Depositional Model of Marine Organic Matter Coupled With a Stratigraphic Forward Numerical Model (DIONISOS): Application to the Devonian Marcellus Formation
Benoit Chauveau, Didier Granjeon, and Alain Y. Huc
IFP Energies Nouvelles, Rueil-Malmaison, France
Predicting the occurrence and organic attributes of source rocks is mandatory for modern 3D basin models, for exploration and for optimal production of shale gas. We propose a numerical model that integrates recent advances in marine biology and organic sedimentology. This approach consists in using simple laws to mimic the main processes which control the quantity and quality of organic matter within marine sediments. Knowing the spatial and temporal distribution of the organic flux exported from the euphotic zone, the model simulates the degradation of organic matter in the water column and its preservation during the early stage of marine burial diagenesis. The exported organic flux is directly related to the production rate of organic matter, namely the primary productivity. This parameter can be tentatively extracted from Global Climate Models (GCM) or from dedicated databases. In a contrasting approach, it can also be adjusted by inversion of the results of the numerical model.
In the model, we assume that the primary controls on the degradation of organic matter are the residence time in the water column and the oxygen content at the sea-sediment interface. Because of the organic matter low density, its transfer to the sea floor required a ballast effect from the associated minerals. In this respect, organo-mineral transport vectors are mainly fecal pellets and marine snow aggregates. The organic flux decreases with depth following to a power law which has been calibrated from measurements in many areas of the oceans.
Recent observations have shown the role of the lateral transport in the final distribution of the organic matter. Hence, a fully predictive model of organic matter distribution requires to include a transport model. Regardless of the quantity of oxygen, fecal pellets and aggregates are rapidly disentangled due to the destruction of the chitineous peritrophic membrane packing the fecal pellets and of the polysaccharide exudates of the phytoplankton trapping the organic and mineral particles within aggregates. This biological driven control is relayed by sedimentary processes since the released organics can recombine with fine grain minerals forming "floccules". They are considered to play a major protective role for the adsorbed organics and to be the main form of transportation of marine organic matter within the bottom boundary layer. Due to their low density, these floccules behave as fine grain sediments. Their displacement proceeds by suspension, advective processes and traction transport currents following water currents, induced by large scale oceanic circulation or storms. These floccules are mandatory to explain the avalanche process implied by the recent observation of organic deposition in small scale foresets instead of large range parallel laminations. The organics follow water energy gradients and consequently have the tendency to settle down in bathymetric lows of epicontinental seas, carbonate platforms, continental shelf and continental slopes. This water energy control explains also the shift of the organic depocentres in a sequence stratigraphy framework.
After its transport, a part of the organic matter is degraded and the residual part is buried and then preserved in sediments. The preserved quantity depends on the oxygen content. In particular, the final preservation is favored by anoxia which occurs when the oxygen demand (oxidation of the organic matter) is greater than the oxygen supply (controlled by the oceanic circulation). In the absence of oxygen, microbial activity is less efficient and metazoan benthic life is precluded. The present model predicts oxic and anoxic zones as a function of (1) the deep sea topography (2) the value of the organic flux at the water-sediment interface. The preservation rate, or "burial efficiency", is linked to the amount of dissolved oxygen. When the deposition occurs in an oxic zone, the burial efficiency is extremely low in many cases, except when the sedimentation rate is strong enough to rapidly bury the organic matter, isolating it from oxidants. When the environment becomes anoxic, the organic matter is better preserved, although a certain quantity is still degraded by anaerobic reactions.
The degradation/preservation model of organic matter is coupled to the 3D stratigraphic forward numerical model DIONISOS that simulates the transport of the organic matter. The validation of the coupled model is firstly done using synthetic cases, to isolate the effect of the different parameters involved. Then, we simulate the deposition of the Devonian Marcellus black shale (a part of the Hamilton group). These black shales deposited during the early stage of the Acadian orogeny. The depositional environment is a shallow stratified water, involving an anoxic bottom layer. In this basin, plankton is the main source of organic matter, which produces a type II kerogen. Our model describes the source rock by giving the quantity of organic carbon (TOC) and the quality of the kerogen (HI).
AAPG Search and Discovery Article #120098©2013 AAPG Hedberg Conference Petroleum Systems: Modeling the Past, Planning the Future, Nice, France, October 1-5, 2012