## 5 Hydrocarbon Accumulation Rules
### Faulted Depression Basins  
Faulted depression basins can form a \"double-source and double-combination type\" hydrocarbon accumulation model (Figure 10). In addition to the structural plays formed in rift sequences, the hydrocarbons generated by marine source rock in depression sequences can directly migrate to the stratigraphic traps related to deep-water gravity flow sand bodies to accumulate, and the upper and lower sets of source rocks can supply hydrocarbons to the sand bodies of the same depression sequence to form reservoirs. Taking the Tanzanian Coast Basin as an example, in addition to the two stages of rifting source rocks that have been confirmed to be the same as those in the above rifted basins, there is effective source rock in the Lower Cretaceous in the depression sequences. At present, it is confirmed that the Lower Cretaceous source rock is mainly of Type III, with a TOC value between 1-7.4%, a HI value of 17-688 mg/g, and a Ro value of 1.2%. The reservoir is mainly composed of shallow sea, delta, and gravity flow elastic sandstones, with porosity generally between 12 and 30%. The cap rock is transgressive shale, and the traps include faulted blocks and composite traps. The source rock of the onshore EI Kuran gas field discovered in the Coriole Basin is Jurassic Hamanlei shale, which is generally less than 3 m thick. It reached the peak of hydrocarbon generation during the Oligocene. Most structures had been formed during the Late Cretaceous--Paleogene. It is speculated that hydrocarbons had been generated at this time, and the geothermal gradient was generally lower (<15 \({}^{\circ}\)C/km). Its reservoir is the Middle-Upper Jurassic Hamanlei carbonate rock, in which the grainstone and dolomite have good physical properties. The Hamanlei reservoir is sealed by the shales of the Hamanlei and Uarandab Formations, with good regional scope and sealing capacity. In terms of hydrocarbon accumulation characteristics, the only effective source rock in the gentle slope belt of the upper shelf (onshore--shallow water) was deposited during the rift period. If the faults are connected, various related faulted block traps can be formed in the rift sequences and depression sequences. The discovered Song Song gas field has confirmed that hydrocarbons from the Jurassic source rock migrated along faults to the lower Cretaceous shore-shallow sea sandstone, forming a faulted block trap. It is  
Figure 10: Hydrocarbon accumulation model of passive continental margin basins of “faulted depression type”. Double-source and double-combination types. speculated that larger slip-collapse-related stratigraphic traps can be formed in the upper part of the continental steep slope belt; in the lower part of the continental steep slope belt, the major traps are stratigraphic and composite traps related to deep-water gravity flow sedimentation (presumably dominated by clastic flow rocks). At present, 10 gas fields discovered in Blocks 1-4 of Tanzania, ranging from the Upper Cretaceous to the Miocene, all belong to the lower composite trap of the steep slope belt. At present, there are no drilled wells on the gentle slope of the lower continental rise. Seismic data reveal that there are more submarine fans (mainly turbidite deposits), thus it has the conditions to form large-scale lithologic traps. Due to the stable geological environment and abundant source supply in the passive continental margin of East Africa, the clastic flow rock deposits (mainly sandstone) eventually developed into large and medium-sized fans, which can be used as high-quality oil and gas accumulation zones. Because the fault of the lower rift layer does not break through the upper depression, the oil and gas connectivity of the rift layer is poor, and the oil and gas connectivity of the depression sequences is good. Different trap types can be found in different structural belts of such basins. The onshore-shallow-water area is dominated by structural traps of rift sequences.