全球生物气藏分布特征及成藏条件

为了明确未来大型生物气藏的勘探方向,通过对全球18个重点生物气藏发育区的基本情况和其中14个早期生物成因的原生型常规生物气藏发育区油气地质条件的统计,分析探讨了原生型生物气藏分布特征及其与成藏要素的关系,归纳总结了大型原生型生物气藏形成的宏观地质条件。研究结果表明:①生物气藏储量分布具有储层时代新、埋藏偏浅、深水超深水的残余洋盆地为主的分布特征;②全球超过70%储量的早期生物成因的原生型常规生物气藏分布在具有低地表(海底)温度、低地温梯度条件、与古三角洲河口区和水下扇特别是大型深水扇相关的沉积环境中;③上述地区发育的巨厚砂泥岩具有较高的沉积速率和较低的砂地比,形成了一定丰度的(TOC 0.5%)、以腐殖型有机质为主的烃源岩层系,既可以为微生物提供长时间、大厚度范围活动空间和养分,同时对生物气的散失也可以起到抑制作用。结论认为,未来的大型生物气藏的勘探方向应集中在具有低地表(海底)温度的冻土区和深水区、低地温梯度类型的盆地、古三角洲河口区—水下扇相关沉积环境、构造活动强度适中的正向构造带的四要素叠合领域:①具有大型物源基础的被动大陆边缘盆地外带深水区,如南大西洋两岸的尼日尔三角洲盆地深水、刚果扇、亚马逊扇以及一些古三角洲发育区;②具有大型水下扇的残余洋盆地,如孟加拉扇、印度河扇等;③具有深前渊特征前陆盆地的前渊区,如巴布亚盆地东部前渊;④冻土地区的克拉通盆地内坳陷区。

Distribution characteristics and hydrocarbon accumulation conditions of biogenic gas reservoirs all over the world

In order to clarify the future exploration direction of large-scale biogenic gas reservoirs, this paper reviewed the basic information of 18 global major development areas of biogenic gas reservoirs and the petroleum geological conditions of 14 development areas of primary conventional biogenic gas reservoirs of early biogenesis. Then, the distribution characteristics of primary biogenic gas reservoirs and their relationships with the hydrocarbon accumulation conditions were analyzed. Finally, the macroscopic geological conditions for the formation of large-scale primary biogenic gas reservoirs were summarized. And the following research results were obtained. First, the reserve distribution of biogenic gas reservoirs is characterized by young reservoir age, smaller burial depth and deep-water and ultradeep-water remnant ocean basin. Second, over 70% reserves of the primary conventional biogenic gas reservoirs of early biogenesis in the whole world are distributed in the sedimentary environments related to palaeodelta river mouth or submarine fan (especially large-scale deepwater fan) with low surface (sea floor) temperature and low geothermal gradient. Third, the thick sandstone–mudstone strata developed in these areas have a high sedimentation rate and a low sandstone/formation ratio. Therefore, the source rocks dominated by humic organic matter with a certain abundance (TOC>0.5%) are formed, and they can not only provide large space and nutrient for the microbe activity in the long period, but also inhibit the dispersion of biogenic gas. In conclusion, the future exploration direction of large-scale biogenic gas reservoirs focuses on the areas with the following four conditions, including a permafrost region and a deepwater zone with low surface (sea floor) temperatures, a basin with a low geothermal gradient, sedimentary environment related to palaeodelta river mouth–submarine fans and positive structure belts with moderate tectonic activity intensity. Specifically, exploration direction is as follows: (1) The deepwater zones at the outer belt of passive continental margin basin with large provenance, e.g. the deepwater of the Niger Delta Basin, Congo fan, Amazon fan and some palaeodelta development area at both sides of the South Atlantic; (2) The remnant ocean basins with large submarine fan, e.g. Bengal fan and Indus fan; (3) The foredeep zones of foreland basin with the characteristics of deep foredeep, e.g. the eastern foredeep of Papuan Basin; (4) The sags inside the craton basins in those permafrost areas.