G. Pyke1, J. Redfern2, R. Lander3, and L. Bonnell3
1Occidental Oil & Gas, Houston, Texas
2University of Manchester, Manchester, UK
3Geocosm LLC, Austin, Texas
The Ordovician sandstones of North Africa contain in excess of 5 billion barrels of oil equivalent in more than 50 separate accumulations across a broad region that spans from the Murzuq Basin of southwest Libya to the Ahnet Basin of central Algeria.
The sediments of the Mamuniyat Formation (Upper Ordovician) were deposited during the Hirnantian Glaciation of Western Gondwana at the northern margin of a large continental ice-mass, similar in size to the present-day Antarctic ice sheet. As is typical of ice-marginal depositional settings, a crude proximal to distal trend in sedimentary facies, and associated sandstone textures is observed. Within this trend however, outcrop and core analysis reveal a locally complex arrangement of depositional facies and textures, punctuated by a series of local, and regionally extensive unconformities, associated with multiple phases of ice sheet advance and retreat.
Petrographic analysis reveals that the sediments of the Mamuniyat Formation are dominated by quartz arenites and sub-arkoses. Petrophysical analysis highlights a complex distribution of porosity and permeability, caused by the combined effects of the inherited depositional fabric and a significant diagenetic overprint. The primary authigenic phases comprise quartz overgrowths and kaolinite. The extent to which each has developed is a function of rock texture, composition and the elevated temperatures and pressures encountered during progressive burial. As they develop, quartz overgrowths progressively restrict the dimensions of the pore network, reducing both the porosity and the permeability of a sandstone. Therefore, being able to predict the extent of their development is fundamental in predicting the regional distribution of porosity and permeability.
Because of the complex arrangements of sedimentary textures, composition affects and a variable burial history, great uncertainty exists in deterministic models of both the evolution, and present-day distribution of porosity and permeability within the Upper Ordovician interval. Previous assessments of reservoir quality within the Ordovician interval of the Murzuq and Ghadames Basins have made unreasonable assumptions regarding either the textural and compositional properties of the sandstones, or the burial conditions to which they have been subjected.
A subsurface database comprising approximately 400 wells and 100 thin sections was used to generate a 3-D burial model for the Mamuniyat Formation, and calibrated to the available geochemical data. A numerical forward (Touchstone) model, calibrated to the available petrographic and petrophysical data, was then generated to predict the effects of compaction and quartz cementation on the evolution of porosity and permeability through geological time. By employing stochastic techniques, a probabilistic model of reservoir quality evolution that accounts for the natural variability in sandstone texture and composition, was generated as a function of the temperature and effective stress encountered during burial.
AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.