柴达木盆地新构造运动与第四纪气田的形成

柴达木盆地第四纪坳陷形成于上新世晚期到全新世早期的新构造运动期,面积3.7万km²,是欧亚板块与印度板块相碰撞导致祁连山、昆仑山和阿尔金山逐渐上升,柴达木盆地东部相对下降的结果,坳陷内沉积物厚达3200m。 第四纪早期湖深水淡,晚期湖浅水咸。生气层由暗色泥质岩和碳质泥、页岩组成,厚约1500m。生气母质以腐殖型为主。天然气中CH₄/C₂H₆+C₃H₈〉150,δ¹³C〈-60‰。天然气化学成分表明,其为成岩作用早期生物化学作用所形成。储层为细砂岩和粉砂岩,主要为湖滩相、小型河流三角洲相及筛积扇—泉水河—三角洲体系的水下部分。由于尚未充分压实,岩性疏松,孔隙度平均为32％,渗透率60.8～619μm²。盖层为湖相泥岩和含膏泥岩。天然气赋存于自生自储体系之中。主要生气凹陷及生气层殴控制天然气的分布,雁列式背斜带上的局部构造控制天然气的富集。

NEOTECTONICS AND THE FORMATION OF QUATERNARY GAS FIELDS IN QAIDAM BASIN

The Quaternary depression in Qaidam Basin which covers an area of 3700 km2 was formed by Neotectonics during late Pliocene to early Holocene. The collion between Indian Plate and Eurasian Plate resulted in the gradual uplift of Qilian Mountain, Kunlun Mountain and Aerjin Mountain and the relative subsidence of the eastern part of Qaidam Basin. The thickness of sediments filled in this depression may reach to 3200 m. In early Quaternary the lake was deep and was filled with freshwater. In late Quaternary the lake shallowed and its water became salt-water. The gas source rock consisting of dark mudstone and carbonaceous shale and argillite is about 1500 m in thickness. The gas predecessor is mainly humic matter. CH4 / C2H6+C3H8 ratio in gas is greater than 150, and I3C values are less than -60%o. The chemical composition of the gas indicates that it was formed by biochemical action in the early stage of diagenesis. Fine sandstones and siltstones which constitute the reservoir were deposited in lacustrine banks, small fluvial-deltas and the subaqueous part of the sieve depositional fan-spring river-delta system. The average porosity is 32%. The permeability ranges from 60.8 to 619 because the rock are only subjected to slight compaction. The cap rocks are lacustrine mudstone and gypsiferous. The reservoir rock is identical with the source rock, i.e. gas accumulated in the strata where they were formed. The distribution of gas is controlled by depression and intervals gas-producing. The most abundant gas is accumulated in local structures on anticlinal belts in echelon form.