SHALE HYDROCARBON RESERVOIRS: CONTROLS ON DEPOSITION

Upwelling and anoxia are invoked frequently to explain generation and preservation, respectively, of marine organic carbon in Paleozoic and Mesozoic shale source rocks. Alternatively, the broad correlation of continuous hydrocarbon accumulations in self-sourced shale with specific combinations of macroevolutionary trends in land plants and marine metazoans suggests the following process was responsible: Consumption of organic matter by benthos declined in marginal-marine basins dominated by unstable, muddy substrates during periods characterized by elevated organic productivity. Fundamental to these processes, however, was active tectonism.

The tectonic histories and paleogeographic settings of sedimentary basins ultimately controlled the complex paleoecological changes that augmented the preservation of thick accumulations of organic carbon. Tectonism and paleogeography also exerted important controls on sea level and climate. A conceptual model integrating tectonic histories, paleogeographic reconstructions, and eustatic curves is useful for evaluating the distribution of continuous hydrocarbon accumulations in shale reservoirs and for predicting new organic-rich shale targets suitable for resource development.

Basin geometry can commonly help distinguish potential self-sourced shale from other types of hydrocarbon source rocks. Examples in the U.S. include the Paleozoic Marcellus and Barnett Shales and Mesozoic Haynesville–Bossier Formations and Eagle Ford Shale. Their deposition was associated with the assembly and breakup of Pangea. Flooded foreland basins along collisional margins were the predominant depositional settings during the Paleozoic, whereas deposition in semirestricted basins characterized the rifted passive margin of the U.S. Gulf Coast. Tectonism during deposition of stratigraphic units of this shale type helped confine (re)cycling of terrestrial and marine organic materials to relatively closed, marginal-marine systems. This resulted in a positive feedback on marine primary productivity and the accumulation of Type II kerogen. During eustatic flooding, thixotropic substrates, poor in coarse siliciclastics, limited the diversity of effective feeding strategies, which expedited the preservation of this kerogen.