Pore Characterization and Classification in Carbonate Reservoirs and the Influence of Diagenesis on the Pore System. Case Study: Thrombolite and Grainstone Units of the Upper Jurassic Smackover Formation, Gulf of Mexico

Abstract

The grainstone and the thrombolite units of the Smackover Formation at Little Cedar Creek Field, in Alabama, USA, were analyzed to determine their reservoir characteristics. The Smackover Formation reservoirs in this field have only minor dolomitization, and most of the depositional texture of the reservoirs is preserved, making Little Cedar Creek Field 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 distinct 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.

Samples of the dolomitized Smackover Formation thrombolite unit from Appleton and Vocation fields were analyzed and compared to Little Cedar Creek Field thrombolite samples. Porosity, permeability and capillary pressure analysis was completed on thrombolite samples with no dolomitization and samples with distinct degrees of dolomitization. The dolomitization, associated with dissolution of calcite, created an intercrystalline pore network in the thrombolite, increasing porosity and pore connectivity (permeability), and usually reducing pore size. These processes also caused the high petrophysical heterogeneity of the thrombolite to decrease laterally and vertically, resulting in a more homogeneous pore system.

In this study a new pore characterization applied to carbonate rocks was developed. It encompasses pore geometry, pore connectivity and the influence of diagenesis in the pore system by generating a quantitative result in order to identify and map reservoir flow units and diagenetic trends. This new pore characterization is based on features observed in thin sections, being a fast and less expensive method to evaluate porosity characteristics.