Evaluate the geological structure , petroleum potential by interpretation the 2 D seismic data of Phu Quoc basin

So far, the understanding of Pre-Tertiary formations in the Phu Quoc basin is eliminated due to lack of drilling and the absence of outcrops except on Phu Quoc Island and some other isles. By analogue with stratigraphic information from surrounding areas, seismic horizons may be correlated with known-age strata. Strata of pre-Cambrian and Early Paleozoic ages have not been discovered yet in SW Viet Nam but by analogue may consist of metamorphosed rocks such as chloritized phylite, sericite schist, and quartzite sandwiched with ultra basic rocks. Therefore, PetroVietnam assigned PVEP to perform acquisition of 2D seismic on the area of blocks 4144 in order to get additional data for more precise evaluation of the petroleum potential and defining the exploration strategy in the Phu Quoc area.Based on seismic data interpretation in block 42 of Phu Quoc Basin, the Isodepth maps were constructed by approach of conversingtime to depth from isochrone maps. The results of interpretation allowed to identify many seismic closures particularly on the isodepth map of the Brown section, but these closures are located deeply above 5000m, which reliability is fair and economic effect is weak. Hence, the Brown horizon did not considered as structures identified on and concentrate to describe those determined in the Orange and Purple horizons. Result of interpretation indicated 8 closures, which could be identified in the Orange and Purple horizons

Evaluate the geological structure, petroleum potential by interpretation the 2D seismic data of Phu Quoc basin

ABSTRACT
So far, the understanding of Pre-Tertiary formations in the Phu Quoc basin is eliminated due to lack of drilling and the absence of outcrops except on Phu Quoc Island and some other isles.By analogue with stratigraphic information from surrounding areas, seismic horizons may be correlated with known-age strata.Strata of pre-Cambrian and Early Paleozoic ages have not been discovered yet in SW Viet Nam but by analogue may consist of metamorphosed rocks such as chloritized phylite, sericite schist, and quartzite sandwiched with ultra basic rocks.Therefore, PetroVietnam assigned PVEP to perform acquisition of 2D seismic on the area of blocks 41-44 in order to get additional data for more precise evaluation of the petroleum potential and defining the exploration strategy in the Phu Quoc area.Based on seismic data interpretation in block 42 of Phu Quoc Basin, the Isodepth maps were constructed by approach of conversingtime to depth from isochrone maps.The results of interpretation allowed to identify many seismic closures particularly on the isodepth map of the Brown section, but these closures are located deeply above 5000m, which reliability is fair and economic effect is weak.Hence, the Brown horizon did not considered as structures identified on and concentrate to describe those determined in the Orange and Purple horizons.Result of interpretation indicated 8 closures, which could be identified in the Orange and Purple horizons

INTRODUCTION
Phu Quoc basin is considered as a Mesozoic basin developing on the Paleozoic basement and spread entirely on area of Blocks 41-44 which are located at the south of Phu Quoc island and stretched southward to offshore of West Viet Nam with area of 19.420 km 2 .Seismic data of these blocks are still few and limited only with 1087 km of 2D seismic lines MH-96 of grid 32 x 32 km [1,2,3,4,5].No exploratory well was drilled yet in surrounding area.Therefore, PetroVietnam assigned PVEP to perform acquisition of 2045 km of 2D seismic lines PQ-05 on the area of blocks 41-44 in order to get additional data for more precise evaluation of the petroleum potential and defining the exploration strategy in the Phu Quoc area.[4,12] Blocks 41-44 are located in offshore of West Viet Nam at the south of Phu Quoc island (Fig. 1).
The Phu Quoc basin is a forearc basin of Mesozoic age.It developed on the plate margin of an ancient Cratonic block -the Indosinia microplate, in contact with the Sibumasu microplate, both which are considered to be fragments that split from Gondwanaland during the Paleozoic Era.The basin is elongated meridionally with a width of 50 km and a length of more than 100 km, and is covered by a Paleozoic-Mesozoic sediment complex of more than 8 km thickness at depocenters.[1,6,7,8,9,10,11,12,13,14,15] Methodology Based on the seismic data build up the appropriate maps, interpret the seismic profile, hence identify the potential closures

Results and Discussion
The seismic data quality it needs to remark that the reprocessed 1.087 km of 2D seismic lines "Minh Hải-96" at Golden Pacific center has a medium quality, while the 2.045km of 2D seismic lines acquired in 2005 and reprocessed by Fairfield Việt Nam in 2006 are qualified very worse particularly in the eastern area with many diffractions and multiple reflectors (Fig. 2; Fig. 3) [4].

Correlation and Interpretation
Based on existed 2D-seismic data (Fig. 2) and correlating with those from Malay -Tho Chu basin three (03) seismic markers were identified in the studied area including -the Orange, the Purple and the Brown markers.(Fig. 4) These boundaries are used in this report for correlation and interpretation.They coincide with those identified before by others authors in pre-existed reports.
The brown seismic marker: This marker characterizes the acoustic basement which is the deepest boundary enable to be observed on the seismic section.The brown marker is characterized by seismic reflectors of low amplitude and weak to very weak intensity.In some places the interpretation needs to draw by analogue with overlain horizons (Fig. 4).The Brown horizon is identified as the basement of sedimentary basin and dated as Paleozoic (Devonian).It is the main study target of the project.The correlation reliability of the Brown horizon is quite low.Compiling of isochrone and isodepth maps with 1: 200,000 scale All isochrones maps were constructed on software CPS3 Geoframe and illustrated on figures 5 for isochrones map of the Orange, Purple, and Brown sections.After converting to depths isodepth maps were constructed in scale of 1.200.000for all seismic boundaries using software CPS-3.Figures5 -10 illustrated isodepths maps of the Orange, Purple, and Brown sections.Isodepth maps were constructed basing on the conversion Time to Depth method of isochrone maps.The result of interpretation allowed to identify many seismic closures particularly on the isodepth map of the Brown section but these closures are located deeply above 5,000m, which reliability is fair and economic effect is weak.So, we did not consider structures identified on the Brown horizon and concentrate to describe those determined in the Orange and Purple horizons.Result of interpretation indicated 8 closures, which could be identified in the Orange and Purple horizons and numbered from N 0 01-to N 0 08 (Fig. 11).depth; the largest closure of 63.8 km 2 is at 3,400m depth.On the Purple horizon the structure top locates at 3,600m with largest closure at 5,000m depth and area of 74.79 km 2  (Fig. 11; 12; 20).

Conclusions
Although seismic data quality is not good enough, seismic interpretation 03 floors showed geological structure of block 42 to the direction of the main structure is dominated by fault system western north-east south: -There are 08 sedimentary structure have been discovered in Paleozoic period.

Figure 1 .
Figure 1.Location of studied blocks

Figure 14 .Figure 15 .
Figure 14.Seismic section cross structure 02 Structure/ Closure 3 Structure 03 locates nearly the Central Uplifted Zone of Phu Quốc basin and at the center of block 42.Structure represents an anticline of 3-way dip closure cut by fault system of NW-SE trend.Anticline is closed to

Figure 18 .
Figure 18.Seismic section cross structure 06 Structure / Closure 07 The structure 07 locates at the south of structure 06 closely to the Western trough of Phú Quốc basin.The structure has anticline form cut by a fault system of NW-SE direction.It is a 2-way dip closure and closed to fault.Structure is identified on both horizons -Orange and Purple.On the Orange horizon the top of structure is determined at 2,400m depth with largest closure at 2,900m and area of 21.28 km 2 .On the Purple horizon the structure top is drawn at 4,200m depth with largest area of closure of 27.24 km 2 at 4,200m.(Fig. 11; 12; 19) Structure / Closure 08 The structure 08 is located southerly of block 42, and bounded northwesterly by the Western trough and by the Southern trough of Phú Quốc basin at the south.It represents an anticline closed to fault in form of 2-way dip closure.Structure is identified on both horizonsthe Orange and Purple.On the Orange horizon the top of structure is determined at 2,200m ) POS = 0.154 have been appreciated over structures remaining in the area (POS = 0:12 to 0:14).

Figure 19 .Figure 20 .
Figure 19.Seismic section cross structure 07 Faculty of Technical Geology and Petroleum -Ho Chi Minh city University of Technology, VNU-HCMC(Manuscript Received on August 10 th , 2015; Manuscript Revised on October 20th , 2015)