The tectonic evolution and new exploration results onhydrocarbon potential , cased study in the NE-Eastern part , Cuu Long basin , Vietnam continental shelf

New exploration results proved the presence of Tertiary sedimentary basins with hydrocarbon potential in the boundaries of Vietnam continental shelf. These basins were perhaps formed in the Early Oligocene on the Mesozoic basement of continental crust and were filled with mainly deltaic clastic sediments. Since very early Miocene these basins have linked together and extended wider. Sediments of marine and prograding delta facies accumulated and widespreaded on the whole continental shelf. At the Middle Miocene to present-day the opening of the Bien Dong Sea (Vietnam East Sea), the regional tectonic subsidence and the eustatic fluctuation had created sedimentary formations of open and deep marine facies with turbidite shale, platform carbonate including reef build-up. Andesite and basalt extrusion occurred in form of dykes resulting from tectonic inversions in Late Oligocene, in the beginning of Middle Miocene, particularly in Late Miocene-Pliocene. These Tertiary sedimentary basins are expected to have hydrocarbon potential and need to be investigated and explored adequately. The main risk is the drilling technology, the exploitation in the deep sea and high investment cost. The recently exploration results in illustrated a better potential in Lower Oligocene reservoir in South East margin of Cuu Long basin and many new signs. These signs need to be highlight researched for predicting accumulation distribution in study area, in purpose of oil and gas exploration for next stages. Keyword: Boundaries of continental shelf, prograding, retrograding, petroleum system, potential play, South East margin, lower Oligocene reservoir. TAÏP CHÍ PHAÙT TRIEÅN KH&CN, TAÄP 19, SOÁ K12016 Trang 181

The tectonic evolution and new exploration results onhydrocarbon potential, cased study in the NE-Eastern part, Cuu Long basin,Vietnam continental shelf

ABSTRACT
New exploration results proved the presence of Tertiary sedimentary basins with hydrocarbon potential in the boundaries of Vietnam continental shelf.These basins were perhaps formed in the Early Oligocene on the Mesozoic basement of continental crust and were filled with mainly deltaic clastic sediments.Since very early Miocene these basins have linked together and extended wider.Sediments of marine and prograding delta facies accumulated and widespreaded on the whole continental shelf.At the Middle Miocene to present-day the opening of the Bien Dong Sea (Vietnam East Sea), the regional tectonic subsidence and the eustatic fluctuation had created sedimentary formations of open and deep marine facies with turbidite shale, platform carbonate including reef build-up.Andesite and basalt extrusion occurred in form of dykes resulting from tectonic inversions in Late Oligocene, in the beginning of Middle Miocene, particularly in Late Miocene-Pliocene.
These Tertiary sedimentary basins are expected to have hydrocarbon potential and need to be investigated and explored adequately.The main risk is the drilling technology, the exploitation in the deep sea and high investment cost.
The recently exploration results in illustrated a better potential in Lower Oligocene reservoir in South East margin of Cuu Long basin and many new signs.These signs need to be highlight researched for predicting accumulation distribution in study area, in purpose of oil and gas exploration for next stages.

INTRODUCTION
Boundary of Vietnam continental shelf exist the Tertiary sediment basins in which the oil and gas potential proved through exploration and production results.
The basins recognized generate from early Oligocene above continental foundation with Mesozoic age content deltaic clastic facies.At begin of Miocene the basins have been extent, correlated together with prominent marine and prograding delta facies covered throughout continental shelf.
Since Middle Miocene up to recent, associated with the extension of Eastern Sea, the subsidence of region and sea level variation raised the favorable conditions to develop open sea and deep water settings in which mostly content turbidite shale and terrace carbonate intermix with reef build-up.The andesite and basaltic extrusive may generate in dyke form and surface layers which related to tectonic reversed activities at the late Oligocene and early Middle Miocene and especially at the end of Miocene-Pliocene.

THE TECTONIC EVOLUTION HISTORY
The open sea and deep water regions of Vietnam continental shelf include areas of Bien Dongsea surrounding basins such as PhuKhanh basin, Eastern shelf of Nam Con Son basin, and TuChinh-Vung May basin group (figure 01).These basins been generated and developed in structural geology region complicatedly, the vestiges of micro MezozoicIndosinia plate which partly oceanismed during Bien Dong Sea widespreaded, happened mostly in Oligocene.These basins to be considered rift basins, which formed synthetic with rift and back-arc basins in South-East Asia (SEA), have similar development stages.

PETROLEUM SYSTEM IN BOUNDARIES OF CONTINENTAL SHELF SEDIMENTARY BASINS.
In rift basins of continental shelf boundaries belong to deep water, fared edge have not anypetroleum exploration wells.The commercial hydrocarbon just discovery in shallow shelf with the depth <500m sea level, such as PhuKhanh basin, Nam Con Son basin in coral build up, clastic with ages of middle and lower Miocene.Based on seismic results and correlate with blocks which had the exploration well data could build up the petroleum system point of view for the offshoreregion (fig 08).

The Source Rock:
The seismic interpretation results and tectonic evolution presented that the deep water and fared edge have the similar evolution history in the whole basin.The Paleogene and Miocene basins were buried by subsidiary thermo shrinkage in late Miocene and Pliocene-Pleistocene.The shale lacustrine sediment with abundant of organic matters and coal matter aged Oligocene as well shale of flooding plain are concerned of source rocks with have potential to generate hydrocarbon for deep water and fared edge regions at PhuKhanh, Nam Con Son and Tư Chính -Vũng Mây basins as well.The deep buries with fast subsidiary velocity of this source rock accelerate the maturation degree of organic materials and expelled the oil and gas out of the source rocks.The results of geochemistry modeling built up in PhuKhanh and Nam Con Son basins presented the organic materials those accumulated in syn-rift stage is falling in oil windows.The hydrocarbon expellee is continuously in Quarterly.The thick shale settings have 200-300 m thickness and distributed at 600-800m deepare also determined have gas hydrate generating potential.The Seal: Lacustrine shale facies in Oligocene syn-rift, which generated the thickness cap throughout the region, the potential seal on pre-Tertiary elevated fractured basement.
The Trap: The buried raised blocks and elevated blocks with twisted and compressed along faults.
Built up carbonate and reef play (figure 11) The Source rocks: The abundant organic material shale, the Oligocene lacustrine coal shale, the shale of late Miocene seashore flood plain, the fared edge submarine fan; The Reservoirs: Bio-carbonate, coral, dolomized carbonate aged of Middle-late Miocene.
The Seal: Shaly thickness cap in open sea, deep water, shale in rear of submarine fan of Pliocene-Pleistocene sedimentary settings.
The Trap:The built up carbonate with cap and wisp form, the reef, the dune of leaf-cuticle waxes aged Miocene, mainly middle-upper Miocene; Elevated-fault play (figure 12) The Source rocks: The Oligocene thickness coal-mud with lacustrine facies and Miocene marine shale abundant of organic materials.
The Reservoirs: The interbed sandstone in the Oligocene-early Miocene,syn-ryft clastic formation.
The Seal: The shale with restrict, locally distribution in Oligocene syn-ryft clastic formation, the moderate shale layers in early Miocene coastal sedimentary settings, the potential cap are also thickness marine shale of deep water settings in post-rift early Miocene-Pliocene.The faults not only take the role of migration path but also prevent the migration of oil and gas.
The Trap: The elevated blocks divided by sealed faults.
Turbiditeplay (Figure 13) The Source rocks: The shale stratum in which abundant organic materials interbed in to turbidite formation.
The Reservoirs: The sandy lens and thin layers interbed in to turbidite formation, sub marine fan and submarine channels.
The Seal: The shaly stratum with sub marine fan and deep water facies, interbed in to Pliocene and early Pleistocene turbidite formation.
The Trap: Normally are thin lens, multilayers, fold and stratigraphy traps in prograding wedges.Recent wells of PVEP POC in Kinh Ngu Trang, Kinh Ngu Trang Nam area as well as Thang Long wells of Lam Son JOC also show that sand bodies in Oligocene E are the main reservoirs.From these wells, bituminous shale layers can be firmed as the seal of these sand bodies with thickness from 15 to 70mTVD (figure 14).

Stratigraphy and tectonic setting[4]
Lower Tra Tan Formation -E/F (?) Sequence (Thickness: 0 -1,500 m) Generally, in Cuu Long basin, the "E/F?"Sequence underlies the organic rich dark yellowish brown shaly, claystone and is distinguished by the coarse grained and breccias/conglomerate of the sandstone with granitic fragments and above the weathered basement.It is generally thin or absent in most parts and is only present on the flank of the structures.Main characters of log curves of E sequence are relatively high density and resistivity, while DT is low.Trang 186

Bituminous shale
The South-Eastern Cuu Long depression encompasses block 09.2 and 09.3.The main structural features of this depression comprise NE-SW, E-W and NW-SE trending faults.

Petroleum system[4]
The Source Rock: In the Southeastern flank, in KNT area, Bitumen shale had been encountered in E lower section.This shale has thickness variety from 15 to 60m, it was deposited in deep lacustrine/lake condition (Fig. 2.10).Geochemistry analysis for Bitum exhibit good to excellent organic richness (TOC~11%) and show very good to excellent hydrocarbon generation potential (S2~63Kg/T) (figure 16).
The Seal: The regional transgressive shale (BH shale) at the most upper part of BI sequence is also known as an effective seal in the Cuu Long basin.

The Reservoirs:
Similar to Bach Ho North -East; Su Tu Nau; Diamond fields, in the study area the preserve porosity is moderately good, effective porosity in E sequence varies from 11 to 15%, up to 18% at some wells.Base on the core and petrophysical analysis results the relation of porosity versus depth was conducted as figure 17.The porosity decreasing versus depth chart (Figure 18) indicate the porosity of E sequence still well until penetrate at depth around 3,900 vertical meter.The effective porosity was preserved around 10 percent at depth 4,000 mTVD.

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Figure 18.The porosity decreasing versus depth -E sequence Equalize of porosity with permeability supported to fair quality of permeability, in the study area, the permeability could up to hundreds mD/cP at some wells as figure 19.

CONCLUSIONS AND RECOMMENDATION
The survey results in margins of Vietnam continental shelf indicated, from geologic structure point of view, they are suture zone to deep sea area of PhuKhanh, Nam Con Son basins and develop the TuChinh-Vung May basin group.These basins were formed in the Late Eocene-Early Oligocene on the Mesozoic basement of continental crust and were filled with mainly clastic lacustrine, deltaic intermix with coastal sediments.Since Miocene, these basins have become extent and linked together with apparently sediments of marine and prograding delta facies accumulated and widespreaded on the whole continental shelf.At the Middle Miocene to present-day with the opening of the Bien Dong Sea, the regional tectonic subsidence and the eustatic fluctuation had created sedimentary formations of open and deep marine facies with turbidite shale, stacked sandstone.The rapid rate of deposition in the early Miocene-Pliocene accelerated the mature range of organic materials and the hydrocarbon expulsion at the beginning of early Miocene to the present-day.The organic material range is determined at the principle zone of oil and gas generation.
In addition with the conventional hydrocarbon resources, the unconventional hydrocarbon resources such as gas hydrate are also the potential aspect and needed to concern for research.
The main risk is the drilling and exploitation technology in the deep sea, high expenditure and investment cost.

Figure 3 .
Figure 3.The cross section of generation and evolution model in boundaries of continental shelf sedimentary basins.

Figure 2 .
Figure 2. The evolution model of tectonic-facies Tertiary rift basin in boundaries of Vietnam continental shelf.

Figure 7 .Figure 8 .
Figure 7.The evolution in the boundaries of continental shelf sedimentary basins in post rift stage Late Miocene-Pliocene.

Figure 9 .
Figure 9.The distribution of hydrocarbon traps.

Figure 10 .
Figure 10.The point of view to petroleum play pre-Tertiary elevated fractured basement.

Figure 11 .
Figure 11.The point of view to lower-middle Miocene carbonate play.

Figure 12 .
Figure 12.The point of view to elevatedfault block in rift generation Oligocene-Early Miocene play

Figure 14 .
Figure 14.The seismic section shows the E Oligocene sequence distribution in NE-Eastern part of Cuu Long basin

Figure 19 .
Figure 19.The permeability versus depth relation chart-Oligocene sequence Exploration in 2005 -2014 illustrated a better potential in Lower Oligocene reservoir in South East margin of Cuu Long basin and many new signs.These new signs need to be highlight researched for predicting accumulation distribution in study area, in purpose of oil and gas exploration for the next steps.
Faculty of Geology and Petroleum Engineering, Ho Chi Minh city University of Technology, VNU-HCM (Manuscript Received on August 10 th , 2015; Manuscript Revised on October 20 th , 2015)