古湖缺氧条件是控制富生油凹陷形成的重要因素—来自珠江口盆地的证据

IMPORTANT FACTOR CONTROLLING THE DEVELOPMENT OF RICH OIL KITCHEN SAGS: ANOXIC CONDITIONS-Evidences from the Pearl River Mouth Basin

珠江口古湖是中国东部最大的早第三纪裂谷湖之一，在珠江口古湖发育的鼎盛期－始新世，湖面宽阔，湖水较深，湖泊具欠补偿沉积条件，其营养和浮游植物生产力都达到较高水平。含有丰富浮游藻类化石的文昌组暗色泥岩即形成于这一时期。文昌组浮游藻类相对丰度高，表明始新世珠江口古湖的表层水初始生产力高。这意味着古湖沉积物具有良好的富氢有机质物源条件。文昌组暗色泥岩有机相横向变化明显，表明始新世珠江口古湖湖底缺氧和充氧条件并存。形成于缺氧水体下的无定形有机相在盆地大部分地区均有发育，其TOC值和IH值较高；反映充氧底部水环境的陆源植物有机相分布于HJ15－1－1井所在的盆地东部边缘，其TOC值和IH值较低。具有无定形有机相的文昌组蝉色泥岩是珠江口盆地良好的湖相油源岩。中国东部陆上和近海早第三纪盆地各凹陷的湖相油源岩发育差异甚大，其控制因素是由水体中溶解氧含量变化所造成的有机质保存条件的差异。富生油凹陷形成于缺氧的古湖区。

The Pearl River Mouth Lake is one of the biggest Palaeogene rift lakes in the east and offshore of China. During its flourishing stage of the Oligocene, the Pearl River Mouth Lake was in lake level highstands which had maximum subsidence rates and came up to an advanced level of lake maturity with respect to nutrient input and phytoplanktonic productivity. The dark mudstones of the Wenchang Formation, containing commonly abundant fossil planktonic algae, were formed in this stage. The high relative abundance of the fossil planktonic algae indicates the high primary surface productivity of the palaeo-lake within the Oligocene. It suggests that the sediments of palaeo-lake bottom had potential source of organic matter rich in hydrogen. Distribution of the organic facies in the dark mudstones of the Wenchang Formation shows distinct lateral variability. It demonstrates that there were both anoxic and oxic lake bottom conditions in the palaeo-lake during the Oligocene. The amorphous organic matter (AOM) facies formed under anoxic water, with high TOC (Total Organic Carbon) and HI (Hydrogen Index) values, were developed in the Wenchang Formation of the majority of the basin area. The land-plant-derived organic facies reflecting oxic bottom water conditions, with low TOC and HI values, were recognized in the samples of dark mud-stones of the Wenchang Formation from Well HJ15-1-1 located in the eastern margin of the basin. The dark mudstones of the Wenchang Formation with AOM organic facies are potential lacustrine oil source rocks in the Pearl River Mouth Basin. There are considerable variations in the development of lacustrine oil source rocks of the sags in the Palaeogene basins of the east and offshore of China. Difference in organic preservation efficiency resulting from variability in dissolved oxygen content of lake bottom water is an important controlling factor on the development of lacustrine oil source rocks. Rich oil kitchen sags were usually developed in the anoxic environments of palaeo-lake.