QUANTITATIVE COLOR ANALYSIS OF DRILL CORE – AN EXAMPLE FROM THE MIDDLE PENNSYLVANIAN PARADOX FORMATION, PARADOX BASIN, UTAH

Color is one of the key characteristics used to identify rocks, as it is often an indicator of a rock’s mineralogical composition. In recent years, an increasing amount of oil and gas has been produced from continuous hydrocarbon accumulations, which are commonly contained within shale-rich reservoirs. The darker colors of these reservoir rocks can obscure subtle mineralogical differences within and between well cores. The U.S. Geological Survey (USGS) has developed a non-destructive method for quantitatively analyzing color variations in well cores that may provide an additional tool that can be used to describe and correlate well core, particularly in darker lithologies.

We discuss an example from the Paradox Basin of southeastern Utah and southwestern Colorado, an asymmetric basin that formed along the southwestern edge of the Uncompahgre uplift during the Pennsylvanian – Permian Ancestral Rocky Mountain orogeny. The Paradox Basin has a long history of geologic research due to excellent outcrop exposures combined with abundant natural resources. The USGS completed a geologic assessment of undiscovered, technically recoverable oil and gas resources in the Paradox Basin. As part of that effort, a detailed examination was undertaken of the Delhi-Taylor Oil Company’s 1 Cane Creek and Shafer No. 1 cores, which were drilled near Moab, Utah. The cores contain the Middle Pennsylvanian Paradox Formation, which is a thick succession of interbedded halite, sylvite, anhydrite, siltstone, dolomite, and black shale.

Analysis of these cores indicates that at least five scales of nested cycles are represented. This cyclicity is likely due to changes in local and regional paleoclimate, which affected temperatures and salinity of basin waters. A preliminary quantitative color analysis of thick halite intervals indicates that the amount of sylvite increases towards the upper-middle portion of each halite section and then abruptly decreases. This trend in increases sylvite is consistent with dry climate conditions that became even more arid during precipitation of the middle interval of each halite section. Quantitative color analysis of the darker clastic portions of the cores, which can serve as analogs for shale-rich, continuous reservoirs, also reveals subtle variations within similar intervals between cores.