Characterization of a Karsted, High-Energy, Ramp-Margin Carbonate Reservoir: Taylor-Link West San Andres Unit, Pecos County, Texas

A waterflood was begun in 1985 in Taylor-Link West San Andres unit (10 million barrels cumulative production) on the south margin of the Central Basin Platform to capture a remaining mobile oil target estimated at 20 million barrels. From the onset of injection, oil-water ratios of 0.01 or less were recorded, which indicated that a simple, layered reservoir model was inadequate to describe observed performance. An integrated geologic-engineering model that used 12 continuous cores, petrography, gamma-ray-neutron wireline logs, and capillary pressure data from the upper San Andres Formation instead revealed a complex karst-modified ramp-crest grainstone reservoir. The bimodal fracture-interparticle pore system of this reservoir promoted extensive water cycling and poor sweep efficiency. The Taylor-Link West San Andres reservoir contains, in ascending order, five intervals: (1) basal bryozoan-brachiopod-crinoid packstone-grainstone, (2) crinoid-brachiopod wackestone, (3) mudstone, (4) fusulinid wackestone, and (5) capping grainstone. The grainstone interval makes up more than 80 percent of the San Andres reservoir. Critical diagenetic events influencing reservoir development were (1) early reflux dolomitization and evaporite emplacement and (2) post-San Andres subaerial exposure and karst development. Core and thin-section descriptions showed that porosity could be divided into two groups of pore types: (1) a matrix group composed of intergranular, intercrystalline, and separate-vug pore types related to depositional and early diagenetic events and (2) a touching-vug group composed of fracture, microbreccia, and large-vug porosity related to karstification. Unique relationships between interparticle porosity and water saturation for intergranular and intercrystalline pore types were used to calculate original water saturation because no reliable resistivity logs exist. Volumetric calculations showed that the reservoir originally contained 48.2 million barrels of stock-tank oil. The reservoir has produced 10 million barrels of oil and has estimated reserves of 1.5 million barrels. This leaves 36 million barrels remaining in the reservoir, of which 20 million barrels is mobile oil. The richest area is in the ooid-peloid grainstone facies of the grainstone interval, which has 7.1 million barrels of stock-tank remaining mobile oil in place. Fluid flow was characterized by two basic flow units. Flow unit A has interparticle flow in the ooid-peloid grainstone facies of the grainstone interval and contains most of the remaining oil saturation. Flow unit B contains little oil saturation and has flow through the fracture, microbreccia, and large-vug pore system in the fusulinid-wackestone interval and the fusulinid-peloid packstone facies of the grainstone interval. Engineering analysis of the waterflood performance indicated that injection water is cycling through flow unit B, Because that unit has little oil saturation, oil cut is about 0.01. A recently acquired sponge core in the ooid-peloid grainstone facies shows that the mobile oil has migrated into the upper part of the grainstone facies.