Fractures and In-Situ Stresses in Fold Traps, Eastern Saudi Arabia: Predominance of Regional Over Local Patterns

Mohammed S. Ameen
Exploration Technical Services Department, Structural Geology & Rock Mechanics Group, Saudi Aramco, Dhahran, Saudi Arabia

Structural traps in Eastern Saudi Arabia are dominated by gentle, basement-rooted forced folds and to a lesser extent by salt-rooted domes. These anticlines have N-S trends, gentle flanks, amplitudes varying up to a few thousand meters, and axial lengths reaching up to a few hundred kilometers. The structures affect the Paleozoic to Cenozoic sedimentary sequences which are up to ca.7000 meters in thickness. Previous studies on fractures assumed local folding strain, and used curvature analysis and similar techniques to predict fracture distribution in anticlinal traps. This paper presents results of fracture and in-situ stress characterization in 15 major oil and gas fields using cores and borehole image logs. Tens of thousands of fractures were measured and analyzed in several reservoirs over a depth interval of 1500 to 5000 meters. The most frequent and open, tectonic fractures are subvertical with NE to ENE strikes. Furthermore, the maximum horizontal in-situ stress is regionally oriented in NE to ENE direction, nearly parallel to the dominant open tectonic fractures. It shows little deflection around local structures and with depth, between different reservoirs. The regional fracture and maximum horizontal in-situ stress pattern appears to be orthogonal to the Zagros fold belt, and is attributed to regional stresses arising from the collision of the Arabian plate with Eurasia.

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