塔里木盆地寒武-奥陶系白云岩储层类型与分布特征

塔里木盆地寒武-奥陶系白云岩是台盆区最重要的储层之一,发育4种类型:①潮坪白云岩。以含膏泥晶白云岩为主,石膏溶孔及白云岩砾间孔发育,发育于潮间-潮上坪蒸发环境。白云石表现为MgO-CaO呈线性正相关、低Mg/Ca值及高∑REE值、锶同位素值分布在0.7085～0.7100之间,略高于同期海水值0.7090、阴极发光不发光或暗色光。储层分布主要受沉积相控制,发育于中下寒武统地层;②蒸发台地白云岩。以藻丘及颗粒灰岩选择性白云石化为特征,发育铸模孔、膏溶孔和残留粒间孔,白云石Mg/Ca值变化范围大、δ13C、δ18O值相对偏正、分别大于2‰和-4‰、阴极发光发较亮红光。储层主要发育于台内靠近台缘一侧;③埋藏白云岩。发育细晶、中晶及粗晶白云岩,以晶间孔及晶间溶孔为主,δ18O值偏负在-5‰～-10‰(PDB)之间,87Sr/86Sr值相对较大,为0.7090～0.7110,阴极发光以发暗棕褐色、紫色光为主。埋藏白云岩储层发育主要受成岩相控制,但也表现出与沉积相具有相关性,这是因为物性好的台缘、台内礁滩体及有裂缝沟通构成的开放体系更有利于埋藏白云石化作用发生;④热液白云岩。以受热液改造的结晶白云岩为特征,往往伴生热液矿物,白云石δ18O值异常偏负、一般小于-9‰(PDB)、阴极发光多发明亮红光、稀土元素标准化配分曲线中Eu出现正异常、出现高于地层背景值的异常高温包裹体;主要发育在具有上覆隔挡层的不整合面之下地层及大断裂发育带附近。上述四类白云岩在规模与分布上有不同,但都可预测。埋藏和热液白云岩规模较大,受原始沉积相带和成岩流体来源双重约束。潮坪和蒸发台地白云岩规模可变性较大,可由沉积环境重建,结合成岩相研究预测评价。

Types and distributional features of Cambrian-Ordovician dolostone reservoirs in Tarim Basin, northwestern China.

The Cambrian-Ordovician dolostones are important reservoirs for oil & gas accumulations in the Tarim Basin,northwestern China,four types of which are recognized:① tidal flat dolostone or sabkha dolostone,which is dominated by dolomicrite with relatively well developed gypsum-dissolution and interbreccia porosities and was deposited in intertidal to supratidal evaporitic environments.The dolomite show linear correlation of MgO and CaO,low Mg/Ca ratios,high ∑REE with none to dull cathodoluminescene.87Sr/86Sr ratios range from 0.7085～0.7100 and are slightly higher than those of the contemporaneous seawater.The reservoir occurs mainly in Middle-Lower Cambrian and is mainly controlled by sedimentary facies;② evaporitic platform dolomite or seepage-reflux dolostone,which is characterized by reef mound and grainstone fabrics being selectively dolomitized with well developed moldic,gypsum-dissolved and intergranular porosities,shows the wide range of Mg/Ca values,δ13C and δ18O with the values of higher than 2 ‰ and-4 ‰,respectively,and bright red cathodoluminescene.The reservoir occurs mainly in the part of evaporitic platform or lagoon close to platform margin;③ burial dolostone,composed of fine to coarse-crystalline dolomites with well developed inter-crystalline(dissolved) pores,shows relatively negative δ18O with a range from-5‰～-10‰(PDB),higher 87Sr/86Sr ratios ranging from 0.7090 to 0.7110,and dark brown and purple cathodoluminescene.The development of this kind of reservoir was controlled mainly by diagenetic facies,however,tends to be related to the sedimentary facies,because platform margin and inner platform reef and shoal and other open systems with fractures are propitious to the burial dolomitization;④ hydrothermal dolostone,characterized by crystallized dolomite modified by hydrothermal fluid and significantly negative δ18O values less than-9‰(PDB),bright red cathodoluminescene,positive Eu abnormity of REE with homogenization temperatures of fluid inclusions 5 to 20℃ higher than the surrounding strata and irregularly developed enlarged intergranular pores and dissolved vugs and caverns.Hydrothermal dolostone is commonly accompanied by barite,fluorite,pyrite and other hydrothermal minerals.The reservoir occurs mainly along the deep-seated faults under the unconformities.The scale and distribution for the four types of dolostones are different,but they can be predicted using integrated methods.The distribution of burial and hydrothermal dolostones is constrained by both initial depositional facies and diagenetic fluid source.Sabkha dolostone or tidal flat dolostone are variable in scale but can be predicted with the reconstruction of sedimentary environment,diagenetic facies analysis and seismic inversion interpretation.