也门1区块白云岩成因及其对油气成藏的影响

Genetic analysis on the dolomite formation and its influence on hydrocarbon accumulation in Block1,Yemen

通过对也门1区块白云岩分布、岩性特点、颜色以及白云岩矿物电镜扫描特征的分析,研究白云岩化过程对油气成藏的影响。也门1区块白云岩具有厚度大、分布范围狭窄的特点,主要分布在Imad-1井附近。白云石晶体多呈白糖状,具有鞍状白云岩的主要特征,虽见介壳化石,但仅保留了原来的痕迹,且在岩心和薄片中未见生物成因白云岩的球状、不规则针刺状或菱形晶体特征。也门1区块白云岩颜色较深,呈灰—深灰色,表明其富含铁、锰元素。上述特征均指向也门1区块白云岩为热液成因,推测热液主要源自深部富含镁离子的液体,沿不整合面或者断层渗入到相关地层中,与地层中富含钙离子的灰岩、石膏等进行交换,最终形成了分布局限、厚度较大的白云岩。白云岩在形成了有利储集空间的同时,也对盖层和保存条件造成了破坏,加上断层的影响,研究区东南部区域形成规模油气藏的潜力不大,建议未来也门1区块的勘探应集中在西北部的玛格拉夫劳达构造带。

With the analysis of the distribution,lithology,color and mineral features under SEM of the dolomite,the origin of the dolomite in Block1 in Yemen and the impact of dolomitization on oil and gas accumulation were studied. The thickness of dolomite in Block1 is large but it distributes in a very limited area near Well Imad-1. The crystals of dolomite look like sugar with main features of saddle dolomite. Only the traces of shell fossils are observed. There is no sphere,irregular thorns and rhombic shapes observed in dolomite cores and thin sections. And the dolomite is grey-dark grey,the dark color of which indicates that the dolomite contains high contents of ferrum and manganese. All these feature points to hydrothermal genesis of the dolomite of Block1 in Yemen. It is inferred that hydrothermal fluid comes mainly from the deep area withabundant Mg2+ ions. It flowed upward to the upper strata through the faults or disconformity and exchange between Ca2+ fromthe limestone and anhydrite and Mg2+ from the hydrothermal fluid occurred. Finally,some thick dolomite rocks developednear the faults(or disconformity)with limited lateral distribution. The formation of dolomite can increase the pore spaces ofthe reservoir,but it can damage the cap rock and preservation condition. Considering the influence of faults as well,it is believed the southeast area in Block1 has lower potential of oil and gas accumulation,but the Magraf belt in northwest area is suggested as future exploration target in Block1.