With the highest normalized recoveries and lowest breakevens, the Marcellus play in the northeastern Pennsylvania region has long been the crown jewel of shale gas plays in the Lower 48. This region of the play has the unique advantage of offering stacked intervals with the primary target being the Lower Marcellus. Remaining high-quality inventory in the Core NE region is becoming a growing concern, and some operators have begun developing the Upper Marcellus interval more regularly as a result.
The upper interval may not feature the same eye-popping 3+ Bcf/1,000’ recoveries compared to its lower, deeper sibling, but it still ranks next in-line in the L48 with average 30-year estimated ultimate recoveries (EURs) around 2 Bcf/1000’. So where is the Upper Marcellus viable and what drives the difference in recoveries?
The high-quality region of the Upper Marcellus is similar to the Lower interval except it is limited in the west by the thickness of the underlying Cherry Valley Limestone. The limestone separates the intervals and needs to be at least 15 feet thick to act as a frac barrier so that both can be developed without vertical interference (Figure 1). To the south and east, the rock approaches the "Line of Death" where it is overly mature and can no longer generate gas. To the north, shallower depths and lower pressures result in lower recoveries.
The main rock quality differences between the stacked zones are showcased in Figure 2. The purple electrofacies, which are concentrated in the Lower Marcellus, highlight superior rock quality in comparison to the Upper based on higher gamma ray and porosities plus lower water saturation. Higher gamma rays are important in the Marcellus as there exists a very strong positive correlation between gamma ray values and total organic carbon in the Marcellus (Figure 3). This is significant as the elevated thermal maturities in the NE region has led to incredible amounts of gas generation over time from the organic carbon. All the above translates into why we see higher recoveries coming out of the Lower Marcellus.
FIGURE 1 | Cherry Valley Isopach (star indicates location of type log in Figure 2)
FIGURE 2 | Marcellus Type Log From Susquehanna County
FIGURE 3 | Correlation Between Gamma Ray and Total Organic Carbon in the Marcellus