3D Geological Models of Submarine Lobes from Borehole Data, Permian Tanqua-Karoo Basin, South Africa
The Permian Skoorsteenberg Formation in the Tanqua-Karoo basin in South Africa provides excellent exposure of submarine basin-floor fans. Because of this, the European Union sponsored various outcrop studies, drilling as well as the data acquisition of the research boreholes in the NOMAD project. It was a unique opportunity to study a submarine fan system by combining widely exposed outcrop and research borehole data. Because both data sets are available, this MSc. project aims to acquire bed thickness data from core and borehole image (Fullbore Formation Micro Imager) analysis and to extrapolate these data to 3D in a variety of methods based on the results of first objective. Analysis has been done on the turbidites of Fan 3 in wells NB-4, NB-3, NB-2, and NS-2.
The cumulative distributions of turbidite bed thicknesses in the studied wells were found to follow a power law. Therefore, the cumulative bed thicknesses plot can be used for the following purposes: (i) to derive certain parameters for the bed geometries and distributions; (ii) to calculate turbidite volume connected to the well; (iii) to suggest accommodation space degree of confinement; and (iv) to derive qualitative information on the extent of erosion and bed amalgamation (thus, may suggest depositional setting).
The change of slope (the change of exponent) in the cumulative bed thicknesses can be interpreted due to confinement or alternatively, due to the variation of the flow rheology. The clustering of data in the cumulative bed thicknesses plot may suggest the flow rheology affecting the turbidite deposition in particular time or location, which may represent a shift in the lobe depocenter.
The turbidites volume connected to the wellbore can be calculated using three different methods, these are: using the mathematical model developed by Malinverno (1997) (volume is calculated from the bed thickness distribution); using the facies model developed utilizing Petrel 2010.1 software; and using the discrete convolution method (volume is calculated by relating the experimental data and the well data). The application of each method has to be done with care, taking into consideration the data availability and the limitation of each method. Moreover, information of the lobe internal geometries is needed in the volume calculation.