PHAN THÞ KIM V¡N

Works of several scientists whose papers appeared in the 1990s has brought about an increasing interest in the application of magnetic methods to oil and gas exploration. It was suggested by Goldhaber Saunders and Andrew Reynolds and others that the sedimentary rocks in oil-bearing basins may contain epigenetic magnetic minerals [5]. These authors assume that hydrocarbon contained in the deposit may escape along fault discontinuities and, by causing alteration or by supporting biogenetic processes, produces magnetic minerals in the overlying rocks. An indication of their presence can be obtained by magnetic methods, which yield useful information in the search for hydrocarbon.

Because numerous recent papers propose that the presence of sedimentary magnetic minerals can be an indicator of hydrocarbon traps, this urges us to carry out magnetic data processing for purpose of hydrocarbon exploration in the Hµ Néi Basin.

The basic problem in the application of magnetic method for the purpose of hydrocarbon exploration to directly or semi-directly determine the oil and gas field is based on the determination of diagenetic magnetite, caused by hydrocarbon seepage.

In the recent years, there are some theories concerning the formation of this diagenetic magnetite. According to D.F. Saunder and S.A. Terry, there are three ways for hematite in the sediments overlying petroleum accumulations converted to magnetite, as follows [3]:

1. Chemical reduction due to the formation of hydrogen sulfide by sulfate-reducing bacteria in the presence of hydrocarbon gases. Hydrogen sulfide can be generated in shallow basins by alteration that results from the participation of anaerobic bacteria in the conditions of meteoric water descent. The bacteria select their food from the hydrocarbon and deliver oxygen by the reduction of sulfate ions in the invading water.

2. Combination of reduced iron in solution with hematite and water to form magnetite.

3. Upward migration of ferrous ions produced at some depth into an oxidizing zone. Slow oxidation may directly produce magnetite.

About this problem, the viewpoint of Russian scientists has a little difference. They connect the appearance of magnetite with the reorganization of siderite. They have also a different point of view on the role of microorganism in the formation of magnetite. They feel the main role played by the microorganisms in the oxidation of hydrocarbons, which leads to the formation of the simplest organic acids, carbonic acids, etc, which, in turn, chemically react with minerals in surrounding rocks.

The presence of the magnetic bodies over oil and gas accumulations has been established in many producing areas. Some of these bodies are shallow (less than 350 m) some are rather deep (greater than 1000 m) [3]. Near-surface magnetic anomalies often are seen as associated with oil-producing regions [1]. The research results have been showing that both abiologic and biologic mechanisms could produce magnetic sulphide minerals (pyrrhotite, greigite, etc.) in some zones of hydrocarbon seepage.

Alternative hydrocarbons can reduce magnetization by replacing detrital magnetic minerals by nonmagnetic sulphide minerals. Thus, a direct connection has been established between the presence of hydrocarbon and the nature of near-surface magnetic anomalies in sedimentary basin.

The conversion of nonmagnetic hematite to magnetite creates anomalous "ripples" on the aeromagnetic total field record, which can be readily identified in the data processing [3].

Magnetic minerals (magnetite) in oil and gas deposits may indicate directly or semi-directly the presence of oil and gas deposits, therefore magnetic methods are applied to hydrocarbon exploration in oil-bearing sedimentary basins.

The basic problem in the application of magnetic methods is the isolation of weak magnetic anomalies caused by low concentrations of the magnetic minerals. These weak anomalies are often masked by much stronger magnetic anomalies caused by underlying magnetic rocks and/or by rocks in the sedimentary basin. The weak anomalies can efficiently isolated by applying selective bandpass filters.

To solve this target, we suggest a procedure of processing including two steps as follows:

The first step is to isolate the weak magnetic anomalies caused by low concentrations of magnetic minerals. In this step, selective bandpass filters can be applied to isolate the weak magnetic anomalies, which are often masked by much stronger magnetic anomalies caused by underlying magnetic rocks and/or by rocks in the basin. In this work, we use the program FFTFIL of Hildenbrand [4], which performs various two-dimensional filtering operations on standard grids using fast Fourier transforms. The results obtained by this method named the filtered field. It has been checked by data obtained through the use of simple models of magnetic anomaly sources.

The next step is to determine the spatial positions of magnetic sources, such as the horizontal coordinates of edges of magnetic sourced, the source depth. In this final step, the Boundary method of Blakely and Simpson has been used for localizing the horizontal coordinates of the edges of magnetic sources; the Werner deconvolution method of Hartman, the signal analysis method of Nabighian, the autocorrect method of Phillips and the Euler deconvolution method of Thompson have been used for estimating the source depth of magnetic bodies [2].

Next, we will present the results obtained by above methods in the theoretic model and practical data from Hµ Néi Basin.

To investigate the possibility of isolating weak anomalies by bandpass filtering, a simplified three-dimensional model of magnetic sourced was designed. This model include 7 prisms with parameters in the Table 1 and the parameters of Earth's magnetic field: T_o = 48000 nT, I_o = 20^o, D_o = 0^o. The first body with its size and magnetic susceptibility symbolizes the region source; and all of the rest prisms of this model symbolize the local caused involving the oil and gas deposits.

By applying various two-dimensional bandpass filtering operations, the weak

anomalies were separated, that is the filtered field. The Fig. 1a presents the observation field of magnetic anomalies of this model; the Fig. 1b presents the filtered field involving the shallow bodies.

The research results have been proving the ability of applying the bandpass filter method to isolate the weak local anomalies from the observation field.

The Fig. 1c presents the results of determining the horizontal coordinates of the edges of the local bodies by the Boundary method of Blakely and Simpson.

The Fig. 2 presents the results of estimating the depth of local caused by applying the Werner deconvolution method of Hartman, the signal analysis method of Nabighian, the autocorrect method of Phillips and the Euler deconvolution method of Thompson for the case of observation field and filtered field. The results obtaining from the theoretic model have been showing that the source depth in the case of the filtered field is more convergent and more exact than the ones in the case of observed field.

The obtained results from the theoretic models have been proving the ability and efficiency of the applied procedure of these methods. They are the basis for carrying out their application to the factual data from the Hµ Néi Basin, where specialists are interested in their shallow gas potential.

Table 1. Parameters of the magnetic sources in the theoretic model

The obtained results have been showing the situation of three small structures distributed in the southwest, the center and the north of the studied area (Fig. 3c).

To estimate the source depths of those structures, the above depth determining methods have been used at the profiles crossing them. The profiles T2283, T2285 and T2287 cross the structure lying in the southwest of the study area; the profiles T2293, T2295 and T2297 cross the structure lying at the center of the study area; and the profiles T2325, T2327 cross the structures lying in the north of the study area.

The Fig. 4 presents the results of estimation of the source depths at the profile T 2295 for the case of the observed field and filtered field by depth determining methods: Werner deconvolution method, the analytic signal method, the autocorrection method and the Euler deconvolution method.

 At the profiles T2293, T2295, T2297, T2299, T2301, the results of determining the horizontal coordinates of this magnetic source have been showing the structure position lying in the distance between of the landmarks 670 and 685 with the source depths ranging from 300 m to deeper 1000 m.

At the profiles T2325, T2327 in the north of the study area, these results are: in the distance between of the landmarks 660 and 675, with their depths from 500 to 1400m.

At the profiles T2285, T2287, T2289 in the southwest of the study area the results are: in the distance between of the landmarks 640 and 660, with their depths from 300 to 900m. The obtained results correspond to informations from other methods.

The results obtained from the theoretic model and the factual materials from the Hµ Néi Basin have been proving the possibility of using magnetic methods for hydrocarbon exploration in ViÖt Nam.

 III. CONCLUSIONS

Beside the use the magnetic methods in the investigation on major fault zones, the crystalline basement or in the determination of the basement structures controlling the emplacement of hydrocarbon in overlying sedimentary basins, in recent years, the new advances have been showing that, under favourable conditions and especially, in combination with other geophysical and geochemical methods, the magnetic

techniques can play a bigger role in the localization of oil and gas fields. The results obtained in this work form the first step of the procedure of using magnetic methods in the hydrocarbon exploration in ViÖt Nam. However, in order to enhance the efficiency of these methods it is necessary to use them in combination with other methods, such as seismic and geochemical methods in the exploration process.

1. Carol A. R. and R. J. Blakely, 1991-1994. Crustal magnetic anomalies. U.S. Nat. Report to IUGG,

2. Cordell L., J. D. Phillips, and R. H. Godson, 1992. U.S. Geological Survey Potential-Field geophysical software. Version 2.0. Open File Report 92-18.

3. Eventov L., 1997. Applications of magnes of magnetic methods in oil and gas exploration. The leading edge.

4. Hildenbrand T.G., 1983. FFTFIL: A filtering program based on two dimensional Fourier analysis: U.S. Geol. Surv. Open-File Report 83-237, 31 p.

5. Milenko B. and Milinko G., 1983. Magnetic data processing for the purpose of hydrocarbon exploration in the Pannonia Basin, Yougoslavia. J. of Petrol. Techn. 35/12: 110-118.