莺歌海盆地高温高压天然气地球化学特征及底辟翼部高效成藏模式

莺歌海盆地是新生代高温高压盆地,底辟与热流体活动强烈,深部的天然气在在底辟构造的浅层圈闭中聚集成藏。2010年前钻探8个底辟构造共11口井,没有发现商业气藏,故底辟构造带中、深层能否寻找到大中型优质天然气田一直是争论的焦点。为此,对该区天然气生烃动力学、储层分布及封盖条件进行了分析研究。结果认为:①该区存在浮力作用下游离相缓慢充注成藏、水溶相缓慢充注析出成藏和混相幕式聚集成藏3种天然气充注机理;②底辟核部高温高压带早期成藏经后期多次改造,形成以烃类为主的大型天然气藏的可能性小,底辟翼部则有早期成藏晚期保存完好的原生型天然气藏,非底辟区推测为水溶气析出成藏区;③存在底辟核部区及其周缘半封闭超压系统"混相改造型"成藏、底辟翼部"气相渗滤型"成藏、非底辟带封闭型超压系统"水溶相脱溶型"成藏等3种模式。结论认为,之前失利的主要原因是钻在了底辟构造的核心部位,并用地球化学特征进一步佐证了底辟构造翼部才是高温高压天然气成藏的良好场所。经过勘探实践,最终发现了东方13-1/13-2大气田。

Geochemical behaviors of HPHT gas reservoirs in the Yinggehai Basin and the efficient gas accumulation mode at the basin's diapir flanks

The Yinggehai Basin is a Cenozoic high-temperature and high-pressure basin where diapirism and thermal fluid activities were so strong that the deeply-sourced natural gas accumulated in the shallow traps in the diapir structures. A total of 11 exploratory wells were drilled in 8 diapir structures before 2010, but no commercial gas discoveries were made, provoking a hot debate on the possibility of discovering medium-to-large quality gas reservoirs in the middle and deep layers of the diapir structures. A comprehensive analysis of hydrocarbon generation dynamics, reservoir distribution and sealing conditions revealed the following findings. (1) Three gas-charging mechanisms were identified in the study area, namely slow free gas charging and accumulation under buoyancy, slow charging, evolving and accumulation of water soluble gas, and mixed phase episodic accumulation. (2) In the high-temperature and high-pressure zones in the core of the diapir, the early gas reservoirs experienced multi-stage modification at later periods, thus the possibility of forming large gas reservoirs dominated by hydrocarbon is small; In the flanks of the diapir, the early gas reservoirs may be well preserved at later stages, thus it is possible to discover primary gas pools; In the non-diapir zones, water soluble gas reservoirs may occur. (3) Three gas accumulation models may exist in the study area, namely "the mixed-phase modification model" of semi-closed over-pressured system in the core and periphery of a diapir, "the gas-phase seepage model" in the flanks of a diapir, and "the water-phase desolvation model" of a closed over-pressured system in the non-diapir zones. The analysis revealed that the failure of the previous exploratory drilling was caused mainly by emplacing the wells in the core of the diapir structures. The geochemical behaviors further support that the flanks of a diapir is the favorable place for high-temperature and high-pressure gas accumulation. Exploration practices based on these understandings finally led to the discovery of the large Dongfang 13-1/13-2 gas fields.