47th Annual AAPG-SPE Eastern Section Joint Meeting

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Spatial distribution of chlorite in the Marcellus Formation and its relationship with static reservoir properties and well production


The objective of this study is to evaluate the relationship of chlorite to static reservoir properties and horizontal well production in the Marcellus Formation. A series of multivariate petrophysical models were built to predict chlorite content (as measured in weight percent (wt%) from x-ray diffraction (XRD) of core samples) from 68 wells in Pennsylvania, West Virginia, Ohio, and New York using basic wireline log data (gamma ray, neutron porosity, bulk density, deep resistivity, photoelectric effect). The petrophysical models were high-graded using residual error analysis and blind testing, resulting in two separate optimized models for use with laterologs and induction logs, respectively. The models were applied to 300+ wells without XRD data, and the modeled results were mapped across the spatial extent of the dataset. The petrophysical models and the XRD data reveal similar trends in chlorite wt% in most areas. The models over-predict chlorite in distal parts of the basin, particularly the West Virginia panhandle and surrounding areas, which is attributed to a hydrocarbon phase change from dry gas to wet gas and its associated impact on resistivity readings. In general, chlorite wt% increases up-section in the Marcellus and decreases basinward. Chlorite wt% is highest in central Pennsylvania in the proximal part of the basin. While chlorite in the Union Springs (basal Marcellus) can average >10 wt% in central Pennsylvania, it is virtually absent from the main play fairways of southwest Pennsylvania and northeast Pennsylvania. Chlorite is present in the Oatka Creek (upper Marcellus) across the entire study area, gradually transitioning from an average of 1 wt% in the west to 20 wt% in the east. Increased chlorite wt% in the Oatka Creek relative to the underlying Union Springs records progradation of the Acadian clastic wedge during Middle Devonian. Cratonward migration and uplift of the Acadian orogenic belt led to unroofing of Lower Paleozoic and Precambrian greenschist facies, sourcing an influx of chlorite into the basin. Statistical analysis reveals a significant inverse correlation (p-value=0.003) between chlorite wt% and total organic carbon (TOC) in the southwest Pennsylvania play fairway. This corroborates the prevailing view that high clastic influx dilutes TOC and increases clay content. Hence, the relationship between chlorite wt% and TOC is correlation, not causation, resulting from allocyclic controls. No significant relationships exist between chlorite wt% and other static reservoir properties (water saturation, permeability, and porosity). Well productivity (720-day cumulative production, normalized for lateral length) significantly correlates with TOC (p- value=0.048) and has no relationship to chlorite wt% (p-value=0.649).