Sedimentological analysis of borehole images

Borehole images can capture detailed sedimentological information, with the detail only limited by tool resolution, borehole coverage and data quality. Sedimentary facies of the logged formation can be interpreted in a similar way to core facies analysis. As these data are captured digitally, thickness distribution and statistical analysis are readily generated. Palaeotransport trends are determined through analysis of depositional features such as cross-bedding, after structural de-rotation. In the deep marine realm palaeoslope orientation can be determined through the analysis of sedimentary remobilisation fabrics such as slumps and slides.

Facies analysis

• Lithology identified from borehole images, openhole logs, core, SWC’s and ditch cuttings.
• Sedimentary fabrics identified from borehole images.
• Dip data that has been corrected for structural tilt used for analysis.
• Sedimentary features identified and classified from the borehole images, including cross-bedding and various macroforms, data resolution permitting.
• Each image facies type allocated a colour code and interactively picked downhole.
• Data stored on flat bed database ready for export to other applications.
• Better and more accurate results are achieved where core is available for image facies calibration.

Sediment dispersal

• Understand general nature of depositional environment prior to dip picking during reconnaissance pass of image analysis. Establish appropriate dip picking scheme. Use hierarchy approach if data and succession suitable.
• Use dip-picking scheme to allow classification of cross-beds and a hierarchy of associated bounding surfaces and macroforms.
• Subtract structural tilt and re-classify dip types based on palaeohorizontal inclination.
• Zone the section into distinct sedimentary packets (e.g. parasequences, bed sets, individual channels or sand packets), tied-in with well stratigraphy and structural zonation.
• Examine bedding surfaces on interactive stereonets with azimuth and strike histogram overlays. In horizontal wells it may be possible to establish dune migration direction where whole trough sets are sampled, using advanced stereographic analysis.
• In fluvial sections an estimation of mean palaeo-river depth can be made using mean macroform thickness.
• Determine relationship of bounding surfaces to each other and cross-bedding.
• Use peak counts from contours, vector means and eigenvector methods to establish measures of central tendency.
• Interpret orientation fabrics for cross-bedding to assess palaeotransport trends within sedimentary packets.
• Assess orientation of macroforms in order to interpret large-scale sedimentary attitudes, where present (e.g. axial bar forms, point bars or elongate sand bars).
• For deep marine sections use deformed bedding & slumps/slides to estimate palaeoslope strike and up-slope direction where block rotation is recognised.

Results and presentation of data

• Manually drafted pseudo-core log along with lithofacies descriptions.
• Sedimentological interpretation presented in a 1:500 scale summary plot for the study interval. Larger scale plots (1:50; 1:100) may be generated in more detailed studies.
• Colour-coded drag and drop facies packets presented alongside borehole images.
• Full statistical analysis of thickness data for each image facies type; can be by zone.
• Sediment dispersal data presented by zone, spreadsheets, stereonets and azimuth frequency data.
• Successions interpreted in terms of facies associations and facies tracts.
• Significant surfaces identified and detailed.
• Potential correlative features reported.
• Unconformities isolated and detailed.
• Possible bed geometries and dimensions calculated.
• Potential baffles and barriers identified.
• Markov chain analysis completed on appropriate lithofacies data sets.
• Stick plots of apparent dips in any azimuth.
• All the results are detailed in a written report incorporating tables, figures and appendices.

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