Character And Effect On Exploitation Of Ordovician Carst Type Reservoir In Tahe Oilfield

Tahe oilfield is the first large-scale Paleozoic marine oil field of mainly Ordovician carst type reservoir in China.It is charactered with low matrix porosity and permeability(matrix without ability of reservoir),deep embedding(94 percent of oil pool under 5400 meters),difficult to definite(It is a world-class problem to understand and appraise this kind of reservoir).By means of systematacially and integrally studying the deposit,karst,reservoir,cell of sew-hole and exploitation character,the author consider:(1)Middle-low Ordovician and Lianglitage group of upper Ordovician in Tahe oilfield are mainly open-platform and platform margin,platform slope are only located in the southeast;Qiaerbake group of upper Ordovician mainly is open sea shelf facies in shallow sea;Sangtamu group is mainly mix-sediment shelf facies.(2)Hypergene , thermal fluid and buried karst etc. are developed in Ordovician of Tahe oilfield. Hypergene karst is the most important among them,and respectively related with early Hercynian and middle Caledonian.(3) Caledonian karst is mainly distributed in the south of the wedge out of Sangtamu group,and mainly developed anlong with middle Caledonian disconformity contact between Qiaerbake and Yijianfang group,Sangtamu and Lianglitage.Continental facies quartzite or silt can't be seen among matrix. Isotope value of strontium is relatively low and negativeδ¹⁸O, positiveδ¹³C, overly indigent lanthanon. Homogeneous temperature of the inclusion of two faces aqueous solution and hydrocarbon is lower.The inclusion colour of liquid hydrocarbon is dust-colour to black-brown,no or weak yellow fluorescence. Hydrocarbon gas inclusion is gray to plumbeous.(4)Early Hercynian karst is mainly distributed in the north of the wedgeout of Sangtamu group.The filling is abundant of continental facies of quartzite and silt,with Devonian and Carboniferous welwitschiopsida. Isotope value of strontium is distinctly on the high side and high negative value ofδ¹⁸O,mainly negative value ofδ¹³C,rich of lanthanon. Homogenizing temperature of the inclusion of two faces aqueous solution and hydrocarbon is higher.The colour of liquid hydrocarbon is brown yellow,yellow, pale yellow,with pale yellow,yellow, pearl opal fluorescence.The inclusion of gaseous hydrocarbon is gray to plumbeous.(5)Fissure type, cavern-fissure type,fissure-cavern type,fissure-solution hole type,filling pore type in large cave are developed during early Hercynian karst period.They are mainly controlled by structure, surface of unconformity, paleogeomorphy and lithology.Among them, structure is the most important controlling factor.Fissure type, cavern-fissure type,fissure-cavern type,fissure-solution hole type, high bed fissure-pore type are developed during middle Caledonian.They are mainly controlled by structure, surface of unconformity between middle Caledonian episode I and II and lithology etc.,with obvious character of fault control and layer control.(6) Judge of middle Caledonian and early Hercynian karst by stages, the whole were given according to control factors of karst reservoir.It is more credible than general appraisement without karst period.(7)Six credible and five irresponsible fissure-cave cells were divided from each other in main body of Tahe oilfield according to carbon isotope, light hydrocarbon of natural gas, carbon isotope, light hydrocarbon, saturated hydrocarbon, aromatic hydrocarbon of crude oil and trace element, rare earth element, Strontium isotope,etc. of oil field water.Compared with former fissure-cave cell ,it can be concluded that microcosmic information of crude oil,natural gas, formation water can be nicely used as parameters to divide the fissure-cave cell.(8)Reservoir character of Ordovician in Tahe oilfield observably affect on exploitation.It mainly be represented in partition of flow unit,system of well pattern, exploitation index,mode and measure of well completion etc.