Тип бассейна: Платформ
Подтип бассейна: Внутриплатформенный (интракратонный)
Класс бассейна: Синеклизный
Возраст бассейна: Древний - Палеозойский
Тип полезных ископаемых:
Геологический возраст начало:
Геологический возраст конец:
Площадь: 263866.62 км²
Gourara basin
Ahnet- Gourara basin is located in the central part of Algeria, 600 Km to the south from Hassi R’mel plant and covers 12 1260sq.Km included 17 blocks.
The exploration in this part of the Saharan platform began in 1950’s but initial efforts were stopped following the oil discoveries in Illizi basin Eastern Sahara and hampered by the no existence of facilities at that time . The majority of exploration activity in the Ahnet - Gourara basin has taken place in the last 20 years . The hydrocarbon environment of Ahnet - Gourara basin is enhanced and will continue with In Salah Gas development project .The pipe line built is already used to forward gas from In Salah to Hassi R’mel..
A proven gas in Ahnet –Gourara basin is from 26 TCF shared by 40 discoveries.The upside gas potential of the remaining exploration area is very high. Expected risked gas resources for economic accumulations can reach more than 20 TCF.
The main reservoirs in the area are the lower Devonian and Ordovician sandstones.
The Ordovician gas bearing produce mainly by fractures .The well test recorded that the flow rates reach up to 8MMCF/D.Ordovician reservoir compartimentalisation is strongly controlled by small faults. Using imagery and well log data such minor faults would be simulated by fracturing job and opened to enhance gas production. The other way to improve the production is to drill a deviated wells which would across a major faults and enhance the productivity. It has been used in Ahnet –Gourara basin and extended well test on Ordovician shows the best flow rates.
Facies & thickness changes are the main characteristics of the lower Devonian reservoirs . It’s due to their depositional environments i.e fluvial and shallow marine . Such environment allowed a development of stratigraphic and combined traps. These combined traps are confronted to seismic resolution problems which still partially inadapted to define a limits of lenticular reservoirs.
A new kind of field must be discovered and conventional structures could be completed by unconventional gas accumulations. Exploration must be more focused on risky zones but using new concepts of exploration.
Some of new concepts of exploration deals with BCGA , asphaltic sands analysis and deep offshore.
Regarding the high costs of investment in gas exploration , it becomes more safe to use new concepts in exploration where the results are economically interesting. Ahnet -Gourara basin could be considered as new hydrocarbon gas province of the 21th century.
Principal trends in Ahnet and Gourara basins
The Ahnet and Gourara basins lie on the site of a former Pan-African orogenic belt which separated the West African from the Saharan craton. The basin appear to represent a modified post rift –fill to an infracambrian rift system. Both basins preserve a thick CambroOrdovician to Carboniferous , largely clastic sedimentary fill which was deposited on the northern margin of the african continent.They were severely inverted during the late Carboniferous Hercynian orogeny to generate the large anticlines which have formed the principal exploration targets Three main trends are obvious : a North –South ; a Northwest-Southeast and a Est West trend which is more subtile.
The main reservoirs are Ordovician ,Lower Devonian and lower Carboniferous sandstones.
Concerning the geochemical aspect , the main source rocks are the Silurian and upper Devonian shales, the maturity is supposed to be reach at the end of Devonian and early Carboniferous. The degree of fill is ranging from 0.15 to 1. The critical factor still existing is the timing.
Stratigraphic column
Ordovician gas potential
The Ordovician deposits are fluvial to glaciomarine sandstones, a reservoir quality depends on fracture distribution. The porosity is extremly low in this section. Gas trapped in this pore system would not be able to flow on its own unless there is network of fractures present.
The productivity and sustainbility of each layer is not well understood. Simulation through fracturation is expected to increase well productivity.
Induced fracturing is believed to have contributed to the good gas rate in the production test.tough natural ones are confirmed to be present in the Ordovician reservoirs from an FMI logs may have plugged the fractures explaining the poor performance of some tests production before job fracturing. These natural fractures are believed to provide the essential permeability for gas flow while the pore system of the matrix provides the porosity for gas storage.
The target is to increase well productivity by fracturing job or to drill deviated well. These methods used in the area show an important results as the flow rate was around 2MMSCF/D and reach 10 MMSCF/D after fracturing.
The Basin Centred Gas (BCGA) production from ‘’unconventionnal’’ tight sands is a huge resource that if applied ,could significantly revive exploration in Ahnet-Gourara basin. This methods will be very interesting in the future as it’s not well developped on the tights gas reservoirs in USA. Using this method needs more studies on Ordovician reservoirs in Ahnet and Gourara basins.
Example of EWT on Ordovician reservoir in the Ahnet-Gourara basins
Lower Devonian and Lower Carboniferous gas potential
A Lower Devonain sandstones are fluvial to marine deposits , the reservoir quality depends on relation Quartz/Chlorite within the grains.The Devonian have higher porosities than the Ordovician.
Primary or intergranular porosity appear to be better preserved in Devonian reservoirs .The degree of preservation of primary porosity varies between fields. This variation is due to depositional history and a diagenesis events.
The vertical distribution of gas in the Devonian section appears to behave as stacks of reservoirs with multiple systems.It indicates that these reservoirs are not sharing the same Gas Water Contact (GWC).
Flow rates in Lower Devonian can reach 4 MMSCF/D . However during tests there are some failed , it’s unclear whether the test failed due to mechanical reasons or formation related. The critical factor is thereservoir presence and quality.
Lower Carboniferous sandstones are deltaic deposits, their distribution and extension is limited. The critical factors for these reservoirs are their lateral extension and reservoir quality .
Ahnet and Gourara basins could be considered as a new gas pool. Most of the well penetrated Ordovician reservoirs but some tests still not conclusives and evaluations used are not adapted to this kind of tight sand reservoirs.
Lower Devonian and Lower Carboniferous reservoirs show good potential . Although additional appraisal wells are needed to control the reservoir extensions. The discovered ressources are designated as potential recovry, redefining these resources to the ultimate recovery category would require additional technical work integrating new concepts of gas exploration.
Data source: Gas potential and prospectivity in Ahnet-Gourara Basin, Algeria. Mme F.Z Mahsas Senior Geologist & A.Rahmani Exploration Manager, 2005
Следующий Бассейн: Kufrah