Oil and Gas Production from Carbonate Reservoirs: ABSTRACT

Abstract

Non-reef carbonate reservoirs include some of the most famous and prolific oil and gas reservoirs in the world, despite the relative emphasis on reefoid masses in recent research and development programs. Approximately 20-25 per cent of the world's reserves are believed to be contained in non-reef carbonate rocks. For general purposes, carbonates are classified broadly as of chemical, evaporative, biochemical, organic, and clastic origin, with examples cited. Although there are classic examples of limestones and dolomites which preserve their lithologic and stratigraphic character for thousands of square miles, probably the majority will exhibit some type of facies differentiation within relatively short distances, both vertically and horizontally. These facies changes provide excellent stratigraphic trap possibilities, even in non-reefed rocks. Oil and gas reservoirs in carbonates may be caused by the structural deformation of rocks having widespread, relatively uniform, intergranular porosity; by the development of fracture and fissure systems at favorable positions with respect to fluid migration; and by permeability pinchouts in connection with facies changes. Potentialities of carbonates as oil sources are not fully understood, but oil in minor quantities at least may be generated from certain facies. Development practices in carbonate reservoirs are influenced by the characteristic irregularity of pore space, the chemical sensitivity of the rock to both natural and artificial leaching, and such physical characteristics as structural strength, rigidity, and competency. Although internal differences from conventional sand reservoirs are marked, nevertheless it has been demonstrated that production histories of carbonate reservoirs provide almost identical patterns with sand reservoirs of comparable drive. Primary recovery estimates by volumetric methods necessarily contain large factors of error. Secondary-recovery programs are proving successful in carbonate reservoirs as well as in sands, but the effect of fracturing and channeling must be considered to avoid by-passing large reservoir volumes. Reservoir-heating techniques may be proved to be applicable. In future prospecting, applied sedimentology may have as large a field in the carbonate rocks as it has had in the clastics. Practical trendology will also be useful, in addition to the conventional fold-finding techniques which have dominated exploration methods in the past. End_of_Article - Last_Page 2607------------