This paper describes the interpretation of compressional and shear wave seismic data, recorded in a multicomponent ocean-bottom cable seismic survey in the Arabian Gulf, to define individual present-day fluid contacts in stacked limestone reservoirs in a partially-depleted oil field.
KJO recorded a multicomponent 3D-4C seismic survey over a dormant, partially-depleted oil field using ocean-bottom cable technology in 2012. Processing of the P-wave, shear-wave and reservoir characterization volumes provided unprecedented resolution of the reservoir architecture, rock and fracture properties, and reservoir fluid distribution to assist in the field redevelopment plan. The reservoir characterization results provided a variety of porosity, fluid & lithology indicators. Vp/Vs ratio was identified as the most reliable fluid indicator. This is expected to be low above the oil-water contact (OWC) and high below the contact.
The field produced oil for many years from five limestone reservoirs. After a multi-year shut-in period, engineering assessment of fluid contacts concluded that the four upper limestone reservoirs, of late-to-middle Cretaceous age, are in vertical communication but also compartmentalized by faulting. Similarly the lower limestone reservoir (early Cretaceous) is also compartmentalized by faulting. Structural interpretation of the new data provided detailed depth maps of each reservoir top together with an updated pattern of axial and wrench faults. Reservoir characterization processing of the new multicomponent seismic data delivered a number of potential fluid indicator seismic attributes: Poisson's Ratio, Vp/Vs ratio and AVO gradient. After evaluation of 16 different seismic volumes, the Absolute Vp/Vs ratio from the PPPS Joint Inversion was selected as the most robust fluid indicator. Structure maps, overlain with the Vp/Vs ratio seismic attribute, showed amplitude shut-offs consistent with structural contours, varying from fault block to block. These maps were used to define present-day fluid distribution for each reservoir level. The results confirmed a) the presence of remaining hydrocarbons in most of the late Cretaceous reservoir compartments; b) middle Cretaceous limestones are largely depleted except for isolated attic volumes, and c) limited depletion in the early Cretaceous reservoir.
This was the first multicomponent reservoir characterization project carried out by KJO. The Vp/Vs ratio results confirmed the fluid distribution as determined by the engineering assessment; identified the sealing faults responsible for compartmentalization of the field, and supported the engineering assessment conclusion that the shallow reservoirs are in vertical communication.
