杭州湾地区浅层生物气田的古地理环境及其特征

PALEOGEOGRAPHIC ENVIRONMENTS AND THEIR FEATURES OF SHALLOW BIOLOGIC GAS FIELDS IN HANGZHOU BAY AREA

论述了杭州湾地区第四纪末次冰期后期的初、早、中、晚四期古地理环境,分析了浅层气田的天然气化学成分、生储盖特征、圈闭条件和气藏类型、气田的规模及分布规律等。本区第四纪浅层天然气为生物成因的甲烷气,主要赋存于末次冰期以来的全新世地层中。流水切割的古地形、海平面升降、以及物源供应量的变化,对该区第四纪的沉积分布起着控制作用。全新世古河谷是研究的主要古地貌单元。浙江萧山夹灶浅气田夹4井的岩性和岩相特征可为本区典型代表。生气层主要为近岸浅海灰黑色、灰色淤泥粘土层,全区分布广泛,厚约10～30m;生气层亦系良好的区域盖层,具较强的自封闭能力;储层绝大多数属河漫滩相砂质透镜体;以透镜体岩性圈闭为主,也存在少数复合圈闭。单个气田的地质储量在千万立方米数量级。现今的河流与古河谷地带是找气的主要场所。

The Quaternary shallow gas in the Hangzhou Bay area is biogenic methane in origin. The gas commonly occurs in Holocene sediments after the last glaciation, during which the former river valley is one of the main paleogeomorphic units. Distribution of Quaternary sediments was dominated by river-cut paleotopographic relief, fluctuation of sea level and varying terrigenous supply. One typical example is represented by the lithology and lithofacies in Jia-4 Well of shallow Jiazao Field in Xiaoshan, Zhejiang. The post-glacial paleogeographic environments of the last glaciation during Quaternary, which are divided into the initial, early, middle and late periods, are studied in detail. What are also studied include the chemical compositions of the gas, the features of gas source, reservoir and cap rocks, the gas-trapping conditions, and the types of gas reservoirs, as well as the scale and distribution of some fields. There are extensive gas source rocks in this area. The source rocks may also serve as regional caps with self-enclosing capability. Most of the reservoirs are sandy lens of alluvial flat facies. The majority of traps are lithologic and a few of combined traps may exist. One field commonly comprises several small-scaled isolated reservoirs and a single field generally has geologic gas reserves of 10,000,000 m^3 or more in order of magnitude. It is suggested that the nowadays river and former river valley belts are the good places for prospecting shallow gas in this area.