Abstract
This paper considers erosion variables and their uncertainties in terms of their influence on outputs of interest in basin analysis calculations. While one often knows with some considerable precision the initial deposition time and final end time of an erosion event, one does not know (as opposed to surmise) when erosion commenced during that time interval, nor does one know the amount of material that was deposited and later removed, nor does one know the lithologic properties of the eroded material. All three of these factors can be of importance in basin development. Using a recently developed extremely fast Excel burial history program, in conjunction with the very fast Monte Carlo program Crystal Ball, one can now rapidly sort out various uncertain factors for their influence on burial history outputs.
Two sets of examples illustrate how one can determine which parameters associated with an erosion event dominate in their impact on basin analysis uncertainties. The first example allows the amount of material to be eroded and the time of erosion onset to be variable, while retaining the lithological properties of the eroded material, and investigates which of the two factors has the most dominant influence on oil and gas charge with time. Cumulative probabilities of obtaining oil and gas charges are given as well as relative contributions to variance from the two variables. By changing the underlying distributions from which the erosion amount and the onset timing are chosen, one also changes the cumulative probability of oil and gas charges as well as the relative contributions. This relative sensitivity is also examined.
The second illustration sets the erosion amount and onset timing at fixed values but allows uncertainty in lithological properties of the material to be eroded, extreme examples use uncertainty in permeability representative of typical shale and sand lithologies, as well as the power index connecting permeability to porosity. The main concern in this example is to investigate the uncertainty in depth to overpressure isobars immediately after erosion, and to identify which of the lithological parameters is causing the largest effect. This problem is related to the fracture development of a basin with concomitant loss of generated hydrocarbons by such fracturing.
The illustrations have been chosen for their simplicity of presentation so that one can see immediately the effects of uncertainties in parameters related to erosion events to the relative importance, relative contributions and relative sensitivity of basin outputs of interest, such as hydrocarbon charge and overpressure development and associated fracturing.
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