“源热共控”中国近海天然气富集区分布

Distribution of gas enrichment regions controlled by source rocks andgeothermal heat in China offshore basins

西湖凹陷、白云凹陷、崖南凹陷和莺中凹陷是中国近海盆地外带中已被证实的富生气凹陷,其主力气源岩为海陆过渡相地层,烃源岩包括煤系三角洲和浅海相泥岩。前者多分布在凹陷的缓坡区,烃源岩包括煤层、碳质泥岩和暗色泥岩,沉积有机质都属于陆源,丰度高,是好—极好的烃源岩。后者分布在洼槽区,多分布于深凹陷部位,水生生物欠发育,沉积环境处于半封闭的背景,因而有利于有机质的保存,主要发育泥岩,有机质属混源成因,多来自陆源,也有海生藻类,丰度属于较好到好,总体属于好烃源岩。结论认为:煤系三角洲区潜在生气量大,可能形成大中型气田,气田充满度高;浅海相泥岩烃源岩区潜在生气能力不及煤系三角洲,也能够形成大中型气田,但气田充满度偏低;煤系三角洲区是近海最有利区,浅海相泥岩区是较有利区。据此指出:①中国近海煤系三角洲区分为勘探成熟区和潜在区两大领域,其中成熟区包括西湖凹陷西部斜坡、白云凹陷北部斜坡、崖南凹陷西北部、莺歌海盆地莺西斜坡,潜在区包括荔湾凹陷北部三角洲、琼东南盆地松东凹陷北坡三角洲、东海盆地丽水凹陷丽水三角洲等;②浅海相泥岩有利区包括西湖凹陷洼槽区、白云凹陷洼槽区、崖南凹陷洼槽区、荔湾凹陷洼槽区、莺歌海盆地莺中洼槽区、琼东南盆地中央凹陷洼槽区等。

This paper aims to offer a good map for the distribution of gas enrichment regions in China offshore basins by use of the theory of hydrocarbon generation controlled by source rocks and geothermal heat flow. The Xihu, Baiyun, Yanan and Yingzhong sags are proved gasrich in the external behs of China offshore basins, the dominating source rocks of which are mainly marine-continental transitional facies and are distributed either in coal bearing delta depositional system or in the depositional environment with neritic facies mudstones. The former is mainly distributed in the slight slopes of the sag with coal beds. carbonaceous mudstones, and dark mudstones as good to extremely good source rocks with a high abundance of terrestrial organic matters. The latter is mostly distributed in the deep subsag area, where hydrobios is not abundant but the semi-closed sediment environment is good for the preservation of organic matters belonging to a mixed type with a moderate to better abundance, mostly from the terrigenous plants or animals, and some from marine algae; the main moderate source rocks are mudstones. Thus, it is concluded that the coal bearing delta has a high potential to form medium and large-sized gas fields with high gas charge, while a shallow sea mudstone environment has a low potential to form small- and medium-sized gas fields with low gas charge. Thence the gas pooling in Cenozoic offshore basins are distributed firstly in coal-bearing delta with a high enough maturity and its hydrocarbon kitchens lying on the slope area, and secondly in a neritic facies mudstone environment with a high enough maturity and its hydrocarbon kitchens lying in the subsag. Coal-hearing deltas in China offshore basins can be divided into mature exploration areas and prospective areas. The mature exploration areas include the western slope in the Xihu sag, the northern slope in the Baiyun sag, the northwestern of the Yanan sag, and the Yingxi slope in the Yinggehai Basin; the prospective areas include the northern delta in the Liwan Basin, the northern slope delta in the .Songdong sag, Qiongdongnan Basin, and the I.ishui delta in the l.ishui sag, Donghai Basin. Plays in the neritic facies mudstone environment include sub sags in Xihu, Baiyun, Yanan, I.iwan sags, and those in the Yingzhong sag, Yinggehai Basin, and in the central sag, Qiongdongnan Basin.