## 5 Hydrocarbon Accumulation Rules
### Reformed Delta Basins  
Due to its unique structural architecture characteristics, this type of basin has a \"multi-source and multi-combination type\" hydrocarbon accumulation model (Figure 11). Taking the Rovuma Basin as an example, the passive continental margin drift stage since the Cretaceous is characterized by the formation of open sea. The continental coast rivers with varying sizes bring terrestrial organic matter to the sea to be rapidly deposited and preserved, forming the organic matter dominated by Type III kerogen, with HI generally lower than 250 mg/g and TOC between 0.3 and 12.2%. In addition to the drift sequences rich in organic matter that mainly generate gas, the source rocks of the underlying two periods of rift sequences still mainly supply natural gas upwards due to the large overlying sedimentary thickness (generally 4000-7000 m) and high geothermal gradient (average more than 30 \({}^{\circ}\)C/km), with thermal evolution at the highly mature to overly mature stage. The reservoirs consist of marine clastic rock and carbonate rock. The former reservoirs are sand bodies in coast, delta, and turbidite channel facies with good physical properties and a maximum porosity of up to 30%. The latter reservoirs have been confirmed to be a few dolomite reservoirs. The cap rocks have been confirmed to be all marine shale, among which the interlayer shale in the Paleogene is the main cap rock for discovered deep-water oil and gas, and the thick rock salt of the Lower Jurassic can serve as a potential cap rock [17]. In addition to the widely developed effective source rocks in the underlying two periods of rift sequences and the Lower Cretaceous, the marine source rocks of the Upper Cretaceous--Paleogene have also basically entered the oil generation threshold. In terms of reservoir types, except for the same type as the \"faulted depression type\" basins mentioned above, the delta sedimentary sequences and the foredeep gravity flow sand bodies are all high-quality reservoirs, and transgressive shale can effectively seal hydrocarbons regionally. In addition to the rift and depression sequences that can form the same plays as \"faulted depression type\" basins, the four major ring structural belts formed on the plane of the delta sequences have unique hydrocarbon accumulation characteristics. Taking the Rovuma Basin as an example, multiple sets of ultra-large gas-rich gravity flow sedimentary sand bodies are developed in the ultra-deep-water region of the basin. One faulted block trap has been found on the inner ring growth fault belt, and a large traction anticline gas reservoir has been discovered in the outer thrust fold belt. While in the foredeep gentle slope belt, 11 large gas fields have been found in the slope fan that may belong to debris flow, with recoverable reserves of more than 130 TCF, mainly distributed in the Paleogene. Its offshore exploration degree is relatively higher, and several commercial gas fields (such as Golfinho, Atum, and Tubarao Tigre) have been successively discovered. The Windjammer well encountered 23 m and 146 m of gas layers in Paleocene and Oligocene, respectively, and the reason for such rich oil and gas is due to its favorable accumulation conditions. The source rocks of the three prototype basins may provide oil and gas sources, and the resources are abundant. Large delta sand bodies and turbidite sand bodies not only form excellent reservoirs but also large traps such as rolling anticline, compressive anticline, and stratigraphic structure, which can develop into extremely thick Marine drift layers over a long period of time. Due to the formation and regression of the sea, multiple sets of favorable reservoir-cap assemblages can be formed, and the preservation conditions are obviously better than the two underlying rifts, and oil and gas enrichment is inevitable. In addition to the Rovuma Basin, the East African sea area also develops the Juba Basin in the south of Somalia.