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

关键词:  羌塘盆地  石油地质条件  初步评价

Key words:  Qiangtang Basin  petroleum geology  assessment

摘要:

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(J₃ s)、夏里组(J₂ x)、布曲组(J₂ b)和上三叠统肖茶卡组(T₃ x),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,J₃ s和J₂ b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。J₂ x和T₃ x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,J₂ x和J₃ s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、中侏罗统雀莫错组(J₂ q)、上侏罗统夏里组和雪山组(J₃ x)及古近系双湖组(E₂ s);岩石类型以细砂岩、粉砂岩为主,次为中砂岩和粗砂岩。碳酸盐岩储层主要分布于肖茶卡组一、二段,布曲组和索瓦组;主要岩石类型有泥灰岩、粒屑灰岩、鲕粒灰岩、生物礁灰岩和白云岩等,其中白云岩储层具有特殊的意义。火山岩分布于二叠系和上三叠统。盆地内白云岩储层分布较广,在平面上分布与古隆起有关,已发现的白云岩中,以隆鄂尼布曲组的白云岩规模最大,并部分地区储有轻质原油。
盆地内存在多层盖层岩石类型,主要有泥质岩、蒸发岩、碳酸盐岩、致密砂岩、火山岩、硅质岩和煤岩等,其中前三种分布广泛,各层系均较发育,且封闭性能较好,是盆地内主要的盖层岩石类型,尤以泥质岩和蒸发岩更为重要。
羌塘盆地具备了形成大、中型油气田的石油地质条件,是青藏地区最具油气希望的盆地,展现了美好的勘探前景和开发潜力。

Abstract:

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 (J₃ s), Middle Jurassic Xiali (J₂ x) and Buqu (J₂ b) Formations, and Upper Triassic Xiaochaka Formation (T₃ 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.