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

Julien Moreau,Jean-Bernard Joubert

doi:10.1190/int-2015-0161.1

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.

Highlights

Well petrophysical data interpretations are crucial in petroleum exploration and production
High-resolution borehole image log coupled with conventional logs provide an image sedimentary facies close to the geologic facies calibrated to the outcrop analogs
The facies association succession and the glacial surface characteristics present a similar evolution as the Ghat area succession (Figures 2, 3, and 12; Table 1)

Summary

Introduction

Well petrophysical data interpretations are crucial in petroleum exploration and production. The realization of this study necessitates sev- sheets were similar to the Pleistocene ice sheets in that eral steps starting from the calibration of the tool to the they contained fast and slow ice-flow areas, ice paleodepositional environment reconstruction This streams, and ice domes (Moreau et al, 2005, 2007; work aims at presenting a comprehensive workflow Ghienne et al, 2007b; Le Heron and Craig, 2008; for the analysis of glaciogenic succession from borehole Moreau, 2011). The glacial ridge situated directly to the northwest of the town of Ghat constitutes the closest analog of the studied well data (Figure 1; Moreau, 2005, 2011; Moreau et al, 2005; Joubert et al, 2007) The description of this outcrop analog intends to image the variability of the sedimentary units to capture all the possible units drilled and imaged (Figure 3; Table 1). The sequence is locally interrupted by meter-thick tabular fine-grained sandstone beds

Ash Shaybiyat Formation

Results

Conclusions