Subsurface Laboratory Methods 239 



materials must be expressed as a range rather than as a definite value. 

 Such variations in chemical composition with resulting changes in lattice 

 dimensions cause differences in line intensities, shifting of lines, appear- 

 ance or disappearance of lines, and other changes in X-ray-diffraction 

 patterns. Consequently, it is not possible to establish a standard pattern 

 for some minerals, as has been attempted in the card-index system. For 

 proper identification of compounds with variable chemical composition, 

 isomorphism and phase relationships must always be considered. For 

 such compounds, complete knowledge of the identity and structure will 

 be obtained only from simultaneous consideration of chemical compo- 

 sition, crystallography, physical and physical-chemical properties, and 

 X-ray-diffraction data. 



Occasionally, evidence of atomic replacement within crystals is hardly 

 detectable in the X-ray-diffraction patterns, ^specially where the unit-cell 

 parameter is dependent on a certain kind or kinds of atoms or ions which 

 form a rigid geometric-space packing, with the other atoms or ions fitted 

 loosely into the holes of the structure. Substitutions of the latter type of 

 atoms or ions may cause little if any change in the lattice parameters of 

 the crystal. 



Applications 



The X-ray-diffraction methods of analysis of geologic materials can 

 be used in subsurface investigations to supply the geologist with infor- 

 mation not otherwise obtainable, to furnish the petroleum engineer with 

 precise knowledge of the composition and certain properties of reservoir 

 rocks, and to trace mineralogic and structural changes of importance in 

 problems of sedimentation and sedimentary petrology. 



The precise identification of mineralogic composition made possible 

 by the X-ray-diffraction method will permit the correlation of formations 

 where other data are lacking, or may prevent erroneous correlation based 

 on unreliable information. Identification of the kind and amount of minor 

 constituents in apparently homogeneous, thick formations may subdivide 

 the sequence in such a manner as to demonstrate the stratigraphic rela- 

 tionship to similar formations occurring elsewhere. 



The analysis of reservoir rocks by X-ray-diffraction may reveal de- 

 tails of composition otherwise overlooked. In particular, the kind and 

 amount of interstitial clay may critically control effective porosity and 

 permeability of formations by changes in hydration and degree of floccu- 

 lation, as a consequence of change in the solutions saturating the rock. 

 Flocculation or deflocculation and hydration or dehydration of clays are 

 controlled by their mineralogy as well as by their environmental changes. 

 Hence, the susceptibility of clays to change during the water-flooding or 

 other secondary-recovery programs can be detected by X-ray-diffraction 

 analysis of reservoir rocks. 



The geologist and engineer will find that X-ray-diffraction methods 



