Organic petrology and clastic depositional environments with special reference to Australian coal basins

Abstract

Two questions are addressed in this paper: (1) What are the types of organic matter that accumulate as coal seams or are dispersed in shaly sediments to form potential petroleum source rocks in clastic fluvial, lacustrine, deltaic and nearshore marine environments?; and (2) What are the systematic variations that exist between the petrographic compositions of related coals and dispersed organic matter (DOM), irrespective of environment? As few published studies combine sedimentology and quantitative organic petrology, most of the data considered, especially for DOM, are from Australian occurences. Tectonic setting, geological age and flora appear to be the major determinants of organic matter type, but atypical occurrences of coals and DOM suggest that their influence can be overridden by unique events or the depositional conditions. From the small amount of information available, no particular coal type was found to be overwhelmingly associated with a specific paleodepositional environment. Systematic and, thus, predictable relationships between coals and related DOM of Permian and Jurassic ages, are: 1. vitrinite (DOM) = vitrinite (coal) − ∼20% (1) 2. inertinite (DOM) = inertinite (coal) + 25% (±5%) (2) 3. inertinite (COAL) = inertite + durite + 0.5 intermediates (3) 4. inertinite (DOM) = inertite + durite + intermediates (4) 5. vitrinite ( DOM) = 0.6 (vitrite + clarite) (5) Relationships(1) and (2) do not hold for some Jurassic data, and (4) and (5) do not apply to the Permian restricted-sea environment, where inertinite (DOM) = inertinite (coal) + ∼40%.