Silurian, Middle

The BSC (part of the Dunnage tectonic Zone) formed in Late Ordovician to Early Silurian (i.e., during the Salinic Orogeny), and contains the accreted and subducted remnants of the Middle to Late Ordovician Tetagouche-Exploits back-arc basin. Consequently, the rocks of the BSC have... 

The study area is situated in the Tobique-Chaleurs Zone, (TCZ) which contains early to middle Silurian sedimentary and volcanic rocks (Chaleurs group) that are disconformably overlain by Lower Devonian volcanic and sedimentary rocks of the Dalhousie and Tobique groups (Wilson 2004) and is bounded by the Rocky Brook-Millstream Fault to the northeast and the Mackenzie Gulch Fault to the north (Fig.1). The McCormack Brook Fault (a splay of the Rocky Brook-Millstream Fault) lies to the north of the... 

Geologic Age Lower Ordovician Lower Ordovician Middle Ordovician Middle Silurian... 

Bituminous Substances. The distribution of hydrocarbons and other bitumens in some of the Paleozoic rocks of the area is shown in Tables I and II, the latter summarized after Swain (7). The presumed saturated hydrocarbons of the Ordovician, Silurian and Lower Devonian samples are measurably less than in the Middle and Upper Devonian samples. The presumed aromatic hydrocarbons are not as well differentiated. The pyridine-plus-methanol-eluted chromatographic fractions, arbitrarily taken as asphaltic material, also appear, like the aromatic fractions, to be controlled more by local variations than by geologic age. 

In general these substances are widely distributed in the acid distillates but appear to increase in number at two stratigraphic levels (1) in the middle Silurian McKenzie Limestone and (2) in the middle Devonian shales. Furthermore, possible two- and three-ring aromatic or heteroaromatic structures show similar increases. There is also a rough correlation with the Kjeldahl nitrogen content of the samples (Table VI). 

The relative increase in carbohydrates in the Middle Silurian and Middle Devonian is suggested as resulting from a period of increased organic productivity during the deposition of those rocks. 

Rosehill Formation, Middle Silurian, north Side of Juniata River, 4 miles east of Mt. Union, Mt. Union Quadrangle. 

Mobile Producing Company No. 1 Long Well, Centre County, Pennsylvania core samples from Lower Ordovician, Middle Ordovician and Middle Silurian (not shown on map). 

The Woodlawn volcanogenic Zn-Pb-Cu deposit is situated on the flank of a folded Middle to Upper Silurian volcaniclastic pile within the Lachlan Fold belt of eastern Australia (Malone et al., 1975). Primary ore comprises some 6 million tonnes of finely-banded complex massive sulphide ore (average grade 14% Zn, 5% Pb, 1.6% Cu and 88 g/t Ag). Ryall (1979b) compared the distribution of Hg with that of Cu, Pb, Zn, Ag, As and Sb in massive sulphide ore and wall rocks. The Hg content of massive sulphide ore ranges from <100-18,000 ppb and thermal release curves indicate Hg resides principally... 

To briefly summarize, the Late (and Middle) Devonian source shales are potentially of commercial interest in the Ghadames, South Timimoune and Reggane, whereas the Silurian source rocks are potentially of commercial interset in the basins of the Triassic Province, North Sbaa and North Timimoune. In these regions, these two source horizons display a similar potential for hydrocarbon generation. 

Thus, by the end of the Carboniferous the average depth of subsidence was about 3.0 km for the Ordovician, 2.8 km for the argillaceous Silurian, 2.5 km for the Early and 2.2-1.5 km for the Middle and Late Devonian (see Fig. 5.5). The Hercynian orogeny entailed a very significant uplift of the western and northern parts of the depression, which resulted in the formation, in the central part of the area, of a depression which was progressively subsiding in the south-east direction to form a saddle in the SED-i area. The Paleozoic sediments were highly eroded in the northern and western areas where the Ordovician, Silurian and Early Devonian deposits were exposed directly beneath the Hercynian unconformity. 

Organic Matter and Hydrocarbons. At the end of the Mesozoic subsidence, the organic matter, deposited in the Paleozoic, once again became subjected to high temperature and pressure and suffered alteration. In the intensely subsided zone, the Ordovician and Silurian sediments reached a depth of 3.8-5.5 km. Lower Devonian 3.4-4./ km. Middle and Late Devonian z.8-4.2 km and Carboniferous z.2-3.5 km. 

Present-day temperatures, R, and TTI in the Ordovician, Silurian, and Devonian layers (Table 6.7) suggest that today Early Ordovician shales at a depth of about 3 800 m are generating dry gas, and the top Ordovician shales are generating wet gas and condensate. The generation of liquid hydrocarbons in the Silurian, Early Devonian, and Middle Devonian shales to a large extent must be exhausted. 

Measured TOC ranges from 2 to 8% in the Middle Devonian shales, and from 1 to 5% in the Late Devonian shales, but with maximum decreasing concentrations westward from 1.5 to 5.0% in the Illizi Basin, 1.0 to 3.5% in the Mouydir basin, and 1.0 to 1.8% in the Timimoune basin. These variations are likely linked to changes in transport direction and in the provenances of detritus as compared to Silurian shales. The level of organic matter maturation in the Devonian sources in these basins is higher (Ro = 1-4%) than it is in the equivalent sources of the Triassic province (Oued el-Mya, Ghadames, and Trias basins) due to differing sedimentation, burial, and tectonic histories. 

Zumberge J. E., Macro S., Engel M., Johansson P., Schiefelbein C. and Brown S. (1996) Silurian shale origin for light oil, condensate, and gas in Algeria and the Middle East (abstract). AAPG/SEPM Anna. Meet. Abstr. 5, 160 p. 

Modzalevskaya, T.L. (1997) The Middle Ludfordian event in the Silurian evolution of brachiopods of the European province. Stratigraphy and Geological Correlation, 5(3), 205-211. 

Stratigraphic Range. 0 Lower Cambrian I Middle Cambrian 2 Upper Cambrian 3 Lower Ordovician 4 Middle Ordovician 5 Upper Ordovician 6 Lower Silurian 7 Middle Silurian 8 Upper Silurian 9 Lower Devonian A Middle Devonian B Upper Devonian C Lower Carboniferous D Upper Carboniferous E Lower Permian F Upper Permian G Lower Triassic H Middle Triassic I Upper Triassic. 

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