沉积盆地盐构造热效应及其油气地质意义

Geothermal effects of salt structures on marine sedimentary basins and implications for hydrocarbon thermal evolution

海相沉积盆地广为发育盐岩,盆内油气藏多与盐岩有关.塔里木盆地是中国最大的海相沉积盆地,该区以往的盐构造研究主要集中于其作为良好盖层的封堵性能和作为构造圈闭的盐相关构造.实测表明,盐岩热导率远大于普通沉积岩(约2~3倍),盐岩这一强烈的热物性差异必然会对盆地地层温度分布及烃源岩热演化产生显著影响,但该问题尚未引起重视.本文基于系列理论模型和库车前陆盆地盐构造地震解释剖面,采用二维有限元数值模拟实验,定量探讨了盐构造的热效应及其对烃源岩热演化的影响.研究表明,盐体会造成盐上地层显著增温(3%~13%)和盐下地层降温(11%~35%),进而分别加速盐上烃源岩和抑制盐下烃源岩的热演化过程.盐体的热导率、几何形态、厚度和埋深是控制地热异常幅度的主要因素.异常范围与盐体尺寸有关,横向上可达盐体宽度的2倍、垂向上为盐体厚度的2~3倍.盐构造使得库车前陆盆地盐下地层温度显著降低,造成盐下侏罗系烃源岩镜质体反射率(Ro)降低约18%,从而有利于盐下深层油气的保存.库车前陆盆地东、西部的盐构造在埋深、厚度、成分和构造变形样式等方面均存差异,这可能是造成该区中生界烃源岩有机质成熟度时空差异分布的原因.上述盐构造的热效应对中国海相沉积盆地深层油气资源潜力评价与勘探具有重要意义.

Evaporitic salt is prevalent in marine sedimentary basins, and many discovered hydrocarbon reservoirs are generally associated with salt structures in the world; accordingly salt structures have attracted much attention from academia and industry during the past decade. The Tarim Basin, located in northwest China, is the largest marine sedimentary basin in China with great hydrocarbon resource potential. Previous studies of salt structures in this basin mainly focused on its strong sealing capacity and structural traps created by salt structures. However, besides its extreme impermeability and low viscosity, rock salt has other unique thermal properties, including a large thermal conductivity as high as 5-6 W/（m K）, usually 2-3 times greater than that of other common sedimentary rocks, but a relatively low radiogenic heat production. This strong contrast in thermal properties could change the evolving thermal regime and associated thermal history of the source rocks around salt bodies, but it has not been understood well. Herein based on the theoretical models and interpreted salt-bearing seismic profiles from the Kuqa Foreland Basin, northern Tarim Basin, we use 2D finite element numerical experiments to investigate the impacts of salt structures on the basin geothermal regime and associated hydrocarbon thermal evolution. Our results show that, owing to their high efficiency in heat conduction, the salt rocks would result in obviously positive temperature anomalies （3%-13%） above the salt body and negative temperature anomalies （11%-35%） in the subsalt, enhancing and restraining the thermal maturation of source rocks above and below the salt body, respectively. The amplitude and extent of geothermal effects of salt structures depend on the thermal conductivity, geometry, thickness and burial depth of the salt bodies. The thermally affected area around the salt body can be 2 times the salt radius laterally and 2-3 times the salt thickness vertically. Salt structures in the Kuqa Foreland Basin can prominently cool the subsalt formation temperature and accordingly reduce the thermal maturity （Ro） of Jurassic source rocks as much as 18%, enabling the source rocks to stay in the range of gas generation rather than reaching an over-mature stage as expected previously; this situation is favorable for deep hydrocarbon preservation below salt. Because the salt structures in the west and east Kuqa Foreland Basin show strong differences in their thickness, geometric pattern, burial depth and composition, the thermal effects of salt structures on thermal maturation of subsalt source rocks should differ accordingly, which is supported by the observed temporal-spatial variation of Ro for Jurassic source rocks in this basin. Finally, we propose that the geothermal effects of salt structures will be of great importance in the deep hydrocarbon resources potential assessment and exploration in marine sedimentary basins in China.