A comparative study of visually estimated mineral composition, X-ray diffraction, and trace element determination from cores across the Khuff-C carbonate in Hawiyah, Ghawar field, Saudi Arabia
Kamal, Rami A. (Saudi Aramco - [email protected]), Ghazi Al-Eid (Saudi Aramco), Bruce W. Sellwood, (U Reading) and Edward A. Clerke (Saudi Aramco)
A 180-ft-long cored interval through the Khuff-C depositional cycle, which includes the Khuff-C reservoir, was subjected to a key study to compare four techniques of mineral composition determination of the same rock, namely from: full core description, thin section descriptions, trace element analyses, and X-ray diffraction analyses. Cored footage from the Khuff-C carbonate in a northeastern Hawiyah area well, was veneered along a length of 180 ft. The thin veneered slabs were ground in one-foot length batches. Homogenized samples from the batches were then X-ray analyzed for mineral composition. Separately, visual estimates of mineral composition were made for the same rock from thin sections taken from core plugs taken at 6-inch intervals in addition to mineral composition estimated from the regularly performed core descriptions. Finally, small samples were acquired from the same plug ends and were pulverized and run through automated trace element detection processes. Data from all four methods were tabulated, plotted and compared. Visual determinations of bulk mineral compositions portray the lithology trends in the rock, expressed in the X-ray determined mineralogy and trace element analyses. Detailed comparisons showed that calcite percentages compared the best. Dolomite percentage compared fairly. Anhydrite percentage trends were correct but values were systematically low compared to mineralogy. The anhydrite discrepancy can be explained by observing that a large percentage of the Khuff anhydrite is concentrated in small to medium sized nodules. Hitting or missing a nodule during the core plugging process can make the difference between a high visually estimated percentage and an almost negligible percentage of the mineral. Although the anhydrite trend is not jeopardized, X-ray diffraction is recommended for studies where accurate anhydrite percentages are necessary. X-ray diffraction revealed the occurrence of minor and trace elements, namely: feldspar, pyrite, siderite, fluorite, and celestite (0.1 Ã 2.9 percent of bulk rock); that were not recognized by the optical petrographers. Minor element recognition can play an important role in correlation, determination of environments of deposition, and diagenetic sequences. This comparative study reveals the variance between visually estimated mineral composition and what is instrumentally detected. Secondly, the study has shown the variance between geochemical mineral analyses taken from a homogenized ground sample taken across a foot of core and spot measurements taken six inches apart along the same foot of core.
Kamal, Rami, Al-Eid, Ghazi, Sellwood, Bruce W., and Clerke, Edward A., 2004, A comparative study of visually estimated mineral composition, X-ray diffraction, and trace element determination from cores across the Khuff-C carbonate in Hawiyah, Ghawar field, Saudi Arabia: Geo 2004, Bahrain, Gulf Petrolink.