Journal of Stratigraphy and Sedimentology Researches (Mar 2023)
3D organic matter modeling: a novel tool in forward stratigraphic modeling
Abstract
Abstract The source rock characteristics (thickness, quantity and quality) can be varied both in the deposition of time and space. Currently, the distribution map of organic matter is estimated by simplified extrapolate methods using observed data on well location. There is a high degree of uncertainty as a result of using this method especially in the area with a lack of well data. The aim of this study is to intreduce a novel tool in DionisosFlow software for three-dimensional (3D) simulation of the distribution of total organic carbon in a source rock. In this method, a stratigraphic modeling approach is used to mimic the production and preservation process of the organic-rich interval. The main parameters in this module will be bathymetry, sedimentation rate, primary production, carbon flux and oxygen condition of the sedimentary environment. In this study, we constructed an artificial model to investigate the efficiency of this process-based algorithm. Furthermore, the 3D simulation result of a real model is illustrated. This study indicates a close relationship between depositional conditions on the one hand and the production and preservation of organic matter on the other hand. Also, simulation results pinpoint which anoxic condition could be one of the main parameters in maintaining and distribution of organic matter. Keywords: Forward stratigraphic modeling, Organic matter modeling, Sedimentary environment, Organic matter production and preservation. Introduction In exploration, the prediction of hydrocarbon accumulation and quality variations within a prospect, prior to drilling, is of great economic importance. Recently, developed 3D modeling methods are gaining significance concerning volumetric hydrocarbon predictions. Nevertheless, in basin modeling studies, source rocks often have higher uncertain input parameters, even though the source rock is the first prerequisite for a hydrocarbon accumulation. Often a conceptual approach or simple models applying average geochemical values describing source rock properties are used. This often is insufficient, particularly in areas with heterogeneous geological conditions and/or variable depositional environments. Recently, a novel method is evolved from the needs of the petroleum industry to obtain a better estimation of the spatial and temporal distribution of source rocks and variability in the basin. This method describes the characterization of source rock as a function of the sedimentation environment and displays a relationship between rich organic layers and accommodation space. The purpose of the approach is a 3D estimation of source rock characterization (Thickness, Total organic carbon (TOC), Hydrogen index (HI)) to utilize in the petroleum system modeling. Material & Methods The 3D forward stratigraphic modeling is a process-based approach simulating sedimentary and tectonic processes both in carbonate and clastic environments (Granjeon, 2014; Granjeon and Joseph, 1999). The main sedimentary processes considered include: 1) accommodation space through time; 2) sediment supply and in situ production in clastic and carbonate sediments, respectively; and 3) sediment transport using Macro-Scale diffusion equations (Csato et al. 2012; Granjeon 2014; Al-Salmi et al. 2018; Hawie et al. 2015, 2021). For each cell of the numerical model, environment properties including a fraction of sediments, thickness, palaeobathymetry, sedimentation rate and energy are estimated. Finally, modeling results are compared with observed data at a well location to validate the constructed model. The stratigraphic forward modeling approach provides most of the parameters required for organic matter simulation (e.g., water depth, basin morphology, sedimentation rate). New parameters are added into classic forward modeling to simulate all the processes needed for organic matter modeling (Granjeon and Chauveau 2014; Bruneau et al. 2016) as follows: primary productivity, carbon flux, organic matter transport, dissolved oxygen level and burial efficiency (which corresponds to degradation within the topmost meter of burial). All these processes and their parameters are based on empirical equations or observations. The production of organic matter by photosynthesis is called primary productivity because it is the first stage in the marine food chain. After primary production, the organic particles sink to the seafloor. The amount of exported production, which reaches the sediment/water interface, is determined by the Martin equation (Martin et al. 1987). The oxygen level in sediments is one of the most important factors controlling the preservation of organic matter since it determines the nature of respiration of benthic organisms. After sinking from the sea surface and transport at the sediment/water interface, the organic matter is finally buried. The amount of organic matter preserved after the first few meters of burial is called burial efficiency; this parameter is mainly controlled by the sedimentation rate (Betts & Holland 1991) and local redox conditions (Tyson 1995). The Total Organic Carbon (TOC) is eventually determined by the amount of preserved organic matter diluted depending on the sedimentation rate. In this study, an artificial model is constructed to determine the efficiency of the new method in the simulation of TOC. Defined initial palaeobathymetry varied between 10 to 700 meters. Sediment was assumed to be composed of two main sediment classes: shallow carbonate and deep carbonate sediment. In this study, the primary productivity, the carbon flux and its variation are used by the default of the software. Discussion of Results & Conclusion The results of the model show the optimal depth for maintaining organic matter in such a way that with the increase of the palaeodepth, first an increase and then a decrease of TOC is observed. One of the reasons is the reduction of carbon flux concerning to depth. The results of this study indicated that the optimal depth of preservation of organic matter is less than 500 meters. Modeling results demonstrated that there is a rather poor spatial correlation between areas of high marine primary productivity and areas of organic-rich sediment deposition. The absence of correlation is due to the combined influence of the other key factors such as oxygen level. The results indicated that the lowest amount of oxygen is observed in the depth range between 200 and 500 meters and will be increased in much more depth. Because increasing depth leads to a decrease the amount of organic particles and oxygen consumption. Therefore, with the increase in depth, the oxygen level of sea water gradually increases. The depth corresponding to the highest value of TOC was observed between 200 and 400 meters. The modeling results showed that the effective parameters in the distribution of TOC are the oxygen level and accommodation space variation through time. This study demonstrated that the use of this new tool is a very suitable method in the 3D estimation of source rock characteristics in order to use petroleum system modeling. One of the strengths of this method is the use of close relationship of the sedimentary environment with the value of TOC. Therefore, this tool can be very suitable and practical in modeling deep source rocks or areas with a lack of data.
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