苏北盆地天然气成藏特征及选区评价

CONDITIONS OF FORMING GAS RESERVOIRS IN SUBEI BASIN AND ITS REGIONAL SELECTION EVALUATION

苏北盆地面积３．８×１０４ｋｍ２，天然气资源丰富，发育了海相下、中、上古生界三套巨厚烃源岩系和陆相早第三系及局部地区中、下侏罗统两套富含有机质的暗色泥岩。海相地层中由于裂隙、晶间孔、溶孔、溶缝及晶洞发育，弥补了原生孔隙储集能力的不足，使其变为有效的储集岩。陆相早第三系储层平均孔隙度大于２５％，渗透率大于１×１０－３μｍ２。盆地内纵向上发育四套区域盖层，自上而下为上第三系盐城组厚层泥岩、白垩系浦口组泥岩与膏盐层、二叠系龙潭组泥岩与煤层系、志留系高家边组泥岩，这些盖层分别封隔了新生界、中生界、上古生界及下古生界储层中的天然气，配置成四套有利的储盖组合。该盆地天然气类型主要有三种：生物气———低温热解气，如刘庄气田；油田伴生气———凝析气，如周庄、永安气田；多源混合气（含幔源气），如黄桥气田。该盆地成藏控制因素有四：多气源、高生气强度；有利构造带；晚期断裂活动；地质背景不同，圈闭类型不同。综上所述提出苏北盆地今后天然气勘探的方向有三：小海—海安区块、淮阴区块和泰兴—如皋区块。

The Subei Basin with an area of 38 000 km2 is rich in natural gas resources.In the basin,there are three sets of thick hydrocarbon source rock series of marine Lower,Middle and Upper Paleozoic and two sets of dark mudstones rich in organic matters of the continental Paleogene and local MiddleLower Jurassic.In marine strata,they may be changed into the effective reservoir beds,owing to the fact that the development of the fractures,intercrystal pores,solution pores,solution fractures and drusy cavities makes up the storage capacity's weakness of their primary pores.In the continental Paleogene reservoirs,the mean porosity is more than 25% and the mean permeability 110-3m2.Vertically,there are four sets of regional cap rocks in the basin.From top to bottom,they are the thick mudstones of Yancheng Formation of the Neogene,the mudstones and anhydrocks and salt beds of Pukou Formation of the Cretaceous,the mudstones and coal measures of Longtan Formation of the Permian and the mudstones of Gaojiabian Formation of the Silurian.The gas reservoirs in the Cenozoic,Mesozoic,Upper Paleozoic and Lower Paleozoic are sealed by these cap rocks by which four sets of beneficial sourcereservoirseal assemblages were constituted.There are three categories of natural gas in the basin:biogaslow temperature pyrolysis gas,as Liuzhuang gas field;associated gascondensate gas,as Zhouzhuang,Yongan gas fields;polygenic mixed gas (including mantle source gas),as Huangqiao gas field.There are four factors controlling the gas reservoir formation in the basin,i. e.multiple gas source,high source potential;beneficial structural belt and late fracture activity.Different traps can be found in distinct geological backgrounds.According to those mentioned above,it is proposed that there are three tracts for natural gas exploration in the future in Subei Basin,i.e.XiaohaiHaian tract,Huaiyin tract and TaixingRugao tract.