WALLING, SUZETTE D., and THOMAS T. TIEH, Texas A&M University, Department of Geology, College Station, TX

ABSTRACT: Stratigraphic Variations of Authigenic Clay Minerals: Guadalupian Delaware Mountain Group, West Texas

Authigenic clay minerals in reservoir sandstones of the Guadalupian Delaware Mountain Group in West Texas comprise up to 13% of the bulk rock volume. These minerals, which line, fill and dissect pores created by dissolution of cement and grains, adversely affect reservoir quality and influence production performance. The mineralogy, distribution, and occurrence of the authigenic clays in selected Waha field wells have been characterized by optical and electron microscopy, x-ray diffraction methods, and wet chemical analyses. Samples cover a stratigraphic interval of approximately 1650' (505 m) representative of the Bell Canyon and Cherry Canyon Formations.

Chlorite and interlayered chlorite/smectite (C/S) dominate the clay mineral suite, and occur with lesser and variable amounts of illite and illite/smectite (1/5). Interstratification in the mixed-layer phases is random and minor, less than 20%. Chlorite-to-smectite ratio increases with depth, as indicated by changes in expansiveness and distribution of Fe within the chlorite structure. Textural and morphological details show three distinct types of chlorite development: (1) poorly developed, thin (<2 micrometers) grain coatings, (2) straight, bladed crystals forming dense, 3-micrometer-thick layers on grain surfaces, and (3) coarsely crystalline, interlocking platelets which dissect much of the pore space. The structural and morphological variability of chlorites appears to be stra igraphically controlled, and to represent a diagenetic progression beginning with a smectitic precursor, through an interstratified intermediate, to a more ordered form.

Petrophysical properties of the sandstones depend on the nature of occurrence of the clay minerals. Each of the three chlorite types in this sequence shows a range in porosity and permeability distinctive from the others. Though lateral extent of these stratigraphic variations has not been established beyond the Waha field, correlations of clay distribution and well log data suggest that the acoustic log may prove a useful indicator of chlorite distribution and occurrence.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.