The Marcellus Shale is the most productive shale formation in the United States, but like all other shale plays, it has a low recovery rate (∼20%), partly because of the incomplete understanding of the factors controlling gas storage and producibility. Variations in porosity, pore structure, permeability, and brittleness of two ∼15-cm core fragments from depths of 2158.8 and 2180.4 m (typical producible horizon of the Marcellus Shale) from the Coldstream 1MH well in northern central Pennsylvania were studied.
The results indicate that the porosity of shales is critically controlled by shale composition. Quartz (especially biogenic SiO2), carbonate minerals, and organic matter all contribute to porosity more than clay minerals. Pores with a throat size <3 nm account for approximately 50% of the total pore volume calculated from helium porosity, most of which are contributed by organic matter. These small pores contribute to the methane storage capacity, and more so in organic-rich shales. Porosity has a strong control on matrix permeability through a connected pore network, suggesting good pore connectivity in these shales.
Based on mineralogical composition, porosity characteristics, permeability, and brittleness, it is concluded that in addition to better methane storage capacity, the organic- and biogenic-SiO2−rich deeper core has porosity and permeability attributes that make it a superior target for horizontal drilling and producibility compared with the clay-rich shallower core. Moreover, because the underlying favorable qualities are best developed in the transgressive systems tract, there is a rationale for in-depth, sequence stratigraphic appraisals of black shale successions.