海相烃源岩发育环境与岩石的沉积序列——以鄂尔多斯盆地为例

Development environment and lithological sequence of marine source rock:Taking Ordos Basin as an example

国内外海相大油气田的烃源岩多为黏土矿物含量较高的泥页岩、油页岩和泥灰岩,形成于由台地向海槽过渡的深水斜坡环境,而碳酸盐岩形成于与大气交换条件较好的浅水环境,是有机质丰度低和干酪根类型差的根本原因。从机理上讲,碳酸盐岩排烃效率低于泥质岩,并且不存在有机碳含量随成熟度升高而有规律降低的现象,不能成为有效烃源岩。鄂尔多斯盆地西缘中奥陶统平凉组自东而西发育的碳酸盐岩—泥质岩、灰岩—硅质页岩和海底碎屑岩反映了海相沉积因水深变大而形成的岩性序列,含有机质丰度高的泥质和泥灰质烃源岩形成于深水斜坡相的古地理环境中。

Most source rocks of the large marine oil-gas fields in domestic and overseas are graptolite shale,oil shale and marl with great amounts of clay mineral,which formed in the deepwater slope environment transferred from tableland to trough.But the carbonate rock is formed in the shallow water environment under good exchanging condition with atmosphere,which causes the low abundance of organic matter and poor-type kerogen.Hydrocarbon-expulsion efficiency of carbonate rock is lower than that of shale,and the TOC content in carbonate rock can not be reduced when the maturity increases.So,carbonate rock can not become the effective hydrocarbon source rock.The carbonate rock-argillaceous rock,limestone-siliceous shale and sea-bottom clastic rock developed from east to west in Pingliang Formation of the western Ordos Basin,which shows the lithological sequence of marine sediment resulted from increasing water depth.The graptolite shale and marl with high-abundance hydrocarbon are formed in the palaeogeographic environment of deepwater slope.