羌塘盆地石油地质条件与初步评价

The preliminary study of petroleum geology of the Qiangtang Basin in northern Xizang

羌塘盆地是我国西部尚待进一步勘探与开发的最具前景的中、新生代大型含油气盆地。盆地内发育有10套烃源层,其中主要烃源层有4套,即上侏罗统索瓦组(J3s)、夏里组(J2x)、布曲组(J2b)和上三叠统肖茶卡组(T3x),除中央隆起带西部缺失外,在盆地其他地区有广泛分布,累积厚度大于3000m。主要烃源层由碳酸盐岩和泥质岩两类岩性组成,J3s和J2b主要为碳酸盐岩烃源层,有机质含量普遍不高,有机碳含量为0.15%～0.25%,多属差—中等烃源岩。J3x和T3x主要是泥质岩烃源层,有机质含量较高,有机碳含量为0.6%～1.2%,属中等—好烃源层。此外,J2x和J3s还有一套油页岩烃源层,不仅有机碳含量特别高,而且可溶有机质也特别丰富,是盆地内最有价值的烃源层。4套主要烃源层均为海相沉积,有机物质以海相低等生物为主,形成以混合型(Ⅱ型)为主的有机质类型,具有较强的成烃能力。盆地中间有机质热演化处于成熟阶段,盆地周边和中央隆起带附近处于高成熟阶段,再向外达到了过成熟阶段。推测,盆地中部为原油分布区,向外扩展为凝析油 湿气区,再向外为干气分布区。其中以索瓦组的原油分布范围最大,而自夏里组→布曲组→肖茶卡组,原油分布区的面积逐步缩小。盆地储集岩类有碎屑岩和碳酸盐岩以及少量火山岩。碎屑岩主要分布于上三叠统肖茶卡组三段?

Ten successions of source rocks are developed in the Qiangtang Basin, of which four ones appear as important source rocks, including the Upper Jurassic Suowa Formation (J3s), Middle Jurassic Xiali (J2x) and Buqu (J2b) Formations, and Upper Triassic Xiaochaka Formation (T3x). The source rocks occur almostly throughout the basin besides the central uplift, with a total thickness of more than 3000 m, and consist chiefly of carbonate rocks and argillaceous rocks. The former are common in the Upper Jurassic Suowa Formation and Middle Jurassic Buqu Formation where the lower organic carbon contents ranging between 0.15% and 0.25% suggest the poor to moderate source rocks. The latter are observed in the Middle Jurassic Xiali Formation and Upper Triassic Xiaochaka Formation where the higher organic carbon contents ranging between 0.60% and 1.20% indicate the moderate to good source rocks. An additional sequence of oil shales has been found to be hosted in the Middle Jurassic Xiali Formation and Upper Jurassic Suowa Formation. The oil shales behave as the valuable source rocks due to their extraordinary high organic carbon and soluble organic matter contents. The abovementioned four sequences of source rocks, without exceptions, have a marine origin; the organic matter is made up of lowgrade marine organisms. The mixed (Ⅱtype) organic parent material is interpreted to have good reservoir quality. The thermal evolutionary history shows that the organic matter is now in the maturation stages in the central part of the basin; the high maturation stages in the peripheral part and central uplift, and supramaturation stages outside the basin. It is estimated that the crude oil area appear in the central part of the basin, followed by condensateswet gas area, and dry gas area towards the margins of the basin. The extents of the crude oil areas reach the maximum in the Suowa Formation. There is a gradual decrease in crude oil area from the Xiali Formation through the Buqu Formation to the Xiaochaka Formation.The reservoir rocks in the basin are assembled by clastic rocks and carbonate rocks, with subordinate volcanic rocks. The clastic rocks occur mainly in the third member of the Upper Triassic Xiaochaka Formation, Middle Jurassic Quemocuo Formation, Middle Jurassic Xiali Formation, Upper Jurassic Xueshan Formation, and Paleogene Shuanghu Formation. The rock types consist of finegrained sandstone and siltstone, with subordinate medium and coarsegrained sandstone. The carbonate rocks are defined to the first and second members of the Upper Triassic Xiaochaka Formation, Middle Jurassic Buqu Formation and the Upper Jurassic Suowa Formation. The rock types consist of marls, grainstone, oolitic limestone, organic reef limestone and dolostone. The volcanic rocks are observed in the Permian and Upper Triassic strata. The dolostones as reservoir rocks are widespread in the basin, and their planar distribution is believed to be related to the paleouplift. Several sequences of cap rocks appear in the basin, including, evaporates, carbonate rocks, condensed sandstone, volcanic rocks, siliceous rocks and coalbearing series. The former three types, especially mudstone and evaporates, widespread in the basin have good sealing capacity, and thus may serve as the principal cap rocks in the basin. In sum, the Qiangtang Basin has favourable geological conditions for the generation of the large or mediumsized oil/gas fields, and thus is interpreted to be highly prospective.