Results: Biofacies – Outer Ramp (deep water carbonate setting)

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Results: eg. Biostratigraphy Models

Preliminary Biofacies studies of the Ratawi, Minagish and

Makhul formations, Kuwait

Stephen Crittenden and Maha Al-Baghli

Introduction The Tithonian stage of the Late Jurassic with the Berriasian and Valanginian stages of the Lower Cretaceous in Kuwait comprise a proven hydrocarbon bearing interval.(Ref. 1) • Producing hydrocarbons in Kuwait since mid-20th Century.

• Fields are „giants‟ and include Minagish Field (1958/9) (Ref. 2), Uum Gudair Field (1962) (Ref. 3) and Wafra Field (1953). (Ref. 4)

• Large structural 4-way closure Traps at multiple stratigraphical levels. (Ref.

5)

• Predominantly shallow marine platform carbonate reservoirs (Ref. 1, 2 & 3)with minor clastic reservoirs – secondary (eg. Sabriyah Field).(Ref. 5)

• Three conventional lithostratigraphical units: Ratawi, Minagish and Makhul formations. (Ref. 6)

• Hydrocarbon shows in the three formations regionally in Kuwait. (Ref. 5)

OBJECTIVE: Evaluate lithostratigraphy and biostratigraphy data. Establish a predictive Holistic Stratigraphy framework (eg.

Ref. 7 & 8) useful in the subtle trap exploration play concept.

A: Biostratigraphical / Biofacies Data Source and collect existing “in-house” well & outcrop data.

Search published literature. (eg. Ref. 1) Evaluate existing Lithostratigraphy, Biostratigraphy, Chronostratigraphy data and frameworks.(eg. Ref. 9)

Generate new data: biostratigraphy, biofacies, sedimentology, lithofacies, petrography.(eg. Ref. 10)

Evaluate, Integrate & Interpret all data sets.

B: Holostratigraphy Develop a Biostratigraphy and Chronostratigraphical Model.

Establish age-dating of the reservoirs and formations. Calibrate to regional chronostratigraphy – time lines.

Palaeoenvironment of reservoirs and formations. Develop a Depositional Sequence Stratigraphy Model.

Correlate wells (iterative process). Prediction of potential Source and Reservoir intervals.

Selected References 1. Al-Fares, A. A., M. Bouman and P. Jeans, 1998, A new Look at the Middle-Lower Cretaceous Stratigraphy, Offshore Kuwait. GeoArabia, v. 3, pp. 543 -

560. 2. Youash, Y. Y. & Mukhopadhyay, A., 1982, Geology of Minagish Oil Field, Kuwait. AAPG 66 (5), pp. 645. 3. Davies, R., Hollis, C., Bishop, C., Gaur, R. & Aziz Haider, A., 2000. Reservoir geology of the Middle Minagish Member (Minagish Oolite), Umm Gudair

Field, Kuwait. In: Middle East Models of Jurassic / Cretaceous Carbonate Systems. SEPM Special Publication No. 69, pp. 273 – 286. 4. Longacre. S. A. and Ginger, E. P. 1988. Evolution of the Lower Cretaceous Ratawi Oolite reservoir Wafra Field, Kuwait-Saudi Arabia Partitioned

neutral Zone. in Lomando, A., and Harris, P. M., eds., Giant Oil and Gas fields: SEPM, Core Workshop 12, pp. 273-331. 5. Carman, G., 1996. Structural elements of Kuwait. GeoArabia, v.1, pp. 239 – 266. 6. Owen, R. M. S. & Nasr, S. N. 1958. The Stratigraphy of the Kuwait- Basra area. In: Habitat of Oil, AAPG Memoir, 1252 – 1278. 7. Sharland, P., et al. 2001. Arabian Plate Sequence Stratigraphy. GeoArabia Spec Publ. 2 8. Davies, R. B., Casey, D. M., Horbury, A. D., Sharland, P. R. and Simmons, M. D. 2002. Early to Mid Cretaceous mixed carbonate-siliciclastic shelfal

systems: Examples, issues and models from the Arabian Plate. GeoArabia, v. 7, 3, pp. 541-598. 9. Al-Rifay, I. A. & Lemone, D. 1987. Calpionellids and the late Jurassic and early Cretaceous stratigraphy of Kuwait and the Gulf Region. Marine

Micropalaeontology, 12, pp. 383-388. 10. Banner, F. T. and Simmons, M. D. 1994. Calcareous algae and foraminifera as water – depth indicators: an example from the Early Cretaceous

carbonates of northeast Arabia. In Simmons, M. D. (ed.). Micropalaeontology and Hydrocarbon Exploration in the Middle East. British Micropalaeontology Society Publication Series. pp. 243 – 252.

11. Al-Husseini, M. & Matthews, R. K., 2008. Jurassic-Cretaceous Arabian orbital stratigraphy: The AROS-JK Chart. GeoArabia, v. 13, pp. 89 – 94. 12. Hughes, G. W. G. 2005. Calcareous Algae of Saudi Arabian Permian to Cretaceous Carbonates. Rev. Esp. de Micropal. V. 37, 1 pp. 131-140. 13. Sharland , P., et al. 2004. Chrono-Sequence Stratigraphy of the Arabian Plate. GeoArabia, v 9, 1, Enclosure 1.

Acknowledgments: This poster is presented with the

approval of the Ministry of Oil of the state of Kuwait and the Kuwait Oil Company. The encouragement, assistance and advice is acknowledged of all members of the Exploration Studies Team of KOC. Some primary data has been gleaned from proprietary “vintage” service company reports held in the archives of KOC authored by: Dr Osman Varol, Dr Peter Morris, Dr Mike Simmons, Fugro Robertson, Halliburton, Corelab , Weatherford and Badley Ashton.

Concluding Remarks The keys to well based sequence stratigraphy modelling for exploration are:

• A robust biostratigraphical model generated from integrated well data sets.

• Evaluate “old” biostratigraphical data gives value.

• Biostratigraphical control and identification of time lines eg. FS, = Chronostratigraphy.

• Beware of “shoe-horning” data to fit a preconceived model. A model is a guide only.

• The ”holistic approach” to stratigraphy has a predictive use for exploring new play concepts.

• Data from High Resolution Field Reservoir case studies contribute to development of a regional well-based predictive sequence stratigraphy.

Results:

• provide an enhanced understanding of the distribution of Lowermost Cretaceous reservoir potential lithologies eg. Shoal barriers.

• add value to an exploration strategy seeking the subtle stratigraphical trap - Risk Reduction.

This poster introduces some initial results from a biostratigraphical methodology that embraces well material generated data (both vintage and new) and models, to assist in evaluating the prospectivity of the lowermost Lower Cretaceous in Kuwait.

Materials and Method

Results:Biofacies–Inner Ramp shoal & Outer Ramp(eg. Ref.10)

Results: Biostratigraphy - Biofacies

Haq *Sharland et

al

2005 2004

139ma 136ma K30 Valanginian

141ma 138ma K20 Berriasian

144ma 143ma K10 Berriasian

147ma 147ma J110 Tithonian

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Late Jurass ic Unconformity (SB 149.0 Ma)

3rd Order MFS Position

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Stratigraphy and simplified lithology of the Makhul, Minagish and

Ratawi formations

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EAGE 2nd Exploration Workshop, Cairo. Dec 2010.

EXPO419

3rd Order Maximum Flooding Scheme for the latest

Jurassic - earliest Cretaceous (Ref. 7)

Lowermost Cretaceous Conventional Stratigraphy Model (Ref. 6)

?? Possible 3rd Order Flooding Event, perhaps equivalent to K20 fs of Sharland et al., 2001, 2004. Difficult to reconcile due to lack of / or sparse, age

diagnostic fossils.

Cyclical deposition in a relatively deep water distal part of a large carbonate ramp during an overall transgressive to highstand sea-level. These cycles are interpreted as high order changes in relative sea level that in a more proximal situation may result in greater lithological diversity eg. “oolitic / grainstone” shoals.

Outer ramp, 25 – 70m water depth indicator

Common Lithocodium 10 – 30m water depth indicator. (Ref. 12)

Calpionellids: characterise deep water, low energy environment. Mostly within argillaceous rich, laminated calcareous mudstones and wackstones. Proximal and distal outer ramp setting. These pelagic, calcareous unicellular organisms are studied in thin section hence re-study of petrographical thin sections provides good data. They are however prone to diagenetic obliteration and reworking (in clasts).

Chronostratigraphy: Integrated Biostratigraphy uses all available data from all groups studied: eg. calcareous nannofossils, dinoflagellate cysts, spores and pollen, calpionellids, radiolaria, ostracods, calcareous algae and foraminifera. All have been used to date the well sections, either in isolation or in an integrated fashion depending on data available. The proviso is that all are facies dependent / controlled – diachronous nature of local stratigraphical ranges within a shallow marine environment, and a detailed high resolution chronostratigraphic biozonation is not possible.

9454 ft FDO Abundant Calpionellids, plus Calpionella alpina

Candidate high order SB from core at top of cleaning-up cycles

The model acts as a broad guide. Observed data should not be “shoe-horned” to fit the model. The biostratigraphical data should be used to modify the model if necessary.

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PS = Polycostella senaria 13360ft

A maximum flooding based stratigraphy of Sharland et al indicates four 3rd order depositional sequences within the 2nd order sequence (duration 16 my). Evaluation of data indicates up to six 3rd order depositional sequences may be present as implied by AROS (Ref. 11). Maximum flooding “intervals”: either deeper water outer shelf shales or shallow water limestones. Well F is in an outer ramp deep water carbonate setting. The Minagish Formation is developed as non-reservoir facies.

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Calcareous nannoplankton: Due to shallow water palaeoenvironments and post depositional diagenesis, nannofossil assemblages in samples from the studied wells are of low abundance, low diversity and usually poorly preserved. Many of the expected stratigraphical marker species commonly reported in deep-sea sediments of Lower Cretaceous age are absent or only occur sporadically. Semi-quantitative analysis provides a means for assisting in the identification of marine flooding intervals – floral acme / abundance. FDO (extinction) of Polycostella senaria: is it a correlatable Time-event (local extinction level) in shallow water shelf carbonates? PS

Results: Palaeoenvironment Models

Major SB

Major SB

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?

?

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PS

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Microfossils and biofacies associations for the Makhul, Minagish and Ratawi formations indicate depositional environments ranging from shallow marine to deeper shelf / basin. An environmental history for the carbonate deposition can be determined that can help assist i) the resolution of layering / stratigraphical tiering in a reservoir and ii) regional correlation. Some biota events are interpreted as transgressive pulses that may equate to time lines. These constrain the reservoir lithofacies sequence stratigraphy interpretation.

Integrated Biostratigraphical Model - numerous fossil groups. Diachronous stratigraphical ranges of key taxa in shallow marine

carbonates – environmental control. Biofacies from prepared Petrographic thin sections.

Correlate wells (iterative process). ? Palaeogeographical Mapping.

Identification of potential Source and Reservoir intervals.

Offshore Kuwait

Minagish Fm as non reservoir facies

Minagish Fm as Oolitic / grainstone shoal reservoir facies

Well B Well A

Exploration Play Concept. The depositional regime for prospective reservoir development is the shoal barrier environment of the inner ramp in well B.

South Kuwait