东海西湖凹陷深盆气勘探前景探讨

A discussion on the exploration potential of deep basin gas in Xihu sag,East China Sea

"深盆气"是一种"气水倒置"的特殊气藏,构造平缓、储层致密和持续供气的气源是其形成的主要条件。通过对西湖凹陷地质结构、沉积充填特征及成藏地质条件的分析,探讨了研究区深盆气藏发育的有利地质条件及勘探前景。西湖凹陷平湖组总体沉积特征为宽缓地貌下形成的海相、陆相复合沉积体系,振荡式海退背景下宽广的浅水低能环境形成了多套交互式生储盖组合;广泛分布的潮控三角洲和浅水三角洲等细粒砂岩经强烈压实和胶结作用形成了致密砂岩,是深盆气藏有效的储层;煤系气源岩分布广、厚度大、生气潜力大,且从古至今持续生排烃,为深盆气藏形成提供了充足的气源;煤系气源岩与致密砂岩体紧密相邻呈"千层饼式"交互接触,有利于深盆气的聚集成藏。研究区气藏纵向上可划分为深盆气饱含气带、气水过渡带和常规油气藏带等3个带,具有深盆气藏的特征。

Deep basin gas is a special type of gas ac- cumulation with an upside-down relation between water and gas, and gentle structure, tight reser- voirs rock and sustainable gas supply are the main conditions to form this type of gas accumulation. In Xihu sag, based on an analysis of the geological framework, depositional filling feature and hydro- carbon accumulation condition, the geological fac- torts favorable to the deep gas accumulation and its exploration potential were discussed. The overall sedimentation pattern of Pinghu formation in the sag is characterized by the complex depositional system of marine-terrestrial facies over a wide and gentle topography and the multiple interbedded as- semblages of source, reservoir and seal, which formed in a wide, shallow and low-energy environ-ment during an oscillatory marine regression. The fine-grained sandstones developed in widespread tidal deltas and shallow-water deltas have become the tight sandstones by severe compaction and ce- mention, which may serve as the effective reser- voirs of deep basin gas. The source rocks of coal measures are wide in distribution, large in thick- ness and great in gas generation potential, and have maintained hydrocarbon expulsion up to now, provi- ding a sufficient gas source for the deep basin gas ac- cumulation. These source rocks of coal measures are closely intercalated with the tight sandstones in a layer-cake mode, which is favorable to forming the deep basin gas accumulation. There are three zones vertically in the gas accumulation in Xihu sag, i. e. the saturation zone of deep basin gas, the gas-water transition zone and the conventional hydrocarbon ac- cumulation zone, indicating the features of the deep basin gas accumulation.