Disequilibrium Compaction, Fluid expansion and unloading effects: Analysis from well log and its pore pressure implication in Jay Field, Niger Delta

Abstract

Disequilibrium compaction, sometimes referred to as under compaction, has been identified as a major mechanism of abnormal pore pressure buildup in sedimentary basins. This is attributed to the interplay between the rate at which sediments are deposited and the rate at which fluids associated with the sediments are expelled with respect to burial depth. The purpose of this research is to analyze the mechanisms associated with abnormal pore pressure regime in the sedimentary formation. The study area “Jay field” is an offshore Niger Delta susceptible to abnormal pore pressure regime in the Agbada –Akata formations of the basin. Well log analysis and cross plots were applied to determine the under compacted zone in the formation since compaction increases with burial depth. It was observed that porosity and permeability of the deeper depth (3700 m to end of Well) are higher than those of the shallow part (3000 – 3700 m). This is against what is expected from normal compacted sediment, demonstrating disequilibrium compaction in deposition. Furthermore, it reveals that sedimentation rate was high, making it unable for the sediments to expunge its fluid as expected. Density and acoustic wave increase with depth in normal compaction trend. However, the reverse that was identified in the mapped interval is attributed to disequilibrium compaction, unloading, clay diagenesis, and fluid expansion. The cross plot divulges sediments at the deeper depth had lower density and acoustic wave value with increased porosity when compared to those at shallow depth. This forms the basis that the sediments from this mapped interval experienced disequilibrium and unloading traceable to clay diagenesis during and after deposition, respectively.