calcite cement precipitation. Little or no fracturing is observed in the downdip carbonates.
Diagenesis of the more basinward carbonates is comparable to that of the southern diagenetic zone of Louisiana-Arkansas, with the exception of the presence of some replacive baroque dolomite. In addition, there is no evidence of late, solution-enhanced porosity development in East Texas.
Porosities and permeabilities are highest in the dolomitized updip Smackover grainstones; oomoldic limestones have good porosity but often low permeability. In general, downdip and basinward grainstones possess lower porosities and permeabilities.
Figures & Tables
Carbonate Sands-A Core Workshop
Carbonate sands, both skeletal and non-skeletal, have been studied by geologists as intensely as carbonate buildups. The underlying reason for the studies is the importance of those sands as significant hydrocarbon reservoirs. This core workshop is intended to provide a “hands on” look at the subsurface geologic record of carbonate sands with emphasis on lithofacies, stratigraphy of the sands and surrounding deposits, geometry of the sand deposits, diagenesis and porosity evolution, and wireline log data.