东海西湖凹陷盖层条件及对油气藏的控制作用

Cap Rocks and Influence on Hydrocarbon Accumulations in Xihu Sag of East China Sea

盖层是西湖凹陷油气成藏的关键控制因素之一,其微观性质和区域分布特征的研究目前还比较薄弱。在统计了西湖凹陷24口钻井的泥岩厚度数据和30个泥岩样品微观物性分析数据的基础上,从宏观和微观尺度上对平湖组和花港组盖层性质及分布开展了研究。研究表明,西湖凹陷存在平湖组和花港组上段两套区域盖层,具有平面连续性好、单层泥岩厚度大、泥岩含量高等特征,其排替压力较高,微观封闭性好。研究发现,单个油气层的直接盖层厚度一般为1～5 m,说明较薄的泥岩也可以作为局部盖层有效封盖油气。断层对区域盖层的影响主要受控于地层中单层泥岩厚度和泥岩含量,当二者都较高时,断层起封堵作用,与区域盖层一起形成断圈油气藏,而当二者较低时,断层对区域盖层产生破坏,只能形成自圈油气藏。

The cap rock was one of key factors for hydrocarbon accumulations in Xihu Sag. The study on the microscopic characteristics and lateral distribution is not enough. The data of mudstone thickness from 24 wells and physical property data from 30 mud stone samples of Xihu sag were analyzed. On the basis of the analyzing results, study on Pinghu Fm and Huagang Fm cap rocks and their distribution have been conducted at macro-scale and micro-scale. The study results indicated that there are two sets of regional cap rocks in Xihu Sag, including Pinghu Fm cap rock and the upper Member of Huagang Fm cap rock. The cap rocks are character ized by good lateral continuity, large mudstone thickness of individual layer, high content of mudstones. The displacement pressure of the mudstone cap rock is high. In a word, the microscopic sealing properties of cap rocks in Xihu sag are very good. It is found that the direct cap rock of individual hydrocarbon reservoir ranges from 1 m to 5 m, indicating that thin mudstone layer could act as local cap rocks, sealling hydrocarbon effectually. The effect of fault to regional cap rocks was controlled by single mudstone thick ness and percent content of mudstone in formation. When single mudstone thickness and percent content of mudstone in tbmaation were high, the faults could seal hydrocarbons, forming fault trap reservoir with regional cap rocks. On the contrary, the fault would damage regional cap rocks that could only form self-trap reservoir.