Diagenetic Characterization and its Influence on Petrophysical Properties of the Jurassic Smackover Formation Thrombolite and Grainstone units of Little Cedar Creek Field, Alabama

Sandra Tonietto¹ and Michael Pope^2
1Petrobras / Texas A&M University Rio de Janeiro, Brazil; [email protected]
²Texas A&M University College Station, TX

The grainstone and the thrombolite units of the Smackover Formation at Little Cedar Creek Field (LCCF), in Alabama, USA, were analyzed to determine their reservoir characteristics. The Smackover Formation reservoirs in this field have only a minor amount of dolomitization, and most of the depositional texture of the reservoirs is preserved, making LCCF a unique location to study facies distribution and diagenetic alteration of these reservoirs. Depositional facies define good quality reservoirs of Smackover Formation, but diagenesis plays an important role on enhancing or reducing their porosity and permeability. Thrombolite and ooid-oncoid-peloid grainstone are the most prolific reservoir facies of the Smackover Formation , whereas dolomitization and dissolution are the main diagenetic processes improving porosity and permeability.

A paragenetic sequence based on petrography, cathololuminescence, and minor and trace elements analysis was determined on both reservoirs types. Image analysis of scanned thin sections calculated the percentage of grains, pores and cements in the samples. Both reservoirs record distinc early diagenetic events, but similar late diagenetic evolution. The microbial thrombolite was exposed only to marine diagenesis, but the ooid-oncoid-peloid grainstone also was exposed to meteoric phreatic waters.

The ooid-oncoid-peloid grainstone can be subdivided in two sub-units: oolitic grainstone and peloid-oncoid grainstone. The lateral distribution of these two sub-units shows that the oolitic grainstone prograded over the peloid-oncoid grainstone to the southeast. The areas where the oolitic grainstone dominates present the greatest porosity and permeability values.

A pore size map of the thrombolite indicates the biggest pores (vugs from 2 to 6 cm wide) occur in the areas where the microbial thrombolite is thicker, indicating that dissolution process follows depositional features. In the thicker portions of the bioherm primary porosity also was higher, facilitating dissolving fluids to percolate and create larger vugs.

Dolomitization occurs only in the south portion of the field. In the grainstone reservoir, dolomite occurs in a very small amount, usually less than 3%. In the thrombolite reservoir dolomitization is more intense, and dolomite can compose up to 30% of the rock. Dolomitization associated with dissolution significantly enhances permeability in this reservoir, from tens of millidarcys (usually 20 to 80 md) to hundreds of millidarcys (150 to 850 md). Blocky calcite cementation is more intense in the north portion of the reservoir, where dolomite is absent. Calcite cement can reach up to 75% of the rock, significantly reducing porosity. As the reservoir dips southwest, this heterogeneous late burial calcite cement and dolomite distribution indicates that the deeper portion saw a distinct pore water chemistry, different than the shallower northeast portion of the reservoir.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63^rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013