陆相断陷咸化湖盆地层水化学场响应及与油气聚集关系——以渤海湾盆地东营凹陷为例

东营凹陷古近系水化学场分布与湖盆演化有着良好的对应关系,矿化度从深到浅逐渐降低,体现出沉积环境开放性的增强。高矿化度分布是咸化湖盆重要特征,盐岩溶解、扩散、运移和断裂沟通是其主要成因。在这种高矿化度地层水化学场中主要发育CaCl_2水型,在盆地边缘低矿化度地层水中,以NaHCO_3水型为主,同时发育有少量低—较高矿化度的Na_2SO_4水型和MgCl_2水型。对东营凹陷大量实测地层水及油气性质资料分析表明:东营凹陷CaCl_2水型指示地层封闭性条件较好的还原环境,在该环境中油气藏分布最为广泛,目前发现的高矿化度(100 g/L)油藏与CaCl_2水型相关;低矿化度(10 g/L)NaHCO_3水型指示封闭性条件差、水交替强烈的水文流畅环境,对油气聚集不利,可形成次生油气藏;高矿化度(30～100g/L)的NaHCO_3水型对油气聚集也有利,在该水型中多形成原生油气藏,其成因与地幔高浓度的CO_2运移及断裂通道的沟通作用有关;研究区高矿化度(30 g/L)Na_2SO_4水型的形成与膏盐有关,其中可形成原生油气藏;低矿化度Na_2SO_4水型指示封闭性条件差的氧化环境,对油气聚集不利。研究结果表明,在东营凹陷断陷咸化湖盆中,含NaHCO_3水型的地层环境更能代表地层水交替强烈的开放性沉积环境,而Na_2SO_4水型分布多与硫酸盐有关;东营凹陷表现出高矿化度的NaHCO_3水型和Na_2SO_4水型等异常地层水环境,但在各类型地层水环境中油气藏都有所发现,显示出典型断陷咸化湖盆中水化学场分布特征及与油气成藏之间独特的相互关系。

Response of formation water chemical fields and its relation to the hydrocarbon accumulation in the saline faulted-basin:A case study of Paleogene system in Dongying sag

The formation water chemical characteristics in the Paleogene system had good response to the sedimentary conditions and its evolution in Dongying sag. The content of total salinity and chloride ion decreased from the deep depth to the lower,which was good response to the evolution of the Dongying sag from salted basin to fresh water basin. High salinity displayed the main characteristics of the saline lake-basin,and dissolution of salt rock,diffusion and migration of formation water,and the communication by faults were main reasons. In the formation water with high salinity,there mainly developed CaCl_2 water type,and in the formation water with low salinity,there mainly developed NaHCO_3 water type,and the NaHCO_3 water type was substituted by the CaCl_2 water type with the content of total salinity increased. There were also developed some Na_2SO_4 and MgCl_2 water types with low to high salinity. The study by the analysis on a great deal of measured data related to formation water and oil or gas demonstrated that,The CaCl_2-typed formation of water in Dongying depression indicated quasi-closed and reductive conditions,in which the oil and gas reservoirs were most widely distributed,and the reservoirs currently found in high salinity(>100 g/L)were mainly associated with formation water in CaCl_2-type; The NaHCO_3-typed formation water with low salinity(<10 g/L)suggested opening and strong alternation in water flow,which was unfavorable for oil and gas accumulation,though some secondary oil and gas reservoirs were found in such environment; The NaHCO_3-typed formation water with high salinity(>30 g/L)suggested a favorable condition comparatively for oil and gas accumulation,in which primary oil and gas reservoirs were found,and its cause of formation was recommended to be connected with the migration of CO_2 in high concentration from the mantle along the channels of faults. The Na_2SO_4-typed formation water with high salinity(>30 g/L)was related to gypsum-salt,in its conditions primary oil and gas reservoirs can be formed,and the Na_2SO_4-typed formation water with low salinity suggested oxidative condition that was also unfavorable for oil and gas accumulation. In the saline faulted lake-basin of Dongying Basin,NaHCO_3 water type can representative the openly sedimentary conditions with strong alternation of formation water,and Na_2SO_4 was relative to the distribution of halite and gypsum-salt. High salinity formation water of NaHCO_3 type and Na_2SO_4 type showed the abnormal formation water field. Some oil reservoirs were found in every condition with low salinity although most oil reservoirs formed in the formation water conditions with high salinity,which displayed the representative characteristics of formation water field in the saline faulted-basin and unique relation of formation water chemical field in salted basin with hydrocarbon accumulation.