dang nhap 188bet Reservoir characterisation of deep water deposits: A case study from MMF30 interval, Hai Thach field, Nam Con Son basin

Summary

A good understanding of dang nhap 188bet complexity and architecture of a deep water reservoir type becomes ever more important to optimise its development and production. Recent technology advances such as higher resolution 3D seismic data combined with sequence stratigraphy in deep water reservoir settings and well log evaluation have resulted in enhancement of dang nhap 188bet understanding of turbidite complex reservoirs. In MMF30 reservoir, dang nhap 188bet turbidite sand is of good quality with sand thicknesses between 31m and 52mTVDSS, dang nhap 188bet average effective porosity between 14% and 16%, and shale volume between 22% and 29%.

Key words: Turbidite, reservoir characteristic, seismic attributes.

1. Introduction

Hai Thach field is located in Block 05-2, Nam Con Son basin. It was discovered by dang nhap 188bet exploration well 1 drilled on dang nhap 188bet Hai Thach horst structure. dang nhap 188bet well found gas and condensate accumulations in multiple stacked sand in Upper Miocene (UMA10), Middle Miocene (MMH10), and Lower Miocene (LMH10, LMH20 and LMH30). Well 1 has not met MMF30 reservoir. An appraisal well (well 2) was then drilled on dang nhap 188bet eastern flank of dang nhap 188bet structure and encountered additional hydrocarbon-bearing Upper Miocene (UMA15) and Middle Miocene (MMF10, MMF15 and MMF30) sandstones.

dang nhap 188bet MMF30 reservoir was deposited in dang nhap 188bet deep marine environment during dang nhap 188bet syn-rift of Nam Con Son basin. This reservoir has then been confirmed by three more producing wells, namely well 3, well 8, and well 9. According to well evaluation, MMF30 interval is a good reservoir with 14 - 16% average effective porosity and dang nhap 188bet gross sand thickness from 31m to 52mTVDSS. dang nhap 188bet distribution of this reservoir was also mapped by using seismic attributes such as AVO and Spectral-decomposition (Discrete Fourier Transform). This attribute mapping shows dang nhap 188bet fan size of approximately 2km in width and 5km in length.

dang nhap 188bet research on MMF30 reservoir characterisation and implication was carried out to understand dang nhap 188bet reservoir quality, distribution, and connectivity in order to optimise dang nhap 188bet field development and production.

2. Geological setting

Block 05-2 is located in a rift basin, named Nam Con Son basin, which overlapped dang nhap 188bet Indochina platform structure. Rift pulses occurred from dang nhap 188bet Palaeogene to dang nhap 188bet earliest Late Miocene. dang nhap 188bet general stratigraphy and structure were recorded as dang nhap 188bet pre-rift stage during Eocene to Early Oligocene, dang nhap 188bet first syn-rift stage from late Early Oligocene to Late Oligocene, dang nhap 188bet subsidence stage during Early Miocene; dang nhap 188bet second syn-rift stage up to Middle Miocene and dang nhap 188bet post-rift from Upper Miocene to recent time [1,2]. dang nhap 188bet rifting period contains dang nhap 188bet study interval (MMF30 reservoir).

Depositional environment during rifting phase were predominantly shelfal to basin floor with both clastic and carbonate sediments preserved, dang nhap 188bet upper part of this composite sequence is characterised by dang nhap 188bet preservation of deeper water clastic sediments. There is an overall back- stepping of facies belts observed from T40 to T60 (MMU), and an increased variation in facies, palaeoenvironments and thickness. dang nhap 188bet uplift and/or magnitude of erosion on some footwall highs have resulted in dang nhap 188bet absence of T60 (MMU), either through non-deposition or erosion. dang nhap 188bet top of MMU is dang nhap 188bet most prominent unconformity in dang nhap 188bet Nam Con Son basin. It is characterised by local erosional truncation, and by a basinward shift of facies in deep-water settings. Structural discordance is locally developed, and is observed best in dang nhap 188bet southeast where it is interpreted to result from footwall uplift and mild inversion. dang nhap 188bet unconformity was enhanced in deeper marine settings by submarine channelling which eroded underlying sections, and may itself have been further modified by subsequent younger channelling events. Nannofossils and foraminifera date T60 as Middle-Miocene [1].

3. Database and methodology

Datasets used in this study include high- resolution 3D seismic data in both time and depth domain, four (4) wells with full log curves, FMI (1 well), conventional core images (1 well) and some results of petrographic analysis of wells (4). dang nhap 188bet 3D seismic data covering an area of approximately 1,000km2 are of very good quality with high resolution, inline and crossline spacing of 12.5 x 12.5m. dang nhap 188bet dominant frequencies are from 30Hz to 60Hz. dang nhap 188bet seismic attributes including AVO and Spectral-Decomposition were used to interpret and predict distribution of turbidite MMF30 reservoir. dang nhap 188bet workflow of this research is defined in dang nhap 188bet following steps:

• Step 1: Sequence stratigraphy and paleoenvironment study;
• Step 2: Seismic attribute analysis;
• Step 3: Well log evaluation;
• Step 4: Structural sedimentology interpretations.

Step 1: Sequence stratigraphy and paleoenvironment study According to seismic sequence stratigraphy, it can be seen
clearly that MMF30 interval was formed within dang nhap 188bet period of strong development of turbidites/slumps as downlap and onlap terminations (Figure 1) [3]. This interpretation has been supported by other data, such as biostratigraphy and sedimentology features. 

Step 2: Seismic attribute analysis

Based on well logs, parameters of Vp, Vs, RHOB and NPHI of dang nhap 188bet four wells 2, 3, 8 and 9, AVO classes could be predicted as class 2 or 3 mainly (Figure 2).

Both AVO and Spectral decomposition attributes have been carried out to map MMF30 reservoir distribution.

Spectral decomposition is a relatively common processing technique that allows anomalies to be identified corresponding to not only dang nhap 188bet sand distribution but also dang nhap 188bet presence of hydrocarbon in a reservoir (Figure 3) [4].

Step 3: Well log evaluations

Integrated data of log analysis and petrography for MMF30 reservoir qualitatively and quantitatively for four wells were evaluated to improve dang nhap 188bet understanding of dang nhap 188bet reservoir characterisation and implication. dang nhap 188bet results of well-log interpretations were demonstrated and compared to each other (Figures 4 and 5).

Step 4: Structural sedimentology interpretations Generally, Bouma sequence is a favourite sequence deposited within deep marine environment. Each single Bouma sequence contains specific sedimentology structures. Ta has massive sand graded feature, Tb has plane parallel laminae feature, Tc has ripple, wavy or convoluted lamilae features and other upper layers (Td, e) are massive shale graded. According to conventional core images of well 8, dang nhap 188bet base of turbidite and its features of each sequence can be observed and interpreted as well (Figure 6) [5].

4. Results and discussion

According to published studies, MMF30 reservoir is deposited in deep water environment and interpreted as turbidite deposits. Moreover, based on conventional cores taken from well 2 and well 8, turbidite deposits cycles named Ta, Tb and Tc can be observed clearly. They were known as cycles of Bouma sequence (Figure 6). In addition to seismic sequence stratigraphic features, MMF30 interval was recognised as downlap and onlap terminations (Figure 1). These features were formed during sedimentation of turbidite deposits. dang nhap 188bet paleo-thrust submarine fan was seen clearly on seismic sections as truncational facies.

Integrated with FMI interpretation, dang nhap 188bet direction of dang nhap 188bet main sediment supply is from SW to NE (Figure 7) [6].

Distribution of MMF30 reservoir can be seen and mapped on both attribute maps (AVO and Spectral Decomposition). Figure 3 of Spectral Decomposition map shows dimensions of dang nhap 188bet MMF30 sand fan approximately 2km in width and 5km in length. dang nhap 188bet dim area to dang nhap 188bet South - South West of well 2 could be related to lithology change from sandy to muddy facies. dang nhap 188bet red line in dang nhap 188bet North East reflecting bright to dim amplitudes could be linked to fluid phase change (gas to water contact). Fault 2 as labelled on dang nhap 188bet map is post-depositional fault, therefore dang nhap 188bet lithology on both sides could be similar. Thus, dang nhap 188bet difference in AVO/Spectral Decomposition map in both hanging wall and footwall likely relates to fluid types (gas reservoir on dang nhap 188bet footwall and water wet on dang nhap 188bet hanging wall).

dang nhap 188bet cross-plots of petrophysical parameters of MMF30 reservoir can be evaluated and verified by dang nhap 188bet results of log interpretation as well [7, 8]. Average porosity varies from 13% (well 2) to 16% (well 3) with gross sand thickness variation from 31 to 52 metres. This reservoir is a gas bearing zone with 60% to 70% of gas saturation. These results show that dang nhap 188bet quality of MMF30 at well 2 has not reflected so well as others (Figures 4 and 5).

Sedimentological analysis of well 2 cores indicates dang nhap 188bet lower slope to inner submarine fan depositional setting for MMF30 sequence (Figure 8). Further to dang nhap 188bet North East of well 2, dang nhap 188bet core features of well 8 show numerous stacked turbidite deposit cycles with thick massive sand in dang nhap 188bet lower part indicating rapid deposition and it is overlain by lamination sand and shale layer which can be contributed to different portions of a Bouma sequence. This is reflected by dang nhap 188bet sedimentological structures of core samples at well 8 (Figure 6). dang nhap 188bet debris-flow deposits which are indicated in well 2 by abundant mud-clast with different orientation cannot be clearly recorded in well 8. This suggests that dang nhap 188bet progradation of shelf-slope during formation of MMF30 interval was limited around well 2 and it did not extend to well 8. Well 8 is therefore mostly located in inner to mid submarine fan. dang nhap 188bet well log curves of well 3 and well 8 show amalgamated stacking patterns on GR curve indicating their formation during dang nhap 188bet regressive period. dang nhap 188bet formation of transgressive shaly layer in dang nhap 188bet middle part of well 2 could be related to a rapid flooding period which cannot be observed in dang nhap 188bet other two wells. In general, there are series of SW-NE elongated stacked fan-lobe turbidites in Hai Thach field which are probably resulted from syn-rift activities of southwest-northeast trending faults during dang nhap 188bet Middle Miocene.

According to sedimentological analysis, MMF30 interval is mostly fine to very fine sandstone with moderate to well sorted, which are subjected to Lithic Arkorse/Feldspathic Litharenite in Fold’s classification (Figure 9). Classification of sandstone with more than 15% fine grained matrix among these wells indicates lower proportion of quartz contents of well 2 than that at wells 3 and 8. This could be interpreted that sediments at dang nhap 188bet well 2 location were deposited closer to dang nhap 188bet source supply than that at wells 3 and 8 due to dang nhap 188bet presence of less amount of any stable materials (quartz). Moreover, petrography analysis shows that dang nhap 188bet high cement content of calcite is an important key factor to reduce porosity. In addition, some thick calcite veins observed from log curves and sample analysis could be deposited from high carbonate structures formed in previous time (Early Middle Miocene) nearby Hai Thach field (Figure 8).

By integrating all data, it aims to define detailed subsurface for correlation of MMF30. As mentioned, some clear subsurfaces can be recognised to separate MMF30 interval into at least 3 main different zones. According to dang nhap 188bet correlation of MMF30, dang nhap 188bet conceptual model environment for this interested interval was constructed to demonstrate dang nhap 188bet MMF30 deposits. Figure 10 represents two main stages of MMF30 turbidites formed. Some slumps locally could be formed during dang nhap 188bet syn-rift period from local high structures.

It can be seen that a strong linkage between physical stratigraphy, depositional environments and reservoir architecture has been found in dang nhap 188bet MMF30 reservoir, turbidite sandstone. dang nhap 188bet advantages and limitations of development and production management at MMF30 reservoir provide a unique chance to test many of these deep-water reservoir/turbidite sandstone concepts, assumptions and interpretations regarding reservoir heterogeneity, connectivity and lithofacies distribution. These understanding and learnt lessons will be critical to dang nhap 188bet successful pursuit of future deep-water exploration, development and production opportunities.

5. Conclusions

Integration of geological and geophysical data showed dang nhap 188bet quality of MMF30 reservoir around dang nhap 188bet well 8 area is dang nhap 188bet best and decreasing to dang nhap 188bet North West (well 2) direction. Cores and log signatures around well 8 and well 3 show an amalgated sheet sand features (Mid fan) while around well 2 in upper most of fan with sand connectivity reducing around upper and outer fans. Some calcite veins could be considered as good markers and correlated from well to well.

References

1. S.J.Matthews, A.J.Fraser, S.Lowe, Simon P.Todd, Frank J.Peel. Structure, stratigraphy and petroleum geology of dang nhap 188bet SE Nam Con Son basin, offshore Vietnam. Geological Society London Special Publication. 1997; 126(1): p. 89 - 106.

2. Phung Van Phong, Nguyen Thanh Tung, Khuc Hong Giang, Nguyen Xuan Phong. Pliocene - Pleistocene sequence stratigraphy in Block 05-2, Nam Con Son basin, offshore Vietnam. Petrovietnam Journal. 2016; 10: p. 14 - 19.

3. Paul C.H.Veeken, Bruno Van Moerkerken. Seismic stratigraphic techniques. In “Seismic stratigraphy and depositional facies models”. 2013: p. 105 - 214.

4. Paul C.H.Veeken, Bruno Van Moerkerken. Seismic interpretation and reservoir characterization. In “Seismic stratigraphy and depositional facies models”. 2013: p. 375, 377 - 410.

5. Arnold H.Bouma. Sedimentology of some Flysch deposits: A graphic approach to facies interpretation. Elsevier. 1962.

6. Julian D.Clark, K.T.Pickering. Submarine channels, processes and architecture. 1996.

7. J.Ardill, R.Jensen, S.Evans, T.Garfield, R.Beaubouef. Deepwater reservoir characterization and implications for hydrocarbon production - An example from dang nhap 188bet Zafiro field, Equatorial Guinea. Offshore Technology Conference, Houston, Texas, USA. 2 - 5 May, 2005.

8. B.Doligez, H.Beucher, F.Geffroy, Eschard Rémi. Integrated reservoir characterization: Improvement in heterogeneous stochastic modeling by integration of additional external constraints. 1998.