东非坦桑尼亚盆地深水区天然气地球化学特征与成因

Geochemical Characteristics and Genesis of Natural Gas in Offshore Area of Tanzania Basin,East Africa

近几年东非大陆边缘盆地(坦桑尼亚盆地、鲁武马盆地)海域发现了一系列大型气田,可采储量达38000×10^8 m^3。为了了解气田的地球化学成因,研究了盆地的构造演化特征,即坦桑尼亚盆地经历了卡鲁裂谷期、马达加斯加裂谷期和马达加斯加漂移期三期构造演化。其中,马达加斯加裂谷期形成了下侏罗统-中侏罗统局限海相优质烃源岩烃源岩,其生、排烃期在早白垩晚期-渐新世。该文详细分析了坦桑尼亚盆地深水区天然气组分、天然气碳、氢同位素特征,探讨了天然气的成熟度,判识了天然气的成因类型,研究结果认为深水区天然气为高成熟-过成熟油型气,天然气成熟度Ro处于1.2%~2.6%之间,与侏罗统-中侏罗统烃源岩成熟度相匹配。因此,认为天然气的气源为下-中侏罗统烃源岩。

In recent years,the great breakthroughs have been made in gas exploration in East Africa.A series of large gas fields have been discovered in the offshore of Tanzania basin and Ruvuma basin,with a total recoverable reserves of 38000×10^8 m^3.In order to understand the geochemical genesis of gas fields,we studied the structural evolution characteristics of the basin.The basin has gone through three stages of tectonic evolution,which includes the Karoo rift,Madagascar rift and Madagascar drift.The Lower-Middle Jurassic restricted sea high-quality source rocks formed in the Madagascar rift period and the hydrocarbon generation and expulsion stages are from late Early Cretaceous to Oligocene.In this paper,the gas composition and carbon isotope characteristics in offshore of Tanzania basin are analyzed in detail,the gas maturity is discussed,and the gas genetic type is identified.The results show that the gas is oil-formed gas with high maturity,and the value of gas maturity Ro is between 1.2%and 2.6%,which is in line with the Lower-Middle Jurassic marine high-quality source rocks.Therefore,it is presumed that the gas originated from the Lower-Middle Jurassic marine high-quality source rocks.