EVALUATION OF HYDROCARBON RESERVE IN AD FIELD, OFFSHORE NIGER DELTA

Abstract

Exploration and production activities in the oil industry involve huge financial resources and in order to reduce associated economic risks, adequate uncertainty quantification and reserve evaluation are required. This study was carried out to quantify the uncertainty in the reserve estimate of hydrocarbon in the reservoirs of AD Field, offshore, Niger Delta. Three Dimensional (3D) seismic data, log suites from seven wells (AD1 to AD7), consisting of gamma ray, resistivity, neutron and bulk density logs, well deviation and checkshot data in AD Field were acquired from oil producing companies in the area. Faults and horizons were identified and picked across the 3D seismic record for structural horizon. The gamma ray log was used to delineate the reservoirs while neutron and bulk density logs were used to identify the fluid contacts. Seismic and well data were tied after which seismic reflections corresponding with the reservoirs’ surfaces were picked across the seismic volume. Static models were generated using the identified seismic structure, lithofacies and petrophysical information. Monte Carlo Simulation for stochastic was carried out to evaluate the reserve estimates. Twelve faults (Fault1 to Fault12) were identified from seismic structural interpretation. The faults were predominantly elongate listric normal growth faults trending from East to West. Fault2 and Fault3 were identified as the regional faults with roll-over anticlines responsible for hydrocarbon trapping. Six hydrocarbon-bearing sand intervals (Sand A - F) were delineated from the petrophysical analysis. The sand intervals were observed to thin-out basin wards, suggesting a prograding sequence. The facies model indicated that the reservoirs were predominantly coarse sands with shales, which were deposited in a North-South orientation, signifying a transgressive marine with minor influence of tide. The porosity of the sand intervals ranges between 0.19 and 0.32, implying good to excellent porosity. The water saturation values ranged from 0.19 to 0.39, indicative of prospective accumulation of hydrocarbon. Sand A reservoir had the largest accumulation of hydrocarbon in-place with hydrocarbon pore volume of 2,343 106 RB, Stock Tank Oil-Initially-In-Place (STOIIP) of 175 MMbbl and gas initially-in-place of 0.30 TCF. The stochastic reserve estimates of the field showed that P10, P50 and P90 for the STOIIP were 482 MMbbl, 554 MMbbl and 636 MMbbl respectively. The coefficient of variation in the reserve estimates of the reservoirs ranged from 0.09 to 0.15 indicating very low uncertainty. Sensitivity analysis based on Monte Carlo simulation showed that porosity, gross rock volume, net to gross and saturation have an increasing order effect on the reserve estimate of the sand intervals which have a low coefficient of variation ranges from 9% and 14% suggesting that the uncertainty of the values are low.