Integrated Biostratigraphical Model - numerous fossil groups which are all environmentally sensitive. For example the diachronous stratigraphical ranges of key taxa in shallow marine carbonates is a result of environmental control.

The distribution and number of the bioclasts identified contribute to the recognition of biozones and biofacies – LK1, LK2 and LK3. These are useful for well correlation and the identification of potential Source and Reservoir intervals.

Semi-quantitative micropalaeontological biofacies analysis of more than 500 thin sections of core samples from more than 30 wells provides an independent insight into the environmental history of deposition of the interval.

The thin sections were not taken at regular spaced intervals in the cored sections and are randomly orientated with the result that species diagnostic features of the taxa recorded are often not visible. Accordingly, a broad approach had to be taken for discriminating various species and for the recognition of vertical stacking / cyclicity of the observed biofacies.

The recorded bioclast data together with core-facies, microfacies and textural data provide an invaluable technique for determination of both gross and subtle variations in the depositional environment and for recognition of sea-level changes and associated flooding events and surfaces.

Biofacies, Palaeoenvironment and Stratigraphy of the Ratawi, Minagish and Makhul formations, Kuwait.

S. Crittenden1*, M. Al-Baghli1, G. Gega1, A. P. Kadar1 and P. Clews2.

The Makhul, Minagish and Ratawi formations form the oldest lithostratigraphic units of the Thamama Group and represent a major 2nd order depositional cycle spanning the latest Jurassic (Tithonian) and Early Cretaceous (Berriasian and Early Valanginian) of Kuwait (Figures 1, 2 & 3).

The three formations in Kuwait comprise shallow marine carbonates and are “informal” lithostratigraphical units defined and dated by calibration with surface exposures and wells in neighbouring Saudi Arabia, Iraq and Iran.

OBJECTIVE

To present the applied micropalaeontology technique of biofacies analysis (thin sections of ditch cuttings and core plus routine washing preparation of cuttings) for the lower part of the Thamama Group in Kuwait.

Biofacies Analysis makes a significant independent contribution to regional stratigraphical resolution, to understanding depositional continuity and to supporting the recognition of vertical stacking patterns and depositional cyclicity observed in the core description and petrographic microfacies studies of this important hydrocarbon bearing interval.

Materials and Method

Results: Bioclasts - Biofacies - Biostratigraphy

The study interval top is a major temporal hiatus separating the Early Valanginian from the Late Hauterivian = KSB40. 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.

Fig 3: 3rd Order Maximum Flooding Scheme

for the latest Jurassic - earliest Cretaceous (Ref. 7)

Fig 1: Lowermost Cretaceous Conventional Stratigraphy Model

The maximum flooding based stratigraphy of Sharland et al (2004) indicates four 3rd order depositional sequences within the 2nd order sequence (duration 16 my). Our evaluation of data indicates up to six 3rd order depositional sequences may be present as implied by AROS (Ref. 1). Maximum flooding “intervals” are either deeper water outer shelf shales or shallow water limestones. Well F is in an outer ramp deep water carbonate setting.

Fig 2: Well F , Offshore Kuwait (Ref. 1)

Carbonate facies are defined on textures and palaeoecological factors, in predominantly shallow marine conditions within the photic zone on a flat, laterally extensive carbonate platform. There is a complex mosaic of facies and differentiation is based upon the degree of restriction of the shallow marine environment that controls the distribution of texture and grain size and the biotic assemblage (the biofacies). The biocomponents are dominated by benthonic foraminifera (lituolids, miliolids and small calcareous genera), echinoid and mollusc debris (rudists, bivalves and gastropods) and calcareous algae fragments. Minor components include sponge spicules, radiolaria, ostracods, calpionellids and macrofossil debris (corals, serpulids, bryozoans).

Three major bio-assemblages (defining local biozones - Kuwait) can be identified and approximate the Makhul Formation (LK1: radiolarite assemblage of restricted platform muddy limestones), the Minagish and Ratawi Limestone Member (LK2: calcareous algae/foraminiferal assemblage of shallow shelf, clean carbinates including shoals) and the Ratawi Shale Member (LK3: foraminiferal / ostracod and calcareous algae assemblage of a mixed clastic and carbonate environment).

Each of these local biozones can be subdivided in to subsidiary biofacies /biozones, laterally and in time.

1 KOC Exploration Studies Team, 2 Independent Consultant. 74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

Poster P053, Exploration & Plays session in Poster box 11, June 5, 2012

Biofacies Observations

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Introduction

Part 1 of 2

BIOZONE LK1 (Makhul Formation facies): radiolarite facies-calcitised radiolaria, calcispheres and calcitised sponge spicules. Rare Bositra ?buchi and Saccomma spp. Significant organic matter. Muddy carbonate.

Relatively deep marine water assemblage. Euxinic, low energy stratified below storm wave base of an intra-carbonate shelf / embayment. In wells west of the Kuwait Arch calpionellids are rare (Fig 4 & 5 Well A & B). In wells east of the kuwait Arch in a presumed down-dip depositional regime (deeper water more open marine) calpionellids are common.

Associated foraminfera are rare small agglutinants: Textularia spp., Ammobaculites spp., & possible cyclamminids. Allochthonous shallower marine taxa associated with thin wackestone/packstone beds (tempestites) include miliolids, lenticulinids & algae, bivalve, echinoderm fragments

Well A

Well B

BIOZONE LK2 (Minagish Formation and Ratawi Limestone facies):diverse assemblage of echinoid and bivalve debris, calcareous algae fragments, rare radiolaria, rare ostracods, rare calcispheres, few sponge spicules, rare to common calpionellids and common benthonic foraminifera (including rare miliolids). Shallow open marine carbonate.

Comprises lime mudstones, packstones, wackestones and developments of higher energy regime, clean well winnowed oolitic grainstone shoals, particularly in the Minagish Formation, and skeletal debris shoals.. Dolomitisation has destroyed contained flora / fauna in some cases.

Species recorded: benthic foraminifera Trocholina spp (high spired and low spired), cyclamminids such as Pseudocyclammina spp., Praechrysalidina spp., small Textularids, Vercorsella sp., Cuneolina sp. , Charentia sp., Nautiloculina sp., rare miliolids and small gavelinellids and frequent small Lenticulina spp.

Sub-biozones and sub-biofacies of various resolution scale can be identified in some wells where there is sufficient data and indicate subtle cycles of deposition and ariation in assemblage components associated with differences in water depth and energy on a carbonate shelf; eg. an outer ramp low energy setting biofacies (Fig 6 Well C) and a shallow water, high energy shoal biofacies (Fig 7, 8: well D & Fig 9 Well E)

Fig 4

Fig 5

Fig 6

Fig 7

Fig 8

Fig 9

Well D

Well D

Well C

Well E

Fig 10 Well F

BIOZONE LK3 (Ratawi Shale Member):The Ratawi Shale Member is characterized by a basal calcareous claystone / argillaceous carbonate to clastic sedimentation transition interval that is rapidly replaced upward by siliciclastic deposits (Fig 10, Well F).

Onshore Kuwait the lower part of the Ratawi Shale Member contains microfaunal assemblages comprising benthonic foraminifera including abundant Cyclammina / Everticyclammina spp ., Recurvoides sp., Trochammina sp., Protopeneroplis spp., Trocholina spp. and common small calcareous benthonic foraminifera including Lenticulina spp., together with common ostracods, microgastropods , mollusc debris, rare calpionellids, calcispheres, dinocysts and calcareous nannoplankton consistent with a shallow marine, inner ramp environment. Calcareous algae are absent except in the thin limestone beds. The recorded foraminiferal assemblage decreases in number into a low diversity assemblage in the overlying sandier and siltier non-calcareous shales. This permits division into upper and lower sub-biozones.

A sub-biofacies division reflects a progressive east to west change in lithofacies of this member; west of the Kuwait Arch the interval comprises the biofacies already described. East of the Kuwait Arch, the calcareous shale & minor siltstone / limestone sequence yields a similar biofacies but includes Gavelinella aff. barremiana, Lenticulina cf heiermanni, L. munsteri , L. macrodisca and Epistomina caracolla typical of outer ramp, deeper fully marine water. Sparse to common miliolids are recorded from the Ratawi Shale Member in south Kuwait suggesting a shallower water depth compared with wells in the north.

Conclusions. The biofacies study provides an important contribution to the regional palaeoenvironmental understanding of the Makhul, Minagish and Ratawi formations in Kuwait. Higher resolution biofacies studies of particular ‘time slices’ assist in stacking pattern recognition, sequence definition and correlation in order to aid exploration.

S. Crittenden1, M. Al-Baghli1, G. Gega1, A. P. Kadar1 and P. Clews2.

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

Poster P053, Exploration & Plays session in Poster box 11, June 5, 2012

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.

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?

Calpionellids: Characterise deep water, low energy environment. Mostly within laminated calcareous mudstones and wackstones. Proximal and distal outer ramp setting. Can they be used to indicate marine flooding events? 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).

Acknowledgments:

This poster is presented with the approval of the Ministry of Oil of the State of Kuwait and Kuwait Oil Company. The encouragement and advice are acknowledged of Abdul Aziz Al-Fares Team Leader of the Exploration Studies Group in KOC, Ghaida Al-Sahlan (Stratigraphy Sub-Team unit head) and team members Irene Truskowski and Abdel Kadar H. H. Youssef.

Results: e.g. Biostratigraphy Models

I. compactus N. quadratus N. Kampt. minor

N. Stein. minor M. obtusus ?P senaria

R. wisei ?T. sarmatus Z. xenotus

Results: e.g. Palaeoenvironment Model

Results: Depositional Models

T. longa

R. cf. cadischiana L. cf. hungarica

C. cf. simplex C. darderi

C. elliptica C. alpina

T. carpathica

Data and figures from Al-Rifaiy and Lemone, 1987. Bars below figures = 50 microns.

Selected References 1. Al-Husseini, M. & Matthews, R. K., 2008. Jurassic-Cretaceous Arabian orbital stratigraphy: The AROS-JK Chart. GeoArabia, v. 13, pp. 89 –

94. 2. 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. 3. 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. 4. 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. 5. Carman, G., 1996. Structural elements of Kuwait. GeoArabia, v.1, pp. 239 – 266. 6. 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. 7. Dunnington, H. V., Wetzel, R. and Morton, D. M., 1959. Iraq: Mesozoic and palaeozoic. Lexique Stratigraphique International., Asie, Volume III, 10a CNRS. Paris. 8. Hosseini, S. A. & Conrad, M. A., 2008. Calcareous algae, foraminifera and sequence stratigraphy of the Fahliyan Formation at Kuh-e-surmeh (Zagros Basin) SW Iran. Geologia Croatia, 61/2-3, 215-237. 9. Hughes, G. W. G. 2005. Calcareous Algae of Saudi Arabian Permian to Cretaceous Carbonates. Rev. Esp. de Micropal. V. 37, 1 pp. 131-140. 10. Owen, R. M. S. & Nasr, S. N. 1958. The Stratigraphy of the Kuwait- Basra area. In: Habitat of Oil, AAPG Memoir, 1252 – 1278. 11. Sharland, P., et al. 2001. Arabian Plate Sequence Stratigraphy. GeoArabia Spec Publ. 2 12. Sharland , P., et al. 2004. Chrono-Sequence Stratigraphy of the Arabian Plate. GeoArabia, v 9, 1, Enclosure 1. 13. Steineke, M. & Braamkamp, R. A. 1952. Mesozoic rocks of eastern Saudi Arabia (Abstract) American Association of petroleum geologists, 6, 909.

1 KOC Exploration Studies Team, 2 Independent Consultant.

Biofacies, Palaeoenvironment and Stratigraphy of the Ratawi, Minagish and Makhul formations, Kuwait.

Part 2 of 2

* Dr Stephen Crittenden is currently a Senior Geologist with Addax Petroleum Ltd, Geneva, Switzerland.