DEPOSITION OF "BLACK SHALE" FACIES: RELATION TO POTENTIAL SELF-SOURCED MUDSTONE RESERVOIRS

A pioneer in studies of “black shale,” M. A. Arthur contributed powerful insight on the depositional conditions necessary to form such a unique type of stratum. Water depth during deposition, oxygenation of the water column and the role of biological productivity remain noteworthy topics of debate. Arthur has been a strong advocate of interdisciplinary studies necessary to resolve the complex processes responsible for the accumulation and preservation of marine organic matter, as the setting(s) do not simply mimic suggested modern analogues.

From a resource perspective, “black shale” can serve as hydrocarbon source rock. In addition, unconventional oil and gas accumulations reservoired in “shale plays” are attractive to hydrocarbon producers. Exploitation of this resource has flourished in the past decade, and new mudstone research by both academia and industry is supplementing earlier models.

As an initial ‘thought exercise,’ a conceptual model for deposition of this type of mudrock constrains it to discrete periods of time, as already noted by Arthur and colleagues. Atypical combinations of geological processes were responsible for the deposition and preservation of thick accumulations of Type II organic matter. The model considers tectonics, eustasy, paleogeography, climate, seawater chemistry and paleobiological trends. For example, basin restriction during periods of cratonic flooding was necessary to confine (re)cycling of nutrients and organic matter to the depocenter. Ultimately, the production of organic carbon in the water column may have outpaced its consumption along the seafloor, suggesting that explanations other than anoxia may possibly be invoked to account for “black shale” facies.