Relaciones en la configuración de autodescripciones

Demaison (1980) reported that there are several biological, physical and physico-chemical factors can influence the accumulation of organic matter in aquatic environments. Biological factors include primary biological productivity and biochemical degradation. Physical factors include the modes of transit of organic

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matter to depositional sites, sediment particle size, and sedimentation rate and physico-chemical factors include the redox conditions under which the sediments were deposited. These factors interact to determine the qualitative and quantitative preservation of organic matter in sediment (Demaison, 1980).

Bordovskiy (1965) demonstrated that the main source of aquatic organic matter is phytoplankton, which are mostly comprised of single-cell microscopic algae residing in the uppermost layers of water illuminated by sunlight, or the euphotic zone. Also, the availability of mineral nutrients, and particularly nitrates and phosphate, is considered to be another principal limiting factor in addition to light in the euphotic zone. The other source of organic matter in the aquatic environment is terrestrial material transported from rivers and streams. Land plant productivity is largely dependent on the amount of rainfall on the supporting landmass (Demaison, 1980).

Tissot and Welte (1984) reported that the biological productivity of the marine aquatic environment represents the most important factor in the potential for source beds, despite the abundance of organic matter in non-marine aquatic environments varying in subaerial environments as a result of the widespread occurrence of land plants. The chances of the preservation of organic matter in subaquatic environments is far greater.

In the past few decades, the frequent association of source rocks with the early part of transgressive sedimentary cycles has remained a major stratigraphic enigma (Wignall and Hallam, 1991). In this context, several of the world’s best hydrocarbon source rocks belong to the category of transgressive black shale deposits, with organic facies AB being typical (Huc, 1988; Creaney and Passey, 1993). However, marine rather than terrestrial organic matter tends to be abundant in organic matter detritus, while amorphous organic matter is the typical maceral (Pasley et al., 1991; Gregory and Hart, 1992).

Black shales were deposited over large regions of the ocean floor several times during the Phanerozoic period. These strata have attracted interest from geologist, petroleum geologist and petroleum geochemist for many reasons, not least of which is their great economic important as represent the source of more than 90% of global recoverable oil and gas reserves. There is more than one factor has been put forward to control the genesis of the black shales, which are including, plate tectonic configuration of

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continents and the opening and closure of marine seaways; sea level rise and the flooding of shelves; the structure of the basins in which deposition occurred; and the evolution of marine and terrestrial biota and thus climate changes. In general, during the Cretaceous times, several tectonic and climatic factors favoured the development of anoxic conditions in the ocean. For the cretaceous a total of six ocean anoxia events have been recognized representing episode of globally widespread black shales deposition in marine environments that correlate closely with transgression.

Deposition of black shales strata in most of the area was restricted to a short period envelope termed the Campanian-Maastrichtian oceanic anoxia event. During this periods, a favourable combination of factors existed which led to the development of exceptionally strong oxygen-depletion in the North African Tethys. The Campanian-Maastrichtian black shales in North Africa are laterally discontinuous and their distribution and thickness were controlled by the palaeorelief. The thickest and regionally most extensive Campanian-Maastrichtian organic-rich shale in North Africa occur in Libya, namely in the Sirt Basin. Between 1 and 6% of the organic carbon burial during the Campanian-Maastrichtian was deposited in the relatively small restricted troughs in the Sirt Basin.

Many black shales are deposited on erosive or hiatal surfaces where they mark the initial stages of transgression. More commonly, black shales occur at the peak of transgression at the time the maximum shoreline retreated (Wignall, 1994). In this situation they represent the condensed section associated with the maximum flood surface (Creaney and Passey, 1993).

In fact it has only recently been appreciated that two distinct types of transgression black shales may be present in the stratigraphic record (Wignall and Maynard, 1993;

Wignall, 1994). These are basal transgressive (BT) black shales and maximum flooding (MF) black shales.

The Sirt Basin is considered to be basal transgressive (BT) black shales since the Sirt Basin is silled basin. The presence of local topographic depressions during the initial stages of transgression appears to be the crucial factor in the formation of basal transgressive black shale. Hallam and Bradshaw (1979) have recognized the

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irregularities of the bottom topography for black shale formation that would have locally inhibited bottom water circulation and allowed isolated pockets of stagnant water to persist, as shown in the model in Figure 1.7. Hallam’s irregular bottom topography model was renamed as the puddle model for BT black shales. Huc (1988) also illustrated that, rather than merely restricting bottom water circulation, hollows would also have served as traps for fine-grained sediments and organic detritus. Tyson and Pearson (1991) suggested that the bottom water oxygen demand may have been high, further enhancing oxygen restriction. Hence, the low sediment influx may have caused the most rapidly subsiding depocentre to become emphasised. Sliter (1989) reported that the black shales actually accumulated during a series of relatively brief events commonly associated with intervals of sea level rise, for instance throughout Cenomanian-Turonian and Campanian-Maastrichtian times, which are termed oceanic anoxic events (OAEs). These phenomena are discussed with reference to the Sirt Basin below.

The Cretaceous period is characterized by a serious of marine anoxic phases (Schlanger, 1976); associated with wide speared organic matter burial and black shales deposition. During the Cenomanian-Turonian and Campanian-Maastrichtian oceanic anoxia events, organic-rich strata was deposited in the rift shelf basin and slope across North African. High sea level during the ocean anoxia events enhanced increased surface water productivity on a global scale throughout increased sea-surface area availability for marine phytoplankton colonization, large scale nutrient supply leaching of flooded lowlands probably have caused anoxic and maybe even euxinic conditions in water column. The grabens within the Sirt Basin may have been separated from the open sea by sills (Hallett, 2002). The Sirt Basin was a basin with unique geometry with an opening into the Mediterranean Sea during Campanian times, when marine transgression occurred from the north side. The presence of planktonic and benthonic foraminifera throughout the formation in the northern Sirt Basin, are indicative of an open marine and outer neritic environment (Barr and Weeger, 1972). Longitudal trough areas, however, gave rise to restricted marine environments further into the interior of the Basin (El-Alami et al., 1989). These carried tracts of inactive mixing of water masses with a tendency to the development of stagnant waters. The lower part of the Sirte Shale barring the northern part of the basin contains bulimines (Foraminifera)

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(Barr and Weeger, 1972), while the upper part of the section contain both planktonic and benthonic foraminifera. The presence of the bulimines indicates the prevalence of a restricted marine environment. Such kinds of environments lead to the depletion of oxygen. This indicates the tendency towards the development of stagnant waters as it becomes a zone of insufficient mixing of the water mass (Demaison, 1984).

Furthermore, the bulimines are recognized to tolerate such stressed environments in stagnant conditions with low oxygen levels (Haynes, 1981). Subsequently, bulimines develop small, smooth and thin skeletons in the marine environment. At the upper formation, a tendency towards the amelioration of the marine reducing environment occurs as indicated by the presence of both planktonic and benthonic foraminifera (Cushman, 1974). Furthermore, based on benthonic foraminiferal analysis, the lowest Sirte Shale is thought to represent a middle neritic setting, while the middle and upper Sirte Shale mark a further deepening and an increase in oxygen-deficiency (El-Alami et al., 1989). At the same time, oxygenated water dominated over the platform areas where deposition of carbonate sediments took place. Generally, during the deposition of the Sirte Shale rocks there was continuity in the growth of the tectonic structure into a horst-grabens fabric in a shallow marine basin. Therefore, a huge amount of subsidence and transgression may have occurred alongside the tectonic development (El-Alami et al., 1989). The flow current of the surface was persistently towards the basin, which may have caused a continual supply and concentration of nutrients (El Alami, 1989). A fluvial drainage system may have curried and deposited the plant nutrients in the graben areas (El-Alami et al., 1989). These systems transported solutes leached from soil in the horsts to grabens. However, mineral nutrients with the presence of light may lead to prolific primary biological productivity, resulting in the depletion of oxygen, and also in the graben areas, the semi-enclosed confined seas may have promoted water stratification to increase the oxygen depletion (El-Alami et al., 1989).

In the Sirte Shale there is good evidence of the presence of anoxic conditions associated with the continual supply of nutrients, which are in the form of laminations, the attainment of brownish-black colours, the presence of phosphate nodules and pyritic concentration, non-bioturbated sequences and the absence of macrofossils (El-Alami et al., 1989).

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The Sirte Shale Formation was deposited during a major sea level rise during Campanian times. This transgression also caused local anoxity in the rapidly subsiding and/or partially restricted areas. The anoxic sea bottom conditions and consequent preservation of organic matter makes it an excellent source rock in the Sirt Basin.

However, mineral nutrients with the presence of light may lead to prolific primary biological productivity resulting in the depletion of oxygen. Also in the graben areas such as the Hameimat and Agedabia Troughs, the confined semi-enclosed seas may have promoted water stratification to increase oxygen depletion, primarily created by a sluggish deep basin circulation in restricted basins. In the Sirte Shale Formation there is good evidence of the presence of anoxic conditions associated with a continual supply of nutrients, in the form of laminations, the attainment of a brownish-black colour, the presence of phosphate nodules and pyritic concentrations, non-bioturbated sequences and the absence of macrofossils (El-Alami et al., 1989). It is therefore probable that anoxia rather than organic productivity exerted the major control on the accumulation of organic-rich marine sediments and the development of organic-rich petroleum source rocks in the Sirte Shale Formation.

Figure 1.7: Puddle model for basal transgressive (BT) black shales for Sirte Shale, after Hallam and Bradshaw (1979) and Wignall (1994).

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1.8 Campanian-Maastrichtian Organic Shales deposits in North Africa and