* INPEX
H-CoreW-18-Inpex-Abadi
PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Core Workshop, Jakarta 16-17 October 2018
SEDIMENTOLOGICAL CORE DESCRIPTION AND DEPOSITIONAL FACIES INTERPREATION IN RELATION TO RESERVOIR PROPERTIES
IN THE ABADI GAS FIELD Tsuyoshi Shikama*
Keitaro Kojima*
Koichi Kihara*
Kentaro Hasebe*
ABSTRACT
An extensive core observation was conducted for the Middle Jurassic Upper Plover Formation in Abadi gas field. The result has been used as the fundamental inputs for constructing the geological/reservoir models.
Based on the core observation, 9 major lithofacies were defined based on lithology, grain size, visually estimated sand-silt content and the presence or absence of primary sedimentary structures and bioturbation.
Ichnofabric analysis was also conducted in order to predict the paleo depositional environment. In the Upper Plover Formation, trace fossil assemblages are capable of providing sensitive information on fluctuations in depositional energy levels, water depth, sedimentation rates, salinity, storm, water turbidity, substrate consistency and dissolved oxygen levels.
By integrating the stratigraphic unit, lithofacies, the ichnofacies and petrophysical properties (i.e., porosity, permeability, etc), the Facies Association (FA) was defined in order to construct the depositional model and to evaluate the distribution of reservoir rock properties in the reservoir (i.e., porosity, permeability and Sw, etc.).
This paper summarizes the process of FA definition in relation to reservoir properties in shallow marine siliciclastic strata based on the detailed core observation in the Abadi gas field.
INTRODUCTION
The Abadi gas field is located in the Masela PSC Block, c.a. 300km to the east of the Timor Island in the Arafura Sea (Figure 1), on the edge of the Australian continental shelf with water depths ranging from c.a. 400m to 800 m. The field was discovered in 2000 by the first exploratory drilling.
3D seismic survey was carried out in 2001 and total of 9 appraisal wells have been drilled to date, which confirmed a substantial gas accumulation in the sandstone reservoir of Jurassic Plover Formation with thickness of 100 – 150 m.
Palyno-stratigraphic interpretation indicated reservoir section (Upper Plover Formation) at Abadi wells were dated as probably Bathonian to Callovian.
Fine to medium-grained quartzenites show variety of reservoir properties with porosity ranging from 1 to 18% and permeability from 0.001 to 2,800 md (average 300md). In general, FA which is defined by the combination of lithofacies and ichnofacies within stratigraphic framework, as well as grain size and related diagenesis provide principal control on reservoir property.
DATABASE
The datasets of this study are summarized table in below:
Data Quantity
Core sample c.a. 400m
Core plug + side wall core
c.a. 1300 point
Thin section c.a. 570 point
Borehole image with other wireline log
10 wells x reservoir interval (100-150m)
STRATIGRAPHY
The stratigraphic (palyno-stratigraphic) interpretation was performed for cores and sidewall core samples in all Abadi wells over the Plover Formation to confirm the palyno-stratigraphy and establish a stratigraphic framework for the Abadi field by referring the scheme introduced by Foster (2001, within the preface of AAP Memoir 24). The reservoir section (Upper Plover Formation) at Abadi wells were dated as probably Bathonian to Callovian.
The reservoir interval is subdivided into five stratigraphic units, Zone A to E, defined based on sequence stratigraphic interpretation as well as palyno-stratigraphic correlation.
Figure 2 shows the typical relationship between the palyno-stratigraphy, sequence stratigraphic interpretation and reservoir zonation.
LITHOFACIES
Total 9 lithofacies were classified by lithology, grain size, visually estimated sand-silt content and the presence or absence of primary sedimentary structures and bioturbation intensity, shaliness and cementation. Main character of each lithofacies is summarized in Table 1.
In many case, cryptically bioturbated sandstone (Sc) is difficult to distinguish from massive sandstone (Sm), so that wireline formation borehole image and permeability profile are used for facies determination.
Core GR information was used for consistent classification of bioturbated sandstone (Sb), bioturbated muddy sandstone (SMb), cross laminated sandstone (Sx) and cross laminated muddy sandstone (SMx).
The core-based lithofacies indicated robust consistency with the interpretation of wireline borehole image log, hence the lithofacies was extended to the whole reservoir section.
ICHNOFACIES
Ichnofossil provides useful information in order to distinguish between open marine environment and brackish (estuarine) environment. Detecting ichnofossil appearing in specific environment is important. Diversity, physical appearance and size of ichnofossil is also support us to interpret the paleo depositional environment.
Total 8 ichnofacies were defined based on the ichnofossil assemblage (Table 2). The ichnofacies analysis results indicated that depositional system of Abadi Field is dominated by a marine to brackish water deltaic-shore continuum, possibly crossed by channel. In general, under brackish environment, morphology of ichnofossil tends to be simple and their diversity is generally low (Figure 3). Especially U-shape or simple vertical burrow and or horizontal deposit feeder trace are common in brackish environment. On the other hand, marine ichnofossil tend to be larger and robust appearance.
The Abadi cores although showing, the most part, a marine ichnofossil assemblage, the ichnofacies are small and dominated by morphologically simple structure. The sandstones are fine to very fine which are carbon rich and do not display prominent wave or tidal sedimentary structures. This indicates that we are dealing with a relatively low energy environment such as tide dominated estuary i.e. incised valley system (Figure 4). This system indicates a protected embayment marine that may similar to the modern portion of Norton Sound on west coast of Alaska or Chesapeake Bay along the east coast of the United States.
FACIES ASSOCIATION
In this study, the Facies Association (FA) was defined by combination of the stratigraphic unit, lithofacies, the ichnofacies and petrophysical properties (i.e., porosity and permeability, etc.).
The FAs were utilized to construct the depositional model and to evaluate the distribution of reservoir rock properties.
Three main depositional environments were identified i.e., i) estuary embayment, ii) open marine shoreface and iii) delta influenced open marine.
These environments were subdivided into certain depositional regions, which were represented by the FAs. Total 13 FAs were defined for the Abadi reservoir (Table 3).
Figure 5 shows the porosity vs permeability colored by FA. FA reasonably captures petrophysical properties. The FA definition and depositional model were utilized as a foundation of the geological model construction.
CONCLUSION
• Total 9 lithofacies were defined based on the core observation (i.e., grain size, sorting, intensity of bioturbation, sedimentary structure, trace fossil, etc.). As an advantage, these lithofacies could be predicted by wireline borehole image log.
• A practical ichnofacies classification (8 ichnofacies) was proposed based on the detailed ichnological study. The ichnofacies is helpful to interpret depositional environment and to understand the complex coastal geography in Abadi reservoir.
• Total 13 FAs were defined by having the combination between lithofacies and ichnofacies.
The FAs are useful to reconstruct the depositional environment for the Abadi reservoir.
Also, FA and their distributions facilitate the reservoir properties (i.e., porosity and permeability, etc) prediction.
ACKNOWLEDGEMENTS
The authors would like to express their sincere thanks for INPEX Corporation, Shell acceptance to the publication of this paper. MIGAS generously permitted our publication of the data from MASELA PSC.
REFERENCES
FOSTER, C.B. (2001). Introduction to Studies in Australian Mesozoic Palynology ll. Eds: Laurie, J.R
& Foster, C.B., Assoc. Aust. Palaeont. Mem. 24. i-iii.
MacEachem, J.A., Pemberton, S.G., Gingras.,M.K., Bann, L.B., Ichnology and facies models, Facies Models 4, p. 19 – 58.
Pemberton, S.G., 2014, INPEX internal workshop material; Ichnological dynamic of Abadi cores, Offshore Indonesia.
Kojima, K., 2014, INPEX internal technical note;
2014 Facies Study.
Hasebe, K., 2014, INPEX internal technical note;
2014 Borehole Image Analysis.
TABLE 1 LITHOFACIES CLASSIFICATION
Lithofacies Code Description Possible depositional
Environment Massive
Sandstone Sm
Consists of quartzarenite, no lamination, commonly well sorted, pebbly sandstones and are classified as Sm facies.
Upper shoreface, delta front, bay head delta, tidal channel and tidal inlet
Cryptically bioturbated sandstone
Sc
Fine to medium grained, well sorted, consists of quartzarenite, faint lamination in some samples, remarkable mosaic pattern in FMI image, homogeneous property in profile permeability, only severely bioturbated interval should be assigned to Sc facies (ex. weakly cryptic-bioturbated sand stone with weak lamination will be assigned to Sx.)
Tidal inlet, upper shoreface
Cross laminated sandstone
Sx
Consists of quartzarenite, commonly low or mid angle of planar bedding or trough cross bedding, mud drape (partly double mud drape) in some case, hummocky cross stratification or swaly cross stratification in some case.
Tidal channel, lower to upper shoreface, tidal inlet, delta front
Bioturbated
sandstone Sb
Fine to course grained, basically clean, moderately to abundantly bioturbated (BI ≧ 3), lamination in some case.
Lower – middle shoreface, shoreface in sheltered embayment and tidal flat
Cemented
sandstone Cem
Basically clean, severely cemented, regardless of cementation minerals, no visual porosity, discernable in FMI image due to high resistivity, likely to appear near boundary of TSE, highly cemented but shaly interval will NOT be included in Cem facies.
Variable environment with above lithofacies
Cross laminated muddy sandstone
SMx
Very fine to fine grained, quartzarenite, alternated with mud stone, commonly mm - few cm current ripple, higher GR than Sm, Sb, Sx, Sc, Cem.
Tidal flat, central basin, storm dominated shelf and pro-delta
Bioturbated muddy sandstone
SMb
Very fine to fine grained, poorly sorted and argillaceous, moderately to abundantly bioturbated, higher GR than Sm, Sb, Sx, Sc, Cem.
Lower shoreface, offshore, central basin or tidal mud flat
Laminated
mudstone MI Laminated mudstone Pro-delta and central basin
Massive
mudstone Mm Bioturbated/massive mudstone Offshore environment,
central base.
TABLE 2 ICHNOFACIES CLASSIFICATION
TABLE 3 FACIES ASSOCIATION DEFINITION BASED ON COMBINATION OF LITHOFACIES AND ICHNOFACIES
Sm Sc Sx Sb Cem SMx SMb Ml MmCrypticMacaronichnus Skolithos Cruziana Zoophycos Skolithos(L) Cruziana(L) Brackish
c c c m c m m m
m c c c m m c
c m c m m m m c
m m c m m c m m c
c m m
m m m c c m c c c
c c c m c c c c
c c c m m c c c
m m c m m c c c m m m
c m c c c
c m m m m c
c c m m m c c
c m c m c
Offshore
Proximal Delta Front Delta front
Prodelta
Sheltered embayment
Open marine
Delta influence
Tidal flat
Lower Shoreface Facies Association
Tidal Inlet
Tidal Channel Esturary Shoreface
Bay Head Delta Central Basin
Shoal Upper Shoreface
Lithofacies Occurrence Ichnofacies
Figure 1 - Location Map
Kupang Dili
Darwin Saumlaki
Evans Shoal South Sunrise-Troubadour
Elang-Kakatua Corallina Laminaria
Buffalo
Jabiru Challis Cassini Talbot
Skua Petrel
Tern
Crux Cornea
Aru Islands
Tanimbar Islands
Timor
East Timor
Arafura Sea Kai Islands
Blacktip
Caldita Barossa
WA 285P
Evans Shoal
Ichthys 0 200km
Bayu-Undan Banda Sea
Indonesia Australia Abadi
Figure 2 - Stratigraphic analysis
Relative
sea Level
Figure 3 - Physical appearance of ichnofossil in brackish environment (left) and open marine environment (right)
Figure 4 - Analogy and modern model of depositional environment
