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SEDIMENTOLOGICAL ASPECTS OF TURONIAN RUDIST-RICH PROVEN RESIRS IN CENTRAL TUNISIAERVO

BOUGHALMI Senda, NEGRA Mohamed Hédi, Ben Alaya Sonia, Zijlstra Hedia Bessaies, 2014

Tenth International Congress on Rudist Bivalves, 1, 10, 3, Juin 2014

Résumé

In Tunisia, Middle Turonian carbonates are mainly depos1ted in Central and South Eastern parts of Tunisia, along a trend extended from Kasserine to the northern part of the Gulf of

Gabes. This trend 1llustrates an extended platform occupying the Central South-Eastern

Tunisia and favouring the deposition of shallowmarine carbonates locally rich in entire and fragmented rudists. However, from the Western part to the SouthEastern part of this platform, series exhibit lateral changes in facies, depositional environments, diagenetic features and reservoir properties. These carbonates are oil and/or gas-bearing in several onshore and offshore fields. They are producing Oil in Rhemoura, Gremda, Guebiba (in the onshore of Sfax area) and gas in Miskar field (Gulf of Gabes; (1] [2]).

The present study is based on core petrographical analyses of a proven Turonian reservoir producing 0il in the Sfax area (CentralEastern Onshore).

Sedimentological studies show that these EarlyMiddle Turonian carbonates are organised into pluridecimetric shallowing upward cycles each comprising three units corresponding, from base

to top, to. - Bedded shghtly argillaceous wackestones containing floating rudist debris and sometimes calcispheres.

- Massively bedded partly dolomitised packstones rich in entire joined rudists.

- Bedded wackestones-packstones containing miliolids, oncoliths and gastropods.

Comparatively to the Jebel Bireno section (3), surveyed at Kasserine area, including relatively high energetic cycles starting with bioclastic limestones rich in sorted rudist debris and some geniculate forms [4], (5] and ending entire rudist rich lithosomes (6), {3], the South-Eastern part of the Turonian carbonatic platform appears relat1vely more restricted. In terms of reservoir properties, the highest values of helium porosity (reaching 27%) and permeability (reaching 700mD) were measured in rudist-rich carbonates. Pores which are partly filled with oil mainly correspond to the Radiolitids cells. In addition, some diagenetic features such as dolomitisation have created additional pores and have clearly enhanced the reservoir potential of these rudist-rich carbonates.