Integrated Geological and Petrophysical Characterization of Permian Shallow-Water Dolostone
- Mark H. Holtz (Bureau of Economic Geology, U. of Texas at Austin) | R.P. Major (U. of Mississippi)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2004
- Document Type
- Journal Paper
- 47 - 58
- 2004. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.6.1 Open hole/cased hole log analysis, 6.5.4 Naturally Occurring Radioactive Materials, 1.6.9 Coring, Fishing, 5.1 Reservoir Characterisation, 5.8.7 Carbonate Reservoir, 6.1.5 Human Resources, Competence and Training, 2.4.3 Sand/Solids Control, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 1.14 Casing and Cementing
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The complex interplay between depositional facies and diagenesis in dolostones makes calculating petrophysical properties from wireline logs challenging. Complex pore geometries and mineralogies control rock petrophysical properties. The complex mineralogy of some dolostone reservoirs, moreover, has profound effects on wireline-log measurements. Therefore, equations for calculating porosities and saturations must be tailored to specific pore-geometry/mineralogy combinations. If dolostone reservoirs are divided into petrophysical/mineralogical facies of similar depositional and diagenetic textures - and, thus, similar pore geometries and mineralogy - empirical equations that apply specifically to that geologically identified facies can be developed. These equations yield more accurate calculations of porosity and water saturation.
Our examples from Permian shallow-water dolostone reservoirs of the Permian Basin, southwestern United States, demonstrate analytical approaches for calculating petrophysical properties in these complex rocks. Four general petrophysical/mineralogical facies characterize Permian shallow-water dolostone reservoirs: (1) subtidal, mud-dominated dolostone; (2) subtidal, grain-domi-nated dolostone; (3) dolomitic and siliciclastic peritidal rocks; and (4) diagenetically altered, subtidal dolostone.
Multiple pore types and associated pore-throat geometries, as well as variations in siliciclastic and calcium-sulfate content, characterize complex dolostone reservoirs because these reservoirs are a consequence of both depositional and diagenetic processes. Petrophysical calculations from wireline logs must therefore be tailored to specific rock types when these reservoirs are analyzed. Simply put, "standard" rock equations generally yield unreliable calculations of porosity and saturation.
Pore types and their corresponding pore geometries in carbonate reservoir rocks control petrophysical properties such as porosity, permeability, and saturation.1-5 Complex mineralogies in some dolostone reservoirs also can have a tremendous influence on wireline-tool response.6,7 Therefore, petrophysical characterization and, more specifically, accurate wireline-log analysis of dolostone reservoirs require an understanding of both complex pore geometry and mineralogy.
We describe herein methods of determining petrophysical properties in complex dolostone reservoirs by discussing the key rock properties of mineralogy and pore and pore-throat geometries that are a result of depositional and diagenetic textures. We incorporate all of these rock properties into calculations by grouping parts of the reservoir into petrophysical/mineralogical facies. We draw on examples from Permian (Guadalupian and Leonardian) reservoirs of the Permian Basin in the southwestern United States, although the principles and procedures presented have application to dolostone reservoirs worldwide.
Influence of Pore Geometry on Petrophysical Properties
Petrophysical properties in complex carbonate rocks are mutually interdependent, hence our reliance on porosity/permeability crossplots and the Archie equation,8 which expresses formation resistivity factor (FRF) as a function of porosity. If permeability and FRF vary with porosity, then FRF must vary with both porosity and permeability. Similarly, capillary pressure characteristics depend on both porosity and permeability, as seen in the Leverett j-function. Thus, the interdependence of petrophysical properties must be considered for making accurate models of porosity and permeability in complex dolostone reservoirs.
Pore geometries control the interrelationship of petrophysical properties, particularly (1) number and types of pores or shapes, (2) interconnectedness of pores (tortuosity), and (3) size of interconnecting pore throats. These three pore-geometry characteristics control FRF, pore capillarity, porosity, and permeability, all of which are critical in determining net pay.
These three characteristics are commonly a function of multiple pore types - such as fracture porosity, intergranular (or intercrystalline) porosity, microporosity, and vuggy or moldic porosity - and pore geometry can be described qualitatively by determining the relative abundance of these pore types. Rocks dominated by the geometrically simplest of these pore types, fracture porosity, generally have low tortuosity, low and uniform FRF, and low porosity and high permeability relative to porosity. In contrast, rocks containing a mixture of fracture, intergranular, micro-, and vuggy porosity have high tortuosity, high and variable FRF, and higher porosities and moderate permeabilities relative to porosity (Fig. 1).
Covariance of porosity and permeability depends on pore geometry, with permeability varying over several orders of magnitude for a given porosity 4-6,9 as a consequence of multiple pore types and their associated pore-throat size. Relatively high permeabilities at low porosities occur in rocks containing fractures and touching-vug pores; moderate permeabilities at moderate porosities occur in rocks containing interparticle pores; and relatively low permeabilities at high porosities occur in rocks containing intercrystalline, micro-, and separate-vug porosity. This pore-type effect on the porosity-permeability relationship is caused by a measurement of pore geometry called aspect ratio, which is the ratio of pore radius to pore-throat radius. The greater the aspect ratio, the lower the permeability at a given porosity (Fig. 2). Thus, establishing a porosity/permeability relationship requires knowledge of existing pore types, which are a consequence of depositional and diagenetic textures.
|File Size||22 MB||Number of Pages||12|