莺歌海盆地乐东斜坡区乐东A构造储层超压形成机制及其对天然气成藏的启示

莺歌海盆地乐东斜坡区乐东A构造深层超压天然气资源丰富，查明超压形成机制对于认识乐东斜坡区天然气运移成藏过程和地层压力准确预测至关重要。根据超压测井响应、数值模拟及天然气成藏条件解剖讨论了乐东A构造储层超压成因机制，分析了储层强超压形成与天然气成藏的关系。结果表明：乐东A构造中新统储层普遍发育超压，压力系数最大可达2.27，接近地层破裂压力梯度，纵向上压力系数发生突变。富泥地层快速埋藏导致的机械压实不均衡作用可使压力系数达到1.5～1.6左右。强超压（压力系数>1.6）表现出明显的卸载响应特征，表明储层压力远高于原位泥岩压力。储层中卸载型超压主要是由三亚组超压沿断层垂向传递作用引起的。储层强超压的形成与天然气运移成藏过程密切相关，深层成熟—高成熟的三亚组烃源岩提供了强超压传递的流体来源，烃源岩与储层之间发育的隐伏断层/微裂隙系统为天然气垂向运移及超压垂向传递提供了有利通道，天然气晚期充注有利于透镜状砂体中传递超压的保存。研究认识将为钻前压力预测和安全钻井设计提供有效参考，并为莺歌海盆地非底辟区高温高压天然气成藏过程提供新的见解。

Generation mechanism of overpressure and its implication for natural gas accumulation in Miocene reservoir in Ledong A structrure,Ledong slope,Yinggehai Basin

The Ledong A structure is located in the non-diapir zone of the Ledong slope in the southeast of the Yinggehai Basin with plentiful overpressured natural gas resources. It is important to identify the origin of the overpressure for the understanding of the migration and accumulation process of natural gas, and as well as for the pre-drilling pressure prediction. Based on the overpressure logging responses, numerical modelling and the investigation of natural gas accumulation conditions, the overpressure generation mechanisms in Ledong A structure are discussed, and the relationship between high-magnitude overpressure formation and natural gas accumulation is analyzed. The results show that the overpressure commonly occurred in Miocene reservoirs in the Ledong A structure, with a maximum pressure coefficient of 2.27, which is close to the fracture pressure gradient, and the pressure coefficient abrupt elevated vertically. The disequilibrium compaction caused by the rapid deposition and burial of the mud-dominated layer is the most common cause of overpressure, which can make the pressure coefficient reach about 1.5~1.6. The high-magnitude overpressures (pressure gradient>1.6) in the reservoirs show obviously unloading response, indicating that higher reservoir pressure than the in-situ mudstone pressure. The unloading overpressure is mainly vertically transferred of the deep overpressure via the opening fault. The formation of high-magnitude overpressure in the reservoir is closely related to the process of natural gas migration and accumulation. The overpressured natural gas came from the underlying mature-highly mature Sanya Formation coal-type source rock, and the fluid flow from the source rock vertically through the hidden faults/microfractures to the shallower reservoirs, which caused the vertical transfer of the overpressure. The natural gas charging time is relatively late, so it is conducive to the preservation of the transferred overpressure in the lenticular sandbodies. The results will provide effective guidance for predrilling pressure prediction, and provide a new insight into the migration and accumulation of natural gas in the Yinggehai Basin.