Understanding, quantifying and predicting inputs of terrestrial organic carbon delivered from the continents to the oceans is a classic but still topical problem in marine biogeochemistry [1] which has direct implications for petroleum systems analysis. The fate of terrestrial organic carbon within the earth system governs our understanding of the global carbon cycle and is critical for constraining biogeochemical models and predicting past and future atmospheric CO2 levels. From a petroleum systems perspective, the composition of the source rock, and hence the contribution of terrestrial versus marine organic matter, is one of the main factors controlling whether gas and/or oil can be generated [2]. Classical biomarker analysis of crude oils (hopane and sterane distributions) often fails to adequately identify terrestrial inputs and other molecular compounds need to be considered (e.g. semi-volatile aromatics and diamondoids), especially in accumulations where there is a mixture of low and high maturity fluids. Here we This study compares the geochemical characteristics of four Australian petroleum systems ranging from the Ordovician to Early Cretaceous that are typified by source rocks deposited in marine environments with varying contributions of terrestrial organic matter.
The Ordovician, Upper Devonian and Lower Carboniferous source rocks of the Canning Basin, Western Australia were deposited under marine conditions but contain petrological, palynological and molecular evidence of terrestrial inputs (Fig 1A). The Middle Ordovician upper Goldwyer Formation records the earliest occurrence of land-plant microfossils (cryptospores and trilete spores) in Australia. The higher-molecular-weight n-alkane distributions and isotopic compositions recorded in the upper Goldwyer Formation show resemblances to those derived from modern day bryophytes and aquatic macrophytes. Upper Devonian source rocks were deposited under stratified, photic zone euxinic (PZE) environments; however, a terrestrial input is indicated by high abundances of perylene and methyltrimethyltridecylchromans, which potentially played a key role in the development of PZE [3]. The carbonates and shales of the Lower Carboniferous Laurel Formation were deposited in a transgressive shallow marine to open shelf environments. Petrological analyses show a pre-dominant contribution of land-plant remains in these rocks. Despite the terrestrial markers identified in these Paleozoic source rocks, these signatures are not obvious in the generated hydrocarbon accumulations. This may be possibly due to either the local origin of the terrestrial organic matter and/or the relatively small terrestrial contribution in the case of the Ordovician and Devonian source rocks, and perhaps by the lack of hydrogen-rich exinite macerals in the in the Lower Carboniferous source rocks.
The Early Cretaceous Echuca Shoals Formation shales from the Australian North West Shelf are typical passive margin sediments, ranging from shallow marine siliciclastics to open marine claystones. Visual estimates of marine (e.g. lamalginite) and terrestrial (vitrinite) macerals in the Echuca Shoal Formation samples indicate 4-8% of the organic matter is terrestrially derived. Oils in the Browse (Caswell 1, 2) and Bonaparte Basin (Elang West 1) both correlate to the Echuca Shoals Formation. Semi-volatile aromatic (SVA) ratios, δ13C of n-alkanes and quantitative diamondoid analysis indicate that the Caswell accumulation exhibits a greater terrestrial contribution than that at Elang West (Fig 1B, C). This terrestrial contribution could originate from either the Echuca Shoals Formation and/or mixing of hydrocarbons generated from the underlying Jurassic fluvial-deltaic source rocks.
References
[1] Blair and Aller, 2012
[2] Pepper and Corvi, 1995
[3] Tulipani et al., 2015
[4] van Aarssen et al., 2007
Presented at the 28th International Meeting on Organic Geochemistry (IMOG) 2017
