Basement Structure of Phukhanh Basin, Vietnam from 3D Gravity Inversion Method Supplymened by...

May 10, 2010 11:58 AOGS - SE 9in x 6in b951-v20-ch17

Advances in GeosciencesVol. 20: Solid Earth (2008)Ed. Kenji Satakec© World Scientific Publishing Company

BASEMENT STRUCTURE OF PHUKHANH BASIN,VIETNAM FROM 3D GRAVITY INVERSION METHOD

SUPPLYMENED BY MAGNETIC DATA

NGUYEN NHU TRUNG

Institute of Marine Geology and Geophysics, VAST18 Hoangquocviet–Caugiay–Hanoinguyen [email protected]

NGUYEN THI THU HUONG

Institute of Marine Geology and Geophysics, VAST18 Hoangquocviet–Caugiay–Hanoi

The Phukhanh basin, a continental margin basin, has been investigated poorly.Most understandings about its basement structure based on seismic datacollected from longitude 110◦E to onshore, so the scale of basin is not clearand under debated. In this paper, the authors present a 3D interpretationprocessing of satellite gravity magnetic data for determining the basementstructure of the basin. The interpretation processing combines the methods ofpower density spectrum (PDS), 3D direct inversion and maximum horizontalgradient (MHG) for estimating the mean depth of the basement, the basementtopography and fault systems, respectively. A structural basement map isconstructed from the interpretation results. The map allows us to define thescale of the Phukhanh basin distributed over 9100 km2 from latitude 11◦–15◦Nand longitude 109◦20′–112◦E. It consists of nine main structural elements:Danang shelf, Phanrang shelf, north trough, south trough, central swell, northswell, northeast swell, east swell and Tuyhoa shear zone. The deepest basementis 9.5 km in the northern trough and 8.5 km in the southern trough. It consistsof N-S, NE-SW and NW-SE fault systems, in which, N-S and NE-SW faultsystems are the main fault systems in the basin.

1. Introduction

Phukhanh basin is a continental margin basin which most basin area lies bythe deep water from 400 to 3000 meter (Fig. 1). Although, the Phukhanhbasin nears the coastline, but the investigation scale is very poor. Mostseismic exploration activities were carried out in the shallow water fromthe longitude 110◦E to the coastline for petroleum exploration only. It

257


258 N. N. Trung and N. T. Thu Huong

Fig. 1. Location of the Phukhanh basin. Satellite free air anomaly11 and its majorstructural elements.

has not yet any exploration drill carried out in this area. So, most ofgeological information of the Phukhanh basin are quite limited, only fromthe longitude 110◦E to the coastline then the estimated potential has stillhigh uncertainty. The recent exploration results in the western area of thePhukhanh basin are optimistically appreciated the oil and gas potential(account for 10% of total petroleum potential of Vietnam).4,5,8,14 In which,the weathered and fractured basement and near basement basalt plays areconsidered as the most important reservoirs in this area. In addition, thePhukhanh basin is also considered as high gas hydrate potential. AlthoughPhukhanh is considered as a high petroleum potential basin, but how scaleof the basin and basement structure have been still open questions and notyet answered adequately. The available exploration data of the Phukhanhbasin show that apart from satellite gravity, there is not any geophysicaldata measured regularly 1′× 1′ grid all over the basin area. This limitation


Basement Structure of Phukhanh Basin 259

is main obstacle to investigate and evaluate totally the petroleum potentialof the Phukhanh basin.

From the above reality, the authors present a new calculated result ofdetermining the basement structure, sediment thickness and fault systemsof the Phukhanh basin by 3D interpretation methods of satellite gravitysupplemented by magnetic data. The calculated results gives independentand objective information of basement structure and reliable basis for thenext exploration step in larger scale.

2. Geological Setting

The Phukhanh basin is classified as a passive extensional margin basin.Its evolution relates closely to the collision of Indian subcontinent andEurasian plate and the opening of the East Sea.5 The tectonic evolutionof the Phukhanh basin is looked at as similar the same as the evolutioncharacteristics of other Tertiary basins in the region. It may consist ofthree main tectonic stages as follows: pre-rift even took place from lateCretaceous to Eocene, synrift even from late Eocene to Oligocene andthe regional subsidence even after rift took place during early Miocene tolate middle Miocene. The recent study results may divide the Phukhanhbasin into the following main tectonic elements (Fig. 1): Danang Shelflocated in the northwest; Phanrang Shelf located in the southwest; TritonHorst located in the east of Quangnghai graben; Phukhanh Deep Troughlocated in the deep water at the center of the basin. The sediment inthis area may be thickest; the Tuyhoa Shear Zone cuts off the south-west part of the Phukhanh Basin and appear as an area with highamplitude faults; and NE-SW Danang fault zone located in the northof the basin. According to seismic interpretation, the Tertiary sedimentthickness changes from 500m along the west margin to 7000–8000min the Phukhanh Deep Trough. According to the seismic and geologyinvestigations in the Phukhanh basin and adjacent, the Pre-Cenozoicbasement consists mainly of magmatic/metamorphic formation havingdifferent ages and compositions. In which, Cretaceous granite formationwas weathered and fractured may be distributed widely in Phanrang Shelf,Danang Shelf and Tuyhoa Shear Zone.5 Overlying unconformity the Pre-Cenozoic basement are raw grate size sediment aged Paleocene-Eocene andthe conglomerate at the bottom lining. According to the recent researchresult’s NOPEC,8 there may be existed a near basement basaltic play that



consisting of high porosity volcanic rock overlain directly the basement. ThePhukhanh Basin consisting of three main fault systems is semi-meridian,NW-SE and NE-SW. The 110◦ meridian fault system maybe the mostimportant fault system making up the deep troughs in the basement andcontrolled the main deposited center of the basin. The NW-SE fault systemin the south of the Phukhanh basin, belong to the Tuyhoa Shear Zone,constrain the SW part of the basin. The NE-SW fault system in the north,belong to the Danang fault zone, constrains the northern part of the basin.

3. Interpretation Methods and Data Sources

3.1. Interpretation methods

The interpretation processing are taken place as follows:

Step 1: we calculate the power density spectrum (PDS) of gravity andmagnetic data to determine the average depth of the basement (d0)and its wavelength band. In this model, we assume that the Earthcrust has four layers: seawater, sediment, crust and mantle. We useof 0.37 g.cm−3 and 0.4 g.cm−3 for the seawater-sediment and crust–mantle density contrasts, respectively.17,18 The gravity effect of theseafloor and Moho topography are calculated by the Parker’s 3Dforward problem.9 Then they are removed from observed gravityanomaly (Fig. 2). Applying the band-pass filtering to this gravityanomaly, we get the residual gravity anomaly produced by thebasement surface. We use the Generic Mapping Tool software(GMT) to calculate the power density spectrum and band-passfiltering.10

Step 2: The 3D direct inversion method based on the Parker’s methods(Parker, 1972) is used to calculate the basement undulation fromresidual gravity anomaly. The 3D inversion formula is given asfollows:9

h(x, y) = F−1

[F [∆γ]

2πG∆ρ exp(−2π|k|d0)

](1)

where: h(x, y) denotes the undulation of the density surface; F−1[ ]is the 2D inverse Fourier transform; F (∆γ) is the 2D forwardFourier transform of residual gravity anomaly; G is gravitationalconstant, ∆ρ is a density contrast through the density surface; d0

is mean depth of the density surface (d0 � h(x, y)).



Fig. 2. (a) The depth of seafloor (in km), (b) gravity effect of seafloor topography(in mGal), (c) The depth of Moho topography (in km), (d) the gravity effect of Mohotopography (in mGal); (e) Free air anomaly minuses the gravity effect of the seafloor and

the Moho topography and (f) the residual gravity anomaly due to the basement afterapplying the band-pass filtering to (e).



The accuracy of the inverse procedure of Eq. (1) depends on (d0)and (∆ρ) as close as possible to the real mean depth of the surfaceand density contract. In our calculation, d0 and ∆ρ are constrainedby the PDS results and the core sample data at the Cuulong andNamconson basins,6 respectively.

Step 3: The fault systems are determined by the maximum horizontalgradient (MHG).2 The fault locations are defined by thedistribution of the MHG. The slop of the faults can be estimatedby the displacement of the position of the MHG point at thedifferent upward continuation levels.15 When the gravity anomalyis continued upward at higher level, the posision of the MHG willdisplace to the slop direction of the faults. The scale of the faultscan be guesstimated by the value and the disappearance of theMHG at the different upward continuation level.

3.2. Data sources

The data set are used in this paper were obtained selectively from somedifferent sources. The bathymetry and satellite gravity grid (1′ × 1′) areobtained from http://topex.ucsd.edu/cgi-bin/get data.cgi.11,12 Magneticdata ∆T are obtained from the Gagarinxkis’s 90–92 cruises consisting of3290 observed points.7 Because of very small-scale magnetic grid comparingto the gravity grid, so the magnetic data are used to determine the averagedepth of the basement only.

4. Computed Results

4.1. Average depth of the Pre-Cenozoic basement

Log of the PDS of the magnetic and gravity anomalies in the Phukhanhbasin are shown in Figs. 3a and 3b, respectively. The PDS of the magneticanomaly can be divided into three straight-line segments correspondingto three magnetization boundary surfaces. The average depths of thesethree surfaces (d) can be estimated 4.5, 12.4 and 28.6 km deep, respectively(with the slop of the straight line equal to −2d2). The PDS of Bouguergravity anomaly is determined as shown in Fig. 3b. The PDS may bedivided into two straight line segments corresponding to two boundarysurfaces and the average depths of these two surfaces can be estimated2.25 and 4.8 km deep (with the slop of the straight line equal to −4πd).



Fig. 3. Power density spectrum of the magnetic anomaly (a) and of Bouguer gravityanomaly (b) in the Phukhanh basin.

We pay attention to the first two boundary surfaces having the averagedepth of 2.25 km and 4.5–4.8km. The first surface (2.25 km average depth)estimated by the gravity PDS was not appeared on the magnetic PDS.It maybe reasonable, because this surface corresponds to the seafloorsurface (the average depth of seafloor is 2.37 km), so it may have too smallmagnetic effect (unconsolidated sediment). The second surface is estimated4.5 km deep by the magnetic data and 4.8 km by the gravity data arealso very reasonable. This small gap may be caused by the accuracy ofthe PDS method or the depth of magnetization and density surfaces arenot exactly the same. According to the previous seismic exploration andthe characteristics of Pre-Cenozoic basement being mainly magmatic andgranite formation,1,5,8,14 this depth could be the average depth of the Pre-Cenozoic basement in this region. In accordance with the PDS segmentof the Pre-Cenozoic basement, the gravity anomaly wavelength of thebasement could be from 40 to 300km. In a word, by calculating magneticand gravity PDS, the mean depth of the basement predicted is 4.5–4.8 kmdeep and its wavelength band of gravity anomaly is 40–300km.

4.2. Calculating the basement depth

To determine the depth of the Pre-Cenozoic basement, we carry out thefollowing steps: eliminating the gravity effects of the seafloor and Moho



topography from the free air anomaly and then applying the band-passfilter with the pass-wavelength of 60–300km and reject-wavelengths of 40–600 km to the just received anomaly, we obtain the residual gravity anomalyof the basement. We make the grid size of 256 × 256 nodes from theresidual gravity anomaly. The initial average depth of the basement (d0)and density contract (∆ρ) are given 4.8 km and 0.27 g.cm−3, respectively. Inaddition, to eliminate the edge effect of Fast Fourier transform, we extendthe calculating area to 1◦ for around. Running inversion to the above dataand take some minor adjustments of density contract and average depth,we finally predict the density contract of 0.25 g.cm−3, the average depth of4.9 km for final inverse calculation procedure. The final depth of basementis predicted as shown in Fig. 4a. We get the sediment thickness (Fig. 4b)by subtracting the seafloor depth from basement depth.

The given result (Fig. 4a) has fairly good agreement with the recentlypublished works.1,4,14 It is shown that the calculation procedure has high

Fig. 4. (a) computed basement depth from 3D gravity inversion and (b) sedimentthickness of the Phukhanh basin.



objective and reliability by the following reasons: The first, the prediction ofd0 was based on the two independent data sources of magnetic and gravityanomalies give all similar results (discrepancy of 6%). The mean seafloordepth was calculated by this method is only 5% different with the availabledata; The second, the gravity data has dense grid and even accuracyover the study area and the wavelengths greater than 40 km related tothe basement are the high accuracy wavelengths of the satellite gravityanomaly.12

4.3. Determining the fault systems

The fault systems are determined by calculating the MHG of the gravityanomaly after rejecting the wavelength less than 40 km and upwardcontinuation at different elevations. According to upward continuation atdifferent elevations, the amplitude of the MHG decrease gradually at thehigher elevation (Fig. 5). At z = 0km the maximum amplitude of the MHGis about 2.3mGal/km. The MHG vectors distribute along the lines of N-S, NW-SE and NE-SW trends. At 10 and 20 km (Figs. 5c and 5d), theamplitudes of the horizontal gradient are mainly less 0.2mGal/km and theMHG are 0.2–0.4mGal/km only. At this elevation we realize that almostMHG are only appeared along the N-S and NW-SE direction. The positionsof the MHG have close correlation with the bathymetry, especially, N-Sdirection around the elongate 110◦ and NW-SE directions along the TuyhoaShear Zone in the southern part of the basin.

Figure 7 demostrates the diagram of the position of the MHG points onthe line 1, line 2, line 12 line 14, line 19 (location in Fig. 5) at the upwardcontinuation level of 0 km (z0), 10 km (z1), 20 km (z2) and 30 km (z3). Onthe line 1 and 11, the positions of the MHG move gradually to the westat the upward continuation level z0, z1, z2 and z3, so the direction of slopof the faults are the west. Similarly, the direction of slop of the faults onthe line 2 and 12 are the east. The direction of slop of the faults on the line14 and 19 are the southeast and northwest, respectively. Doing the samefor the other faults, we also obtain their direction of slop. In case, the slopof the faults is vertical, the position of the MHG point has no change atthe different upward continuation level. Based on the above calculation,the fault map of the Phukhanh basin can be constructed as shownin Fig. 7.



Fig. 5. The gravity anomalies at the upward continuation levels of 0 km (a), 5 km (b),10 km (c) and 20 km (d) are converted to horizontal gradients. Dots and arrows showmaxima and their direction vectors in horizontal gradient.

5. Structural Characteristics of Pre-Cenozoic Basement

5.1. Scale of the Phukhanh basin

The scale of Phukhanh basin is defined by the 1-km contour of the Cenozoicsediment thickness (Fig. 4b). According to this criterion, the scale of thePhukhanh basin is about 9100km2, elongated to the latitude trend from



Fig. 6. Diagram shows the movement posision of the MHG points at the upwardcontinuation level of 0 km (z0), 10 km (z1), 20 km (z2) and 30 km (z3) on the line1, line 2, line 12 and line 13 (location in Fig. 5).

latitude 11◦N to 14◦30′N and from the coastline to the longitude 112◦E. Thenortheastern part of the basin elongates to longitude 112◦E and latitude15◦N. The sediment thickness could be 9.5 km in the central of the basin andthe thinner in the western and eastern sides (1–2 km thick). The Phukhanhbasin succeeds Oceanic Central Basin in the southeast part where thesediment thickness is about 2–3km.



Fig. 7. Map of basement structural elements of the Phukhanh Basin.

5.2. The structural elements of the basement

According to the calculated results, the basement structure of the Phukhanhbasin could be divided into the following structural elements (Fig. 7):

— Phanrang shelf: the basement uplifts to 1–3 km deep. Its structure is inhorsetail fashion, uplifts in the northern and southern parts and low inthe center.



— Danang shelf: it elongates from latitude 13◦N to 14◦30′N and its widthis very narrow (14–40km). It is restricted by the N-S and NE-SW faults.The elevation of the shelf changes abruptly from some hundreds alongthe coast to 5.5 km off the coast. There occurs a graben elongated to N-Sdirection in the Quinhon offshore and the depth of basement is 5.5 km.

— Northern trough: it is located just in the east of Danang shelf fromlatitude 13◦N to 14◦N and its size is 150× 120km. The elevation of thetrough is fair slope in the north and west sides and gentle slope in theeast. The maximum sediment thickness is about 9.5 km in the centralof the trough.

— Central swell: the basement uplifts to 6.5–5.7km deep and creates anE-W uplift zone separated between the North and South troughs. Thewestern part of the swell is about 50 km wide and eastern part is 30 kmwide.

— Southern trough: Its area is smaller than the Northern trough and thedeepest basement is about 8.5 km. The western side of the trough isfairly slope and gentler slope in the eastern side. The southeastern partof the trough succeeds Oceanic Basin.

— Eastern swell: located in the east of the basin where the basementuplifts and creates a 70 km wide terrace with the depth of basementis about 3–4km. The Phukhanh basin end at the longitude 112◦ wherethe basement raises near the seafloor.

— Northeastern Swell: this is a part of the Phukhanh Basin extends to thenortheast where the basement uplifts abruptly from 8–9km to 5 km.There also occurs some local lifted blocks with the depth of 3.5–4km.

— Tuyhoa Shear Zone: this zone located in the south of the Phukhanhbasin and elongates to NW-SE. The basement depth is 3.5 km in theoffshore and 1–2 km in the Phanrang shelf.

— North swell: it is located in the north of the basin, including the Tritonridge. The basement depth changes from 7km to 4.5 km. Here occurssome local lifted blocks with the basement depth of 3 to 3.5 km.

5.3. Fault systems

The fault systems in Phukhanh basin are plentiful and consist of semi-meridian, NW-SE, NE-SW and semi-parallel. In which, the semi-meridianand NE-SW faults affect mainly the basement structural plane of thePhukhanh basin. The NW-SE faults governs the structural characteristicsof the southern part of the basin.



— Semi-meridian fault system: this fault system actes strongest along thelongitude 109◦–110◦E, consists of F1, F2 and F3 faults (Fig. 7). Thereare F4 and F5 faults in the northern part of the basin, F15, F16 andF17 faults in the southwest. The semi-meridian faults are deepest faultsin the region in which the F2 fault is biggest one. The F1, F2 and F3are crossed over by the NW-SE and NE-SW faults. These three faultsgoverns fully the basement structure plane of the western part of thePhukhanh basin.

— NE-SW fault system: this fault system appears mainly in the centraland eastern parts of the basin. The northern trough, northeastern andeastern swell are controlled mainly by the F7, F8, F9 and F10 faults.The central swell is constrained by the F12 and F13 faults.

— NW-SE fault system: this fault system appears mainly along the TuyhoaShear Zone (F17, F18) and control the southern margin part of thePhukhanh basin. This fault system crosses over the 109◦–110◦ meridianfaults at the Phanrang shelf. It is also a deepest fault system in thestudy region.

6. Conclusion

The combination of the interpretation methods have been presentedabove allows to determine rapidly, objectively and reality the structuralcharacteristics of basement. Phukhanh basin is a large basin with thearea of 9100km2, elongates from the latitude 11◦N to 14◦30′N and fromlongitude 109◦20′E to 112◦E. Its major structural direction is N-S. Thecentral part of the basin locates in the longitude 110◦E to 111◦E andlatitude 11◦30N–14◦10′N, the deepest basement is 9.5 km and the sedimentthickness is 7.3 km. Based on the basement structure plane, the Phukhanhbasin could be divided into nine major structural elements: Danang shelf,Phanrang shelf, northern trough, southern trough, central swell, northernswell, northeast swell, east swell and Tuyhoa shear zone. These structuralzones are constrained by the semi-meridian, NW-SE and NW-SE faultsystems. In which the semi-meridian and NE-SW fault systems are two mainfault systems in the basin. The advantage of being need less pre-informationof the basement, the authors believe that the above interpretation procedurecould be very useful and effective to determine the basement of the remotebasins where the scale of seismic investigation is poor. It can be a reliabilitybasic for planning the petroleum exploration projects in detail scale.



Acknowledgments

This paper was supported by the National Basic Scientific ResearchProgram of Vietnam, No. 718806, 2006–2008, the authors thank honest.

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