Glacial sedimentology interpretation from borehole image log: Example from the Late Ordovician deposits, Murzuq Basin (Libya)

Abstract

In the Murzuq Basin, the Late Ordovician glaciogenic succession forms a very complex clastic reservoir system. Although the structural setting is simple, the architecture of the stratigraphic succession is particularly intricate, and conventional wireline logs display rather homogeneous signatures. However, when exposed, the glaciogenic sedimentary succession indicates a very large range of depositional environments and clear stratigraphic changes. Based on high-quality processing and interpretation of wireline microresistivity image logs over a single well, our method allows the precise recognition of the internal sedimentary structures and supports the interpretation of the depositional environments within the Late Ordovician succession. During interpretation, it is possible to draw a descriptive sedimentological log, similar to a standard log from cores or outcrops. The image log is interpreted like a regular sedimentary log and compared to an outcrop analog from the nearby outcrop area of Ghat. The success of the well analysis resides in the quality of the borehole image log, permitting the recognition of sandstone grain sizes, textures (facies), and sorting. In addition, crucial information is extracted from the identification of glacial surface and ice-flow orientations, which, combined with the recognition of major transgressive events, allows the recognition and correlation of glacial-type stratigraphy. As in the modern Pleistocene glaciation, stadial/interstadial and glacial/interglacial stages are identified from resistivity imaging of the Libyan Ordovician succession. In addition to the unprecedented potential of correlation between wells within the basin, the sedimentary information extracted from the borehole image log provides important insights on the paleogeographic context of the basin and thus on the exploration potential of the prolific Ordovician-Silurian petroleum system. IntroductionWell petrophysical data interpretations are crucial in petroleum exploration and production. They are routinely conducted and provide essential insights on the quality of the rocks involved in the petroleum system. However, in some sedimentary environments, the acoustic and the petrophysical measurements are not able to provide sufficient diagnostic information to characterize reservoir properties and geometries. The limits of the use of standard logs arise from (1) the possible confusion between formations with similar sand/shale ratios (e.g., basal lag with large shaley clasts formations and sandrich debris flow) and (2) the limited resolution and the smearing effect of standard logs, which do not provide any detailed sedimentological information of the penetrated strata. On the contrary, bed thickness, bed contacts, or postdepositional events, which are important

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

“…Upper‐flow‐regime bedforms commonly occur within deposits of glacifluvial deltas (Fig. 3; Table 1) (Postma et al ., 1983; Winsemann et al ., 2011, 2018; Girard et al ., 2012a,b; Dietrich et al ., 2016, 2017; Moreau & Joubert, 2016; Nehyba et al ., 2017; Normandeau et al ., 2017; Lang et al ., 2017b; Hanáček et al ., 2018; Leszczynska et al ., 2018; Kostic et al ., 2019). The successions are very similar to those described from non‐glacigenic coarse‐grained deltas (e.g.…”

“…The dominantly glacifluvial and glacimarine successions indicate depositional environments with abundant meltwater and supply of sand‐grade sediment (Le Heron et al ., 2009; Bataller et al ., 2019). These conditions should favour the formation and preservation of upper‐flow‐regime bedforms, which indeed have been documented by a range of studies (Hirst et al ., 2002; Le Heron et al ., 2010, 2013, 2015; Hirst, 2012; Lang et al ., 2012; Girard et al ., 2012a,b, 2015; Moreau & Joubert, 2016).…”