Reservoir Inhomogeneities Deduced From Outcrop Observations and Production Logging

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Abstract
Many fields, where the reservoir is coil/posed of sandy layers, show great complexity because of the lack of continuity which results from a particular type of sedimentation. This complexity may be a factor of prime importance in studying reservoir drainage, especially in the displacement of oil by water or gas. This paper describes work done to study this heterogeneity and its influence in the exploitation of the fields of the lower Devonian sandy shale in the Fort Polignac Basin in Algeria. Two lines of research were followed.On the border of the basin, a detailed sedimentological study established rules for the distribution of the sandy zones, which furnished hypotheses necessary for devising a geologic model of the reservoir. A certain number of sandy zones were recognized and characterized by their sedimentation.In the interior of the basin, production logs determined the productivity profile. Study of these point data led to a description of the distribution of oil and gas phases in the reservoir so that the continuity of different zones between the wells could be deduced. These techniques were applied to the study of the lower Devonian sandy shale reservoir of Zarzaitine. The sedimentological study guiding the correlation work, with the aid of the conventional static data (logs and cores), led to the establishment of a useful geologic model of the reservoir. Production logs, thanks to the contribution of dynamic elements, permitted corroboration of the model. In particular, the logs confirmed separation, as proposed by the sedimentological study, of two comparable sandy groups by revealing the continuity of a thin impermeable layer. invisible on conventional logs. INTRODUCTION The heterogeneity of sandstone reservoirs considerably influences the distribution of fluids. as well as their displacement in the reservoir. Rational development of a field and maximum recovery of hydrocarbons are closely tied to an exact and precise knowledge of the internal architecture. We will try to show how it is possible to refine the knowledge of the reservoir by seeking quite different and varied sources of information. Production geology, the group of techniques which leads to ideas concerning reservoir architecture and its geometric description, furnishes the framework of this undertaking. Of these techniques, those used so far are quite varied. Descriptive as well as quantitative, the techniques partake as much of classical geology as of reservoir science. They includegeologic studies of the entire basin, especially a stratigraphic and sedimentological study of the reservoir member at its outcrop, detailed observations of cores taken from wells and, finally. oriented cores;interpretation of geophysical data;interpretation of electric logs, nuclear and sonic logs and dipmeters:petrophysical study of the reservoir on samples taken from wells;reservoir dynamics (study of the production history and, in particular, interpretation of production logs); andanalysis of the fluids and thermodynamic studies. This incomplete list could also includestudy of the migration of the fluids,geochemistry (chemical characterization of the oils),regional hydrodynamics. andinjection and detection of tracers. If most of these techniques are classic, attention will nevertheless be called to applying two of the techniques in conjunction: the study of the reservoir member at its outcrop and the study of dynamic distribution of fluid phases in the reservoir through use of production logs, used in the study of the sandy shale reservoirs of the lower Devonian of the Fort Polignac Basin, Algeria. In particular, their application to the study of the Zarzaitine field where they have led to a new and satisfactory model of the reservoir will be described.

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