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Western Desert Basin
The Western Desert is located in Egypt and it consists of a few extensional coastal rift-basins. It started as rifts and was formed during the Jurassic time in association with the opening of the Tethys Sea. There were three major tectonic events that occurred in Western Desert within Jurassic to Eocene time and resulted in NE-SW trend and NW-SE trend normal fault, and inversion of Western Desert basin due to rifting of Neo-Tethys followed by South America & Africa Atlantic rifting and Transpressional Syrian Arc event which had contributed to the formation of hydrocarbon trap. The generation, migration and accumulation of hydrocarbon started in the Late Cretaceous (95–90 Ma) and it continues to the present time. There is proven petroleum system named as Khatatba-Khatatba petroleum system within the Western Desert. The source of hydrocarbons is the Middle Jurassic Khatatba organicz-rich shales which contains type II- III and type III kerogen source migrated into Khatatba sandstones reservoir rock. Khatatba sandstones are mostly quartz arenite, which composed mainly of more than 95 % quartz. These sandstones have high porosity and high permeability with well sorted and are mostly subangular to subrounded grains. Masajid carbonate acts as regional seal within the basin. Hence, the Western Desert of Egypt has a significant hydrocarbon potential for exploration or development targeting on inversion structure.
The Western Desert (Figure 1) covers about 700,000 square kilometres which is equivalent to the size of Texas and is about two-thirds of Egypt's land area. Egypt borders the Mediterranean Sea to the north, the Red Sea to the east, Sudan to the south and Libya to the west. Western Desert consists of a few extensional coastal rift-basins including Alamein Basin, Abu Gharadig Basin and Matruh-Shushan Basin.
Figure 1: Location map showing significant sedimentary basins in Western Desert.
The stratigraphic succession of Western Desert can be divided into four tectono-sequences (Figure 2) which included Tectono-sequence 1: Paleozoic; Tectono-sequence 2: Jurassic-Coniacian; Tectono-sequence 3: Santonian–Late Eocene and Tectono-sequence 4: Late Eocene-Pliocene. Shoushan Basin evolved through two cycles namely Cycle 1: Margin Sag Basin (MS) and Cycle 2: Wrench Basin (LL) (Jong, 2019).
Figure 2: Phases and cycles of Shushan Basin on Stratigraphic Column (Jong, 2019).
There were three major tectonic events that occurred in Egypt while another important event was the sinistral or dextral rotation of the North Africa plate relative to Laurasia which had strong modifying effect on the local basin tectonic Extensional rift-basins of Western Desert started as rifts and they were formed during the Jurassic time in association with the opening of the Tethys Sea (Figure 3).
Figure 3: Location of Shoushan basin on a paleotectonic map of North Africa during the Middle Jurassic (Tassy et al., 2015; Ramberg et al., 2008).
Within the overall passive margin sequences, several half-graben basins formed due to extensional faults initiated in the Jurassic continued to subside through Creataceous. Western Desert consists of a few extensional rift-basins including Alamein Basin, Abu Gharadig Basin and Matruh Shushan Basin. The Mesozoic Egyptian margin was the southern margin of the Eastern Mediterranean basin, at the northern African plate boundary. It corresponds to the “Unstable Shelf” (Said, 1962). There are authors who considered the Egyptian segment as a passive margin with a NE-SW rifting (Keeley, 1994; Bevan and Moustafa, 2012). However, another interpretation suggested that North-West Egypt was an oblique margin associated with NW-SE opening direction (Garfunkel, 1998; Frizon de la Motte et al., 2011; Tari et al., 2012; Tassy et al., 2015). In this study, we follow the first interpretation.
Petroleum system types (PSTS) and lithofacies playtypes of Shoushan Basin
Some researchers proposed that by recognizing petroleum system types and reservoir lithofacies play types can facilitate prediction of hydrocarbon prospect (Doust and Simmer, 2007). By using the approach three main petroleum system types were recognized in Shoushan Basin and their classification were related to phases of basin evolution (Figure 4) (Doust and Summer, 2007). They are Early Syn-rift Transgressive Deltaic Petroleum System Type (Khatatba Formation), Late Syn-rift Transgressive Deltaic Petroleum System Type (Alam El-Bueib Member) and Post-rift Sag Shallow Marine Petroleum System Type (Abu Roash-G Member).
Figure 4: Three petroleum system types (PSTs) recognized in Shoushan Basin, Western Desert and their relation to phases of basin evolution (Shalaby et al., 2014).
Early Syn-rift Tansgressive Deltaic Petroleum System Type (Khataba Formation)
Based on the plot of hydrogen index (HI) versus oxygen index (OI) on Van Krevelen diagram, Khatatba shale contains mixed kerogen types II-III. It consists of dark shale which contains TOC ranging between 3.60 and 4.20 wt.%. This indicates an excellent source rock (Peters and Cassa, 1994). The pyrolysis yield S1+S2 varies between 8.00 and 10.65 kg HC/ton rock and the productivity index (S1/S1+S2) of these rocks’ ranges between 1.35 and 1.70. Therefore, the shale rock of the Khatatba Formation has an excellent source rock potential. The data of vitrinite reflectance measurements (Ro%) for the well Shushan-1X were plotted against depth (Figure 5) to indicate the phases of hydrocarbon generation and expulsion.
Figure 5: Idealized geochemical log to the well Shushan-1X, showing Rock-Eval pyrolysis data, total organic carbon and vitrinite reflectance measurements (Younes, 2012).
The maturity profile in the burial history model of the well Shushan-1X (Figure 6) shows the different hydrocarbon bearing rock units. It indicates that the shale source rock of Khatatba Formation has entered the late mature stage of oil and gas generation window during the Late Cretaceous associate with vitrinite reflectance measurements between 1.0-1.3 Ro%.
Figure 6: Burial history model of the well Shushan-1X and stages of hydrocarbon generation windows (Younes, 2012).
Late Syn-rift Transgressive Deltaic Petroleum System Type (Alam El- Buieb Member)
Based on the plot of hydrogen index (HI) versus oxygen index (OI) on Van Krevelen diagram, Alam El-Bueib Member shales contain mixed kerogen types II-III. The shale section of Alam El-Bueib Member contains TOC varying from 1.85 to 2.40 wt.%, indicating a good source rock. The pyrolysis yield S1+S2 ranging between 3.60 and 4.50 kg HC/ton rock and the productivity index (S1/S1+S2) of this rock is generally less than unity, therefore the shale rock of the Alam El-Bueib Member has a good source rock generating potential. The burial history model of the different hydrocarbon bearing rock units indicate that the shale source rock of Alam El-Bueib Member entered the mid mature stage of oil generation window during the Late Cretaceous associate with vitrinite reflectance measurements between 0.7-1.0 Ro%
Post-rift Sag Shallow Marine Petroleum System Type (Abu Roash-G Member)
Based on the plot of hydrogen index (HI) versus oxygen index (OI) on Van Krevelen diagram, Abu Roash-G Member shale contains mixed kerogen types II-III. The organic richness of Abu Roash-G Member varies from 1.10 to 1.50 TOC (wt.%) reflects a medium to good source rock. The pyrolysis yield S1+S2 ranging between 0.85 and 1.10 kg HC/ton rock and the productivity index (S1/S1+S2) of this rock is generally less than unity, therefore the shale rock of Abu Roash-G Member indicates fair source rock generating potential. The burial history model of the different hydrocarbon bearing rock units indicate that the shale source rock of Abu Roash-G Member has entered the early mature stage of oil generation at Late Eocene associate with vitrinite reflectance values between 0.5-0.7 Ro%. Each of the three PSTs outlined in Shoushan Basin has an associated suite of reservoir lithofacies as is discussed below (Figure 7). Useful generalizations can be made about the relationship between reservoir, seal and charge by placing these lithofacies within the context of overall stratigraphic sequence that characterizes the PSTs.
Figure 7: Schematic diagram showing major lithofacies reservoir plays belongs to the three petroleum system types (PSTs) for Shoushan Basin, Western Desert (Shalaby et al., 2014).
Data source: Petroleum system of Shoushan Basin, Western Desert, Egypt. Jong E Cheng, 2020
Следующий Бассейн: Lamu Embayment