NATURAL FRACTURES IN PHANEROZOIC SHALE DUE TO MEMBRANE STRESSES

Natural fractures can be both good and bad for gas plays. Characterizing the orientation, distribution, and scale of natural fractures, whether by microseismic, well-bore, or field techniques, improves economic retrieval of tight gas. Mapping how vertical joint structures may intersect faults that reach near-surface aquifers is important for ensuring environmental containment of resource extraction. Establishing evidence for a unified mechanism to explain the common development of characteristic natural fractures in intracratonic hydrocarbon-bearing shale formations could increasingly optimize exploration. Our tectonostratigraphic and remote sensing analyses indicate that fractures in Paleozoic (Marcellus Shale) and Mesozoic (Pierre Shale) epicontinental North American shales are remarkably similar in stress style and paleogeographic setting despite a ~300-million-year age gap and differing distances from boundary stresses related to background plate tectonics. The fractures are syndepositional because embedded early diagenetic concretions grew in alignment. We speculate that the systematic cracks in the shallow lithosphere can be attributed to ‘membrane tectonics’, whereby the outer shell of the Earth is deformed as a consequence of latitude change so as to preserve hydrostatic equilibrium. Geologically unique, membrane tectonics involve large stresses but small strain, benefiting the ongoing shale gas revolution.