南海高压气田泥岩盖层封闭机制、封气能力及演化过程——以莺歌海盆地X13高压气田为例

近期在南海莺歌海盆地勘探发现了高压大气田和一批含气圈闭,随着勘探的深入,高压领域(高压区、高压层系)天然气规模成藏和气水分布的复杂性日益凸显。利用钻井、地震和分析测试资料,对莺歌海盆地X13高压气田上中新统黄流组一段泥岩盖层的封闭机制、封气能力进行了研究,认为泥岩盖层存在物性封闭和泥岩滞排超压封闭两种封闭机制,并建立了相应的封气能力评价方法。X13高压气田泥岩盖层封气能力形成时间约在6.1Ma;3.6Ma时,X13高压气田构造-岩性圈闭形成,这也是天然气成藏的早期,储层、泥岩盖层为常压,泥岩盖层以物性封闭为主,盖层的排替压力大于储层剩余压力,因而具备了封气能力;1.8~0Ma,是天然气晚期成藏时期,储层、盖层已发育超压,泥岩盖层既存在物性封闭,也存在泥岩滞排超压封闭,泥岩盖层的排替压力与滞排超压之和大于超压储层的剩余压力,其中滞排超压发挥了主要的封气作用。较之天然气成藏早期,成藏晚期的盖层泥岩具备更好的封气能力。提出了"中部滞排欠压实-底部畅排压实型盖层—大型储集体组合"的圈闭(带)是高压领域天然气成藏富集区,这一认识对南海高压领域天然气勘探具有指导意义。

Sealing Mechanism and Sealing Capacity of Mudstone Caprock in the Overpressured Gas Field:A Case Study of X13 Gas Field,Yinggehai Basin,South China Sea

One overpressured gas field and several gas -bearing traps were discovered recently in Yinggehai Basin, South China Sea, and the complexities of gas accumulation and distribution in overpressured domains(high pressure area and high pressure layer) is becoming more and more prominent. Based on the drilling, seismic, and test data, it is concluded that there are two kinds of sealing mechanism of mudstone caprock in the first member of the Upper Miocene Huangliu Fm. in X13 overpressured gas field, i.e., capillary sealing and overpressured sealing. The corresponding evaluation method of sealing capacity is setup, by which if the sum of caprock displacement pressure and overpressure exceeded the reservoir residual pressure, the mudstone had good sealing capacity. Combined with the burial history of the study area, the evolution process of sealing capacity of the mudstone caprock in the Huangliu Fm. is analyzed. The mudstone caprock began to have sealing capability at 6.1Ma. At 3.6Ma, the early charge period of natural gas, the mudstone caprock had capillary sealing capability with normal pressure. During 1.8~0Ma, the late charge period of natural gas, the mudstone caprock had both capillary sealing capability and overpressured sealing capability, in which the overpressure played a major role. The mudstone caprock had better sealing capability in the late charge period than in the early period. It is suggested that the traps(or plays)developed with an assemblage of such as overpressured layers in the middle of the mudstone, and compacted layers in the bottom of the mudstone, and some large-scale reservoirs, would be the favorable targets in overpressured domains.