Stratigraphy and numerical forward modeling

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What is stratigraphy?

Stratigraphy is the study of how rocks are organised in space and time. Hence, stratigraphers are interested not just in what the rocks are made of, but also in what processes led to the depositon of the rocks. Within the carbonate research group, we have studied outcrop and core stratigraphy to reconstruct sea-level changes, past climate change, and the heterogeneities of carbonate rocks at a range of scale.

Numerical forward modelling of stratigraphy

But these geological processes occur over a range of time scales from seconds to millions of years, which makes them difficult to grasp or predict. For instance, think of an earthquake: the quake itself and the destruction associated with the tremor might be done in an instant, but the movement of the tectonic plate that led to the event could have taken millions of years.

In sedimentary geology, of which carbonate sedimentology is a subset, we also deal with processes occuring at a range of time-scales: basin opening and closing controlled by tectonic, global sea-level changes, sediment supply by rivers filling in the space in the

 basin, and in-situ production of (carbonate) sediments. The mathematics controlling the shape and distribution of sediment bodies is thus very complicated and cannot easily be visualised by humans based on the incomplete stratigraphic record that the rocks give us.

Instead, we use a technique called "forward numerical modeling", and a commercial simulation package called "DionisosFlow" made by Beicip, in France. The principle of this tool is that we define a region (a basin) and a time in the geological past, and from there, we can model forward how sediments are being deposited, eroded and transported in the basin, where the main area of deposition are, what the nature of the sediment deposited is, and wether or not it form good reservoirs for oil and gas for instance. Importantly, we can also derive  the uncertainties in our models, e.g. build uncertainty maps. We use diffusion equations to represent sediment flow, an approach that has been shown to work well for sediments modeled over long (>10'000 years) periods.

Examples of this type of work within the carbonate group include understanding the regional 4D (three dimensional, plus geological time) distribution of carbonate sediments in the Early Cretaceous of Oman, a time periods characterised in the Middle East by the deposition of important hydrocarbon reservoirs. This study is now published in the Journal of Marine and Petroleum Geology (see Al-Salmi et al, 2019), and we demonstrate for instance the need for regional differential tectonic in the Early Cretaceous to explain the stacking patterns of carbonate rocks in this area (see figure above on the right, showing facies distribution in the Lower Cretaceous of Oman).

Other examples included the Msc work of Ilyas Kussanov on the sub-salt of Kazakhstan, where we looked at the Serpukovian Stage of the Karachaganak FIeld (Kussanov et al, in prep). There, we worked on a relatively small (10-20 km) carbonate atoll (figure on the left), and tried to understand the constraints from the data on the geologic models. We demonstrated for instance that the seismic horizon picking needed to be re-evaluated in view of our models, and we also were able to model where and why the most promising reservoir facies would be. This has tremendous implication for the extractive industry.

Publications related to stratigraphy and forward modeling: