塔里木盆地奥陶系萤石脉—油气叠合成藏作用——高分辨率成像测井资料提供的证据

根据高分辨率电阻率成像测井解释，结 合岩心观察及薄片资料，探讨了塔里木盆 地塔中X井下奥陶统灰岩段内罕见的萤石脉—油气叠合成藏现象。萤石脉侵位于二叠纪塔里 木盆地中西部区域性火山活动时期，由上侵至地层浅部的低温岩浆热液沿断层、裂隙发育带 充填而成。萤石脉及其附近灰岩围岩中溶洞的形成为后期油气聚集提供了良好的储集条件， 它与充足的油气来源及上覆巨厚中、上奥陶统泥质岩有利的封盖条件配合，促成了油气在萤 石脉基础上的叠合成藏。该油气藏的发现为塔里木盆地油气勘探开辟了一个新领域，并丰富 了传统的油气成藏理论。

Origin of the Oil and Gas Accumulation Related to Fluorite Veins in Lower Ordovician Limestones, Tarim Basin, Northwest China: Evidences from High Resolution Borehole Electric Image Logs

In this paper, we will describe a rare type of oil and gas reservoir, discovered by drilling in well TZx, Tarim basin, northwest China. It was formed in fluorite veins in Lower Ordovician limestones. The characteristics and origin of the reservoir are studied on the basis of interpretation of high resolution borehore electric images in combination with data from cores and thin section observations. Although the fluorite veins and their host limestones are shown as similar high resistive white or bright yellow colors, the images revealed that the veins with structureless features are generally oblique to or even perpendicular to the host limestones developed with horizontal stratification. Irregular dark patches or spots, centimeters to decimeters in size, on images are interpreted as caverns and vugs, which was confirmed by core observations. These vugs mainly occur in fluorite veins and at the contacts between veins and their host limestones. They are the major reservoir space for oil and gas. It is believed that the origin of those fluorite veins is related to the late Early Permian volcanism in the middle and western parts of Tarim basin. They were formed by low temperature postmagamatic hydrothermal fluids that intruded upward into the near surface limestones along pre existing fractures and faults. The oil and gas reservoir was generated after the formation of fluorite veins during the Permian volcanism. Caverns and vugs developed in fluorite veins and their vicinity provided space for the gathering of oil and gas. The Lower Ordovician carbonates, the good source rocks regionally, are proven to have reached their peak maturation after the Paleozoic matched well in time with the formation of caverns and vugs. With a total thickness of about 600 meters, the overlain Middle Upper Ordovician mudstones and marlites efficiently hindered the oil and gas gathered in caverns and vugs from escaping upward. A model on the formation of the oil and gas accumulation in fluorite veins is proposed. It consists of four stages as follows: (1) fractures and faults formed in the Lower Ordovician limestones due to the Caledonean regional compression; (2) fluorite deposited as veins in pre existing fractures and faults of the Lower Ordovician limestones as a result of the volcanism during the late Early Permian; (3) caverns and vugs formed in fluorite veins and their vicinity after the Permian by dissolution; (4) oil and gas accumulated in vugs in the MesozoicCenozoic.