Structural and Sedimentary Characteristics of Passive Continental Margin Basins in East Africa and Their Effect on the Formation of Giant Gas Fields

Abstract

Abstract: In the past five years, a number of giant gas fields have been discovered in offshore East Africa. They are mainly located in Ruvuma and Tanzania Basins, with total recoverable reserves of 3.8?0¹² m³. It is indicated by regional tectonic evolution study and basin structure-sedimentation characteristics analysis that the basins in East Africa mainly experienced three phases of structural-sedimentary evolution, including: (1) Karoo rift period of Late Carboniferous to Early Jurassic, with dominant sedimentary facies of fluvial, lacustrine and delta; (2) Madagascar rift period of Middle Jurassic to Early Cretaceous, with dominantly continental and shallow marine facies, the principal source rocks developed; and (3) Madagascar drift period of Late Cretaceous to Quaternary, with dominantly passive continental margin sedimentation, with turbidite sandstones of abyssal fan, slope fan and gravity flow channel as major reservoirs and the shales as regional cap rocks. Major plays of the giant gas fields are Oligocene-Pliocene, Paleocene-Eocene sandstones in the Ruvuma Basin, and Oligocene and Upper Cretaceous sandstones in the Tanzania Basin. In Ruvuma Basin, the giant gas fields are mainly located at deep-water thrust belts and their fronts. Thrust belts are widely developed at Ruvuma front due to gravity collapse and Jurassic salt diaper, and they act as good migration pathways for oil and gas. Oil and gas migrates upwards from the deep source rocks and accumulates in sandstone reservoirs of thrust belts. In Tanzania Basin, giant gas fields are mainly located at the gravity flow channel sandstones of slopes, and oil and gas accumulation is controlled by the S-N normal faults.

Key words: passive continental margin, deep-water thrust belt, salt diapir, abyssal fan, gravity flow channel, Ruvuma Basin, Tanzania Basin

Zhang Guangya, Liu Xiaobing, Wen Zhixin, Wang Zhaoming, Song Chengpeng. Structural and Sedimentary Characteristics of Passive Continental Margin Basins in East Africa and Their Effect on the Formation of Giant Gas Fields[J]. China Petroleum Exploration, 2015, 20(4): 71-80.

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