Petrographic and geochemical characterization of organic-rich Mississippian black shales in the north of Spain: Vegamián Formation, Cantabrian Zone

Abstract

Black shales (1–60m thick) with minor cherts (radiolarites) and phosphate nodules of Mississippian age (Middle–Upper Tournaisian; Vegamián Formation) occur in the fold-and-thrust belt of the Cantabrian Zone (N Spain) at the base of the Carboniferous succession, which is infilling the Variscan foreland basin. The Vegamián Formation was accumulated in a similar sedimentary and paleogeographic context to the Upper Devonian-Carboniferous black shales of the Appalachian, Arkoma, Fort Worth, Western Canada and Antler foreland basins (North America) and to the Variscan foreland basin in Belgium and Germany. Five stratigraphic sections were investigated in the southern branch of the Cantabrian Zone, corresponding to the shallower sectors of the passive margin of the Variscan foreland basin in N Spain, where this stratigraphic unit reaches a thickness of generally <15m. The Total Organic Carbon (TOC) ranges from 2.81wt% to 7.43wt% with a significant number of values over 5wt% indicating a high level of organic matter preservation. Thermal maturity can be considered to be between the peaks of oil and wet gas generation. The total sulphur (TS) content is generally low with few samples showing values higher than 1.5wt%.The organic matter is dominated by bitumen found either as diffuse masses, or concentrated in spaces and cavities, often parallel to the bedding planes or filling voids within calcite veins and fractures. Generally, the bitumen masses are homogeneous without secondary porosity, but occasionally they show mosaic optical texture indicating thermal transformation. Anastomosing structures, likely related to bituminite, are also common, and exhibit a large range of reflectances. Autochthonous organic matter (algae/bacteria) is seen as scarce Tasmanites and liptodetrinite, whereas allochthonous components (vitrinite, inertinite) are also scarce, except in the Olleros section. The range of reflectances of vitrinite-like particles is similar to that of bitumen but the identification is uncertain and therefore the reflectance of bitumen (BRr) has been used for thermal maturity assessment. A Gaussian-like distribution of bitumen reflectances was found for La Braña section (BRr=0.70%), whereas a bimodal distribution was observed in Olleros (BRr=0.70%) and Vegamián sections (BRr=0.95%) and large scatter of readings in Millaró (BRr~1.55%). The vitrinite reflectance equivalent estimated from aromatic compounds indicates that most of the existing correlations between vitrinite and bitumen reflectances underestimate the maturity of the lowest reflecting sections and overestimate it in the highest reflecting one. TS/TOC ratios, the geochemistry of organic matter and spectral gamma ray measurements suggest a normal marine to dysoxic environment of deposition with only local anoxia at the base of the Vegamián Formation.