Petrophysical Research Of Cap Rock Over The Gas Chimney In South China Sea

With the rapid development of China’s economy,the demand for energy is increasing and the development of new energy sources is becoming more and more important. Gas hydrates which is a new energy has an important significance. China has also actively carried out research work on gas hydrates, and has made some progress in research in the South China Sea.The deep crust fluids pierce the upper portion and migrate to the shallow under ultra high pressure,then format a diapiric structure which is called Gas chimney.The shape of Gas chimney on the seismic section looks like chimney and the Gas chimney has a chimney effect.That’s why we call it Gas chimney. Gas chimney is hydrocarbon migration channel and its associated structures is a good place for hydrocarbon accumulation,so we should do research on it.In Southeast Hainan Basin,a large number of the Gas chimney have been found.On the seismic section,the strong reflections just appear upper the Gas chimney and block the Gas chimney. The strong reflection is called cap rock.In order to figure out the formation of the cap rock in the rock physics research,we use AVO technology.The AVO technology is a new seismic exploration methods which use the relationship of seismic reflection amplitudes and offsets.The AVO technology is the indispensable tool in research of lithology.In this study,we start with the construction of the theoretical model by using the effective medium model.Then we calculate compressional wave and shear wave velocity and density of the area of gas chimney and the area of cap rock.Then we make a layered model by using the model of gas chimney and the model of cap rock.The layered model is just similar to the formation of the actual formation.Then we use Zoeppritz equation to compute reflection coefficients of the layered model and extract the actual reflection coefficients from actual seismic data.We change the pore filler cap and its content in the model of cap rock in order to make the reflection coefficients of the layered model matches the actual reflection coefficients from actual seismic data.Then we could figure out petrophysical composition of the cap rock which is upper the gas chimney.