Effects of structural segmentation and faulting on carbonate reservoir properties: A case study from the Central Uplift of the Tarim Basin, China

Abstract

The largest condensate field of the Ordovician reef-shoal reservoirs, in China, was discovered along the N°1 fault zone in the Tarim Basin. However, current drilling activities are hindered by the low and unstable production from these carbonate reservoirs. A detailed structural analysis of 3-D seismic data identified three segments along the N°1 fault zone characterized by differences in structural geometry and evolution. Segmentation of the N°1 fault zone has resulted in the variation of characteristics and distribution of the Upper Ordovician carbonate reservoirs. Due to fault induced differential uplift before the Late Ordovician, a narrow and thick-rimmed platform margin rich in reefs has developed in the eastern segment, whereas wider and thinner shoals developed in the central and western segments. We clearly identified a fault block uplift related erosional unconformity, and associated karstification at the top of the Upper Ordovician carbonates in the eastern segment. In this segment, the karst caves resulted in the high and stable hydrocarbon production. In the central segment, good matrix reservoirs exist, but due to the lack of fault-related enhancement of the reservoirs quality, only relatively low production is observed. In the western segment, where fracture-vug reservoirs occurred in relation to later fault damage zones, production is much greater but unstable. Large-scale fracture-caves reservoirs related to fault activity, rather than the good unfaulted matrix reservoirs, lead to the high production of oil and gas. We classify these reef-shoal reservoirs as faulting modified reservoirs. Further exploitation should focus more on the fault damage zone beyond the platform margin rather than the microfacies in the Lower Paleozoic carbonates.