4th EAGE North African/Mediterranean Petroleum and Geosciences Conference and Exhibition Tunis 2009

The study concerns the Oligo-Miocene aged Numidian Flysch which forms the highest structural unit in northern Tunisia and consists of an interbedded unit of highly quartzose sand and shale, which are of turbiditic affinities. An integrated petrographic and geochemical study of this formation was carried out to infer the sandstones provenance, and reservoir characteristics. On the basis of the framework composition (point counting) and whole-rock geochemistry (major elements), the sandstones are classified as quartzarenite, and sublitharenite types. The modal analyses imply a recycled orogen provenance. Moreover, petrographic point count data indicate quartz-rich sedimentary (recycled), middle to high-grade metamorphic and plutonic parent rocks for Numidian sandstones. According to the plate and tectonic reconstructions of the western Mediterranean Sea it appears that the kabylean basement terrain cropping out along the Algerian coast and forming part of Calabro-Peloritani-Kabylian zone to be the parental source of the Numidian of both Sicily and Tunisia. Additionally, petrographical study indicates that the reservoir properties of the Numidian Flysch are a direct function of grain size and textural maturity.

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In this paper a strategy is outlined to analyze illuminating beams for optimally locating target zones, and optimally acquiring all necessary seismic data. Illumination analysis of wave propagation through the earth’s subsurface plays a very important role, in locating shadow zones and target prospects, to optimize the success of exploration wells. Using illuminating beams for optimal data acquisition design and data processing provides a unique integrated and cascaded tool for insight and interpretation of target prospects. The quality and accuracy of seismic images is being calculated from weighted energy distributions along areas that are dominantly contributing to potential oil and gas reservoirs. During illumination analysis, migration parameters are being determined to locate target images in shortest time. The a priori integration of seismic data acquisition design and optimal imaging of target zones minimizes the overall expenses for seismic data acquisition, data processing, and high costs of exploration wells.

The onshore part of Sirt Basin is the most prolific petroleum province in whole Africa. It is a large Mesozoic-Cenozoic rift basin. The offshore part has been limited explored without any big success. The main objectives of this presentation are to discuss the possibility that the onshore petroleum systems extends offshore, based on improved understanding of the regional structural and sedimentological settings. 2D seismic interpretation, modelling of gravity and magnetic anomalies, source maturity modelling, structural and sedimentological descriptions have been carried out. Bathymetry and gravity and magnetic anomalies indicate that some areas have been heavily influenced by different volcanic activities (seamounts and sills). The marine Upper Cretaceous Sirt Fm. is expected to be the most probable hydrocarbon source. The thick overburden (2000-7000 meters) in the offshore extension of Ajdabiya Trough indicates that the source, if present, will be gas and/or oil mature in different parts of the area. Lower Cretaceous Nubian sandstones (expected main reservoir) have been penetrated but are heavily calcite and /or quartzite cemented. This study gives a better understanding of the Mesozoic and Cenozoic basin evolution of offshore Sirt Basin. The hydrocarbon potential is good. Reservoir quality is considered as the main risk factor.

Petrophysical properties in the Acacus formation are quite variable and inhomogeneous since the reservoir shows substantial diagenetic overprint and layer heterogeneity. To build a sound property model on such conditions it is imperative to establish a petrophysical rock classification concept first by comprehensive integration of log, core and test data. Due to variable diagenetic effects (mainly chloritization and siderite cementation) there is obviously no simple poro-perm relationship in the Acacus sands. Therefore an empirical rock classification approach based on poro-perm to pore-throat relationship has been applied (Winland-R35). Preliminary results of the Winland-R35 quick-look on core data allow the discrimination of four main petrophysical rock types. Mercury intrusion tests (HPMI) carried out on representative samples show rock class specific shapes of capillary pressure curves which endorse the initial classification by Winland-R35. To apply this rock typing concept to the entire reservoir section the correlation of core permeability to irreducible water saturation from NMR log data has been used to derive a permeability estimate which was finally scaled to the permeability thickness derived from DST. The petrophysical rock classification and its specific parameter sets were used to initialize the geo-model as well as to control the spatial distribution of properties.

The present study deals with Pickett's plot to develop the reservoir petrophysical parameters of Abu Roash Formation in Ras Qattara area, Western Desert, Egypt. These parameters include capillary pressure, pore throat aperture radii, height above the free-water table and bulk volume of water. The method involves the use of log-log plots of the effective porosity vs. resistivity combined with empirical equations for calculating the for-mentioned reservoir characteristics of the considered rock unit. Plot of log Rt vs. log Ф can be drawn in a straight line with a slope controlled by the cementation factor (m). This plot results in a straight line for intervals with constant capillary pressure and constant pore throat aperture radii, where the capillary pressure at any point on the Pickett's plot is consistent with porosity, permeability and water saturation at that particular point. The value of Rw is not known in advance, provided that the reservoir contains some water-bearing intervals. However the integration of permeability, capillary pressures and pore-size classes of the pores on a log-log graph of porosity vs. resistivity gives the importance for Pickett's plot to be used in reservoir interpretation.

The main controls on hydraulic properties and hence the fluid (oil-water) distribution in the porous reservoir media is of major importance for reservoir description. The controls on porosity and permeability in the Lower Cretaceous (Nubian sandstone formation) of study area Sirt Basin, Libya are considered in this study. The primary depositional texture, quartz overgrowth and clay minerals are shown to be the key control to the hydraulic properties. Primary depositional texture results in existence of distinct rock types with similar pore throat attributes described using Global Hydraulic Elements (GHE's) approach.

Understanding the regional events that configured present-day North Africa is a basic step in any hydrocarbon exploration programme. The generalised term of “cratonic basins”, as these basins are often referred to, masks a vast geological history with a sedimentary record that goes back to the Neoproterozoic, i.e. 1000 m.y. ago. Since then, dramatic geological events have alternated with long periods of quiescence that have resulted in basin architecture and hydrocarbon occurrence at present day. Given three outstanding peak periods of this geological history such as the Pan-African Orogeny (1), the Hercynian to Tethys transitional events (2) & the Alpine cycle (3); surface geology, potential data and regional sections can shed light on key exploration issues at different stages of basin exploration.

The study explores the potential of numerical models to predict tectonic stresses and fracture networks on a reservoir-scale. A workflow is presented to build such geomechanical models on the basis of finite element techniques. Input data required are the reservoir geometry, mechanical material properties and boundary conditions. Modeling results are calibrated against observed data, e.g., in-situ stress measurements and fractures from cores and logs. This validated model is then used to predict stresses and fracture properties in the inter-well space. To assess its practical value this workflow for geomechanical modeling is applied to data from a reservoir in the eastern Sirte Basin/Libya.

The careful study of the seismic data shows at the edge of Chott Jerid a flexural fault, dipping to the north, with a general E-W orientation associated with a high amplitude tectonic inversion. This inversion implies the presence of a graben situated between this E-W normal fault and a second major fault, dipping to the south. An N-S extension during Permian and Triassic times has most probably initiated the opening of the graben. Later, during Upper Jurassic and Early Cretaceous, subsidence was very active. The NW-SE compressions, from uppermost Cretaceous to Upper Eocene initially then during Late Miocene, induced the tectonic inversion. Later, an intensive compression phase affected the Mio-Pliocene series. The E-W faults played like reverse faults accompanied by important folding. This is the N-S Villafranchian (Quaternary) compression. Consequently, the northern area affected by Quaternary compression is now considered as of secondary importance to hydrocarbon exploration. Furthermore, the location of the structures identified on both sides of the southern flexural fault is critical for the structures feeding and for the enhancement of the petrophysical properties of the targeted reservoirs.