莺歌海盆地中央坳陷带成藏体系的盖层评价及控藏作用

莺歌海盆地是我国南海重要的天然气探区。通过对莺歌海盆地中央坳陷带不同成藏体系典型气藏和含气构造的精细解剖，评价了盖层的有效性及控制因素，并分析了盖层对天然气成藏的控制作用。研究结果表明：(1)莺歌海盆地中央坳陷带浅层和超浅层成藏体系中，盖层的封盖有效性受到盖层、底辟构造及底辟构造活动伴生断裂的共同控制；中深层成藏体系中普遍存在异常高流体压力，且断裂不太发育，盖层水力封闭是控制油气差异富集的关键因素。(2)浅层和超浅层成藏体系内，脆性盖层保持封盖有效性的临界断接厚度为86～98 m；中深层成藏体系内，盖层的封盖有效性可以通过水力破裂压力系数进行评价，当系数大于1时，意味着盖层已发生破裂或具有极强的水力破裂的风险。(3)整体上中央坳陷带超压诱发的水力破裂是中深层盖层封闭失效的根本原因，深层天然气通过水力破裂通道运移至浅层，最终经断裂调整至超浅层成藏，气源充足的条件下，超浅层及浅层气藏主要集中分布在底辟的顶部，而中深层气藏主要集中分布于底辟翼部及斜坡区。

Caprock evaluation and its reservoir control of different accumulation systems in central depression zone of Yinggehai Basin

Yinggehai Basin is an important natural gas exploration area in the South China Sea. Based on detailed analysis of typical gas reservoirs and gas-bearing structures of different accumulation systems in the central depression of Yinggehai Basin，the effectiveness and controlling factors of caprocks were evaluated，and the controlling effects of caprocks on natural gas accumulation were analyzed. The results show that: (1) In the ultra-shallow and shallow accumulation systems in the central depression zone of Yinggehai Basin，the sealing of caprocks is controlled by the caprocks，the diapir structure and the faults associated with the activity of the diapir structure. In the middle-deep accumulation system，under the background of abnormally high fluid pressure and the absence of faults，hydraulic sealing of caprocks is the key factor controlling differential enrichment of oil and gas. (2) The critical juxtaposition thickness of the brittle caprocks to maintain the sealing effectiveness is 86-98 m in the ultrashallow and shallow accumulation systems. The effectiveness of the caprocks in the middle-deep accumulation system can be evaluated by the hydraulic fracturing pressure coefficient. When the value is greater than 1，it means that the caprocks have ruptured or have a strong risk of hydraulic fracturing. (3) From the overall view of the central depression，hydraulic fracturing induced by overpressure is the fundamental cause of sealing failure in mid-deep caprocks. Deep natural gas migrates to shallow layer through hydraulic fracturing channel and finally to the ultra-shallow reservoir through the faults. Therefore，under the condition of sufficient gas source，ultra-shallow and shallow gas reservoirs are mainly distributed at the top of the diapir，while middle-deep gas reservoirs are mainly concentrated in the diapir wing and slope areas.