New data on the deep structure of the northern part of the Gulf of Tonkin in the South China Sea based on the results of magnetotelluric soundings

Abstract

We present a geoelectric model for the northern part of the Gulf of Tonkin in the South China Sea, which was developed for the first time based on the data of magnetotelluric studies carried out in 2012- 2013. The existence of a regionally developed aniso� tropic conducting fractured horizon and a steeply dip� ping electrically conductive fault zones is a distin� guishing property of this model, which has a layer� block structure. The nature of the zones of electric conductivity is related to the accumulation of a water� graphite mixture during longterm propagation of the flow of deep gases, which include H 2 and CH4. This allows us to consider the outcrops of the deep fluid sat� urated fractures as potential oil bearing regions. In the conditions when traditional oil and gas deposits are being exhausted, the search for new objects of accumulation of hydrocarbons has become one of the present pressing problems. In recent years, research into fissured formations in the upper level of the basement, which is a transition complex between the crystal basement and the sedimentary cover, has become widely developed (1). The rocks related to the transition basement are almost not changed by the processes of regional metamorphism. They are dislo� cated more greatly than the overlying columns of the sedimentary cover, transformed by recrystallization processes and catagenesis, and have a different struc� tural position. The oilandgas-bearing properties of such complexes have been established in different regions of the world: East Siberia (2), Venezuela, US, Libya, Egypt, and India (3). In 1988, a unique deposit was discovered in the granites of the Mesozoic base� ment on the shelf of South Vietnam (4). Deep geophysical processes that give us the possi� bility to make a primary estimate of the structural and material characteristics of the basement in the study regions play an important role in the study of these structures. The estimate includes the existence of fis� sured complexes and locations of permeable zones in the lithosphere that provide pathways for migration of deep fluid fluxes, which are sources of saturations of the fissured complexes with hydrocarbons. All this allows us to forecast the oilandgas-bearing potential of the study region. One of the geophysical methods used for the solution of these problems is magnetotel� luric sounding, which makes it possible to study the differentiation of electric resistivity of the tectono� sphere deep section depending on its structural mate� rial composition.