伊拉克艾哈代布油田上白垩统Khasib组生物扰动对储层非均质性的影响

Effect of bioturbation on reservoir heterogeneity of the Upper Cretaceous Khasib Formation in AD Oilfield,Iraq

生物扰动可改变碳酸盐岩原始沉积组构,影响后期成岩作用,导致生物潜穴与基质间存在物性差异,进而增强碳酸盐岩储层的非均质性。在大量岩心、薄片观察和点渗透率等多种资料分析的基础上,对伊拉克艾哈代布油田上白垩统Khasib组Kh2段生物扰动遗迹组构类型及其对储层非均质性的影响开展研究。结果表明:①Kh2段发育海生迹(Thalassinoides)、蛇形迹(Ophiomorpha)、古藻迹(Paleophycus)等3种主要遗迹组构。Thalassinoides潜穴外壁光滑,不具有衬壁结构,呈三维网状连通管形,单个潜穴直径为5~25 mm;Ophiomorpha潜穴呈“T”形管状,多呈水平分布,部分发育泥质衬壁结构,直径为5~15 mm;Palaeophycus潜穴呈圆形—椭圆形的轻微弯曲—直管,直径为2.5~10 mm,主要呈水平分布,不具有衬壁结构。②由于物性差异导致的含油性差异,岩心具有明显的斑块状特征,同时不同含油级别区域分布与生物遗迹组构的形态及空间展布相关,这表明Kh2段储层非均质性主要受生物扰动作用的控制。③Kh2段不同沉积环境下发育不同的生物扰动遗迹组构类型,导致潜穴与基质的原始组构及沉积介质化学性质发生变化,形成潜穴与基质之间的孔隙结构差异,在此基础上,叠加后期成岩差异(胶结、溶蚀、压实等),进而导致Kh2段不同的储层非均质性类型。Thalassinoides遗迹组构主要发育于中缓坡绿藻生屑滩,潜穴内受到准同生溶蚀而形成大量铸模孔,基质部分受到早期强烈的胶结作用。Ophiomporpha遗迹组构主要发育于中缓坡中—低能浅滩,潜穴内因颗粒充填而发育粒间孔,基质部分灰泥含量高。Palaeophycus遗迹组构主要发育于中缓坡颗粒滩,潜穴内颗粒含量高,且早期胶结作用受到抑制;随着海平面下降,大气淡水优先进入潜穴通道进行溶蚀。

Bioturbation can change the original sedimentary fabric of carbonate rocks,and affect later diagenesis,which lead to the difference of physical properties between biological burrows and matrix,and enhance the heterogeneity of carbonate reservoirs.The Khasib Member 2(Kh2)of Upper Cretaceous in AD Oilfield,Iraq was mainly developed in middle-outer carbonate ramp with upward shallowing cycle,where bioturbation relic fabrics developed widely.Based on the analysis of a large number of core,thin section,point permeability and other test and analysis data,this paper studies the types of bioturbation relic fabric and its influence on reservoir heterogeneity.The results show that:(1)There are mainly three types of relic fabrics:Thalassinoides,Ophiomorpha and Paleophycus in Kh2.The outer wall of Thalassinoides burrow is smooth without lining structure,which is in the shape of three-dimensional network connected tube,and the straight diameter of a single burrow is 5-25 mm;Ophiomorpha burrow is"T"shaped tubular,mostly horizontally distributed,and partially developed argillaceous lining structure,with a diameter of 5-15 mm;Palaeophycus burrow is a circular-oval slightly curved-straight pipe with a diameter of 2.5-10 mm,which is mainly distributed horizontally without lining structure.(2)The core has obvious patch-like characteristics due to the oil-bearing difference caused by the difference of physical properties.At the same time,the regional distribution of different oil-bearing levels is related to the morphology and spatial distribution of biological relic fabric,which indicates that the reservoir heterogeneity of Kh2 is mainly controlled by biological disturbance.(3)Different bioturbation relic fabric types are developed in different sedimentary environments of Kh2,resulting in changes in the original fabric and chemical properties of sedimentary media between the burrow and the matrix,forming the differences in pore structure between burrow and matrix.On this basis,the later diagenetic differences(cementation,dissolution,compaction,etc.)are superimposed,resulting in different reservoir heterogeneity types in Kh2.Thalassinoides relic fabric is mainly developed in green algae shoal of Kh2-3 layer.Due to the change of chemical properties of sedimentary media in the burrow,a large number of mold pore are formed by quasi syngenetic dissolution,while the matrix is strongly cemented in the early stage,which intensifies the physical property difference between the burrow and the matrix.Ophiomporpha relic is mainly developed in mediun-low energy shoal of Kh2-2 layer.Intergranular pores are developed in the burrow due to particle filling,and the mortar content in the matrix is high,which reflected the reservoir heterogeneity.Palaeophycus relic fabric is mainly developed in granular shoal of Kh2-1 layer.The content of particles in the burrow is high,and the early cementation in the burrow is inhibited due to the change of chemical properties of sedimentary media.With the decline of sea level,atmospheric fresh water preferentially enters the burrow channel for dissolution.