生物碎屑灰岩差异成岩及储集层特征:以伊拉克HF油田白垩系Mishirif组为例*

中东地区白垩系Mishirif组以生物碎屑灰岩为主,其形成于温暖潮湿的环境中。综合利用岩心、铸体薄片、全岩分析、常规物性及高压压汞等资料,以伊拉克HF油田Mishrif组为例,开展生物碎屑类型、分布特征、差异成岩及储集层特征等研究。Mishrif组灰岩中生物碎屑以底栖有孔虫、非固着类双壳类、厚壳蛤和棘皮动物为主,含少量苔藓动物、藻类与海绵动物,其含量、类型及大小对沉积环境有重要指示意义。沉积环境决定岩石组分与结构的差异,在此基础上成岩作用控制岩石的孔隙结构与物性特征。生物碎屑主要经历了不同程度海水环境的泥晶化和生物钻孔、大气淡水环境的溶蚀和胶结、埋藏环境的压实压溶和颗粒破裂作用。以底栖有孔虫和非固着类双壳类碎屑为主的低能沉积环境具有“弱溶蚀、强胶结、强压实”的成岩特征,主要发育微孔、晶间孔及粒内孔,孔喉分布呈偏细态细微喉单峰型,物性较差;以厚壳蛤和棘皮动物碎屑为主的高能沉积环境具有“强溶蚀、弱胶结”的成岩特征,主要发育铸模孔、粒间孔及粒间溶孔,孔喉分布呈偏粗态中粗喉极宽峰型,是Mishrif组最有利储集层。以HF油田Mishrif组为代表的白垩系生物碎屑灰岩在中东地区发育广泛,故上述成果对于该地区生物碎屑灰岩油气开发具有重要意义。

Differential diagenesis and reservoir characteristics of bio-clastic limestone: A case study on the Cretaceous Mishrif Formation in HF Oilfield,Iraq

The Cretaceous Mishrif Formation in the Middle East was developed in a warm and humid environment,mainly consisting of bio-clastic limestone deposits. Based on core observations,cast thin sections,whole rock analysis,petrophysical properties,and high-pressure mercury injection tests,bio-clastic types,distribution,composition,differential diagenesis and reservoir characteristics of the Mishrif Formation in HF Oilfield were systematically studied to clarify the relationship between bio-clastic and intrinsic heterogeneity. The Mishrif Formation is mainly composed of benthic foraminifera,non-fixed bivalves,thick crustaceans,and echinoderms,containing a small amount of bryozoans,algae and sponge animals. The distribution characteristics of biological debris are consistent with the living environment. The content,type,composition and size of biological debris are important signs of the depositional environment. The original composition of these bio-clastics includes unstable aragonite,high-magnesium calcite, and relatively stable medium-low magnesium calcite,which mainly experienced various degrees of soft decay,micritization, and biological drilling in the seawater environment,dissolution and cementation in the atmospheric fresh water environment,compaction pressure dissolution and particle disruption in the buried environment. The differences in sedimentary environment,particle size,morphological characteristics,and composition of various types of bio-clastics have led to differential diagenesis,which controlled pore type,physical properties,and pore-throat structure of bio-clastic limestone,forming a complex pore-penetration relationship. As the content of bio-debris increases,the physical properties of the reservoir become better.The low-energy depositional environments mainly developed benthic foraminifera and non-fixed bivalve detritus are characterized by weak dissolution and strong cementation. Micropores,intercrystalline pores, and intragranular pores were mainly developed. The physical properties are poor,and pore throat distribution shows a fine micro-throat single peak pattern. The high energy sedimentary environment with rudist and echinoderms debris is characterized by strong dissolution and weak cementation. The main development of mold pores,intergranular pores, and intergranular dissolution pores with good physical properties,pore throat distribution shows a very broad range mainly with coarse pore throat,which is the most favorable reservoir type in the study area. The Cretaceous bio-clastic limestone,represented by the Mishrif Formation in HF Oilfield,was widely developed in the Middle East. The above study is of great significance to the development of bio-clastic limestone oil and gas in the Middle East.