Ice Sheets and Hot Rocks: Unravelling the Glacial Signature in the Late Ordovician Reservoirs of North Africa and the Middle East
Eni Exploration and Production Division, San Donato Milanese (MI), Italy
The Upper Ordovician reservoirs in North Africa contain at least 5 billion barrels of oil equivalent in more than 50 fields scattered across a broad area from the Murzuq Basin in SW Libya to the Ahnet Basin in central Algeria. Most of these reservoirs were deposited in glacially-influenced, generally shallow marine settings, on the continental shelf, beyond or at the margins of a continental ice sheet. In keeping with their ice-proximal to distal setting, they exhibit complex and rapid changes in facies and reservoir quality, and contain a wide range of glacially-induced syn-sedimentary structures.
Many of the Upper Ordovician glacigenic deposits of North Africa were first documented during the 1960’s and 70’s, but relatively few attempts have been made since then to interpret them in the context of our now much greater understanding of glacial and glacimarine processes. Considering their importance as hydrocarbon reservoirs and their intimate association with one of the main source rock horizons in the region, a re-evaluation is long overdue.
Over the past two decades, Eni, in association with several different academic institutions and industry contractors, has made considerable progress in unravelling the depositional history of these complex rocks. The sedimentology has been re-interpreted, new biostratigraphic data have been collected and a method of correlation has been developed based on the cycles of ice advance and retreat that occurred during deposition. The age of the deposits has been revised and the timing of ice-sheet growth and collapse has been related to plate movements, changes in climate and in oceanic conditions and, ultimately, to the causes of the second largest mass extinction in Phanerozoic times.
The deposits of the late Ordovician glaciation are extensive and occur at outcrop and in the sub-surface in North and South Africa, Arabia, South America, and parts of southwest Europe. The ice sheet was centred over Central Africa and expanded outward onto the surrounding continental shelves. At its maximum extent, it was of comparable size to the present day Antarctic Ice Sheet and may have extended over 65º of palaeolatitude, reaching as far north as 30ºS.
Growth of the late Ordovician ice sheet paradoxically occurred during a period of elevated CO2 levels that lasted throughout most of the Early Palaeozoic. The glaciation caused a eustatic fall in sea level of 50-100 m, produced ventilation of the world’s oceans and triggered a major mass extinction, during which 85% of all extant species were eliminated.
The initial stage of ice-sheet growth was terrestrial and commenced at the start of the extraordinarius Zone of the early Hirnantian (latest Ashgill). This initial stage was coeval with the ‘first strike’ of the late Ordovician mass extinction that resulted in the evolution of the Hirnantia fauna. This distinctive fauna is preserved immediately beneath and within Upper Ordovician glacigenic rocks of North Africa, indicating that the glaciation here is almost entirely Hirnantian in age. In the later stages of the extraordinarius Zone, the ice sheet advanced onto the continental shelf and deposited ‘glacially-influenced’ sediments across most of North Africa.
Unequivocal evidence of the presence of ice on the continental shelf of North Africa is sparse, but includes isolated occurrences of outsized, exotic, faceted and striated (icerafted?) clasts in shales and the presence of locally extensive soft sediment striated ‘icepavements’. The Upper Ordovician glacigenic rocks characteristically exhibit very rapid lateral and vertical changes in facies. These make it notoriously difficult to establish a sound sequence stratigraphic framework for the deposits and to correlate them from one area to another, even within a single field. However, new work has enabled the Upper Ordovician glacigenic rocks of North Africa to be subdivided allostratigraphically into ice-contact, glacimarine shelf, and rebound units, based on an analysis of the facies and facies associations preserved within the Upper Second Bani Formation in Morocco, the Hassi el Hadjar Formation in Algeria, and the Melez Chograne and Memouniat Formations in Libya. Fourteen facies and six facies associations characterise these rocks. Together they indicate ice-contact to distal glacimarine shelf settings on a high-latitude shelf influenced by an extensive grounded ice sheet. The fining-upward transition from ice contact to glacimarine shelf architectural elements characterises a glacial-retreat succession.
Two main glacial-retreat successions are preserved in the Upper Ordovician rocks of North Africa and both are underlain by sub-glacially deformed, coarsening upward, glacimarine shelf allostratigraphic units deposited during glacial advance. Related cycles of eustatic sea-level fall and rise are recorded in age-equivalent sequences in Canada, the Welsh Basin, the Prague Basin and Portugal suggesting that these two major cycles of advance and retreat of the ice sheet produced global effects. The upper glacial retreat succession in North Africa is overlain by a rebound allostratigraphic unit that represents the collapse of the ice sheet from the shelf and the associated isostatic rebound. Full-glacial conditions ended abruptly in North Africa near to the base of the late Hirnantian persculptus Zone. This triggered the ‘second strike’ of the late Ordovician mass extinction when the associated glacioeustatic sea level rise produced a return to normal oceanic stratification and flooded the previously exposed continental shelf with anoxic waters. Organic-rich graptolitic ‘hot’ shales were deposited in isolated topographic depressions in the former glacial landscapes of the continental shelf in the Rhuddanian (earliest Llandovery) during the initial stages of the transgression. These discontinuous basal Silurian black shales are the source of at least 80% of the Palaeozoic-derived hydrocarbons discovered in North Africa to date. The sea level continued to rise through the early Silurian, flooding most of the remaining topography by the Aeronian (mid-Llandovery). Analogy with late Quaternary glaciations and with chronometric data from coeval Upper Ordovician deposits in Australia suggests that the climate (and presumably ice-sheet volume) during the late Ordovician was influenced by Milankovich-eccentricity cycles of approximately 100,000 years. The presence of two main cycles of glacial advance and retreat in the Upper Ordovician successions of North Africa suggests that full glacial conditions may only have lasted about 200,000 years, a much shorter time than previously thought. This accounts for the poor biostratigraphic resolution that exists in the late Ordovician glacigenic sequences in North Africa, and has important implications for extinction rates and global climate change.
The different facies of the Upper Ordovician glacigenic succession have widely differing reservoir properties. Successful exploration for, and development of, these reservoirs requires a detailed understanding of sand body distribution, shape and vertical and lateral continuity. Ice-proximal fluvio-glacial deposits and high-density turbidites typically form the best quality reservoirs, but punctuated coarsening-up shoreface deposits in the postglacial isostatic rebound succession also have considerable potential. Appraisal and development of these glacigenic reservoirs is further complicated by the presence of a wide range of syn-depositional, glacially-induced heterogeneities, including sub-glacial and ice marginal fold-thrust belts, tunnel valleys, soft-sediment load structures, intraformational shear surfaces, dewatering structures and micro-faults, all of which have the potential to act variously as barriers, baffles or conduits for fluid flow on both geological and production time-scales.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013