Abstract

Late-stage authigenic clay minerals are pervasive in the reservoir sandstones of the Guadalupian Delaware Mountain Group, comprising an average of 13% of the bulk rock volume. Reservoir rock properties are, therefore, strongly influenced by these minerals. Samples selected from cored intervals, ranging from 3835′ to 6460′ (1176-1970 meters), were prepared and studied using optical and electron microscopy and x-ray diffraction methods to determine the distribution and nature of occurrence of the authigenic clays.

In thin section, clay minerals are recognizable as grain coatings, with thicknesses varying from a few microns to tens of microns. Details revealed by scanning electron microscopic analyses show three distinct morphologies: featureless to irregular grain coatings, well developed interlocking platelets which line, fill and dissect pores, and delicate “hair-like” growths which are sparse and randomly distributed. X-ray diffraction analyses of clay concentrates show chlorite and an interlayered chlorite/expandable clay to be the most abundant clay minerals, with lesser and variable amounts of illite. Successive heat treatments of samples reveal varying degrees of interstratification in the chlorites as indicated by differences in both the amount of peak shift and intensification of the 14 Å reflection. Differences in the relative intensities of even-versus odd-ordered x-ray reflections combined with energy dispersive spectrometry (EDS) also suggest variable Fe/Mg ratios among chlorites. The structural and morphological variability of chlorites in these cores probably represent a diagenetic progression beginning with a smectitic precursor, through an interstratified intermediate, to a more ordered form. There appears to be no systematic vertical or lateral trends in clay mineralogy.

The importance of a complete understanding of the clay mineralogy and chemistry in these sandstones is evident when considering enhanced recovery procedures. Different clay structures and chemistries may respond differently to production and stimulation techniques. The proposed chlorite diagenetic sequence suggests that drastic changes in borehole fluid chemistry may cause retrogression of chlorite to an expansive form, which may be water sensitive or inclined to migration.