SEISMIC SEQUENCE ANALYSIS AND RESERVOIR CHARACTERIZATION OF ‘GEORGE FIELD’ OFFSHORE NIGER DELTA, NIGERIA

Abstract

Seismic sequence analysis and reservoir characterization of ‘George Field’ offshore Niger Delta were carried out with a view to determining the hydrocarbon potential of the area. The data set used for the study includes three dimensional seismic data and a suite of geophysical well logs from wells A, B, C and D. Lithologies were identified from the gamma ray log, resistivity and the gamma ray logs were used for delineation and correlation of reservoirs across the wells. Petrophysical parameters were determined for five mapped reservoirs across the wells. Well to seismic tie was carried out to identify the reservoir tops and bases on the seismic sections. The mapped horizons were used to generate the time structure maps for the reservoirs. The checkshot data was used to convert time to depth from which the depth structure maps were generated. Two surface attributes and one volume attribute were used to identify the prospects on the depth structure maps. Five reservoirs AB_1, AB_2, AB_3, AB_4 and AB_5 were delineated from the well logs. The petrophysical properties from the five reservoirs show that: the thickness values ranging from 19-75m, volume of shale (Vsh) 0.0041-0.97, porosity (φ) 21- 39%, permeability (k) 50-250md, water saturation (Sw) 6.9- 85% and hydrocarbon saturation (Sh) 15- 93% . Seven faults labeled as: F1, F2, F3, F4, F5, F6 and F7 were mapped. The well-to-seismic ties of the hydrocarbon bearing reservoirs were obtained from the synthetic seismogram and checks shot data and are displayed on the seismic lines they intersected. The depth structure maps generated showed an anticlinal structure at center of the surfaces and mapped faults with three of the wells located on the anticlinal structure. The trapping mechanism is a fault assisted anticlinal closure. The reserve estimates from the hydrocarbon pore volume (HCPV) show that the reservoirs AB_1 and AB_2 have the highest hydrocarbon accumulation estimates of about 398.52 mmbls and 351.38 mmbls respectively, while reservoirs AB_5 has 120.37 mmbls, AB_4 has 71.39 mmbls and AB_3 has 29.96 mmbls. It was observed that the anticlinal structure located at the center of the AB_3, AB_4 and AB_5 surfaces has one of the minor faults (F7) terminates on it which could have caused leakage and thereby reducing the integrity of the major hydrocarbon harbouring structure in the area. Four seismic facies types: I, II, III, and IV were identified and mapped. Seismic facies, coupled with the identification of key basinwide unconformities, were used to separate the strata into four megasequences: SQ_1, SQ_2, SQ_3 and SQ_4. The seismic facies maps of the sequences were generated. Generally, the facies distribution for the sequences revealed that sequence SEQ_1 has low-moderate amplitude continuous reflection facies (Type III facies) and high amplitude continuous and sub parallel reflection facies (Type II facies) are the predominant facies type. In sequence SEQ_2 it could be seen to compose primarily of lowmoderate amplitude continuous reflection facies (Type III facies). The sequence SEQ_3 was observed to consist of low amplitude continuous reflection facies (Type III facies), suggesting low energy turbidities the high amplitude continuous and sub parallel reflection facies (Type II facies) and low amplitude discontinuous sub parallel reflection facies (Type IV facies) suggesting possible mass movement deposit. And sequence SEQ_4 composed mainly of high amplitude continuous and parallel reflection facies (Type I facies). The seismic sequence stratigraphic analysis revealed four depositional sequences (DS_1, DS_2, DS_3 and DS_4) and the accompanying systems tracts were interpreted and mapped in the study area. Depositional systems in the study area comprise lowstand systems tracts (LSTs), transgressive systems tracts (TSTs) and highstand systems tracts (HSTs). The direction of sediments supply within the study area is from the west to east as interpreted by the variations in the seismic facies. The depositional environments within the study area could be interpreted as been controlled by these factors, the amount and type of sediment input, fluctuations in relative sea level, tectonic activity and regional growth faulting. The environment of sediment deposition in this area is delta plain, shelf, slope to toe of slope.