Paleozoic of Northern Gondwana and Its Petroleum Potential A Field Workshop

In the Hazro area of the Turkish border fold zone in SE Anatolia (Fig. 1), the exact position of the Silurian-Devonian (S-D) boundary is still under debate. Biostratigraphic approaches failed up to now (compare Luppold et al., 2012, this meeting: Silurian-Devonian Boundary 2) due to several missing index fossils (e.g. graptolites), revision of index fossils (conodonts) and a partly incomplete distribution of palynomorphs (e.g. chitinozoans and acritarchs). According to the geological information from this area near the north rim of the Arabian plate, the SD boundary should be positioned within the local Upper Dadas Fm. which represents a subunit of the Silurian to Devonian Diyarbarkır Group (Bozdoğan et al, 1987; Fontaine et al., 1980). Based on Acritarch assemblages, the Dadas Fm. is of Wenlockian to Lochkovian age (Bozdoğan and Ertuğ, 1997).

Devonian rocks in the Eastern Taurides are represented by the Ayıtepesi, Şafaktepe and Gümüflali formations. These lithostratigraphic units have been investigated along the measured Halevikdere Section (Sarız region, Eastern Taurides, Turkey). The Ayıtepesi Formation has a thickness of 425 m and is built up by quartzitic, cross-bedded sandstones, well bedded dolomitic layers and some shaly intervals in the Halevikdere Section (Wehrmann et al. 2010). The Şafaktepe Formation is represented by a limestone-dominated facies in the middle part of the section which is highly affected by tectonics. The Gümüşali Formation consists of limestones, shales and siltstones.

‘Paleozoic-type’ shell beds are accumulations dominated by typical members of Paleozoic fauna, such as articulate brachiopods, bryozoans, crinoids, and ostracodes. Articulate brachiopods are the major component of the Paleozoic fauna, and are the dominant members of many shallow-water marine communities (Li and Droser, 1999; Waisfeld et al. 1999; Gourvennec and Hoşgör, 2012). On the other hand, Early Carboniferous shell beds, including bivalve-only and bivalve-dominated beds are very common in the southern Anatolia, Hakkari-Çukurca region (Fig. 1). Bivalve-rich accumulations provide insight into the role of shell composition and taphonomic resilience in the formation of the typical monataxic shell bed types. In the light of the numerous studies from all aquatic ecosystems, bivalves have been used to reconstruct ancient paleoenvironments from Carboniferous strata from Avalonian or Perigondwanan terranes and Gondwana, encompassing marine shelf through to the nonmarine, coal-bearing strata (Okan and Hoşgör, 2007; Hoşgör et al. 2012). Okan and Hoşgör (2007) and Hoşgör et al. (2012) provided a comprehensive summary of Early Carboniferous bivalve paleoecology directly related to the depositional environments. The purpose of this study is to examine the paleontologic, stratigraphic and sedimentologic aspects of such unique accumulations in order to better understand the characteristics of ‘non Paleozoic-Type’ myalinid bivalv shell beds. Fossil deposits were described considering stratigraphic, sedimentologic, taphonomic and palaeoecologic attributes sensu by Kidwell and Holland (1991). Taphonomic features were observed in fossil fragments larger than 5 mm.

In 1987, exploration started in Central Saudi Arabia shortly after the government directive enlarged Saudi Aramco's exploration activities outside the Retained Areas in Eastern Saudi Arabia. In June 1989, the first oil (Arabian superlight), condensate and gas discovery were made in the continental red bed sandstones of the Early- Middle Permian (Kungurian to Wordian from 275.6 to Ma 265.8 Ma) Unayzah Formation at the Hawtah Field, south of Riyadh. The Hawtah discovery provided a clear evidence for a significant hydrocarbon potential in the Paleozoic successions. Since then, Saudi Aramco has made several new oil and gas discoveries in the Khuff carbonates, Unayzah and Jauf sandstones, both in central and eastern Arabia. Intensive drilling that resulted from these discoveries has provided an immense wealth of new subsurface lithological and palynological data to establish the stratigraphy and sedimentology of the entire Paleozoic succession.

Geographically the Central Taurides are defined as the area between the Kırkkavak and Ecemiş faults, whereas the Eastern Taurides are defined as the area between the Ecemiş and Eastern Anatolian faults at the southern part of Turkey. As it was presented by Özgül, 1997, Taurides are divided into several tectonostratigraphical units on the basis of stratigraphical, structural and tectonic features. Of these, Aladağ unit is comprising the Devonian- Upper Cretaceous clastics and carbonates; whereas, the Geyikdağı unit consists of Cambrian to Upper Lutetian clastics and carbonates. Paleozoic sections of these units have been focused on as a result of their source rock potentials (Table 1).

The Taurus Belt is geographically subdivided into three portions by two N-S oriented major faults (Figure 1) The Eastern Taurus region is bordered in the east by the Munzur Mountains and in the west by the Ecemifl fault.

The Late Ordovician glaciation was initially (1960-1990) thought to represent a long-lasting icehouse period. In the nineties, the idea was imposed of a much shorter glacial event, possibly catastrophic at the geological timescale. The corresponding glaciation was limited to the Hirnantian, or even to a part of this 1-2 My stage, as suggested by short-lived isotopic excursions and correlative biological turnovers. The sequence stratigraphy of Late Ordovician successions in the palaeo-high latitude North Gondwana setting reveals that at least 3 glacial event similar to the Hirnantian event occurred in the Katian, as indicated by the stacking-pattern of shelf successions including forced regressive system tracts. The latter seem to have their counterpart in low-latitude settings, either in the form of fully developed third-order regressive-transgressive cycles (Laurentia) or in karst horizons (Baltica). The synchronous character of these inferred worldwide glacioeustatically controlled event is however difficult to ascertain as biostratigraphy is essentially based on endemic (Laurentia/Baltica/Gondwana) faunal assemblages. The view of a long-lasting glaciation (> 25 My) including discrete short glacial events (< 1 Ma, intra-Katian events, Hirnantian, base Wenlock) prevails today. The tempos and forcings of these glacial events are however largely debatable, what made their correlation at continental scales questionable. Until recently, most of the glacial features (tillites, striated surfaces, tunnel valleys) were ascribed to the Hirnantian glacial event(s). Only during the corresponding time interval ice fronts indeed reach sedimentary basins around the Gondwana supercontinent, and hence were related to the widespread and well-known glacial record.

Silurian source rocks are wide spread and charge approximately 9% of the world’s conventionally trapped hydrocarbons. In addition, thick Silurian source beds are seen as unconventional targets for oil and gas and Shell has secured such acreage in China and Turkey. The most prolific source rocks of this age are found in the Middle East and North Africa. They are responsible for numerous conventional hydrocarbon accumulations, including the Arabian ‘super-giants’ (North Dome/South Pars and Ghawar) and major Algerian gas and oil fields (Hassi R’Mel and Hassi Messaoud). In this presentation, the variation in depositional setting for the Silurian source rocks will be illustrated, through a careful reconstruction of the Silurian flood in time and space. Although these source beds typically occur at or near the base of Silurian, some are clearly linked with oceanic upwelling, some accumulated in relatively confined foreland basin settings whereas others were formed on a broad but tectonically and glacially modified Gondwana shelf.

The “Evaluation of Shale Resources” has come a long way in the last 10 years and has brought to the industry a much clearer understanding of these challenging reservoirs. The basic understanding of micro porosity and its relationship to organic matter is important for one having to work with shale. This is a starting point for a better understanding of shale production; however this knowledge clearly leads to the necessary requirement of accessing this micro porosity through the process of hydraulic fracturing. This presentation will give a short overview of the evaluation of “Reservoir Quality” and “Completion Quality” before presenting a more detailed discussion on the issue of Mechanical Properties Anisotropy in Shale.

Rhuddanian transgressive black shales deposited across northern Gondwana include world class hydrocarbon source intervals in North Africa, Arabia and southern Turkey. The highest TOC values are usually concentrated in one (or more) layers in the lower part of the Silurian succession. These source units are commonly referred to as hot shales because high TOC values are often coincident with elevated uranium contents. Hot shales were deposited during the early stages of the Silurian marine transgression and their distribution was controlled by relief on the underlying depositional surface (Lüning et al., 2003). Understanding how glacial and tectonic processes influenced topography on the pre-Silurian depositional surface and how such topography influenced the deposition of hot shales can therefore reduce risk during hydrocarbon exploration.