Walker Creek field in southern Arkansas produces hydrocarbons from oolitic packstones and grainstones of the Jurassic Smackover Formation. The relationships between pore geometry and reservoir quality in these rocks were evaluated using petrographic methods and mercury injection capillary pressure analyses. Results indicate that reservoir quality is controlled by pore geometry, which, in turn, is determined by depositional and diagenetic processes.
Reservoir rocks at Walker Creek were deposited as prograding grainstone shoals in a shallow-water, high-energy environment. Diagenetic processes, including early marine cementation, compaction, and deeper burial pressure solution and calcite cementation, modified the original pore system. Primary interparticle porosity is the dominant effective pore type and is most important in terms of reservoir performance. Secondary microporosity is also abundant, comprising a significant percentage of total porosity (locally up to 100%), but is generally ineffective.
The rocks were subdivided into lithofacies based on their depositional and diagenetic origin. Within each lithofacies, reservoir, marginal reservoir, and nonreservoir rock types are identified based on their pore geometry and characteristic capillary pressure curve. The reservoir rock types are less cemented oolitic and oolitic-skeletal grainstones with steeply sloping capillary pressure curves and flat plateaus indicating relatively homogeneous pore sizes that are well interconnected by large pore throats. The marginal to nonreservoir rock types include mudstones, wackestones, muddy packstones, and cemented grainstones with generally small interparticle and moldic pores. Gently sloping capillary pressure curves with poorly defined or no plateaus indicate variable pore and pore-throat sizes.
A close correlation is found among parts of the capillary pressure curve shape, such as slopes, plateaus, and displacement pressure, and aspects of pore geometry such as pore-throat size, pore-throat size distribution, and pore interconnectivity. In addition, capillary pressure analyses provide guidance in evaluating overall reservoir quality by rock type, as well as answer specific questions regarding net pay criteria as a function of height above free-water level.