Abstract: In this paper, we will describe a rare type of oil and gas reservoir, d iscovered by drilling in well TZx, Tarim basin, northwest China. It was formed i n fluorite veins in Lower Ordovician limestones. The characteristics and ori g in of the reservoir are studied on the basis of interpretation of high-resolut i on borehore electric images in combination with data from cores and thin section observations. 　　Although the fluorite veins and their host limestones are shown as simila r h igh-resistive white or bright yellow colors, the images revealed that the veins with structureless features are generally oblique to or even perpendicular to th e host limestones developed with horizontal stratification. Irregular dark patch es or spots, centimeters to decimeters in size, on images are interpreted as cav erns and vugs, which was confirmed by core observations. These vugs mainly o ccur in fluorit e veins and at the contacts between veins and their host limestones. They are th e major reservoir space for oil and gas. 　　It is believed that the origin of those fluorite veins is related to the lat e Early Permian volcanism in the middle and western parts of Tarim basin. They were formed by low-temperature postmagamatic hydrothermal fluids that intru d ed upward into the near-surface limestones along pre-existing fractures a nd faults. 　　The oil and gas reservoir was generated after the formation of fluorite veins du ring the Permian volcanism. Caverns and vugs developed in fluorite veins and the ir vici nity provided space for the gathering of oil and gas. The Lower Ordovician carbo nates, the good source rocks regionally, are proven to have reached their pe ak matu r ation after the Paleozoic matched well in time with the formation of caverns and vugs. W ith 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 pr oposed. It consists of four stages as follows: (1) fractures and faults formed i n the Lower Ordovician limestones due to the Caledonean regional compression; (2 ) fluorite deposited as veins in pre-existing fractures and faults of the Lower Or dovician 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 Permi an by dissolution; (4) oil and gas accumulated in vugs in the Mesozoic-Cenozoic .