Print this pageReservoir Potential and Compartmentalization In Coarse, Calcitic, Cool-Water Limestones; Mid-Tertiary Te Kulti Group, New Zealand
ANASTAS, ALEXIS S., NOEL P. JAMES, CAMPBELL S. NELSON, and ROBERT W. DALRYMPLE
Primary textural complexity within intercalated horizontally-bedded and cross-bedded carbonate sands strongly influences their reservoir potential, heterogeneity, and fluid compartmentalization. Three important factors influenced the reservoir potential of the Te Kuiti Group limestones-physical processes, biological processes and diagenesis.
Rocks of the horizontally-bedded facies assemblage contain biologically-controlled textures and were formed of sediments deposited on a low-energy, wave-dominated shelf with thorough bioturbation. They are fine-medium grained, moderately-poorly sorted grainstones-packstones with a closed/densely packed framework fabric. They comprise tabular sediment bodies (10's m thick) with little compartmentalization and heterogeneity. Reservoir potential was good until the end of early shallow burial.
Cross-bedded facies assemblage rocks were formed of sediments deposited on a current-dominated shelf and contain textures that are largely the result of physical processes. Sets are extremely thick (50-450 cm) with porous and permeable upper parts (foresets) and much less permeable lower parts (foresets, set boundaries). Rocks of this facies assemblage (grainstones) form sediment bodies which are strongly heterogeneous and anisotropic as well as being both vertically and laterally compartmentalized (3-7 m thick). Due to "propping-up" by early intergranular cements, residual primary porosity and permeability withstood greater burial thus leading to superior reservoir potential. Lessons learned from these calcitic, cool-water limestones are applicable to reservoirs in cross-bedded oolitic/bioclastic limestones and carbonates formed in ancient "calcite" seas.