Abstract Microbial carbonates (microbialites) are remarkable sedimentary deposits for four good reasons: they have the longest geological range of any type of biogenic limestones; they form in the greatest range of different sedimentary environments; they oxygenated the Earth’s atmosphere; and they produce and store large volumes of hydrocarbons. However, they are amongst the most intractable of sedimentary rocks to study, as, being formed by the action or influence of microbes, they do not always preserve direct, or diagenetically robust, evidence for their mode of formation. Despite this, the scientific study of microbial carbonates has seen a significant renaissance in recent years, largely because of their importance as petroleum reservoirs, in both the Proterozoic of the Salt Basin of Oman and, more recently, the discoveries in the Lower Cretaceous pre-salt, Santos Basin, offshore Brazil (Figs 1 & 2). Here, production from the pre-salt reservoirs surpassed 500 000 BOPD in June 2014, and the Basin was estimated to have over 50 billion barrels STOIP (Formigli 2014). However, these are in deep-water sites, hundreds of kilometers offshore, hosted in poorly understood carbonate facies and with no nearby outcrop analogues. Many research programmes by academia and the petroleum industry have been spawned in recent years to investigate possible analogues and to further our understanding of these intractable rocks and their complex pore systems. The results from some of this work are contained in this volume together with the first series of scientific papers on the remarkable pre-salt plays of Brazil. This Special Publication provides significant contributions at a pivotal time in our understanding of microbial carbonates, when their economic importance has become established and the results of many research programmes are coming to fruition.

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.