Depositional Fabrics and Dolomitization In Microbial Reef Reservoirs at Little Cedar Creek Field, Conecuh County, Alabama

The relationships between depositional fabric and diagenetic textures are examined in the Upper Jurassic Smackover Formation at Little Cedar Creek Field (LCCF), Conecuh County, Alabama. Reservoir quality in LCCF is influenced significantly by the modification of depositional fabric types by the digenetic processes of dolomitization and leaching.

Two different microbial fabric types are identified in the thrombolite boundstone in LCCF. They are layered thrombolite (type I) and chaotic thrombolite (type II). Layers in type I thrombolite are thicker and textures of type II thrombolites are similar respective to the type I and type II of the five thrombolite fabric types identified at Appleton and Vocation Fields by Parcell (2002). Both type I and II alternate development over time. Type I is associated with centimeter scale microbial layers interbedded with lime mudstone. High vuggy porosity and high permeability in type II is an indication of equal lateral and vertical thrombolite fabric growth and produces a good reservoir rock. Planar subhedral dolomites replaced the limestone cement in type I and filled the limited pores. Dolomitization in the type II is limited to limestone cement replacement and void boundary fill with polymodal planar subhedral dolomites concentrated in the center part of the field. In the southwest of the field, where the entire thrombolite facies is type II, unimodal planar euhedral dolomite form in voids and void boundaries. In addition, some of the peloids are partial replaced by the dolomite leaving the original moldic texture.