Comparing fracture statistics from outcrop and reservoir data using conventional manual and t-LiDAR derived scanlines in Ca2 carbonates from the Southern Permian Basin, Germany

Natural fracture networks strongly control hydrocarbon flow paths in tight carbonate reservoirs. An improved understanding of their geometries regarding orientations and distribution may result in reduced uncertainties in reservoir modeling and well planning. Outcrop analogs provide supplementary information about fracture networks below seismic resolution. We introduce a suitable analog for a gas-producing Zechstein reservoir in the Stassfurt carbonates (Ca2) in the Southern Permian Basin, northern Germany. Dolomite represents the main lithology in outcrop and reservoir rocks, which were deposited on a similar carbonate platform slope environment, and both locations were subject to the same events of diagenetic and tectonic overprint.

The scope of this study is the evaluation of fracture characteristics by a direct comparison of three similar datasets of manual outcrop and digital outcrop data with borehole resistivity image log data from a horizontal gas development well. Manually measured fracture data of the exact same digital scanlines in the outcrop are used to successfully validate terrestrial laser scanning (t-LiDAR). T-LiDAR data is used to generate a high-resolution digital outcrop model, and we introduce a novel workflow, firstly applied to carbonate rocks, to detect fractures from that data set along artificial horizontal wells following E&P industry best practice.
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Results of both outcrop and subsurface data suggest W – E orientations for well path planning due to dominant northerly striking open tectonic fracture directions. Higher fracture intensities (P10 values describing the amount of fractures intersecting the scanline) of conventional scanline measurements of 4.3 m−1 indicate a bias in the t-LiDAR (P10: 2.6 m−1) dataset inversely related to limitations in fracture detection abilities along the horizontal well. As for the horizontal well (P10: 2.7 m−1), fractures paralleling the borehole appear systematically underrepresented. Fractures with almost perpendicular orientation to the outcrop wall can only be occasionally detected in the t-LiDAR data set due to their limited exposed surface. However, outcrop fracture characteristics are in good correlation with subsurface results and thus, can help reducing uncertainties in reservoir characterization during field development and well planning.