CONSTRAINTS ON DIAGENESIS AND RESERVOIR QUALITY IN THE FRACTURED HASDRUBAL FIELD, OFFSHORE TUNISIA

Abstract

The hydrocarbon reservoir of the Hasdrubal field (offshore Tunisia) lies within the Eocene El Garia Formation. This formation was deposited on a shallow north‐ to NE‐facing ramp in the Early Eocene and is composed of a belt of nummulitic wackestones‐grainstones. The nummulitic fades occupies a range of depositional environments from outer to mid ramp. In addition to Hasdrubal, several other producing oil‐ and gasfields have been discovered in the variably dolomitised El Garia Formation offshore Tunisia. Cores from three Hasdrubal wells were examined. Reservoir quality shows a limited relationship to primary depositional fabric and has been influenced significantly by compaction and later diagenesis. The highest permeabilities are typically developed within a dolomitised zone which occurs near the middle of the reservoir interval across the entire field, and which may follow a primary wackestone lithofabric (typically 20–30% bulk volume dolomite, with porosities of 15–22% and permeabilities of l‐30mD). Fractures, particularly in zones surrounding faults, have resulted in enhanced permeabilities. Combined results of isotope (δ18 O ‐5.0 to ‐7.3%oPDB) and fluid inclusion (Th 80–90d̀C) analyses of dolomites from this dolomitised zone indicate that matrix dolomites are burial diagenesis cements. Dolomitisation of the reservoir was a “closed system ” event and was not the result of major fluid flow or mixing. Magnesium ions for dolomitisation were derived from the transformation of high‐Mg to low‐Mg calcite in nummulite tests within the reservoir fades. Our analyses indicate that calcite cements were precipitated at temperatures of up to almost 150d̀C in primary and secondary pores and in variably‐sealed fractures Fracture lining and filling cements show a range of δ18 O values, which suggests that the fractures acted as fluid conduits over a range of temperatures during burial diagenesis Fracture densities measured in core increase rapidly close to seismically‐resolvable faults in the reservoir facies Fracturing probably resulted in the leakage of hydrocarbons through the Compact Micrite Member seal which overlies the accumulation, as well as facilitating the ingress of hot fluids from stratigraphically deeper levels in the basin

THE PETROLEUM GEOLOGY OF THE EARLY EOCENE EL GARIA FORMATION, HASDRUBAL FIELD, OFFSHORE TUNISIA

“…The unit's basal boundary with the underlying El Garia Formation probably represents a depositional hiatus because the upper surface of the latter formation is frequently burrowed, and the base of the "Compact Micrite" contains abundant evidence for substrate colonization, condensation and a marked increase in water depth. Diagenetic studies in the Hasdrubal field indicate that the El Garia Formation was not subaerially exposed prior to the deposition of the "Compact Micrite" (Macaulay et al, 2001). Nannofossil assemblages over this interval in the Malkadir-1 well are richer and more diverse, supporting the idea that this interval represents a maximum flooding surface.…”

“…Stylolites (often hydrocarbon stained) are common at bed boundaries. Closed-system dolomitisation is prevalent in the wackestone to packstone units and commonly leads to an improvement of the poroperm properties (Macaulay et al, 2001). No meteoric cements have been recorded in Hasdrubal samples, suggesting that the El Garia Formation has not been subaerially exposed at this location.…”

Petrographical and geochemical evidences for paragenetic sequence interpretation of diagenesis in mixed siliciclastic–carbonate sediments: Mozduran Formation (Upper Jurassic), south of Agh-Darband, NE Iran

“…Cross plots of carbon versus oxygen isotope values have been used by many researchers for geochemical interpretation (e.g., Hudson 1977;Anderson and Arthur 1983;Lohmann 1988;Morse and Mackenzie 1990;Nelson and Smith 1996;Rahimpour-Bonab et al 1997;Macaulay et al 2001;Mahboubi et al 2004Mahboubi et al , 2006Adabi et al 2006). Data from the cross plots of the Upper Jurassic limestones (Fig.…”

Sediment production and dispersal on foraminifera‐dominated early Tertiary ramps: the Eocene El Garia Formation, Tunisia

“…, 1989; Moody & Grant, 1989; Loucks et al. , 1998; Anketell & Mriheel, 2000; Macauley et al. , 2001; Racey et al.…”

“…These studies have identified the broad Metlaoui Group facies pattern, the role of topographic and structural highs during deposition, and also the diagenetic sequence and reservoir characteristics (e.g. Fournie, 1975;Bishop, 1985Bishop, , 1988Bernasconi et al, 1987;Moody, 1987;Bailey et al, 1989;Moody & Grant, 1989;Loucks et al, 1998;Anketell & Mriheel, 2000;Macauley et al, 2001;Racey et al, 2001;Jorry et al, 2003;Vennin et al, 2003). Therefore, with the broad palaeogeographic, stratigraphic and structural context already described, and with good outcrop and subsurface data available, the El Garia Fm provides the opportunity to address the role of Nummulites within early Tertiary carbonate deposystems.…”