藏北羌塘盆地侏罗系白云岩类型、成因及油气储集特征

TYPES, ORIGIN AND RESERVOIR CHARACTERISTICS OF THE JURASSIC DOLOSTONES IN THE QIANGTANG BASIN, NORTH TIBET

羌塘盆地地处藏北高原 ,其南北界分别为班公湖—怒江缝合带与拉竹龙—金沙江缝合带 ,盆地面积约 16 0 0 0 0km2 。盆地内发育古生代—中生代海相沉积地层 ,其中尤以侏罗系分布最广、发育最为齐全。在侏罗系的中统和上统下部广泛发育白云岩 ,并构成最具潜力的油气储层。这些白云岩按其成因与岩矿特征可归为两类 ,即I型和II型白云岩。I型白云岩是在蒸发条件下由毛细管浓缩作用形成 ,故与膏岩层关系密切 ,具有白云石晶粒细小、化学组成中相对富Ca2 + 、晶体有序度相对偏低等特点 ;II型白云岩则是在海水与大气淡水的混合水中生成 ,因而与膏岩层无关 ,与I型白云岩相比 ,具有白云石晶粒较粗且自形程度较高、化学组成相对富Mg2 + 及晶体有序度较高的特点。I型白云岩在形成之后基本无明显后期改造 ,因而其在储集物性上保持了原始的中低孔和低渗的特点 ,可构成区内的II—III级油气储层 ;而II型白云岩则在形成后的晚成岩—表生成岩阶段 ,经历了明显的重结晶、溶蚀和方解石充填交代等后期改造 ,由于其 (尤其是地下未经表生成岩改造的部分 )具有较好的孔渗条件 。

The Qiangtang Basin is situated in the North Tibet Plateau. Its north and south boundaries are Lazhulong-Jinshajiang suture belt and Bangonghu-Nujiang suture belt, respectively. The area of the basin is about 160 000 km 2. Palaeozoic to Mesozoic (especially Jurassic) marine strata are well developed in the basin. Dolostones are widely distributed in the Middle Jurassic and the lower part of the Upper Jurassic, constituting the most potential oil and gas reservoirs. The dolostones can be classified into two types (i.e. type Ⅰ and type Ⅱ) according to their origin and the characteristics of petrology and mineral compositions. The type Ⅰ dolostone resulted from the capillary concentration dolomitization. It is associated closely with gypsum and anhydrite. Dolomite crystals in the dolostone are very fine with chemical compositions being relatively rich in Ca 2+ .The ordering degree of its crystals is fairly low. The type Ⅱ dolostone was formed in the mixed marine and meteoric fresh water, thus is not associated with gypsum and anhydrite. Dolomite crystals in the type Ⅱ dolostones are larger and more euhedral than those in the type Ⅰ dolostones. The chemical composition of the type Ⅱdolostone is relatively rich in Mg 2+ . The ordering degree of its crystals is fairly high. The type Ⅰ dolostone was not obviously modified after it was formed, keeping a low to medium porosity and a very low permeability as it was in original. The type Ⅰ dolostone can form the oil and gas reservoirs of ranks II-III in the basin. After it was formed, the type Ⅱ dolostone underwent evident recrystallization and dissolution, and filling and replacement by calcite in late diagenetic to epidiagenetic stages. The type Ⅱ dolostones, especially those not modified by epidiagenesis, can form oil and gas reservoirs of ranks I-II in the basin because of its excellent conditions of porosity and permeability.