A certain minimum energy is required to transport and sort the clastic fraction in a given environment. The energy can be estimated by making detailed textural analyses of the sand distribution and constructing maps of average grain size and sorting, but this takes a long time. An easier and faster method is to use the coarsest size fraction in the sediment as an index of the minimum energy required to transport and deposit the clastic particles. In a diagram devised by Hjulstrom, transport and erosion energies are shown for clastic particles of all sizes in relation to various water velocities. The minimum transport velocities for various grain sizes are used as energy values. The average energy value for a section of rocks is calculated by tabulating the percent of the section that occurs in each arbitrarily chosen velocity group and multiplying this percent by the water velocity value. The resulting values are added to give the average energy value of the rock section. A map is constructed showing the distribution of energy values in the rock unit. One advantage of this method is that the maps can be used to evaluate reservoir characteristics since grain size correlates with permeability. The above procedure does not take sorting into consideration, but in most shelf deposited sediments a correlation typically exists between sorting and grain size.