南海北部叠合断陷盆地海陆过渡相烃源岩及成藏贡献——以珠二坳陷白云凹陷为例

伸展盆地裂谷期为陆相、海陆过渡相和海相沉积充填的叠合断陷盆地,与断陷期完全为陆相地层充填的陆相断陷盆地在形成的板块构造环境、沉积相和烃源岩等方面有显著区别。中国近海叠合断陷盆地一般主要沉积有陆相、海陆过渡相和海相3个层系的烃源层,其中海陆过渡相烃源岩是南海北部断陷盆地珠二坳陷、琼东南盆地的主力烃源岩,该层系生烃成藏均晚于断陷期沉积的陆相地层。该类烃源岩具有陆源生烃母质和海相生烃母质双源输入的混合特征,干酪根类型为Ⅱ—Ⅲ型,以生气(煤型气)为主,但浅部烃源岩有一定的生油能力,其生成的油气亦有双源成因的特征。油气源对比研究表明,白云凹陷番禺地区和LW3-1气田油气成藏的主要贡献者是白云凹陷海陆过渡相的恩平组烃源岩,但也有渐新统珠海组海相烃源岩的贡献。勘探实践证明,这种海陆过渡相烃源岩目前已成为南海北部断陷盆地最为重要的烃源层系,是该区有效的烃源岩。

Transitional source rock and its contribution to hydrocarbon accumulation in superimpose rift-subsidence basin of northern South China Sea: Taking Baiyun Sag of Zhu Ⅱ Depression as an example

The superimpose rift-subsidence basin is filled with the continental, transitional and marine deposits in the rifting stage of extensional basin. The plate tectonic environment, sedimentary facies and hydrocarbon source rocks of the superimpose rift-subsidence basin are obviously different from those of the continental fault depression exclusively filled with the continental strata in the rift period. Three major hydrocarbon source rock strata of continental, transitional and marine facies were deposited in the offshore rift-subsidence basin of China. The transitional source rocks have great contribution to the rift-subsidence basins such as Zhu Ⅱ Depression and Qiongdongnan Basin in the northern of South China Sea. The hydrocarbon-generation time and hydrocarbon accumulation period of the transitional strata are later than those of the continental strata deposited in the faulted period. The transitional source rocks have the mixed characteristics of the land plant and marine bio-matter inputs. The hydrocarbon generated from the source rocks has the double-source genesis. This type source rock with the kerogen of II-III type is prone to coal-type gas,but it is prone to oil in the shallow layer. The oil-gas-source correlation suggests that oil and gas in the Baiyun Sag was derived from the transitional source rocks of Enping Formation. The marine source rocks of Zhuhai Formation have some contribution to LW3-1 gas field. The explore experience shows that the transitional source rock is the most important and effective to hydrocarbon source rock strata of the rift-subsidence basins in the northern South China Sea at present.