南羌塘盆地扎仁地区中侏罗统布曲组沉积环境特征及其对油气地质条件的控制作用

南羌塘盆地扎仁地区地处南羌塘盆地南缘,属油气研究的"空白区",结合羌塘盆地扎仁区块1∶5万石油地质构造详查,笔者重点剖析了扎仁地区布曲组地层中的两个古油藏剖面。依据剖面中碳酸盐岩的岩石类型、结构特征、自生矿物组合、古生物标志以及扎仁地区碳酸盐岩的组合特征,将该区布曲组划分为半局限海台地、局限海台地、开阔海台地和浅滩4种沉积环境。浅滩沉积环境形成的颗粒灰岩在区内广泛分布,沉积厚度大,具有孔隙度好、渗透率高的特征,这些颗粒灰岩的展布决定了区域储集层的规模。局限海台地沉积环境形成的白云岩是区内的另外一套储集层,这套储集层是区内有效的储集层,但这套储集层的有效性与上覆岩层在空间上的展布有关。开阔海台地沉积环境形成的泥晶灰岩夹泥晶砂屑灰岩、泥晶砾屑灰岩的组合为区内较好的生油层,该套烃源岩组合的规模在空间上严格受沉积相的控制。半局限海台地沉积环境形成的泥晶灰岩夹泥晶生屑灰岩、泥晶砂屑灰岩组合是区内有利的盖层,但半局限海台地的性质决定了盖层在区域上的展布。

Sedimentary environment of the Middle Jurassic Bi Qu Formation and its controls on petroleum geology in the Zaring area, southern Qiangtang basin

Located on the southern margin of the southern Qiangtang basin,the area is a "gap area" for hydrocarbon investigation. Detailed reconnaissance of 1:50,000 petroleum geological structure was carried out in the Zaring block of the Qiangtang basin,and according to the reconnaissance results the authors analyze two fossil oil accumulations in the Bi Qu Formation of the study area. Based on the types,textures,authigenic mineral assemblages and fossils of carbonate rocks in the sections as well as the characteristics of carbonate rock associations in the area,the Bi Qu Formation in the area is classified into four major sedimentary environments,namely,semi-restricted platform,restricted platform,open platform and shoal. Grain limestone showing the shoal environment is characterized by widespread occurrence,a great thickness,high porosity and high permeability,and its occurrence controls the size of reservoirs in the area. Dolostone showing the restricted platform environment forms another reservoir in the area. It is an effective reservoir,but its capability is related to the occurrence of the overlying strata. Micritic limestone with micritic calcarenite and calcirudite interbeds that formed in an open platform environment is a better source rock in the area,and its spatial distribution is strictly controlled by sedimentary facies. Micritic limestone with bioclastic limestone and micritic calcarenite interbeds formed in a semi-restricted platform environment is a favorable cover in the area,and the occurrence of the cover is also controlled by the nature of the semi-restricted platform environment.