构造演化对莺歌海盆地天然气成藏差异性的控制作用

尽管两个重要的天然气勘探区"东方区"和"乐东区"均位于莺歌海盆地莺歌海凹陷中央底辟带,但二者天然气的成藏机制却存在一定差异。为此,分析了构造演化对烃源岩发育和底辟活动的控制作用以及对两区天然气成藏差异性的影响:①中新世以来的构造演化导致沉降、沉积中心由东方区向乐东区迁移,这是控制该区中新统烃源岩发育和底辟活动的主要因素,对天然气的生成、成藏和分布产生重要影响;②东方区烃源岩生气时间较早、持续时间较长,而乐东区烃源岩的生气时间则相对较晚、生气更集中,更有利于天然气聚集成藏;③晚中新世以来的底辟活动对天然气的运移、成藏和分布起决定性作用;④东方区底辟构造较少、活动能量较弱,而乐东区底辟构造更发育且活动更为复杂;⑤二者天然气的成藏机制也有共同之处,即均处于高温高压带,天然气成藏均受控于底辟活动。结论指出,该盆地底辟波及区能量相对较低,中深层天然气的成藏时间较早,保存条件较好,CO_2风险较低,是有利的优质天然气分布区域。

Control effect of tectonic evolution on gas accumulation difference in the Yinggehai Basin

Gas accumulation mechanisms of the Dongfang area and Ledong area are different to some extent although both are the key gas exploration areas located in the same central diapers in the Yinggehai Sag, Yinggehai Basin. We analyzed the control effect of tectonic evolution on source rock development and diapirism and further on the differences of gas accumulation mechanisms between these two blocks. (1) The tectonic evolution since the Miocene caused the migration of the subsidence center and depocenter from the Dongfang Block to the Ledong Block, which was a key factor controlling the Miocene source rock development and diapirism and the follow-up gas generation, accumulation and distribution. (2) Gas generation in the Dongfang Block initiated earlier and lasted for a rather long time. In contrast, gas generation in the Ledong Block began later and completed in a shorter period, thus favorable for gas accumulation. (3) The diapirism since the Late Miocene determined the gas migration, accumulation and distribution. (4) The number of diapirs is small and their intensity of activity was relatively weak in the Dongfang Block. In contrast, diapirs were highly developed and their activities were more complex in the Dongfang area. Despite these differences, they have some similarities, such as their locations in high temperature and high pressure zones as well as the control of diapirism on gas accumulation. It was finally pointed out that the diapir distribution areas in this basin, featured by relatively low energy, early gas accumulation in the middle and deep layers, good sealing conditions, and a low risk of CO₂ occurrence, are thus favorable play fairways for gas exploration.