Тип бассейна:
Подтип бассейна:
Класс бассейна:
Возраст бассейна:
Тип полезных ископаемых:
Геологический возраст начало:
Геологический возраст конец:
Площадь: 27220.15 км²
Thrace basin
The Thrace basin is one of the largest Tertiary sedimentary basins in Turkey, and one of the most important hydrocarbon provinces in the country. It is bordered on the west by the Aegean Sea and Greece, on the north-west and north by Bulgaria and the Black Sea, on the east by the Bosphorus, and on the south by the Sea of Marmara (Fig. 1). Topographically it is generally a flat-lying plain; in the centre it is covered with Quaternary to Recent alluvial sediments. The basin margins are areas of outcropping Tertiary sediments, older metamorphic and crystalline basement complexes. Interest in hydrocarbon exploration within the basin began in the late 1950s and still continues at present, although with less euphoria. By the end of 1988, about 235 wells had been drilled by various oil companies, the most active of them being the Turkish Petroleum Corporation (TPAO), the State oil company. Most of the wells are in the north-centre and north of the basin where commercial accumulations of oil and gas have been discovered.
Fig.1 Simplified geological map of the Thrace basin and vicinity, north-west Turkey.
Previous studies have discussed the stratigraphy, facies distribution, and petroleum potential of the basin (Holmes 1961; Kellog 1972; Burke and Uğurtaş 1974; Doust and Arıkan 1974: Keskin 1974; Turgut et al. 1983; Saner 1985). The studies were supported by oil companies evaluating the hydrocarbon prospectivity. Recent drilling and high quality multi-channel seismic data gathered by TPAO have improved our understanding of the basin. The new seismic data shows that basin tectonics were closely related to the evolution and activity of the North Anatolian fault system (Sengör 1979). Our aim in this paper is to demonstrate and describe interrelationships between tectonic styles, basin development, and the hydrocarbon prospectivity of the Thrace basin.
STRATIGRAPHY AND SEDIMENTATION
The overall geometry of the basin is shown by a total sediment isopach (Fig. 2). The north-west and southeast marginal areas show less than average thicknesses, because these areas represent intra-basinal palaeo-highs. They were either positive land masses supplying clastic sediments to the basin or sites of shallow-marine deposition at the initial stages of the basin development. The central basinal areas were largely sites of marine clastic deposition throughout the depositional history of the basin; they form the largest parts of the Thrace basin. The lens-shaped, broad northern shelf and the intra-basinal palaeo-highs form other elements of the present-day basin. The southern shelf has largely been destroyed by the deformative effects of the north Anatolian transform fault in late Miocene to early Pliocene times. The only shelf sedimentation belonging to the southern shelf is observed in the Gallipoli Peninsula, and in several wells drilled along the northern shore of the Sea of Marmara (Fig. 32.3).
Fig.2 Present-day sedimentary provinces of the Thrace basin.
Sedimentary evolution
The basin began its development at the start of Middle Eocene times with rapid subsidence over an older metamorphic and crystalline basement. The positions of the shelf areas and palaeo-current directions indicate a west-southwest to east-north-east progression for the initial transgression. The early basin consisted of a large central trough and limited surrounding shelf regions (Fig. 3).
Fig.3 Middle Eocene palaeogeography of the Thrace basin.
The central trough was the site of rapid deposition of thick sequences of turbidites derived mainly from the western and southern highlands; poorly-sorted, thick and shaley turbiditic sands were laid down. Intermittent andesitic volcanism introduced considerable amounts of volcanic ash into the basin. Early volcanism shows a south-west-north-east to west-east trend, indicating that it was confined to the basin margins where early normal basement faults were active. The basin reached its largest extent in late Eocene times with the development of a broad northern shelf. This period in the basin's development coincides with widespread carbonate deposition. Shallow-marine micritic mudstones, wackestones, and reefal carbonate build-ups were deposited on the northern shelf. The central and north-western parts of the basin were the sites of dark coloured, argillaceous deepwater lime-mudstone deposition. Similar carbonate depositional environments existed in the south and south-east at this time. Reefal carbonates of the northern shelf consisted of framework builders, such as corals, red algae, and also large foraminifers, pelecypods, gastropods, and echinoderms (Fig. 4).
Fig. 4 SW-NE stratigraphical cross-section across the basin, showing relationship of the shelf and shelf-slope sedimentation.
Only a small central area remained as the site of clastic deposition. As a result, the Upper Eocene strata represent a time of maximum transgression and of decreased subsidence. Following widespread carbonate deposition, moderate subsidence and clastic deposition resumed throughout the basin in late Eocene and early Oligocene times. Thick, fine-grained, marine clastic sequences accumulated, containing glassy tuffs which in some areas reached thicknesses of 1000 m. The large volcanic input indicates increased fault activity on the basin margins where the glassy tuffs are thickest. Volcanic input in the central basinal and shelf areas is limited or nonexistent (Fig. 5).
Fig. 5 NW-SE (A-A') stratigraphical cross-section, showing facies relationship in the shelf sedimentation.
The intensity of volcanic input into the sedimentary column decreased in early Middle Oligocene times, showing up as thin beds of volcanics and tuffites, indicating a possible decrease in fault activity.
In late-early to Middle Oligocene times, depositional environments were mostly shallow marine to marginal marine represented by thick sequences of fine-grained clastics, mostly shales interbedded with siltstones and silty sandstones. Late Oligocene to early Miocene conditions were paralic to lacustrine. The whole Oligocene is represented by thick shale-dominant clastic sequences. The late Oligocene and early Miocene sequences contain thick coal deposits throughout the basin.
The basin was subjected to considerable uplift and erosion in Middle to late Middle Miocene times. Much of the sedimentary cover was removed along the margins of the basin. In the late Miocene, a small marine incursion took place on the south-western corner of the basin, depositing thick sections of shallow-marine clastics as interbedded sheet and deltaic sand and claystones (Fig. 6). Deformation along the basin margins continued throughout the Pliocene and Pleistocene.
Fig. 6 Late Miocene palaeogeography of the Thrace basin.
RESERVOIR AND SOURCE ROCKS
Large volumes of clastic sediments in the form of thick turbidite sequences filled the Thrace basin from the start of extension in early Middle Eocene to latest Eocene times in the first phase of rapid subsidence and deposition. The Middle Eocene turbidite sands are widespread and contain thick sand bodies (Figs. 7,8).
Fig. 7 SW-NE structural cross-section across the northern flank of the Thrace basin.
Fig. 8 Schematic palaeo-reconstruction of the northern flank of the Thrace basin.
Although later diagenesis and synchronous volcanism have partly destroyed porosity and permeability, these sands still remain the most widespread and prolific reservoir facies in the basin. The interbedded shales are usually poor in total organic carbon content, and usually overmature. Thick sequences of basinal and shelf limestones, marls, shales, and silty shales were laid down in the basin in late Eocene times (Figs. 4,5). Some of the shelf carbonates are reef build-ups and show porosity and permeability. A thin veneer of basement wash, underlying the shelf facies, was deposited in this period and has fairly good porosities and permeabilities. The rest of the late Eocene sequence is generally devoid of reservoirs and has poor source-rock characteristics.
The rate of subsidence and deposition slowed in Oligocene times. Thich shallow-marine sand and shale sequences occur in the Lower and Middle Oligocene. The Lower Oligocene sequences are shale-dominated, and form the best source-rocks in the basin. The Middle Oligocene sequences are deltaic to near-shore deposits, and are sanddominated. These sands form the second main reservoir facies in the basin. The Middle Oligocene shales are quite rich in their total organic carbon content but they are usually immature. The Upper Oligocene series is mostly paralic to lacustrine and contains abundant lignite coal seams, but is immature.
SOURCE ROCKS AND MATURITY
Figures 32.18 and 32.19 show total organic carbon content and vitrinite reflectance values from two deep wells in the basin. The Lower Oligocene sections contain most organic matter and are the best source facies. The maturation of organic matter in the basin starts at around 2500 m depth.
Fig. 9 Vitrinite reflectance and total organic carbon content with respect to depth, Kumrular-1 well. (See Fig. 2 for location.)
Fig. 10 Vitrinite reflectance and total organic carbon content with respect to depth. Umurca-1 well. (See Fig. 2 for location.)
Therefore, it would be the areas where the lower Oligocene sections have been buried to 2500 m or more that would generate economic quantities of hydrocarbons in the basin. Such areas are located in the centre and north centre of the basin. The time-depth burial history and hydrocarbon generation potential of the various source rocks in the basin has been quantitatively modelled for the location of a deep well in the north central part of the basin. The modelling technique uses a combination of kinetic theory and Lopatin's (1971) concept (Dahl et al. 1987; Waples 1980; Welte and Yükler 1981; Yalcin and Welte 1988; Yükler 1987, Yükler et al. 1987). From Middle Eocene to late Oligocene times, there were continuous and rapid subsidence with the exceptions of short pauses, as explained above. This continuous subsidence seems to have come to an end around 18 Ma when the whole basin started to be uplifted. Uplift was at a maximum around 6 Mа. which corresponds to the late Upper Miocene when the North Anatolian transform fault came into existence and caused a wholesale uplift and deformation in the basin. Maximum oil generation occurs today at around 2750 m to 3000 m depth, depending on the type of organic matter contained in the sediments. The organic matter found in the source-rocks of the Thrace basin is usually herbaceous or land-derived, or a mixture of both. The amount of marine-derived organic matter is usually low. This characteristic has caused the source-rocks to be more gas-prone, with low quantities of paraffinic petroleum generation.
Fig.11 Stratigraphic sections of the Thrace Basin with new and published tuff ages and biostratigraphic data. The stratigraphy is from Siyako (2006b), Erbilet al. (2021), and Okay et al. (2010, 2019) modified to the new age data. The biostratigraphic zonation is after Speijer et al. (2020) and Steininger and Wessely (2000).The Miocene mammal ages are from Ünay & De Bruijin (1984) and Geraads et al. (2005). The inset shows the locations of the stratigraphic sections. The approximatelocations of the biostratigraphic samples that have yielded results, are shown by paleontological symbols
Data source: Evolution of the Thrace sedimentary basin and its hydrocarbon prospectivity Süleyman Turgut, Muharrem Türkarslan, and Doğan Perinçek. 1991
Thrace Basin—An Oligocene Clastic Basin Formed During theExhumation of the Rhodope Complex. Aral I. Okay, Ercan Özcan2, Muzaffer Siyako, Kerem A. Bürkan. 2023
Следующий Бассейн: Antalya