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[[file:geological-heterogeneities_fig2.png|thumb|{{figure number|2}}Typical vertical stratification and permeability profiles of (a) fining- or thinning-upward and (b) coarsening- or thickening-upward sequences. ''Fining'' and ''coarsening'' refer to average relative grain size of individual laminae and beds, and ''thinning'' and ''thickening'' refer to the relative thickness of Individual laminae and beds.]]

[[file:geological-heterogeneities_fig2.png|thumb|{{figure number|2}}Typical vertical stratification and permeability profiles of (a) fining- or thinning-upward and (b) coarsening- or thickening-upward sequences. ''Fining'' and ''coarsening'' refer to average relative grain size of individual laminae and beds, and ''thinning'' and ''thickening'' refer to the relative thickness of Individual laminae and beds.]]

Elements of wellbore heterogeneities include the pore network ([[Pore system fundamentals|pores and pore throats]]), grain size and composition, grain packing, lamination and bedding styles, sedimentary structures, lithofacies, and vertical stratification sequences. These properties can be readily described in a numerical or quantitative fashion because of the usual availability of rock samples and well logs. Rock cores provide the best information on lithofacies and stratification sequences, plug or whole core [[porosity]], [[Permeability|permeability]], and fluid saturation (if oil-based drilling mud was used during coring). The use of log shapes for facies recognition, as well as sidewall samples, micrologs, and [[Dipmeter analysis|dipmeter]] tools can also provide indirect information on lithofacies and stratification types. (For more on lithofacies, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]). Pore networks, grain size characteristics, and mineralogy can be analyzed by routine [[Thin section analysis|thin section petrography]] as well as by [[SEM, XRD, CL, and XF methods#X-ray diffraction (XRD)|X-ray diffraction]], [[Scanning electron microscopy (SEM)|scanning electron microscopy]], [[capillary pressure]] measurements, and petrographic image analysis (see [[Reservoir quality]]). Analysis of all or most of these properties is essential for adequate reservoir description because these properties provide the database and thus the foundation for reservoir description at larger scales. (For information on these types of analyses, see [[Wireline methods]] and [[Laboratory methods]].)

Elements of wellbore heterogeneities include the pore network ([[Pore system fundamentals|pores and pore throats]]), grain size and composition, grain packing, lamination and bedding styles, sedimentary structures, lithofacies, and vertical stratification sequences. These properties can be readily described in a numerical or quantitative fashion because of the usual availability of rock samples and well logs. Rock cores provide the best information on lithofacies and stratification sequences, plug or whole core [[porosity]], [[Permeability|permeability]], and fluid saturation (if oil-based drilling mud was used during coring). The use of log shapes for facies recognition, as well as sidewall samples, [[Basic open hole tools#Microresistivity|micrologs]], and [[Dipmeter analysis|dipmeter]] tools can also provide indirect information on lithofacies and stratification types. (For more on lithofacies, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]). Pore networks, grain size characteristics, and mineralogy can be analyzed by routine [[Thin section analysis|thin section petrography]] as well as by [[SEM, XRD, CL, and XF methods#X-ray diffraction (XRD)|X-ray diffraction]], [[Scanning electron microscopy (SEM)|scanning electron microscopy]], [[capillary pressure]] measurements, and petrographic image analysis (see [[Reservoir quality]]). Analysis of all or most of these properties is essential for adequate reservoir description because these properties provide the database and thus the foundation for reservoir description at larger scales. (For information on these types of analyses, see [[Wireline methods]] and [[Laboratory methods]].)

In clastic rocks, there is usually a direct relationship between primary depositional lithofacies and reservoir properties and performance. For example, sandstones that become progressively thinner bedded and finer grained stratigraphically upward also become progressively less permeable upward ([[:file:geological-heterogeneities_fig1.png|Figure 2]]) so that during [[Waterflooding|waterflood]], both gravity and higher permeability toward the bottom will pull water down. In contrast, sandstones that become progressively thicker bedded and coarser grained upward also become more permeable upward ([[:file:geological-heterogeneities_fig2.png|Figure 2]]) so that during waterflood, gravity still pulls the water down, but permeability pulls the water up, resulting in better vertical sweep.<ref name=pt06r75>Lassiter, T. K., Waggoner, J. R., Lake, L. W., 1986, Reservoir heterogeneities and their influence on ultimate recovery, in Lake, L. W., Carroll, N. B., Jr., eds., Reservoir Characterization: Orlando, FL, Academy Press, p. 545–560.</ref><ref name=pt06r144>van de Graaff, W. J. E., Ealey, P. S. 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1400/1436.htm Geological modeling for simulation studies]: AAPG Bulletin, v. 73, p. 1436–1444.</ref>

In clastic rocks, there is usually a direct relationship between primary depositional lithofacies and reservoir properties and performance. For example, sandstones that become progressively thinner bedded and finer grained stratigraphically upward also become progressively less permeable upward ([[:file:geological-heterogeneities_fig1.png|Figure 2]]) so that during [[Waterflooding|waterflood]], both gravity and higher permeability toward the bottom will pull water down. In contrast, sandstones that become progressively thicker bedded and coarser grained upward also become more permeable upward ([[:file:geological-heterogeneities_fig2.png|Figure 2]]) so that during waterflood, gravity still pulls the water down, but permeability pulls the water up, resulting in better vertical sweep.<ref name=pt06r75>Lassiter, T. K., Waggoner, J. R., Lake, L. W., 1986, Reservoir heterogeneities and their influence on ultimate recovery, in Lake, L. W., Carroll, N. B., Jr., eds., Reservoir Characterization: Orlando, FL, Academy Press, p. 545–560.</ref><ref name=pt06r144>van de Graaff, W. J. E., Ealey, P. S. 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1400/1436.htm Geological modeling for simulation studies]: AAPG Bulletin, v. 73, p. 1436–1444.</ref>