RESERVOIR QUALITY OF THE MARCELLUS SHALE PLAY: DETERMINING MINERALOGICAL AND LITHOLOGICAL PROPERTIES

The Marcellus Formation is a valuable shale gas resource in Pennsylvania. In the southwest region of the state (particularly Washington and Greene counties), shale gas drilling and hydraulic fracturing are common. In 2010, two wells were drilled in Washington County by Antero Resources, the Hill Unit 2H (API# 37-125-23880) and the Hill Unit 3H (API# 37-125-23879). The purpose of this study was to examine the lithology and the mineralogy of the Marcellus Formation in the 2H and the 3H wells using rock cuttings samples and geophysical well log data in an attempt to discern overall reservoir quality in each well. Rock cutting samples from the vertical and lateral portions of the two wells were collected and evaluated for mineralogical content using XRD analysis. In addition, both wells are compared for similarities in lithological compositions based on geophysical log interpretations.

Economically viable Marcellus wells are commonly completed in intervals with >40% quartz and carbonates, and <30% clays. The 2H well contains >40% clay minerals, <10% carbonates, and ~50% quartz. The Marcellus in both the vertical and lateral portions he 3H well is more abundant in quartz and carbonate content than the 2H well. In addition, the clay mineral content in the in lateral portion of 3H well lies is ≤30%. The mineral percentages of the vertical and lateral depths of both wells suggest that the 2H resides in the upper Marcellus while the 3H resides in the lower Marcellus. Given that the 3H well lies within the preferred mineralogy range for quartz, clay minerals, and carbonates, the preliminary mineralogical and geophysical log analysis suggests that the 3H well should be a more effective producer than the 2H well. This hypothesis is supported by production data that indicate since 2010, the 3H well has produced >2974 MMcf of natural gas versus >2247 MMcf for the 2H well, with both displaying similar decline curves. While this may be in part due to potentially enhanced organic content of the lower Marcellus compared to upper parts of the unit, this study supports the idea that the primary mineralogical content of shale gas intervals is a strong control on overall hydrocarbon production.