Application Of Magnetotelluric Sounding To Explorate Petroleum In The Marine Carbonate Distrbution Area Of Southern China

Southern marine carbonatite depositional region is a potential region which is rich in gas-petroleum. However, due to formidable natural environment and complicated geological structure, conventional prospecting method is hardly effective. High-resistance carbonatite layers impede the propagation of seismic wave. As a result, it is difficult to obtain a image of deep underground. At the same time, the complicated topography makes gravity prospecting impossible. Recently, magnetoelectric methods, with Magneto-telluric (MT) as the principle method, has been widely used. Although MT has a high penetrability, and is immune from high-resistance shield, sensitive to low-resistance layer, portable, high-effective, low-cost, adapted to rugged mountain area in southern China, this method plays an important role in the gas-petroleum prospecting in this region. However, the special geological condition of the south brings great difficulty to the acquisition of MT data and the discussion and explanation of it. Therefore, it is crucial to have a review of new technology and methods in order to improve the resolution of investigation and precision of explanation of MT in this region.Sequence stratigraphy is a successful technology in the aspect of gas-petroleum prospecting. In this paper, we take advantage of the geological idea of sequence stratigraphy, in conjugation with the electric character reflected by MT curves and regular patterns of various petrographic profile, to build geoelectric model based on MT result. With reference to this geoelectric model, we can distinguish platform facies, platform-margin facies, basin facies, individual platform facies, figure out the related method, step and rule of applying MT data. And, we point out that based on the joint investigation of geoelectric structure of various facies, and in conjugation of the theory of sequence statum, we can divide facies and trace the symbol strata among various facies and discover tectonic anomaly, its image and its approximate burial depth, so as to provide the objective area of further seismic prospecting.In high-resistance carbonatite-covered rugged southern mountains, MT is a robust substitute and assistant approach in prospecting gas-petroleum with the development of soft and hardware. But to use geoelectric character to divide facies, it is necessary to make clearthe character of regional sequence stratum and the properties of deposit. And, after a joint analysis from the unknown to the known, can we achieve a satisfied result.In this paper, under the guide of theory of MT and sequency stratigraphy, we systematically and strictly process MT data gathered in Nanpanjiang Region, which covers Yunnan, Guizhou and Guangxi provinces, and have a review of several parts as: Pre-processing of MT data, which includes pattern recognition of polarization, noise suppression, and static correction; Character analysis, which is composed of type of curve, total longitudinal conductivity, resistivity, and facies section; Qualitative analysis,which is composed of conventional inverse approach, joint modeling, and imaging technology. And, in conjugation with the data of nearby-bore sounding, we design a reasonable flow of material processing and explanation. And based on these jobs, we divide basin and platform facies in Nanpanjiang region for devonian, carboniferous and permian, triassic period through remodeling, labeling geoelectric layers and structures, inversing typical profiles, analyzing geoelectirc character and its varying rule, in combination with theory of sequence stratigraphy. Meanwhile, after tracing the variation of Paleozoic stratum, we determine and study the objective area for in-depth gas-petroleum prospecting, so as to further broaden the application aspect of MT and sequence stratigraphy. Yanba structure and Zhexiangjie structure is the fruit of our study. As a successful example, the error between the calculated burial depth and the real value of uppermost interface of Carboniferous-Permian and Devonian in Yangba structure is only 8.17% and 1.81%. It states that MT has quite effective in this area.