Abstract

Knowledge of the distribution of rock fractures in the Asmari limestone reservoir rock of the prolific Khuzestan oilfield belt of southwest Iran provides for a better understanding of the production mechanism. Though the high productive capacity of wells in this area has been ascribed predominantly to fracturing of the reservoir rock, quantitative work on this topic has been neglected in the past.

Details of small-scale fracturing have been investigated locally on individual anticlines and regionally in Asmari limestone outcrops over an elongate area of about 2,000 sq mi (5,180 sq km) of the Zagros Mountains foothills. Fracture density has an inverse logarithmic relation to bed thickness, but it is independent of structural setting. Such findings make necessary the rejection of a theory involving a genetic relation of fractures of this scale to the folding process, at least in the area studied. The early formation of fractures is such that their orientations are related to localized irregularities, and their initiation by shock waves is suggested.

Specific values for average fracture spacing in the Asmari limestone beds provide valuable data for the reservoir engineer. Fold formation by the exploitation of appropriate preexisting fracture sets enhances reservoir porosity and permeability in preferred directions.

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