Main favorable factorscreatee oligocene formation become a petroleum prospect in south-east area , Cuu Long basin

The previous researchs almost demonstrated the reservoir in Lower Oligocene sandstone has complicated distribution, probably low porosity and permeability, no nature flow are seen by wells. Based on recent study results of depositional environment of E Oligocene sedimentary in Cuu Long basin and E sand distribution in blocks 01/10 & 02/10, block 09-2/09, block 09-2/10 and block 16-2 area, the paper determines depositional facies according to petrophysic and geophysic data. From Chrono-stratigraphy priciple, as well as interpretation and intergration of seismic data, well data, well log data, core & sample analysis results, petrophysic data, well test data, the paper forcuses on create correlation lines between wells and seismic line for confirming Oligocene E distribution in Cuu Long basin, buildup the gross mapping for depositional environment in sub-sequence Oligocene E upper and E lower and mapping of seismic attributes for sub-sequence Oligocene E upper and E lower.At last successfully predict the sand distribution in Oligocene E upper and E lower sub-sequence in research area and hence determine the stratigraphic traps in Oligocene formation of South-East area Cuulong basin.


GENERAL INTRODUCTION
The main target in Cuu Long basin is Pre-Tertiary fractured granite basement, Lower Miocene sandstone and also Lower Oligocene sandstones.Previously, the fractured basement and Lower Miocene sandstone reservoirs (the traditional targets) are being depleted so exploration is focused on Lower Oligocene sandstone.However, this reservoir has complicated distribution, probably low porosity.Tight sandstone with porosity from 10 to 15%, permeability lower than 10mD, no nature flow are seen by wells.But in some marginal areas such as North East Bach Ho, Su TuNau and KinhNguTrang, there are several fair porosity and permeability reservoirs, 1,000 to more than 3,000 BOPD natural flow [1].Recent wells in KinhNguTrang, KinhNguTrang 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 of 15 to 70mTVD.
In Cuu Long basin, E Oligocene sediment was buried in big depth and large varied from the area to area.In the top of the positive structures, E Oligocene sediment is moderate thin, which distributed in depth of 3,000 to 4,000mss.Meanwhile, in the flanks of these structures, this sedimentary is distribute in depth of 4,000 to 5,000mss and the thickness could be approach to 1,000mTVD.Due to burry in varied depths hence E Oligocene sediment had been altered irregular and moderate complicated

New discoveries in Oligocene E sequences:
There are several fair porosity and permeability reservoirs, 1,000 to more than 3,000 BOPD natural flow (Fig. 01).Recent wells of PVEP POC in KinhNguTrang, KinhNguTrang 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 of 15 to 70mTVD.
In Cuu Long basin, E Oligocene sediment was buried in big depth and large varied from the area to area.In the top of the positive structures, E Oligocene sediment is moderate thin, which distributes in the depth of 3,000 to 4,000mss.Meanwhile, in the flanks of these structures, this sediment is distribute in depth of 4,000 to 5,000mss and the thickness could be approach to 1,000mTVD.Due to burry in varied depths hence E Oligocene sediment had been altered irregular and moderate complicated (   -The trap mechanism: According to Lambiase and Morley (1999) and Olsen (1990), variations in climate, in addition to plate tectonic activities, can significantly impact synrift sedimentation.During long period of wet climate, water level in the eastern sub-basin of Cuu Long basin rose and led to the expansion of the concurring lake (Fig. 05).The result of these changes in environment was the deposition of fine-grained (shalely) sediments on top of the previously deposited coarse-grained sediments.The evidence of this point is the existence of organic-rich (>10% TOC) Carboniferous black shale formations, discovered in recent exploration wells in this area, with thickness of tens to hundreds of meters that deposited and distributed widely across the eastern lake (Fig. 6).Due to of their considerable thicknesses and extensive distributions throughout the eastern sub-basin, these shale units are not only the sources of hydrocarbons but also the regional top seals for Early Oligocene syn-rift reservoirs of the area, resolving the previous skepticisms regarding the existence of such reliable seals.
-The reservoirs: Major exploration targets of recent discoveries in East Edge of CLB are basal sandstone reservoirs that were deposited right on top and shared a similar topography with the underlying Pre-Cenozoic granitic basement.These are fluvial/alluvial deposits of near-source sediments (Fig. 07).Lithologically, these basal are mainly Arkose: Feldspar-rich sandstone composed of granitic fragments with increasing proportion toward the sedimentary source.Qualities of these reservoir are fairly good at such diagenesis depth; some are even better than reservoirs deposited later (Fig. 08 &Fig.09).Porosity/permeability was well preserved partly because the Feldspars hadn't been fully weathered (Morley, 1999;San T. Ngo, 2005).Another reason was the deposition of widely distributed stiff carboniferous shale on top of these reservoirs making it not only a top seal for the underneath reservoirs but also a "stress shield" that absorb and dissipate vertical compaction.-The Seal:On the other hand, during long periods of wet climate, the lake covered area spread out (dashed black line) facilitating lacustrine facies deposition, such as black shale to overlay the whole area making capable seals for underlying reservoirs formed during dry climate periods (Fig. 05).

Internal seismic facies of sequence E:
-Low frequency, high amplitude, discontinuously, nearly parallel to parallel reflection in the lower part of the uplift areas.
-Low frequency, medium to high amplitude, chaotic reflection, hummocky, weakly continuous to demonstrate alluvialfluvial depositional environment.
-Sediments deposited in these sub-garbens are dominantly alluvial fans, so-called eluvia and deluvia sediments.Alluvial fans were deposited in condition of strong currents, which flowed in multi-direction streams, slope terrains, near sediment sources.Therefore, sediments of this Formation are dominantly coarse-grained, poor sorted and angular rounded.-At study area, in the E sequence were encountered mainly shale, silt laminate on sandstone.Porosity value was evaluated by the petrophysical analysis with core correlation.Base on the petrophysical analysis results the reservoir thickness which was distribution in the study area varies from tens to hundreds meters.
In the E sequence exist the bitumineous shale with the thickness around 20 -70 meters as role as overburden gaining, that is possibility reason of porosity preserve in the E sequence.-Similar to Bach Ho North -East; Su TuNau; 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 relationships of porosity versus depth ware conducted as figures 10, 11 bellows.The porosity decreasing versus depth chart 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.
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 12 hereafter.-Before 2005, the exploration results indicated a poor potential in study area except North East part of Bach Ho field; because of low/no nature flow, tiny and regional accumulation.
-Recent exploration achievement 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 step.
-Lithological basement in Southeast margin Cuu Long basin is deeper than acoustic impedance basement; this shows probably "bottom" clastic layers onlap top of granite basement which present in a large area and thicker than previous theories.Therefore, oil and gas potential in Lower Oligocene accumulations are also higher.
-Eocene -Oligocene reservoirs in Southeast marginalCuu Long basin preserved porosity -permeability characteristics and probably got oil and gas industry flow in varied sedimentary facies.They distribute in a large area in study area.
-Accumulations in study area distribute widely in Southeast margin Cuu Long basin and probably related to the existence of half-grabens along Con Son swell.
-Mapping of sedimentary facies distribution in purpose of predicting preserved porosity -permeability characteristics accumulations in study area.
[2].Up to now, there are several different subdivisions of the Oligocene sequence in other block of Cuu Long basin.It is caused by the variety of seismic characteristics in another blocks as well as lithology characteristics [4].The stratigraphy of Cuu long basin is summarized hereafter (fig.a):-Pre-Tertiary Basement -Late Oligocene 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 coarse grained and breccias/conglomerate of the sandstone with granitic fragments and overlies the weathered

Figure 2 .Figure 1 .Figure 3
Figure 2. Seismic section through the ST, block 15.1, shows the distribution of Oligocene sediment in vertical

Figure 3 Figure 4 .
Figure 3.a.The seismic section shows E Oligocene sequence distribution in NE-Eastern part of Cuu Long basin

Figure 5 .
Figure 5. Illustration of Cuu Long Basin Eastern sub-basin through different climate condition

Figure 6 .
Figure 6.The AA' & BB' seismic sections show continuity and widely distribution of the black shale unit (green horizon) in the eastern edge of Cuu long basin.

Figure 9 .
Figure 9.The depositional environment of the Basal unit was interpreted (with Image log) as prograding Alluvial Fan to Fluvial Axial Drainage System.

Figure 7 .
Figure 7.The depositional environment of the Basal unit was interpreted (with Image log) as prograding Alluvial Fan to Fluvial Axial Drainage System.

Figure 12 .
Figure 12.The permeability versus depth relation chart-Oligocene sequence Conclusions: basement.It is generally thin or absent in most parts and only present on the flank of the structures.Main characters of log curves of E sequence are relatively high density and resistivity, meanwhile DT is low.