Borehole Images and Logs: Tools to Constrain the Architecture of a Channelized Slope System, Karoo Basin, South Africa

Borehole Images and Logs: Tools to Constrain the Architecture of a Channelized Slope System, Karoo Basin, South Africa

Kolenberg, Y.

Luthi, S.M. (mentor)

Civil Engineering and Geosciences

Geotechnology

Petroleum Engineering and Reservoir Geology

2011-04-22

The SLOPE3 project investigates the depositional architecture of a Permian age, deep water / complex channelized slope system which is of interest to the oil industry as a potential analogue for real oil / gas reservoirs. This study forms a part of the SLOPE3 project and is focused on an integration of cores and wireline logs from six research boreholes, each forming a 1-D datasets showing a hierarchy of genetic depositional elements: -channels, complexes, complex sets and equivalent hierarchy channel sands, channel margin, levee and overbank deposits. These elements were previously characterized in outcrop by the University of Liverpool in an earlier phase of the SLOPE project. The University of Liverpool presented their data as a high resolution outcrop map / correlation panel some five kilometre long showing a unit truncated by a 120 m deep slope valley (see Figure frontpage). For SLOPE3 a set of six boreholes were drilled along a line sub-parallel and about 300 metres behind that outcrop section. The borehole positions had been picked on the basis of the outcrop correlation panel. All six boreholes were fully cored and five boreholes were logged with GR / Sonic / FMS. The ultimate objective of the project is to provide a set of analogue-based input data for 3D reservoir modelling. The wireline data of GR, Sonic, and FMS borehole images formed the basis for this MSc thesis and was processed and presented as graphics/logs by making use of GeoFrame software. With this software manually dip-picking of planar sedimentary structures was done. With the focus on constraining the 3D architecture of the channelized slope system, bed boundaries as well as the following other dip classes were picked: erosion surfaces, climbing ripples, low-angle cross-bedding, syn-sedimentary faults, and slump folds. By making use of the FMS images and the GR / Sonic log results the following lithofacies associations / genetic sedimentary units were identified in the five logged boreholes: massive sandstone, channel margin, external levee, internal levee, all in a background of shale. All measured sedimentary dip picks were corrected to palaeocurrent / palaeoslope indicators by subtraction of the measured and averaged structural dips. Sedimentary dips were plotted on stereonets for statistical analysis and interpretation in terms of palaeocurrent flow direction / palaeoslope orientation. This combination of a detailed outcrop data set with a suite of boreholes located on the basis of the outcrop data is a near ideal test set for the quality of geological predictions of a 3D reservoir architecture. The different type of architectural reservoir elements each with their specific lithofacies associations, can be characterized by FMS, Sonic, and GR logs when run in boreholes. However in this thesis it is shown that by doing a prediction (‘prognosis’) of the distribution of the lithofacies associations and their thicknessencountered by the boreholes based on an extrapolation from the nearby outcrop, it is rather difficult to predict the ‘actual’ geology away from well control. Moreover the value of palaeocurrent and palaeoslope indicators as identified from the FMS images for the prediction the distribution (orientation and the position) of the different genetic bodies, proved to be surprisingly minor. This is a reflection of the large spread / variation in measured palaeocurrent directions (local variation), and the additional scatter of those indicators away from the boreholes (areal variation). This latter aspect is characteristic for a meandering submarine slope channel complexes. The CD Ridge complex channelized slope system is hence interpreted to be a ‘jig-saw’ to even a ‘labyrinth’ reservoir type. The shale-prone external levee deposits are laterally extensive, thinly bedded sequences have a more layer-cake architecture. Therefore it is suggested to use deterministic modelling for these layer-cake parts, where the ‘jigsaw’ parts require a probabilistic modelling approach. The analysis of borehole images and logs resulted in: A complete input dataset including: • Zonations of the genetic bodies for every well. • A whole bunch of stereonets for 3D analysis. Both could not be achieved without BHI. Value of the use of borehole images is the 3D aspect and to distinguish between different thin-bedded lithofacies associations.

Karoo
Laingsburg
Borehole images
FMS
Slope
Turbidite
Sedimentary

http://resolver.tudelft.nl/uuid:006a4fdb-3281-4afe-baa5-cc4c7b5a5c25

Student theses

master thesis

(c) 2011 Kolenberg, Y.