Assessing Carbonate Reservoir Potential and Improving Reservoir Performance Using Core-Based High Resolution Sequence Stratigraphy

Lindsay, Robert F.¹ (1) Saudi Aramco, Dhahran, Saudi Arabia

High resolution sequence stratigraphy (HRSS), describing beds, cycles, cycle sets, high frequency sequences (HFS), and composite sequences, significantly assists carbonate reservoir potential assessment and improves reservoir performance. Newly discovered fields with a few cored wells (primary recovery) and mature fields with numerous cored wells (secondary [waterflood] and tertiary [CO₂] recovery) benefit dramatically with an HRSS "blue print" of reservoir architecture. The 3-D distribution of porous-permeable grain-rich lithofacies and less-porous to non-porous mud-rich lithofacies, which are potential intra-reservoir baffles (aquatards) and barriers (aquacludes) to fluid/gas flow, together with production and reservoir engineering data can be better understood when superimposed on the HRSS framework.

Understanding flow unit distribution within the HRSS model, in transgressive and highstand systems tracts, will ultimately optimize reservoir production. As flow units can contain variable pore types, with potential variations in matrix permeability, an understanding of flow unit distribution is critical in optimizing reservoir performance and management. Flow units are of variable thickness and, in carbonate ramps to distally steepened ramps, tend to thin up-dip within individual beds/cycles and thicken down-dip within stacked grain-rich cycles.

For conformance workovers, HRSS can improve sweep efficiency by redirecting injection fluids/gases and avoid water/gas cycling, locate high permeability streaks and zones of potential water-cycling, and identify by-passed and/or compartmentalized mobile oil/gas. Porosity pinchouts, due to up-dip loss of accommodation in HFS, provide potential infill drilling plays.

Examples of HRSS applications within reservoirs are provided from the Cretaceous Shu'aiba, Jurassic Arab-D and Hanifa, and Triassic-Permian Khuff (Middle East); Permian Grayburg, San Andres, and Clear Fork (Permian Basin); and Mississippian Mission Canyon (Williston Basin).