Reservoir stratigraphy and architecture of glacial tunnel valleys reservoirs

Abstract

Upper Ordovician glaciogenic rocks from Block-NC186, in the Murzuq Basin, SW Libya, represent important hydrocarbon reservoirs. The producing reservoirs are found in glacial incisions filled by the Melaz Shuqran and Mamuniyat formations. These formations present complex reservoir architecture and high sediment heterogeneity. Understanding the morphology and internal reservoir architecture in these incisions is important in order to improve prediction in such complex reservoir systems. This study describes the Upper Ordovician formations from Block-NC186 (Libya) using an integrated approach based on seismic, isopach maps, well logs, core and chemostratigraphic data. The results are compared and contrasted with the infill of a Pleistocene glacial incision from Lake Simcoe (Ontario, Canada) inferred from high-resolution seismic data. Results show differences in the infilling history of the two systems. Sedimentological records from the Upper Ordovician show different phases of ice-sheet advance, retreat and glacio-isostatic rebound infilling a complex network of mainly fault-controlled incisions, of very variable widths and depths, cut under the pre-glacial Middle Ordovician Hawaz Formation. In the other hand, the Pleistocene infill from the Lake Simcoe shows a similar incised valley, filled during small fluctuations of the ice front in a single episode of ice-sheet retreat within low energy lacustrine environment. The comparison of these two systems highlights the variability of the glaciogenic infill and the need to incorporate influencing parameters such as regional tectonic trend, bedrock types, glaciations amplitudes and geographical environments into the geological models aimed to reconstruct and predict reservoir distribution within these glacial sedimentary systems.