Analysis Of The Main Controlling Factors Of Deep Natural Gas Accumulation In Qibei Slope Area Of Qikou Sag

Qibei slope is located in the south central of Qikou sag,Huanghua depression,with a north-east plunging and an areal extent about 1000km².It is one of the main blocks of oil and gas exploration in Qikou sag.With the extension of exploration to the deeper formation in recent years,the deep natural gas resources in Qibei slope has been becoming an important exploration target.A comprehensive study of the deep gas of Qibei slope is carried out in this thesis based on the theoretical system of petroleum geology,sedimentary petrology,organic geochemistry and structural geology.The geochemical characteristics and genetic types,source rocks and hydrocarbon generation conditions are firstly discussed.Then the reservoir conditions,seal of caprock,trap conditions and reservoir forming stages,and temperature and pressure environment of the study area are presented.Finally,the forming mechanism of the deep gas reservoir in Qibei slope is analyzed and the exploration potential is discussed.According to the research results,the high-quality gas source rocks in the Qibei slope area are widely distributed with large thickness.The source rocks are high in abundance and include mainly?₂-?mixed type of organic matter.The natural gas is typically humic and mature.The reservoir is mainly composed of lithic arkose,and the maturity of composition and structure are relatively low.The diagenesis of the reservoir mainly includes mechanical compaction,cementation,dissolution and metasomatic.Three secondary pore zones are developed in the reservoir,which is characterized with dual-porosity.Three types of reservoir-cap associations can be revealed.The strong micro sealing capacity of the cap rock is related with sealing mechanism of capillary pressure and hydrocarbon concentration.The reservoir-cap associations are characterized by self-generation and self-accumulation.The structural and lithologic traps are well developed.The analysis of inclusion and forming period of reservoir shows that the deep gas is characterized by multi-stage filling and late-stage accumulation.The temperature and pressure play an obvious role in the control of hydrocarbon generation,reservoir physical properties and natural gas preservation,which results in the extending of effective gas well section and the increasing of the depth of gas-generating threshold.The increasing of hydrocarbon generation depth results in the formation of deep organic acids,and increases the depth of effective reservoir,which expands the effective exploration depth of natural gas.