Regional depositional setting and pore network systems of the El Garia Formation (Metlaoui Group, Lower Eocene), offshore Tunisia

Abstract

This paper is part of the special publication Petroleum geology of North Africa (eds D.S. Macgregor, R.T.J. Moody and D.D. Clark-Lowes). The Metlaoui Group was deposited on a broad ramp that deepened to the north-east into the Tethyan Sea. The ramp contains a series of broad facies belts. The inner-ramp facies belt is composed of three facies tracts: (1) Faid sabkha (most landward) characterised by skeletal-poor dolomudstones and wackestones and interbedded evaporites, (2) Ain Merhotta restricted shallow- lagoon facies composed of sparse skeletal wackestones, packstones, and cross-bedded gastropod grainstones; (3) updip El Garia high-energy shoal complex (most seaward) composed of red algal-discocyclinid grainstones and packstones which are locally cross-bedded. The El Garia carbonates in the mid-ramp setting are predominantly composed of large, flat nummulites. Dominant textures are burrowed, mud-poor packstones and poorly sorted or bimodal grainstones. The accumulation was located below fair-weather wave base and above storm wave base, at a water depth estimated to have ranged from 30 to 60 m (middle neritic). The deep-water (deep neritic), outer-ramp Ousselat Member consists of coarse silt- to medium sand-sized nummulithoclastic debris that grades from coarser to finer debris down the ramp. The Bou Dabbous Formation is composed of basinal foraminiferal mudstones, wackestone and packstones deposited by suspension and less commonly by gravity-flow deposition in upper bathyal water depths. Reservoir quality is variable, with most lime packstones and grainstones having moderate to high porosity (related to intraparticle and microporosity), but only poor to fair permeability (generally less than 10 mD). The higher- quality reservoirs have preserved interparticle porosity with permeabilities ranging from tens of millidarcys to several darcys. Permeable nummulitic packstones and grainstones are favoured by the following factors: (1) low abundance of lime mud, (2) low abundance of nummulithoclastic debris, (3) low abundance of echinoderm fragments, (4) moderate sorting, (5) minor precipitation of late burial cements, (6) dolomitization.