Summary

Reservoir performance within the Illizi basin (southeast Algeria) is controlled both by sedimentary and structural factors. Reservoir heterogeneity analysis within several Palaeozoic horizonts is the main objective of this study. The aim is firstly to understand different production trends and secondly to analyze the reservoir potential of new zones.

Using electrical borehole images (FMI) from six wells, a new an extensive sedimentological and structural data set has been gathered and interpreted. This allowed several types of studies to be carried out: palaeocurrents' direction, correlation between different reservoir zones, analysis of fractures and faults, examination of the orientation of the regional in-situ stress field, and the implications for production and future exploration. Information obtained from borehole images has been integrated with core analysis data.

A new correlation based on wireline logs is proposed in this study. It divides reservoir zones into subzones that can be correlated within wells This correlation clearly identifies palaeocurrent orientation variations, changes in reservoir thickness and net-to-gross. In some cases, it also reflects porosity and permeability differences. This subdivision of reservoir zones is useful to identify good potential reservoirs, within both current and not yet developed zones.

The fracture network study shows two major orientations, NW-SE and NE-SW, that are associated with a N-S trend of basement related strike-slip zones. From the FMI data, fractures, microfaults and faults can be characterized and their electrical response correlated with core descriptions. A fracture density index and fracture density curves have been established to characterize the reservoirs.

The in-situ stress field shows a markedly NW-SE orientation of the maximum horizontal stress (Shmax). Local variations in the in-situ stress field occur in the proximity of highly tectonised zones in two of the wells.

Open or partly open conductive fractures, microfaults and faults oriented parallel to the direction of Shmax are expected to be more favourable for production.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah