Devonian Source Shales

The Devonian source shales, when compared with the Silurian, appear to be markedly less mature than the Silurian they have developed to the stage of a gas-and-conden-sate window only in certain locations of the area of their occurrence. This situation is most clearly observed in the Triassic Basin, where the Devonian thickness at depths greater than 3 km exceeds 1.0 km. Here the organic matter maturity in the Silurian and Devonian shales is markedly different. Indeed, in certain regions of the Devonian roof, 

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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. 

Higher temperatures are characteristic of the sedimentary section of the Akfadou area as compared to the Takhoukht area. Temperatures of about 120 °C were reached in Early Devonian sediments. Our model calculations for the end of the Early Carboniferous (about 330 Ma), and before the Hercynian uplift and erosion, give temperatures and maturation levels for the Ordovician, Silurian, and Early Devonian shales consistent with early to main stages of hydrocarbon generation (Table 6.6). Toward the close of the Carboniferous (about 289 Ma), the sapropelic and humic organic matter of the Ordovician, Silurian, and probably Devonian source shales was sufficiently mature to generate both liquid and gaseous hydrocarbons. Silurian and Devonian shales in other areas of the basin are also characterized by considerable potential... 

Middle and Late Devonian source shales in the central part of the Ghadames Basin are currently within the oil- or gas-condensate windows at temperatures of 100-110 °C (Table 6.5, Fig. 6.16). Our model calculations of the quantities of hydrocarbons generated and the generation rate of hydrocarbons in the Late and Middle Devonian shales of the Akfadou region are shown on Fig. 6.18. About 90% of the initial hydrocarbon... 

Fig. 6.18. Hydrocarbon yields (solid line), rates of hydrocarbon generation (dashed line), and expulsion threshold in the geological history of the Middle and Late Devonian source shales of the Ghadames Basin, Akfadou region...

The expulsion threshold for liquid hydrocarbons was attained in Campanian time (nearly 80 Ma) for Late Devonian source shales in the Mereksen region. During the Paleozoic, only some Ordovician and partially Silurian shales could realize their petroleum potential, whereas during the Mesozoic, the process of hydrocarbon generation occurred throughout the Paleozoic shales, including Carboniferous shales in the areas of their maximal subsidence (Fig.6.17). 

Devonian Source Rocks in the Late Triassic. By the end of the Triassic, the Late Devonian shales in the Illizi and Ghadames Basins started generating hydrocarbons which are at present confined to the Late Devonian reservoirs. In the west of the platform, the hydrocarbons migrated to both subsided and exposed zones. The former, in all probability, were degraded, while the latter could be entrapped beneath the Triassic salt-bearing horizons. 

Mesozoic (Triassic) Province. In the region north of the Ain Sefra-Zouia el-Kahla (Fig. 5.4), the hydrocarbon was generated mainly during the Mesozoic. In the case of Silurian source rocks, represented mainly by radioactive shales, thickness lines were drawn with reference to the Ordovician roof. For the Devonian source rocks, the thickness lines were drawn with reference to the Early Devonian roof (Siegenian). The Lower Devonian roof appears to be the only reliable datum mark for intra-Devonian correlations. 

Devonian shales in the Illizi Basin have properties similar to those in the Ghadames Basin but are thinner. According to opr model. Middle and Late Devonian sources in... 

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. 

The C4-naphthalene and C4-benzene data presented in this paper were collected as part of a larger study addressing the influence of pressure, temperature and time on hydrocarbon generation (Hill et al, 1994, 1996, 2(X)3). The goal of the study was to evaluate whether maturity parameters defined during oil pyrolysis could be extended to natural systems. Thus, the Devonian sourced oil from the WSCB was used to define C4-naphthalene and C4-benzene maturity parameters in the oil pyrolysis experiments and Mississippian oils from the Fort Worth Basin were used to evaluate C4-naphthalene and C4-benzene maturity parameters in natural systems. It was not the goal of this study to determine whether results from pyrolysis of a Devonian oil will compare exactly with oils from a Mississippian source. Due to facies differences between Devonian and Mississippian marine shales, the initial distribution of C4-naphthalene and C4-benzene isomers is different as discussed earlier. For this reason, we would also not expect maturity ratios from pyrolysis of a Devonian oil to exactly match maturity ratios from Mississippian Barnett Shale oils. However, if the maturity trends observed from oil pyrolysis results are valid, the C4-naphthalene and C4-benzene ratios should correlate with a well established thermal maturity parameter such as TAS in Mississippian Barnett Shale oils. 

Figure 1. Atomic H/C vs. O/C ratios for asphaltenes from a variety of sources (M) petroleum crude (Alberta, Cretaceous, Carboniferous, and Devonian) ( ) Middle East crude (w) North and South American crude (+) shale oil (+) oil sands and (A) liquids from Kentucky coal at 4000 psi hydrogen and West Virginia coal at (O) 4000 psi, and (U) at 2000 psi hydrogen (21)...

Lambert M. W. (1993) Internal stratigraphy and organic facies of the Devonian-Mississippian Chattanooga (Woodford) Shale in Oklahoma and Kansas. In Source Rocks in a Sequence Stratigraphic Framework, AAPG Studies in Geology 37. American Association of Petroleum Geologists, Tulsa, OK. pp. 163-176. 

Figure 5. Isopach map of total Devonian shale composited from numerous literature sources. (Reproduced with permission from Ref 14. Copyright 1982, Institute of Gas...

The 50 million years of the Devonian Period provided sufficient time for over 11,000 feet of sedimentary rocks to accumulate in the Appalachian Basin this accumulation was controlled by pulses of tectonic uplift to the east. A similar control of a northwestern source area by intermittent tectonic movement is suggested as a source for the upper wedges (B/C) in the Michigan and Illinois Basins. The lower black shales (A) of regional type represent quiescent periods of slow deposition followed by relatively more rapid sedimentation occurring in response to uplift and erosion of source areas and to depositional restrictions caused by rising arches. The total thickness of Devonian shales in either the Michigan or Illinois Basin is less than 1000 feet, so that the sediment volume involved in these two basins is far less than in the Appalachian Basin. Thus there was ample time for... 

The major source rocks in the Oued el-Mya Basin are Silurian and Devonian shales and, to a certain extent, Ordovician shales (Figs. 5.1,5.2,5.3). The organic matter of the... 

Fig. 5.3. Organic isotope and geochemical data, Triassic Province, a Carbon isotope composition of oil and bitumenoid fractions for Paleozoic sediments, Triassic Province b Carbon isotope composition of oil from Saharian fields c Geochemical log for Takhoukht geological section based on shale pyrolysis data d Classification diagram HI-OI for Silurian and Devonian shale sediments, Triassic Province e Distribution of residual oil potentials vs. organic carbon content in Silurian and Devonian oil source rocks, Triassic Province...

The Silurian and Early Devonian organic matter is typically sapropelic, mixed, and humic (Fig. 5.3). It was intensely generating hydrocarbons in the south of the region in the Paleozoic, and in the areas of the other basins in the Mesozoic. At present, the organic matter may presumably persist either at the end of the oil window, or in the gas window and low-molecular hydrocarbons. The radioactive shales of the Early Silurian are good source rocks in the central and north-eastern areas of the region. The Late Devonian and Carboniferous periods have provided satisfactory source rocks in the south-west and north-east of the region. 

The isotopic composition of the bulk carbon in the oils is sufficiently uniform and varies only within a narrow range of -28 to -30%o (Table 5.6). On the whole, the oils are isotopically light which is typical of petroleum genetically related to marine source rocks. For the purpose of a correlation the isotopic composition of bituminoids from Silurian and Devonian shales of a very high hydrocarbon potential has been investigated (Tables 5.5,5.6). 

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