Organic Geochemistry and Depositional Environment of a Hydrocarbon Source Rock: The Middle Triassic Grenzbitumenzone Formation, Southern Alps, Italy/Switzerland

Abstract

The Middle Triassic Grenzbitumenzone Formation is a 16-m-thick sequence of finely laminated, organic-rich dolomites and black shales. TOC contents reach 10% in the dolomites and 40% in the shales. Sedimentological and geochemical data suggest deposition in a shallow marine basin under permanently anoxic conditions. The organic matter is of type II. Based on geochemical parameters the Grenzbitumenzone can be divided into two sections. In the lower part, hopanes are present in very high concentrations (up to one order of magnitude higher than steranes) and the J13C of the TOC varies between -30.0 and -31.8%0 (PDB). Above 13 m from the base hopanes are scarce and the J I3 C of the TOC becomes less negative. Sedimentological and geochemical data suggest that a planktonic "bacterial plate" developed at the oxic-anoxic boundary in the water column and that bacteria were a significant contributor to the primary organic matter production in the basin. Variations in the maturity-dependent Ts{Tm ratio and 20S/20R C29 sterane isomerization ratio occur over thicknesses of a few metres. Their correlation with the carbon isotope composition of the extracts suggests that the scatter may be related to changes in organic matter input and/or bacterial activity during early diagenesis and not to differences in maturity.

Organic geochemical evaluation of the oil/gas-generative potential of organic matter in Cretaceous strata from the Lower Benue Trough, Nigeria

“…The bioturbated intervals represent oxygen-rich periods during which the organic preservation potential was low, while the laminated non-bioturbated sequences represent oxygen-poor periods during which the organic preservation potential was elevated. It has been suggested that the observed variation in organic carbon content is in part the result of carbonate dilution (Bernasconi and Riva, 1993). The higher organic carbon contents are associated with periods of more rapid carbonate accumulation.…”

Mesozoic (Lower Jurassic) red stromatactis limestones from the Southern Alps (Arzo, Switzerland): calcite mineral authigenesis and syneresis-type deformation

“…This appears reasonable in the light of independent evidence. Indeed, the clay mineral assemblage of the underlying Rhaetian limestones and dolomites represents the original detrital assemblage (Dunoyer de Segonzac and Bernoulli 1976), and the organic matter in the underlying Middle Triassic succession is immature and was never buried to temperatures exceeding 70 or 80°C (Bernasconi and Riva 1993). The data set shows some indications for a temporal decrease of d 13 C values separating Sinemurian carbonates from phosphorites and hemipelagic Pliensbachian marly limestones.…”

Cranial anatomy ofBesanosaurus leptorhynchusDal Sasso & Pinna, 1996 (Reptilia: Ichthyosauria) from the Middle Triassic Besano Formation of Monte San Giorgio, Italy/Switzerland: taxonomic and palaeobiological implications

“…The Besano Formation (also known as Grenzbitumenzone) consists of an alternation of variably laminated dolomitic banks and bituminous shales, and sparse cineritic tuffs that are dated as late Anisian–early Ladinian ( Brack & Rieber, 1986 , 1993 ; Mundil et al, 1996 ; Brack et al, 2005 ; Wotzlaw, Brack & Storck, 2017 ). It was deposited in a marine setting with an estimated depth of 30–130 m ( Bernasconi, 1991 , 1994 ; Bernasconi & Riva, 1993 ; Furrer, 1995 ; Röhl et al, 2001 ; Etter, 2002 ). The middle portion of the Besano Formation, which probably yielded all specimens examined herein, was deposited in an intraplatform basin ( Röhl et al, 2001 ) and is characterized by organic-carbon rich layers, with well-preserved macro-lamination testifying very quiet hydrodynamic conditions and a lack of post-depositional bioturbation.…”