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A 4-step procedure has been developed that provides key information for recognizing the reservoir facies in carbonate
sequences. The procedure is used in conjunction with standard methods of environmental and diagenetic analysis, both in petrographic description and reservoir-map construction.
The first step in the procedure is largely petrographic in nature. The pore network is examined to determine the critical rock properties that influence hydrocarbon volume and productivity, pore type, pore arrangement, pore abundance, and pore size. Samples with similar pore networks are grouped together for use in the next step.
In the second step, representative samples are selected from each pore network group for porosity, permeability, and capillary-pressure measurements. The 3-dimensional and quantitative characteristics of the pore network are established in this way and basic data are obtained for reserves calculations.
The petrophysical data are interpreted in the third step by means of porosity-permeability cross plots and capillary-pressure graphs. The use of well logs at this point is recommended as an additional reference base. The output from this step is the identification of the reservoir facies and the determination of its range of quality. The development of a set of reference samples at this time aids cuttings description later on.
Finally, the reservoir and nonreservoir rock groups just identified are linked with their environmental facies counterparts (step 4). When this is accomplished for cored wells, the relations are extended to uncored wells by means of the reference set established previously. Cuttings samples, and even additional cores, can usually be described adequately with a low-power microscope once the reference set is available.
Because the procedure is based on experimental pore-size studies as well as on subsurface and surface studies of several areas and rock types, the system of description should have general application. Studies of fine-grained carbonate sequences are aided particularly with this approach, and helpful information commonly is obtained for evaluating well-test and pressure-production history data.
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