Conceptual Model. While there are many habitats that are associated with the deposition of organic-rich marine and lacustrine source rocks, one important pathway is linked to the onset of increased basin subsidence associated with major tectonic events. A key aspect is that this subsidence is spatially variable, with the uplift of basin flanks contemporaneous with the foundering of the basin center, resulting in a steeper basin profile.
The increase in subsidence-driven sedimentary accommodation is commonly associated with a 2nd order maximum flooding surface (MFS) and condensed section. This enhances the concentration of marine kerogen. Moreover, differential subsidence often results in basin restriction, enhancing organic matter preservation via dysoxia. Steepening of the paleogeographic profile can also increase organic productivity via changes in basin circulation, especially upwelling. Lastly, condensed sections related to rapid subsidence are of long duration, relative to eustatic drivers. Therefore, these intervals can be relatively thick.
Generally, the thickest source rock occurs in the depositional basin axis. While the MFS within the source interval is synchronous, the top and/or base are often diachronous, related to the progressive infill of syn-tectonic sediments. This is especially true for basins with constructional clinoforms, such as the Hue-Torok-Nanushuk and the Bazhenov-Achimov-Vartov systems. In these cases, the most distal source intervals represent more than 50 and 20 million years, respectively.
Examples. Sixteen source rock examples from twelve basins are reviewed that conform to this model. They occur in a variety of basin types, including rift systems, thin-skinned forelands, thick-skinned (“Laramide-style”) flexural basins, and a cryptic sag basin example. Despite differences in genetic origin, all have total subsidence rates 50-350 m/m.y. in the basin center, based on 1D basin models. Most are Type II marine source rocks, but two basins are failed rifts with thick lacustrine source rocks. Half of the examples are “shale reservoirs”, some of significant commercial significance (Wolfcamp, Marcellus, Utica, Niobrara, and Bazhenov).
Four examples are described in detail. The Hue and Kingak shales of the Alaska North Slope are coincident with the onset of increased subsidence related to a linked extensional-contractional system that marks the opening of the Amerasian Basin. The Draupne/Kimmeridge Clay was deposited at the culmination of North Sea rifting and an associated extensional accommodation increase. And the Utica and Marcellus shales of the Appalachian foreland basin were deposited at the onset of flexural subsidence associated with the Taconic and Acadian orogenies, respectively. And the Bazhenov of the West Siberia Basin occurs during an increase in cryptic post-rift “sag” subsidence.
Plate-Scale Factors. Source rocks that have very regional distributions that are not restricted to basin axes are quite different than the examples above. In some cases, these have been attributed to global Oceanic Anoxic Events (OAEs). Speculatively, some of these examples may be related to plate-scale tectonic restriction. Two potential examples are discussed. The widespread Upper Devonian (Fammenian) to Lower Mississippian (Kinderhookian) black shales of North America include the Ohio, Chattanooga, Antrim, New Albany, Woodford, Bakken, Exshaw, and other black shales. This is a time when mountain belts rimmed the Laurentian continent on three sides (Acadian, Antler, and Ellesmerian orogenies). Another possible instance is the Turonian-Cenomanian marine source rocks of the South and Central Atlantic (OAE2). This includes important oil source rocks of Venezuela (La Luna), Guyana (Canje), Ghana, and Angola (Iabe). With the opening of the Atlantic, including the transform margin between Africa and South America, the continental restriction enhanced dysoxia. The Rio Grande-Walvis ridges provided an additional barrier to the south.