Intra-Platformal Organic-Rich Facies of the Alpine Triassic

Mark T. Burchell

During the Middle and Late Triassic, large areas of the Alpine-Mediterranean region were blanketed by carbonate-platform systems that developed prior to the initiation of major rifting that ultimately produced the Ligurian Tethys Ocean.

Field studies reveal that the stratigraphic record of these platforms includes a number of locally developed beds, in particular of Ladinian and late Norian-Rhaetian age, characterized by the preferential preservation of organic carbon in a range of facies, from subtidal right through to stromatolitic units conventionally referred to as inter/supratidal environments. This heightened carbon preservation is most strikingly developed in Ladinian and Norian-Rhaetian deposits of intraplatformal depressions with limited area (up to 1,000 km²). These successions can approach 500 m thick and commonly consist of dark interbedded shales, limestones, and/or dolomite with 1 to 5% organic carbon, depending on carbonate dilution. With hydrogen indices up to 500 mg HC/g and S₂ yrolysis yields in the range of 5-20 kg/MT, they clearly have considerable "bulk" petroleum source potential. In condensed sections these values may be greatly exceeded. The carbon-rich sequences were usually associated with Mesozoic structural highs which received relatively thin post-Triassic cover and as a consequence were commonly pushed into the oil window in the latest flysch phases, i.e., post-Alpine deformation. Thus not only were the source horizons surrounded by abundant carbonate reservoirs, but also any secondary migration should have occurred at a time when potential structural traps were stable.

Such organic-rich facies may have been a ubiquitous feature of many Tethyan carbonate platforms and consequently must be considered as important in the petroleum exploration of the Mediterranean region.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.