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

The cap rock was one of key factors for hydrocarbon accumulations in Xihu Sag. The study on the microscopic character-istics 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 formation 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.