74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012 Copenhagen, Denmark, 4-7 June 2012

P053Biofacies, Palaeo-environments and Stratigraphyof the Ratawi, Minagish and Makhul Formationsof KuwaitS. Crittenden* (Kuwait Oil Company), M. Al-Baghli (Kuwait Oil Company),G. Gegas (Kuwait Oil Company), A.P. Kadar (Kuwait Oil Company) & P.Clews (Independent Consultant)

SUMMARYThree major micropalaeontological biozones / biofacies assemblages are described from the latest Jurassic(Tithonian) to Early Cretaceous (Berriasian to Early Valanginian) lower part of the Thamama Group inKuwait. The thin sections studied and analysed are predominantly from core samples but supplementedwith a small number of ditch cuttings where core was not available. The sample interval was irregular asthe thin sections were originally chosen for reservoir parameter studies that included petrography,microfacies, porosity and permeability.Three major bio-assemblages (defining local biozones) can be identified and approximate the MakhulFormation – radiolarite assemblage of the restricted platform muddy limestones; the Minagish Formationand Ratawi Limestone Member – calcareous algae / foraminiferal assemblage of shallow shelf, cleancarbonates including oolite / grainstone shoals; and the Ratawi Shale Member – foraminiferal assemblageof the mixed clastic and carbonate sedimentary environment.Within these three major bio-assemblages a number of subsidiary biofacies subdivisions, temporal andspatial, can also be identified. This allows the recognition of vertical stacking patterns and depositionalcyclicity, as observed in the core description and petrographic microfacies studies, of this importanthydrocarbon bearing interval.

74th

EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

Copenhagen, Denmark, 4-7 June 2012

Introduction

The Makhul, Minagisha 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.

This study presents the applied micropalaeontology technique of biofacies analysis (thin sections of

ditch cuttings and core plus routine washing preparation of cuttings). It 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.

Lithostratigraphy

The shallow marine carbonate sediments of the Makhul, Minagish and Ratawi formations (Al-Fares et

al., 1998) form the lower part of the Thamama Group as defined in Saudi Arabia (Steineke and

Bramkamp, 1952). It is underlain by the Hith Anhydrite Formation and the study interval top is a

major temporal hiatus separating the Early Valanginian from the Late Hauterivian – the Late

Valanginian Unconformity, KSB 40 of Sharland et al. (2002) (Figure 1).

Figure 1 The stratigraphy of the Makhul, Minagish and Ratawi formations in Kuwait.

The three formations in Kuwait are informal lithostratigraphical units defined and dated by calibration

with surface exposures and wells in neighbouring Saudi Arabia, Iraq and Iran (Steineke & Bramkamp,

1952: Owen & Nasr, 1958; Dunnington et al., 1959: Hosseini & Conrad, 2008).

Previous biofacies work

This study was aided by the substantial number of helpful taxonomic, stratigraphical and

palaeoenvironmental references that reflect the great deal of attention given to carbonate thin section

74th

EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

Copenhagen, Denmark, 4-7 June 2012

micropalaeontology in the Middle East during the last 50 plus years (Simmons (ed), 1994: Hughes,

2005). The available historical biostratigraphical and biofacies data for the Makhul, Minagish and

Ratawi formations of Kuwait however, are not substantial.

Method

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, originally used for carbonate reservoir parameter

characterization, 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.

Bioclast components

The biocomponents identified are environmentally sensitive and their presence or absence, their

distribution and number and stratigraphical variation contribute to the recognition and delineation

of biozones and biofacies.

Figure 2 The depositional model and biozones of the Makhul, Minagish and Ratawi formations in

Kuwait.

74th

EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

Copenhagen, Denmark, 4-7 June 2012

These biocomponents comprise benthonic foraminifera – lituolids, miliolids and small calcareous

genera, together with calcareous algae, sponge spicules, radiolaria, ostracods, calpionellids and

macrofossils (fragments) such as corals, serpulids, molluscs (rudists, bivalves and gastropods),

echinoid debris and bryozoans (Figure 2).

Biofacies layers and palaeoenvironment

The assemblages are dominated by benthonic foraminifera, echinoid and bivalve debris and

calcareous algae. Three major bio-assemblages (defining local biozones) can be identified and

approximate the Makhul Formation – radiolarite assemblage of the restricted platform muddy

limestones, the Minagish Formation and Ratawi Limestone Member – calcareous algae / foraminiferal

assemblage of shallow shelf, clean carbonates including oolite / grainstone shoals, and the Ratawi

Shale Member – foraminifera assemblage of the mixed clastic and carbonate sedimentary

environment. Within these three major bio-assemblages a number of subsidiary biofacies subdivisions

can also be identified.

Biozone LK1 (Makhul Formation facies)

This biozone characterised by a radiolarite facies comprising calcitised radiolaria, calcispheres and

calcitised sponge spicules, rare Bositra ?buchi and Saccomma spp together with significant amounts

of organic matter is typical of the muddy carbonates of the Makhul Formation west of the Kuwait

Arch. This relatively deep marine assemblage represents a euxinic, low energy, stratified marine

environment below storm wave base of an intra-carbonate shelf basin / embayment. The occurrence

of calpionellids, including Calpionella alpina (Early to Middle Berriasian age) and Tintinopsella

carpathica (Valanginian to Tithonian age) indicates a subtle lateral biofacies subdivision. In wells

west of the Kuwait Arch they are extremely rare or absent while in wells in the offshore area east of

the Kuwait arch in a presumed down-dip depositional regime (deeper water more open marine

connection) they are common.

Associated foraminifera in these lime mudstones are extremely rare – small agglutinants that

includeTextularia spp., Ammobaculites spp., questionable Valvulina spp. and Cyclammina /

Everticyclammina spp. Allochthonous shallower marine species, associated with thin wackstone

and packstone horizons (storm generated turbidites – tempestites), include miliolids and small

lenticulinids together with small fragments of algae, bivalves and echinoderms. This biozone, both

west and east of the Kuwait Arch, can be divided into two sub-biozones; a lower sub-biozone

characterized by the virtual absence of an in-situ fauna / flora and dominantly lime mudstones and an

upper sub-biozone characterized by a diverse but sparse in number in-situ benthonic and planktonic

fauna / flora and with an upward increase in the occurrence of wackestones and packstones.

Biozone LK2 (Minagish Formation and Ratawi Limestone)

This biofacies comprises a diverse assemblage of echinoid and bivalve debris, calcareous algae

fragments, rare radiolaria, rare ostracods, a few scattered sponge spicules, rare to common

calpionellids and common benthonic foraminifera including rare miliolids, all typical of an open

marine, relatively shallow, carbonate ramp. Biozone LK2 comprises packstones, wackstones and

developments of higher energy regime, clean- winnowed oolitic grainstone shoals, particularly in the

Minagish Formation, and skeletal debris shoals together with uncommon developments of low energy

regime lime mudstones.

Dolomitisation of some intervals has destroyed the original texture together with the contained fauna /

flora while in other intervals the fossils have been leached and the moulds replaced by calcite.

Species recorded include the 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

74th

EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

Copenhagen, Denmark, 4-7 June 2012

variation in assemblage components associated with differences in water depth and energy on a

carbonate shelf; eg. an outer ramp low energy setting biofacies and a shallow water, high energy shoal

biofacies.

Biozone LK3 (Ratawi Shale)

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.

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.

Acknowledgements

This abstract is published 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-Sahalan (Stratigraphy Sub-Team unit

head) and team members Irene Truskowski and Abdel Kadar H. H. Joussef.

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