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Three triggering factors, including preservation, organic paleoproductivity, and clastic dilution, for the formation of black shale successions during the Ordovician-Silurian transition have been debated for many years. Preserving conditions are controlled by redox conditions and microbial activities which have close relationship with salinity. Five outcrop sites of the Wufeng-Longmaxi black shales located in the southeastern area of Chongqing are analyzed for their elemental geochemistry, total organic carbon, and clay minerals. Clay minerals and boron are used to reconstruct the paleosalinity (S p) of the bottom water during this period. Depending on the clay minerals’ specific combining pattern, the calculation of S p is modified. The results show that the S p concentration ranges from 1.73 to 16.12 ‰, with an average value of 9.68 ‰. This shows an increasing trend from the Late Ordovician to the Early Silurian, indicating oligohaline-mesohaline water. Desalinization of the global ocean in the Late Ordovician was caused by melting of continental glaciers. The S p concentration shows negative correlation with total organic carbon (TOC) content, since the microbial decomposition rates are improved by high salinity. V/(V + Ni) and V/Cr ratios are used to reconstruct the paleoredox conditions of the bottom seawater during the Ordovician-Silurian transition, and the results suggest that the oxygen levels in seawater increased from the Late Ordovician to the Early Silurian and increased landward as a whole. Correlation between redox conditions with the TOC content reveals that anoxic conditions are favorable to the accumulation of organic matter. Ba and (Ba/Al)terr are used to calculate the biogenic Ba, in order to reconstruct organic paleoproductivity. The biogenic Ba shows no apparent relationship with the TOC content, which implies that organic paleoproductivity does not solely control the accumulation of organic matter. Si/Al and Ti/Al ratios are considered as indicators of clastic input. Our data imply that clastic input is quite low in the distal basin around the Ordovician-Silurian boundary, which is consistent with the maximum content of TOC. In the proximal basin, high clastic input dilutes enrichment of organic matter. We may conclude that complex, non-linear interactions among the three factors most likely lead to the accumulation of organic matter. High preservation, low dilution, and low organic production lead to the formation of organic-rich shale, whereas low preservation, high dilution, and high organic production lead to organic-poor shale. Paleoenvironment and organic paleoproductivity of the Yangtze Platform during the Late Ordovician and the Early Silurian are reconstructed. A combination of multiple factors significantly affects the formation of organic-rich shales.

Four recently discovered glacio-fluvial paleovalleys in southeast Jordan and northwest Saudi Arabia are described for the first time. The paleovalleys formed as a result of glacial erosion by series of sub-parallel valley glaciers during the Late Ordovician (Hirnantian) southern hemisphere glaciation on the Arabian Plate. The southwest-northeast orientation of the paleovalleys, Proterozoic erratic clasts and paleocurrent vectors indicate the advance of glaciers and subsequent glacio-fluvial siliciclastics emanating from a paleo-ice sheet located to the south on the Arabian Shield. U-shaped, paleovalley cross-sectional morphologies and gently inclined longitudinal profiles indicate initial glacial erosion of the ‘finger-like’ paleovalleys, probably as wet-based valley glaciers, eroded up to 250 m depth into Late Ordovician marine bedrock formations. Paleovalley-fill sequences comprise a tripartite upwards succession: (a) basal sandstone-dominated tillite with well-rounded, grooved and striated granitoid and metamorphic basement clasts derived from the Proterozoic Arabian Shield together with locally derived, rounded and elongate boulders eroded from the local bedrock at the margins of the paleovalleys (Retrogradational Lowstand Sequence); (b) green chloritic siltstone (Zarqa Formation) deposited during a progradational sea-level rise with marine influence (Transgressive Sequence); (c) coarse-grained, trough cross-bedded sandstone (Sarah Formation) attributed to progradational fluvial sedimentation as glacial outwash. Rapid sea-level rise during latest Hirnantian to Early Llandovery time resulted in marine flooding of the glacio-fluvial alluvial plain and deposition of organic-rich mudstones representing transgressive and high-stand sequences.

The Ordovician Umm Sahm Sandstone Formation of Jordan comprises approximately 200-m-thick succession of fluvial quartzarenites with subordinate claystone and siltstone lithologies of shallow marine conditions. The Umm Sahm Formation is characterized by its dark brown color, frequent jointing, and steep scarps. The Umm Sahm Formation is bounded by the marine claystones of Hiswah Formation at the top and the fluvial sandstones of the Disi Formation at the bottom. The Umm Sahm Formation is composed of two main facies: fluvial facies and tidal facies. The fluvial facies constitutes about 93% of the total thickness. The lower few meters of the succession passes upward from the Disi Sandstone Formation into similar massive white sandstone facies exhibiting similar white color, fine- to coarse-grained sandstone, with round-shaped pebbles. Trough and planar cross-bedding show a northwest unidirectional palaeocurrent trend. Light brown colored quartzarenites similar to those of the Cambrian Umm Ishrin Sandstone Formation are most common in the upper part of the succession. The tidal facies occupies three intervals in the middle part of the succession. It is composed of laminated and thin-bedded sandstones, siltstones, and claystones. They are rippled and varicolored with abundant trace fossils (Cruziana, Harlania, ruzophycus). The presence of hummocky cross stratification indicates the earliest short-lived tempestite conditions during the Paleozoic erathem of Jordan. The first appearance of Graptolites in the Ordovician rocks of Jordan was recorded during this study in the tidal facies of the Umm Sahm Formation. The vertical arrangement of both fluvial and tidal facies indicates three successive short-lived transgressions and regressions. These marine incursions indicate the successive shoreline advances of the Tethys Ocean, which was located northward, and inundated the southern braid plain. The three short-lived transgressive events took place, and the Tethys marine margin was displaced southward, giving rise to deposition of tidal facies in an open coast tidal flat. Following the transgressive events, renewed progradation and strandline retreat took place, fed by large amounts of siliciclastics derived from the continent and transported by braided streams across the intertidal zone.

The Late Ordovician glacio-fluvial Sarah Formation is an important tight gas reservoir target in Saudi Arabia. This study uses statistical methods to characterize the petrophysical heterogeneity of the paleovalleys of the Sarah Formation that crop out in central Saudi Arabia. Four paleovalleys were studied: Bukayriyah, Hanadir, Sarah, and Khanasir Sarah. Several lithofacies were identified in each that vary in texture, porosity, permeability, and facies abundance that reflect periods of ice advance and retreat. The heterogeneity analysis is based on three statistical measures, namely, the coefficient of variation, the Dykstra-Parsons coefficient, and the Lorenz coefficient. The coefficient of variation values is in the 0.62–1.94 range, indicating an extremely heterogeneous distribution. The Dykstra-Parsons coefficient values are in the 0.56–0.88 range, suggesting very high to extremely high heterogeneity in the reservoirs. The Lorenz coefficient correlates well with the Dykstra-Parsons coefficient for paleovalleys of the Sarah Formation. The heterogeneity parameters studied here indicate that the outcrops of Sarah Formation paleovalleys represent heterogeneous to very heterogeneous reservoirs, which may be attributed to complex depositional and diagenetic variations that have affected the porosity and permeability distribution.

This study evaluates the Late Ordovician glaciofluvial deposits of the Sarah Formation and equivalent outcroppings in north, central, and southwestern Saudi Arabia. The Sarah Formation also covers a wide area in the subsurface and is considered as an important target for unconventional tight gas reservoir. Defining the fracture types, nature, and distribution in outcrop scale might help to establish a successful fracture simulation model and behavior for the Sarah tight gas reservoir in the subsurface. This study investigates fracture characteristics for the Sarah Formation at Sarah paleochannel outcrops. The study revealed three sets of fractures, which have EW, NS, and SE-NW directions, and these fractures vary from open, resistive, and filled to resistive fractures. The closed fractures are filled with ferruginated iron oxides and gypsum. The filled fractures (the thrust boundary) are found in the study area at the SE-NW strike fracture set, while open and resistive fractures are found mainly at S-N and E-W fracture sets, respectively. The syn-depositional filled fractures (iron oxides) are considered as the younger fracture sets while the open and resistive fractures are post-depositional fractures which may have resulted from uplift or tectonic movement. A general model representing the fracture pattern and the thrusting boundaries due to glacial movement was constructed. It has been noticed that the systematic occurrence of filled fractures (thrust boundaries) described the boundaries between different glacial events, which act as a fluid barrier (filled fractures) and decrease the reservoir quality. The finding of this study might be utilized as a guide and lead for exploration in the subsurface Sarah glacial deposits. It will also help to understand and speculate the nature pattern and distribution of fractures with the Sarah Formation.

The Middle Ordovician Hawaz clastic reservoir on the Murzuq Basin is a new exploration play of great importance in the NC186 concession. H-field is one of the new discoveries in this concession. The main target of this investigation is to detect and evaluate the hydrocarbon play of Hawaz Formation in H-field. Petrophysical characteristics and seismic interpretations of the Hawaz Formation in H oil field, Concession NC186 of Murzuq basin have been evaluated through the analysis of well-logging records of ten exploratory wells and 3D seismic sections distributed in H oil field. These data have been analyzed and interpreted through utilizing some analytical cross-plots. The lithosaturation analysis indicates that the Hawaz Formation is mainly oil-bearing with some water-bearing sand levels with thickness ranging from 550 ft in the northwestern part of this field and increasing to more than 640 ft. The average cutoff of H-field illustrated that the water saturation is 45 %, the porosity is 12 %, and the reservoir thickness is 190 m. This surface was picked at 840 ms (TWT) in the northwestern part of the study area and increased to 1060 ms (TWT) in the southeastern part of the study area. The Hawaz surface was located at depths ranges from 4360 ft in the H1-NC186 well in the northwestern part and increased to 4611 ft in the H3-NC186 well in the southeastern part of the H-field. The analytical formation evaluation reveals that the lithological facies consists mainly of sandstone and little shale. This sandstone is coarsening upward. The lateral distribution of the petrophysical parameters portrays that the Hawaz Formation is a good reservoir rock where H4 and H5 horizons are the main productive zones.

Across the Saharan platform, mudrocks of latest Ordovician–Silurian age (the Tanezzuft Formation) are a major source rock interval for Palaeozoic petroleum systems, but source rock quality is variable and difficult to predict. In the Kufra Basin of southern Libya, evidence for organic enrichment in this formation is scarce. This paper presents the results of a spectral gamma-ray study of siliciclastic sedimentary rocks in Jabal Eghei at the western margin of the basin. The study spans the Ordovician–Silurian interval together with overlying Mesozoic strata and was conducted at outcrop using a hand-held gamma-ray spectrometer. Element concentrations of potassium (K), uranium (U) and thorium (Th) were collected from mudrock and sandstone successions to identify natural background values of these elements for different formations. Uranium contents were used to identify possible ‘hot shales’ with source rock potential. This study shows that sandstones and mudrocks are clearly discriminated by their K, U and Th contents. Most sandstones have 0.3 % of K, >4 ppm of U and >10 ppm of Th. Based on gamma-ray spectrometry, the Tanezzuft Formation is divisible into lower, middle and upper members. There is a significant difference in K content between the three members. Exposures of the lower member show K contents less than 0.5 %; those from the middle member are between 1.5 and 3.6 %; and the upper member has K contents between 0.5 and 1.5 %. Notably, U values >10 ppm are restricted to the upper member, indicating the presence of ‘warm shales’ in Jabal Eghei. There is a progressive increase of Th/K ratios from the basal part of the Tanezzuft Formation towards the upper member. The decrease in K content and increase in Th/K ratios between the middle and the upper members of the Tanezzuft Formation can be explained by a mineralogical change, confirmed by x-ray diffraction analyses on representative samples. The total volume of clay minerals is 64 % higher in the upper member than in the middle member, with kaolinite predominant. These changes in composition suggest a change towards a more weathered sediment source, which was probably caused by increased continental weathering due to climate change from icehouse to greenhouse conditions at the Ordovician–Silurian transition.

The carbonate unit in the Lower-Middle Ordovician Yingshan Formation in the Tazhong region of the Tarim Basin in western China is partially to completely dolomitised. Three types of dolomite were identified in this unit: microcrystalline (

The Ordivician-Khabour Formation from the Northern Thrust Zone of Iraqi-Kurdistan comprises between 500 and 800 m of alternating predominantly greenish-gray sandstones, siltstones, and shales. The succession has revealed an abundant ichnofossils characterized by 11 ichnogenus, namely: Helminthopsis, Gordia, Cruziana, Rusophycus, Monomorphichnus, Rhizocorallium, Thalassinoide, Planolite, Paleophycus, Deplocraterion, and Skolithose. Ethologically, the ichnofossils are distributed among domichnia and fodinichnia groups. This association of ichnofossils contains elements of the Skolithose and Cruziana ichnofacies. The presence of these ichnofacies indicates variations in energy of the sedimentary environment. High-energy condition with sandy-shifting substrate in foreshore zone is related to the Skolithos ichnofacies, while the Cruziana ichnofacies indicates unconsolidated-soft substrate developed under low-energy condition in the shoreface/offshore zone. These ichnogenera indicate shoreface-offshore zone of shallow-marine environment for the deposition of the rocks of the Khabour Formation.