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Home Studies Southern Vietnam (Nam Con Son-Cuu Long)

 Southern Vietnam (Nam Con Son-Cuu Long)

GIS-based G&G Database and Report

JMJ Petroleum has generated a GIS-based G&G exploration database and accompanying hard copy A3 report for the basins of southern offshore Vietnam – Cuu Long and Nam Con Son Basins. Studies_Southern_Vietnam_01Studies_Southern_Vietnam_02Studies_Southern_Vietnam_03Studies_Southern_Vietnam_04Studies_Southern_Vietnam_05Studies_Southern_Vietnam_06Studies_Southern_Vietnam_07

GIS G&G Project and A3 Hard Copy ReportSouthern Vietnam (Nam Con Son-Cuu Long) Study – A Summary 

Studies Southern Vietnam 08


GIS G&G Project and A3 Hard Copy Report 


A Petroleum Systems Evaluation of the Nam Con Son and Cuu Long Basins. 

In the Nam Con Son – Cuu Long Basins Study, all blocks (open and concession – a total of 62 blocks) have been evaluated and ranked in terms of hydrocarbon potential and risk and uncertainty

The study has now been updated, including more defined prospects and leads, and we recognise several open blocks in the Nam Con Son and Cuu Long Basins that we believe should be of commercial interest. Numerous prospects and leads have been identified in this and adjacent blocks, which we contain in our GIS project. 

It should also be noted that the block application system for direct negotiation has not changed and PetroVietnam still welcomes applications for data room studies with a view to direct application/negotiation for all other open blocks. 

Although the Nam Con Son and Cuu Long Basins are in close proximity to each other (located offshore southern Vietnam), and can be regarded as being genetically-related in terms of overall geotectonic setting and history, in terms of detail, and its relevance to their petroleum systems, the basins contrast greatly. This not only includes their differences in tectonostratigraphy, and its implications, for instance, on source rock, reservoir and seal facies, but also their contrasting overall basin geometries (including a significant difference in total sediment thickness), heat flow and deformational histories. In light of this, a great deal of time has been spent integrating data/information that will provide us with a better understanding of the geotectonic evolution of both basin systems and, in turn, on their depositional systems, the timing of events and, ultimately, on their petroleum systems. 

The first part of the study programme has been to rigorously define the tectonics, structure and geology („basic building blocks‟) of the region in question – in this case, offshore southern Vietnam. Placed in a regional and plate tectonic context, this has been used to generate a dynamic geological model for predictive purposes, and provides a fundamental platform for evaluating the basins and their blocks in terms of structural architecture and geometry, with a key objective being the mapping of their basement (total sedimentary fill). A Base Tertiary Map for the entire southern Vietnam region, constrained by seismic, well, magnetics and gravity data, and public domain sources, including depth converted TWT maps and georeferenced interpreted seismic profiles, has been generated and is included in the GIS project. 

Using a combination of seismic, well and potential field data (gravity and magnetics), the following key elements have been mapped through GIS to produce a detailed structural/geological coverage for the entire region: 

  • Basement (crustal) types
  • Igneous intrusives and volcanics
  • Basement (crustal) discontinuities
  • Principal faults and gravity-defined lineaments
  • Definition of the main Tertiary basins and isolated depocentres on the flanks of each basin
  • Depth-to-basement and other key intervals

Collectively, the seismic, well and potential field data have provided an invaluable insight into the overall shape of the basins, and identification of predominant basement fabrics, and more subtle structural trends, the type(s) of crust that floors the basin (i.e. continental, oceanic or transitional) and the extent geological units such as carbonate and igneous bodies. 

GIS-based qualitative mapping and quantitative 2D modelling of gravity data indicates that the Cuu Long Basin‟s basement (floor) is largely composed of “true” continental crust that has been largely intruded by granitic intrusives; with fractured granite being a principal target, as an established major proven play (e.g., Bach Ho). In contrast, our mapping and modelling suggests that the Nam Con Son Basin is floored by highly attenuated heterogeneous (transitional) crust, whereby, the now deepest part of the basin became extremely stretched resulting in sub-continental mantle to rise quickly to shallow levels and being exhumed beneath extensional detachments. The observation that this, the deepest part of the Nam Con Son Basin, appears to have acted as the truncation point for the South China Sea oceanic crust‟s southwestward progression (spreading ridge‟s southwestern limit) is probably highly significant. This model helps to explain the contrasting basin dynamic histories of the two basins and the significant difference in heat flow regime, with the Nam Con Son Basin experiencing considerably greater amounts of subsidence and much higher heat flow. The two contrasting domains (basins) are separated by the NE-SW trending Con Son Swell, a major tectonically-defined basement high. 

A detailed chronostratigraphic summary chart has been constructed to cross both the Cuu Long and Nam Con Son Basins in order to show the time-space relationships of the sedimentary fill. Six seismically-defined megasequences are recognised, with further divisions either representing sequence boundaries or maximum flooding surfaces. In detail, the two basins show significant differences in terms of their sedimentary fill and structure. For instance, the Cuu Long Basin underwent NW-SE transtensional extension in the Early Oligocene and again in the early Late Oligocene, with minor E-W directed extension in the late Early Oligocene. Whereas, in the Nam Con Son Basin, Early Oligocene extension was N-S orientated, resulting in E-W trending half graben development, and was tectonically quiescent in the Late Oligocene, with a second phase of rifting beginning in the Mid Miocene under E-W and NW-SE extension. The timing of these events and their magnitudes have very important implications on, for example, heat flow and trap forming mechanisms. This information has been used to generate a dynamic (variable) heat flow model through time. 

The distribution and quality of both proven and potential source rocks in the southern Vietnam basins (Cuu Long and Nam Con Son) have been assessed as a means of determining the spatial and temporal distribution of source families and their accumulation environments, with respect to modern-day analogues. This investigation indicates that all three types of kerogens (Type I, II and III) may be present in the offshore southern Vietnam (Cuu Long and Nam Con Son Basins) region. Oil-prone amorphous-rich Type I is almost certainly present within lacustrine settings of Eocene-Oligocene and Lower Miocene age. Within the Cuu Long Basin, the Oligocene Tra Tan “D” Shale Formation probably represents the best oil-source rock potential. These sediments are lacustrine in origin and therefore predominantly of Type I kerogens. In the Nam Con Son Basin, the lacustrine elements of the rift sequence probably occur in the undrilled Pre-Cau Formation, and the Upper Oligocene Cau Formation and Lower Miocene Dua Formation are the result of shallow water, “swampy” sedimentary facies. 

In terms of source rock facies for the syn-rift section of both basins, there appears to be a significant difference with regards to source rock depositional environment and subsequent oil type. For instance, the Bach Ho (White Tiger) oil from the Cuu Long Basin has geochemical characteristics that strongly indicate that it is associated with sources that have developed in deep water lacustrine settings; whereas, in contrast, the syn-rift source sequences of the Nam Con Son Basin are not considered to include source facies that developed in “deep” meromitic, fault bounded lakes, but are the product of fluvio-deltaic to shallow water fluvio-lacustrine environments. Under such conditions several types of hydrocarbons can evolve. 

In both the Nam Con Son and Cuu Long Basins, post-rift sources are commonly associated with fluvio-deltaic environments of deposition. It is anticipated that there will be significant amounts of gas generated from post-rift sources in both basins where the thermal maturity conditions allow. From the maturation modelling carried out on numerous pseudowells, we have identified specific local settings in both basins where post-rift gas-prone sources are likely to have attained sufficient maturity for gas generation. 

A summary of predicted hydrocarbon types – deep water lacustrine, fluvio-deltaic, and shallow water fluvio-lacustrine facies – and general observations have been made in relation to the geochemical characteristics of “Rift” and “Sag” phase source rocks and sourced oils in both basins. 

Non-hydrocarbon gases, particularly CO2, have also been considered in terms of risk, particularly in the Nam Con Son Basin. 

The timings of generation and expulsion of different hydrocarbon phases across both basins have been analysed in detail by constructing numerous burial/maturation history models for wells and pseudowells. A total of eighteen modelling points have been chosen to provide adequate coverage across the two basins, to cover both their depocentres and flanks. Modelled wells utilised to verify the heat flow models, together with observed Vitrinite Reflectance data, are provided; in addition to all burial/maturation models for both wells and pseudowells, showing transformation ratios, timing of maximum hydrocarbon expulsion and the amount of predicted expelled hydrocarbons and residual potential. All the models are presented in full and the results have been summarised in terms of depth of maximum expulsion and timing. 

The results of the modelling have been assessed with respect to key moments in each basin‟s tectonic history, which relate directly to the formation of traps and the possible breaching of traps. The results of the burial/maturation history modelling have also been used to generate expulsion windows for Top Oil (0.8% Ro), Top Gas (1.0% Ro) and Top Main Gas (1.2% Ro) by integrating the results with the Base Tertiary Map. This, together with known hydrocarbon occurrences (producing and non-producing discoveries) and mapped prospects and leads, provides a powerful exploration tool for assessing hydrocarbon potential in different parts of the two basins, particularly, in less well explored areas away from the main depocentres. 

There are essentially nine proven plays within the Nam Con Son-Cuu Long area, five in the Nam Con Son Basin and four in the Cuu Long Basin. Many fields discovered to date are marginal and require multiple reservoir intervals to be economically viable. Potential plays are few in both basins. The remaining potential of both basins is thought to lie within currently known plays in as yet unexplored or under explored acreage, as has been demonstrated fairly recently by the Premier Oil Cá Rong Do discovery in Block 07/03, Nam Con Son Basin. 

For each play, whether it is proven or potential, the essential play elements have been analysed and the results of this assessment have been synthesised under the following headings: Reservoir; Source; Seal; Trap; Migration; Timing; Preservation; Risks/Critical Factors; Possible Reserves. The relationships of the essential play elements are shown through play cartoons

All mapped interpretational elements and underlying datasets are contained within the GIS project. The A3 hard copy report contains 176 colour pages.