Analysis of two-dimensional magnetotelluric inversion for the delineation of the petroleum system in the Kutai basin, East Kalimantan, Indonesia

The Kutai Basin in East Kalimantan is known as a potential area for oil and gas exploration. Two-dimensional magnetotelluric (MT) analysis is applied to investigate the geological structure and distribution of subsurface resistivity. This study aims to delineate the petroleum system using MT data and identify zones with potential for the accumulation of hydrocarbons. MT data has been collected at several strategic locations in the Kutai Basin, and two-dimensional cross-sections have been constructed to obtain vertical resistivity imaging at several depths. In this study, there were nine measurement points located on one line. The data is then inverted to obtain a two-dimensional resistivity model, which qualitatively represents the subsurface structure. The results of this study indicate that there is a low resistivity anomaly zone that identifies the presence of source rock with a resistivity value of 1-12 Ωm. In this line, it is suspected that the petroleum system that allows trapped hydrocarbons is found in the area, below the KT36 and KT13 measurement points. In this area there are folded structures in the form of synclines and anticlines, which raises suspicions about the types of traps formed from structural traps.


Introduction
One of the key components to attaining sustainable development is energy [1].The main energy sources in Indonesia are oil and gas [2].The Ministry of Energy and Mineral Resources (Kementerian Energi dan Sumber Daya Mineral-ESDM) stated that oil and gas in Indonesia reserves can be used only for another 9.5 years, besides that natural gas reserves are only enough for another 19.9 years, by assuming there are no new discoveries related to these supplies [3].As a result, to increase oil and gas production, the government needs new sources of oil and gas [4].The Kutai Basin geological area is one of the basins that has the largest petroleum production potential in Indonesia, with an estimated petroleum production potential of 80.4 Trillion cubic feet (Tcf) [5], [6].
Geophysical methods are used to carry out exploration activities.According to Philip Kearey [7] geophysical methods are classified into four main methods, namely seismic, gravity, magnetic, and electrical methods.In the electrical methods, there are resistivity, polarization induction, self-potential, electromagnetic (EM), and radar methods [7].The EM methods is one of the geophysical methods that is often used for various purposes [8].
Magnetotelluric (MT) is used to assert the potential of the petroleum.MT is the passive EM method that utilize fluctuation of electric and magnetic filed perpendicular to the earth's surface.This method is useful for identifying the conductivity of rocks in the subsurface, in the range from a few meters to kilometers [9], [10].One of the advantages of MT compared to the other geophysical methods, it reaches the significant penetration ability and minimal ambiguity [11], [12].
The MT method is one of various the geophysical methods that can be applied to delineate subsurface structures [13].Delineation is a geologycal methods to identify the geological structures [14].By analyzing and interpreting the MT data, then correlating the results with elements of the petroleum system, it will has a more targeted methodological analisys [15].
Based on this background, it is necessary to know how resistivity anomalies in the subsurface indicate the existence of source rock from the results of 1-dimensional and two-dimensional inversions.In addition, it is necessary to know how hydrocarbon traps in the Kutai Basin through two-dimensional MT cross-sectional analysis.

Regional geology
The research was conducted in Kutai Basin, East Kalimantan as shown in Figure 1.In Indonesia, this basin is the biggest and deepest tertiary basin.In terms of geological structure, in Kutai Basin there are anticline and syncline-shaped folds with northwest (NE) -southwest (SW) oriented axes.The anticlinorium folds are strong, asymmetrical, and have syncline boundaries.This syncline contains Miocene siliciclastic sediments [16].1.In the study area, rock layers from older to younger consist of the Pamaluan, Bebuluh, Palaubalang, Balikpapan, Kampungbaru, and Alluvium Formations.The Pamaluan Formation from the Oligocene to the Early Miocene consists of quartz sandstone with layers of either claystone, black shale, limestone, and siltstone.The Bebuluh Formation is of early Miocene age.It consists of coral rocks that have layers of rough rock and black fragments on top.Greywacke and quartz copper meet at the cross of layers of copper, brick, coal, and tuff in a square formation spanning the medium to late Miocene.Sandstone and claystone meet at the junction of siltstone, ruins, copper, and coal seams in the Balikpapan formation, which ran from the center to the Miocene end tip.Plyocene formations, which date back to the Pliocene era, consist of layers of copper, detritus, tobacco, and lignite mixed with copper quartz.In the environment of rivers, delta dungeons, and beaches, the sunken grass, sand, and mud together form alluvium.The high total organic carbon value and thermal maturity of the black shales of the Pemaluan Formation are thought to make it a source rock for the Kutai Basin petroleum system [17].

Petroleum system
A Petroleum system is a system of working from the formation of hydrocarbons to migrating or being trapped in a rock trap [18].The concept of petroleum system includes a variety of different elements.The existence of key geological factors (source rocks, carrier rocks, traps, a nd overburdens) and petroleum formation processes (trap formation, oil formation, displacement, and accumulation).The oil system involves all naturally connected aspects related to petroleum and indicates the presence of oil, leakage, or accumulation from one active source of rock [19].

Magnetotelluric method
A commonly used geophysical method in exploration is the EM method, which is often used to identify objects by their resistivity properties.The EM method utilizes electromagnetic waves penetrated into the layers of the earth [20].Changes in magnetic fields caused by variations in resistivity are used to identify structures in the subsurface [21].
The MT method is one of several the geophysical exploration methods that uses a natural source electromagnetic (EM) field [22], [23].The natural source of about 1 Hz is thunderstorms, from which lightning emits terrain that propagates over long distances.At frequencies below 1 Hz, most signals are caused by currents in the magnetosphere that are from solar activity.This situation causes the natural EM wave to explore the Earth's subsurface structure with a depth range of tens or thousands of meters [24].
The MT method uses several formulas, including Maxwell's equations.Maxwell's equations are the result of experimental synthesis of electrical-magnetic phenomena by scientists such as Faraday, Ampere, Gauss, and Coulomb, as well as research conducted by Maxwell himself [25].Maxwell's equation for the frequency domain can be described in equations 1-4: where q is the electric charge (Coloumb/meter 3 ), B is the magnetic induction density (Webber/meter 2 ), E is the electric field intensity (Volt/meter), D is the electric flux density (Coloumb/meter 2 ),  is the conduction current density (Amperes/meter 2 ), and H is the magnetic field intensity (Amperes/meter).Bold notation represents the vector in the three-dimensional roar  =  ⃑ = (  ,   ,   ), meaning that components B, E, D, J, and H have components x, y, and z.
In an isotropically homogeneous medium, the relationship of magnetic field intensity to the flux occurring in that medium can be represented by the linear constitution equation:  (7) where  0 is magnetic permeability,  is dielectric permittivity, and  is conductivity [26].So equations 3 and 4 become: From equations 8 and 9, the curl operation is performed to produce the equation, Equations 10 and 11 are also called the Helmholtz equations.Taking into account the general conditions found in geophysical exploration (with a frequency of less than 104 Hz, in the earth medium), terms containing the electrical displacement parameter (ε) can be considered insignificant compared to terms containing electrical conductivity (σ) due to the price με≪μσ [27].In addition, it can be assumed that the EM field depends on time.Basically, the variables E and H are functions of position and time with  is angular frequency,  is frequency, and  is period.So the equation is obtained,  =  0  (−) (12)  =  0  (−) (13) Based on the definition of the function, equations 10 and 11 would be, Thus, obtained the equation of diffusion of electric field E and magnetic field H ∇ 2  =  2 and ∇ 2  =  2  = √ 0  (16) where  is the wavenumber.
Skin depth is the attenuation distance of EM waves in the homogeneous medium [28].The amount of skin depth can be determined based on the real component present in the wavenumber (), which is expressed as where δ is the skin depth (meters), ρ is the resistivity of the medium (Ωm), and  is the period (seconds).Impedance (Z) is the tensor connecting an electric field with a magnetic field [29], expressed as  =  (18) So that resistivity and phase impedance can be connected with impedance, namely In the situation of actual conditions below the surface of the earth which is an inhomogeneous medium, the resistivity in equation 20 will experience variations.into pseudo-resistivity.

1-D and 2-D inversion
In the 1-dimensional medium, there are EM field components in the horizontal direction (Ex, Ey, and Hy, Hx) that vary only with depth (not varying with x-axis and y-axis) and there are no vertical EM field components [30].Based on the orthogonal nature of induction between the electric field and the magnetic field and the same properties in Ex and Ey as well as in Hx and Hy, the discussion of EM components in this medium is one of them in Ex and Hy [31].
In a two-dimensional medium, there are horizontal components of electromagnetic waves (Ex, Ey, and Hx, Hy) varying with lateral conditions and also with depth, and there is no vertical EM component.The polarization orientation of electromagnetic waves can be illustrated in this medium.Such polarization is given in the form of a simple model of two vertical layers with different conductivity, and it is assumed that the conductivity value of each layer is fixed along the strike direction parallel to the x-axis [24].
Polarization is divided into two types, namely Transverse Electric (TE Mode) an d Transverse Magnetic (TM Mode).When current flows perpendicular to the direction of the strike, it is described by TM Mode, whereas TE Mode describes current flowing parallel to the strike (x-axis) [32].
In the process of inversion can be formulated in the form, where   is the measured response of the data, and () is the model parameter, which is a function related to the desired physical parameter, and   is the kernel matrix.Kernel data is data that describes the correlation between the measured data and the model parameters [33].

Data acquisition
This study used Kutai Basin MT data consisting of nine state-owned measurement points.In 2017, a magnetotelluric survey was carried out in the Kutai Basin by a team from the Geological Survey Center.The measurement points are spread over an area of 50 x 35 km 2 , with distances between points ranging from 3-5 km.MT measurements were carried out using MTU5A Phoenix geosystem equipment equipped with MTC50 magnetic sensors that have a frequency range from 320 Hz to 0.00034 Hz.

Data analysis
MT data processing to identify the presence of petroleum systems can be illustrated in the flow diagram of figure 2 using SSMT2000, MTEditor, and WinGlink software.After acquisition, the data in header format contains field data such as measu rement time, coordinates, and other variables, along with time series data from low to high frequencies.Time series data represent fluctuations in magnetic fields and electric fields in the time domain, which need to be converted to frequency domains through the Fourier transform.This process involves determining the parameters of the Fourier transform, including input data type, output data format, frequency band, and processing time.In this study, field data is used, so the type of input data used is measured field.Subsequent processing uses the default settings provided by the application.
Data that has been converted to the frequency domain still contains noise.In order to reduce or filter noise, robust processing is needed.There are 3 types of rob ust processing: robust no weight, rho variance, and ordinary coherency.After applying the three robust methods, the next step is to calculate the coherence value of each robust processing result.From the coherence value, the robust processing upgrade method with the highest coherence value will be selected.Next, the coherence value of the robust processing upgrade results will be recalculated.If the coherence value is ≥ 75%, proceed to the next stage.However, if the coherence value is ≤ 75%, then the combine method will be performed.After the robust processing process is complete, apparent resistivity and phase curves will be obtained that have not undergone crosspower selection for the data.
At the crosspower selection stage, data that deviates from the trend curve and is considered noise will be filtered.This filtering process is done using Mteditor software to increase the coherence number.After that, apparent resistivity and phase curves will be obtained again after the data filtering process (muting), resulting in data with a higher coherence value.
For the inversion stage, WinGlink software is used.The first stage is to create an elevation map and plot the MT measurement point.Then, the D+ smoothing curve is used with a 10% rho error tolerance and a 10% phase, and the visible resistivity curve is adjusted to the phase impedance of the sounding point.The next step is to make a resistivity map at a specified depth using WinGlink software to determine the distribution of petroleum.

Sounding
This section displays the sounding curve in depth as a result of 1-dimensional inversion using the Occam algorithm.This 1-dimensional sounding curve is divided into three modes, namely TE mode, TM mode, and invariant mode.In this study, invariant mode was used because it was more comprehensive in representing the conductivity of the earth [34].

X-section
Inversion consists of multiple executions of a simple task, with new parameters introduced in subsequent iterations in interpretative models.[35].This stage displays the results of the 1-dimensional inversion that has been performed at all measurement stations.Several inversion methods used to solve EM method problems include Occam, Particle Swarm Optimization (PSO), Very Fast Simulated Annealing (VFSA), and Nonlinear Conjugate Gardien (NLCG) [33] [36], [37], [38], [39].In the two-dimensional inversion process, the method applied in this study is NLGC.NLGC is able to reduce the objective functions present in residual data and a special second derivative of resistivity [39].NLCG methods are able to directly reduce non-quadratic problems, freeing up the process of linear iteration and inversion.The model used is a combination or invariant mode of TE and TM.Optimizing the TAU value and iteration is the best way to achieve optimal inversion results.In order to find the lowest error Root Mean Square (RMS) value, 30 iterations and a TAU value of 3 are intended to be employed.The results of data processing get the smallest RMS of 1.2%.Furthermore, the interpretation is adjusted to the geological state of the study area and to the set goals.

Result
Overall, the number of MT data measurement points is 9, which becomes 1 measurement pass with a west-east line direction and a track length of 50x15 km.The nine measurement points are located at stations KT25, KT26, KT36, KT35, KT14, KT13, KT39, KT08, and KT07.
Figures 3 and 4 show the inversion results of the MT method.The result of this resistivity crosssectional interpretation is based on the range of resistivity values possessed by each layer.Each type of layer will be identified based on the type of rock that makes it up.Thus, from the determination of this type of rock, information can be obtained about geological groups and their role in the petroleum system.The distribution of resistivity values on this line is divided into 3 groups based on the resistivity value scale and color contrast visible from the cross section.Areas with low resistivity have a value range of 1-12 Ωm indicated in purple-blue.Areas with medium resistivity have a value range of 13-119 Ωm and are depicted in green-yellow.Meanwhile, areas with high resistivity have a value range of 120-512 Ωm and are marked in an orange-red color.
The resistivity value in one-dimensional inversion only changes vertically or in depth.The various depths of every measurement site are displayed in Figure 3. Low resistivity values, Low resistivity values or often known as conductive zones were detected at a depth of 1 km to 5 km at the KT25 station.Then low resistivity is found at depths of 1 km to 2 km at the KT26 station.Furthermore, low resistivity is found at depths of 3 km to 7 km at KT26 stations and at depths of 2 km to 14 km at KT07 stations.Moderate resistivity levels were also detected at almost all stations.
The results of the two-dimensional inversion show that there is a distribution of low-type resistivity anomalies, namely 1-12 Ωm along the A-B line.The presence of low type resistivity anomalies is not the only sign of subsurface shale gas host rocks.However, zones with low-type resistivity anomaly values can at least indicate regions that have the potential for organic shale accumulation.This is supported by previous studies that have proven that organic shale has a low-type accuracy anomaly value, namely by Brach et al., 2007 [40] and Novelyarisyanti &; Iskandar, 2019 [41].
Based on the research of Hidayat et al., the structure that developed in the Kutai Basin is a faulty anticlinorium [42].One of the migration paths is through these faults.Figure 4 shows that there is a fault between the measurement points KT26 and KT36, points KT13 and KT39.In this line, allegations of petroleum systems that allow hydrocarbons to be trapped are found in areas below the KT36 and KT13 measurement points.In the area, there are folding structures in the form of synclines and anticlines, which raises suspicion about the type of trap formed by the trap structure.
Based on the results of studies that have been conducted, in the research area there is a high potential for organic material traps and the potential to increase the complexity of carbon bonds in shale rocks.These criteria have been sufficiently met in the formation of shames in the Kutai Basin [43].Anticlinorium patterns have also been successfully confirmed to map anomalous resistivity zones in the study area [42].

Conclusion
Based on the research that has been done, the results of 1D and 2D inversions in the A-B line contain low-type resistivity anomalies, representing the presence of source rock with a resistivity value range of 1-12 Ωm below the measurement point between KT25 and KT26, KT14 and KT08.Based o n the results of 2D inversion, the type of petroleum trap in the research area is suspected to be a structure trap in the form of syncline and anticline.In this line, the petroleum system trap is thought to be below the KT36 and KT13 measurement points, with a depth range of 4000-6000 m.

Figure 1 .
Figure 1.Geological map of the Kutai Basin, East Kalimantan