Significance of Stylolite Development in Hydrocarbon Reservoirs with an Emphasis on the Lower Cretaceous of the Middle East

Abstract

Growth of stylolites and related pressure-solution features may adversely affect the continuity of carbonate reservoirs by producing barriers or impedences to fluid flow. The impact of stylolite development on reservoir performance, however, may vary greatly from one part of a reservoir to another. Therefore, it is important to know the distribution and the permeability of stylolites for development of an effective program of reservoir management. The variable impact of stylolite development is illustrated through analysis of a reservoir case history. In this example, stylolite frequency and cumulative stylolite amplitude increase from the crest to the flanks of the anticlinal closure which constitutes the field. These trends are largely controlled by: 1) the pattern of stress concentration developed during growth of the anticlinal structure and by 2) the inhibition of pressure solution by hydrocarbons along the anticlinal crest. Although stylolites occur throughout the reservoir, three principal zones of stylolite development are of concern with regard to reservoir management. Of the three, only the uppermost zone (D 1) is a significant barrier to fluid flow. Variation in permeability among the main stylolite zones probably is related to the timing of stylolite formation with respect to hydrocarbon entrapment. The impermeable stylolite zone (Dl) formed largely before hydrocabon entrapment. In this case, abundant crystal nucleation sites are present immediately adjacent to the pressure-solution feature allowing local precipitation of calcium carbonate derived from dissolution along the pressure-solution surface. The remaining stylolite zones (02 and 03) largely developed during or after hydrocarbon entrapment. Hydrocarbons inhibit precipitation of calcium carbonate by coating many favorable nucleation sites within the reservoir. Thus, the bulk of the calcium carbonate dissolved at pressure-solution surfaces can be trailsported away from the stylolite zone prior to precipitation. Localized cementation and permeability reduction, therefore, will not occur adjacent to the stylolite zone. Consequently, stylolites developed largely after hydrocarbon entrapment, in this example, do not constitute significant barriers to vertical fluid flow.