Effect of diagenesis on compaction of reservoir rocks

Abstract

Predicting sediment porosity-depth trends and ultimately the quality of reservoir rocks requires an understanding of mechanical and chemical compaction mechanisms during diagenesis. In many siliciclastic sediments porosity versus depth profiles can be predicted from the sediment’s stress history. In carbonates such predictions are more difficult because chemical diagenesis is prevalent even within a few meters of the seafloor.

We used a systematic laboratory approach to investigate the influence of early diagenesis in a meteoric environment on compaction of oolitic carbonates. Aggregates were synthesized in an autoclave from loosely packed natural aragonite ooids and fresh water, to mimic phreatic conditions. Time and temperature were used to control the degree of chemical diagenesis. Constant stress-rate, uniaxial strain compaction tests were performed on the aggregates to track mechanical properties as a function of chemical alteration. Samples were characterized before and after compaction with electron and optical microscopy, X-ray tomography, and X-ray diffraction. The aragonite ooids dissolved preferentially inwards from their rims, and blocky calcite precipitated in the original inter-ooid pore space with little change in porosity. This progression results in an inverted structure with moldic pores inside a foam-like structure of calcite. With deformation, all samples exhibited elastic to plastic compaction typical of granular aggregates. With increasing alteration, the elastic moduli appear to increase, the transition to plastic behavior occurs at progressively higher stresses, and the elastic-plastic transition becomes more abrupt. At high stresses the plastic behavior was similar for all samples. X-ray tomography with micron-scale resolution tracks grain displacement and void and cement compaction. These experiments and results help us understand the complexities of chemical-mechanical interactions during diagenesis and improve our ability to predict porosity changes with depth for basin modeling, reservoir quality prediction and reservoir management.