Petrography of the Marcellus Shale in Well WV6, Monongalia County, West Virginia
Well WV6 was drilled as part of the Eastern Gas Shale Project (EGSP) at the National Energy and Technology Laboratory site in Morgantown, West Virginia. The purpose was to provide core of the Marcellus Shale and perform well testing related to the hydrocarbon production potential of the formation. The well was completed in April 1978 and the core was logged and preserved for future studies. Numerous studies of the core have been made with regard to the organic content and production potential, but no detailed petrographic study was ever performed.
The purpose of the current study was to document the determinable mineralogy in thin section and to evaluate textures and potential natural fracturing within the shale to support a program to use depleted Marcellus shale wells for CO2 storage. For this study, 23 thin sections were made from a depth of 7387 ft near the base of the Mahantango Formation to a depth of 7506 ft in the underlying Onondaga Limestone, including the Marcellus Shale, which occurs from 7396 ft to 7500 ft depth. The thin sections were studied using a Nikon Optiphot petrographic microscope and a Leica M-series Microscope.
Results of the study indicate a transitional change between the Onondaga Limestone to the Marcellus shale, with the upper several feet of the Onondaga composed of biogenic and fossiliferous limestone mixed with dolomitic shale. Above the Onondaga, the Marcellus grades from a sequence of interbedded fossiliferous limestones and silty, limy shales to a more clay-rich, illitic mudstone above about 7450 ft depth. The clays and pyrite are fairly consistent throughout the sequence with some variation in the amount of pyrite. Preliminary X-ray diffraction results suggest a high proportion of illite and significant chlorite in the clays.
It is apparent from this study that the lower 50 feet of the Marcellus shale at this location in West Virginia is calcareous, often dolomitic, with significant limestone below about 7480 ft depth. The upper 50 feet still contains significant fossil debris and carbonate grains, but the shale is less limy overall. Most of the fossil debris above about 7420 ft depth is in concretionary limestone units separated by relatively fossil-free and carbonate-free, illitic mudstone. This sequence of lithology, when combined with other data, has implications regarding the optimum production interval in the shale.
The results of this study will be combined with other research to evaluate the potential for carbon dioxide storage in depleted wells in northern West Virginia and surrounding areas. The effect of the introduction of CO2 on the mineralogy is key to CO2 storage and enhanced natural gas recovery potential in the Marcellus shale.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013