中国近海新生代叠合断陷煤系烃源岩特征与天然气勘探方向

Coal measure source-rock characteristics and gas exploration directions in Cenozoic superimposed faulted depressions,offshore China

中国近海新生代叠合断陷主要是指由陆相、海陆过渡相和海相地层充填的古近纪凹陷,这类凹陷主要分布在东海、南海北部大陆架边缘外带,在叠合断陷期形成的烃源岩主要有中深湖相泥岩、海陆过渡相煤系地层和海相泥岩等3种,其中以海陆过渡相煤系地层为主,包括煤、碳质泥岩和暗色泥岩。中国近海新生代叠合断陷煤层主要形成于古新世—渐新世,其识别方法主要有测井资料多方法综合识别和地震多属性线性回归等2个技术系列,其形成主要受古气侯、古地形和补偿沉降等因素控制,主要形成于潮湿气候条件下的三角洲环境,多分布在凹陷的缓坡部位。中国近海古近纪断陷期煤层具有单层厚度薄、层数多、横向变化快等特征,煤组分以镜质组为主;煤系地层和海相泥岩烃源岩有机质类型均为腐泥腐殖型和腐殖型,生烃门限高,R o值一般达1.5%以上才能大规模生气,但生气窗宽,R o值在4.38%以下均能生成大量天然气,所以天然气生烃期一般较晚,主要在中新世及其以后。中国近海新生代叠合断陷天然气成藏模式有垂向运聚、侧向运聚及近距离运聚等3种,其中垂向运聚受断层控制,侧向运聚受构造脊控制。东海盆地西湖凹陷、钓北凹陷、丽水凹陷,珠江口盆地白云凹陷、荔湾凹陷,莺歌海盆地莺中凹陷及琼东南盆地诸凹陷均是重要的天然气勘探领域,其中西湖凹陷、白云凹陷、崖南凹陷、莺中凹陷是已证实的深大叠合断陷,天然气资源潜力巨大,勘探潜力大,是增储上产的主要领域;陵水凹陷、乐东凹陷、丽水凹陷已有天然气突破,勘探潜力较大,属于战略扩展的主要领域;而荔湾凹陷、钓北凹陷、华光凹陷等尚未钻探,但其生烃条件优越,煤系三角洲发育,也具有相当大的天然气勘探潜力。

Cenozoic superimposed faulted depressions, offshore China mainly refer to Paleogene sags filled with sediments of continental, marine and transitional facies, which mainly occur in the outer zones of continental margins in East China Sea and the northern South China Sea. In these sags, main hydrocarbon source rocks deposited during the period of superimposed faulted depression are the mudstone of middle-deep lake facies, the coal measures of transitional facies and the mudstone of marine facies, and the coal measures, including coal seams, carbonaceous shale and dark mudstone, are the most important. These coal seams formed mainly during Paleocene-Oligocene, and under the controls of paleoclimate, paleoto- pography and compensation subsidence, they developed mainly in delta environment with a humid climate, with most of them occurring on gentle slopes of the sags. There are two technical series to identify these coal seams, i.e. the comprehensive multi-tool recognition using log data and thelinear multi-attribute regression using seismic data. These coal seams are characterized by thin sin- gle-bed, numerous beds and rapid lateral variation, with their maceral being predominated by vitrinite. In the coal measures and the marine mudstone, the organic matter belongs to the sapropel-humic type and humic type and has a high threshold of hydro- carbon generation （massively generating gas only when R0, is higher than 1.5%）, but its gas genera- tion window is very broad （capable of generating numerous gas when R0 below 4.38%）, with the gas generation period quite late, i.e. mainly in Miocene and thereafter. In these Cenozoic superim- posed faulted depressions, offshore China, there are three models of gas accumulation, i.e. the vertical migration-accumulation controlled by faults, the lateral migration-accumulation controlled by structure ridges and the near migration-accumulation. There are a series of important gas exploration areas, such as Xihu, Diaobei and Lishui sags in East China Sea basin, Baiyun and Liwan sags in Pearl River Mouth basin, Yingzhong sag in Yinggehai ba- sin and several sags in Qiongdongnan basin. As the deep and large superimposed {aulted depressions that have been confirmed, Xihu, Baiyun, Yanan and Yingzhong sags are great in gas resources and explo- ration potential, and are the major areas to increase reserves and production. Meanwhile, Lingshui, Le- dong and Lishui sags in which ＊here already have gas discoveries are also great in exploration potential and are the major areas of strategic expansion. Although Liwan, Diaobei and Huaguang sags have not been drilled, they should also be quite great in gas exploration potentail, due to their quite developed deltas of coal measures and excellent conditions of hydrocarbon generation.