Fresh Perspectives on Palaeozoic Glaciogenic Reservoir Intervals in the Middle East and North Africa
Study of Palaeozoic glaciogenic deposits over the last 20 years, by multiple research teams, has demonstrated how Late Ordovician and Late Paleozoic glaciogenic reservoir intervals require particular, “out of the box” thinking. Conventional correlation of core intervals and wireline logs using simple sequence stratigraphic concepts is liable to be wrong, because geoscientists must step back and look at the big picture to understand complex architectural heterogeneity, in order to make useful predictions in the subsurface. The last two decades have highlighted how vital it is to fully incorporate analogue datasets- i.e. outcrop analysis alongside thorough satellite image mapping and interpretation and aerial photography from Unmanned Operated Vehicles (UAVs)- to propose the best interpretations. In this presentation, I will draw on multiple examples of glaciogenic reservoir analogues from around the world to highlight this approach. It is hoped that this approach has value in explaining some of the complexities in reservoir intervals such as the Sarah and Unayzah formations (Saudi Arabia), the Al Khlata Formation (Oman), and the Akbra Formation (Yemen). The palaeo-ice stream concept- which has proven to be useful in the understanding of Late Ordovician intervals of North Africa- is demonstrated to be widely applicable to the record of the Late Paleozoic Ice Age (LPIA) in Namibia, Chad, South Africa and elsewhere. At the largest scale, analysis of freely available satellite data in various locations allows: (i) belts of glacial lineations, interpreted to have been produced by fast flowing and erosive ice streams, extending tens of km in length; (ii) crosscutting belts of large meltwater channels, some cut subglacially and others cut proglacially, filled with sandstone. Understanding whether meltwater channels were cut subglacially or proglacially is vital, because the geometry and connectivity differs depending on this context. Proglacial channels have a tendency toward braiding and anastomosing patterns, because the channels are free to avulse without the restriction of an ice overburden; subglacial channels are typically fixed in location and true braiding is inhibited. In greater detail, the deployment of UAVs to map subglacially cut unconformities is now seen as very important. A great deal of information on ice sheet behaviour- which can be used to predict the nature of overlying sediment (e.g. reservoir vs. non-reservoir) downdip in the basin- is archived on unconformities, recording vital steps in ice margin retreat and sand release. A bird’s eye view of a subglacial landscape reveals phases of buoyancy at some ice margins; in others, clues into relative roles of meltwater release and glacial abrasion. In other words, analysis of unconformities at outcrop is expected to provide a very powerful predictor for the nature of sediment downdip in the basin. At the smallest scale, we gain tremendous information on heterogeneity from micromorphology and fabric analysis in this section. Dovetailed with photogrammetry, micromorphological analysis reveals the orientation tendency of e.g. clay seams in sandstones produced through subglacial shearing, allowing the likely trajectory of porefluids through glacial sandstones to be understood. In summary, working together, academic geologists and industry geologists have a good chance of better understanding complex glaciogenic reservoir intervals, but in isolation, the understanding is fragmentary. From the academic side, the community now appreciates that looking at these systems at every scale- from the satellite image to the scanning electronic microscope- is only way to make rapid progress. From the industry side, a wealth of data and outcrops await analysis from these new approaches. A full understanding of the glaciogenic reservoirs of the Middle East and North Africa remains a very exciting prospect for all concerned.
AAPG Datapages/Search and Discovery Article #90360 © 2019 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition of the Siliciclastic Reservoirs of the Middle East, Muscat, Oman, October 28-30, 2019