Glacial Lading: A Cause of Natural Fracturing and a Control of the Present Stress State in Regions of High Devonian Shale Gas Production

Abstract

Abstract It is likely that ice sheets contributed significantly to natural fracturing and hence gas production in low permeability Devonian shale. The large ice sheet of the last ice age caused considerable deflection of the lithosphere. Predicted horizontal tensional stresses south of the glacial limit resulting from this crustal flexing exceed the tensile strength of Devonian shale. Glacially induced fracturing is therefore expected in these regions which correspond with regions of historical gas production. The north-south variation in observed stress ratios is also explained by the glacial model. Lineaments and faults will perturb locally the glacially induced stresses and, when coupled with the glacial model, offer an exploration rationale. Introduction It is generally agreed that natural fractures are essential for gas production from low permeability Devonian shales in the eastern U.S. A number of causes for natural fractures have been hypothesized and these include such diverse mechanisms as tectonics, meteorite impacts, glacial loading, uplift dilatancy, and stratigraphic and diagenetic effects (e.g. Dean and Overbey1). No one has seriously considered the glacial loading mechanism and so in this study this mechanism is analyzed in detail. During the most recent ice age, which ended less than 10,000 years ago, an ice sheet covered the northern half of North America and extended as far south as Ohio, Pennsylvania, and New York. Glaciological reconstructions of the ice sheet indicate that the ice sheet had a maximum thickness over Hudson Bay of 3.5–5.5 km.2,3 This tremendous ice load (40 MPa, 5800 psi) upon the earth's surface caused the lithosphere to deflect by approximately one kilometer and the underlying viscous mantle to flow. Numerous well-documented paleo-sea-level studies, summarized by Walcott,4 indicate that after deglaciation the region around Hudson Bay experienced several hundred meters of uplift with uplift continuing today at rates up to 2 cm/year. More pertinent to this study is the observed sea-level change along the eastern U.S. coastline during the past 5000 years (Figure 1). During this period the volume of water in the oceans remained essentially constant,5 so that the observed sea-level rise, depicted in Figure 1, equals the vertical deformation of the earth's surface. The 2 mm/yr rate of sinking of the east coast shown in Figure 1 is comparable to the vertical rates of movement encountered in the most highly tectonic and seismically active regions of the world (e.g. New Guinea). Furthermore the maximum amount of subsidence occurs beyond the limit of glaciation in southern New Jersey and Delaware, where mantle material flows from regions peripheral to the ice sheet towards uplifting regions once under the ice sheet.5,6,7 This observation is supported in Figure 2 where the maximum present rate of submergence, as indicated by tide gauge records, is south of the glacial limit. Glacial loading is the cause of this differential deformation because a model of glacial isostasy, incorporating a realistic glacial history and spherical self-gravitating Maxwell earth model, explains 90% of the variance in the sea-level data in a region extending from northern Canada to the Gulf of Mexico.8 These large vertical deflections observed at the earth's surface do not necessarily imply that stresses large enough to cause fracturing are induced in the crust. Rigid vertical movement would not cause increased stress. Therefore, the goals of this study are to calculate the stresses resulting from flexure of the lithosphere caused by glacial loading and to relate these stresses to gas production and exploration.