3-D seismic interpretation and reserve estimation of Ossu Field in OML 124, onshore Niger Delta Basin, Nigeria Uzonna Anyiam * , Victor Nduka and Alex Opara, Federal University of Technology Owerri Summary In the Ossu field oil block, the Agbada Formation consists of multiple vertically-stacked reservoir sequences. Multidisciplinary approaches – petrophysical, seismic interpretation and volumetric method were undertaken in this study. The objectives are to show how 3-D seismic data acquired on Ossu field can be used to ascertain if the structure and its closures are favourable for hydrocarbon accumulation; to obtain reserve estimation for determining the field’s fair market; and to illustrate the processes of iterative seismic interpretation and structural modeling in a structurally complex basin. The northern area has predominantly widely spread simple rollover structures, bounded by growth faults, triggered by the movement of deep-seated, over-pressured ductile marine shale aided by slope instability. Faulted rollover anticlines prevail in the middle zone while the southern area is characterized by collapsed- crest structures. The -7480ft sand contains non- associated oil trapped in annealment phase trap. The - 4540ft sand contains associated oil. The oil in Reservoir A is not in commercial quantity, while only about 77.6 million barrels is recoverable from the estimated 1207.5 million barrels for Reservoir I. Key exploration risk is that growth-fault-related anticlinal traps are segmented by normal faults. Introduction The structural setting of the Niger Delta in the study area is predominantly growth faults with associated rollover anticlines, which are generated by rapid sedimentation load and gravitational instability of the Agbada sediment pile accumulating on the mobile undercompacted Akata shales. The essence of the 3-D seismic method is a real data collection followed by the processing and interpretation of a closely-spaced data volume. Because a more detailed understanding of the subsurface emerges, 3-D surveys have been able to contribute significantly to the problems of field appraisal, development and production, as well as to exploration. It is in these post-discovery phases, that many of the successes of 3-D seismic surveys have been achieved. The Ossu field is located onshore some 55 km South of Onitsha in the South-western sector and straddles at deeper levels to the West concession border with Shell. The major part of the field is located in a drawn river valley affected by seasonal flooding which limits the access to the field during the rainy season (Figure 1a). The Ossu field rollover structure is situated in the distal part of the northern depositional belt of the Niger Delta. The producing sequence (4500 – 8500ft Tvdss) consists of fourteen stacked hydrocarbon reservoirs characterized by large gas caps with underlying thin oil rims (10ft – 70ft). A system of antithetic and synthetic normal faults compartmentalizes the reservoirs below 5700 ft Tvdss into several blocks of variable size. In this paper integrated seismic interpretation method, petrophysical evaluation and volumetric approach are used to delineate the structures and estimate the possible reserves contained in the field. About 22 seismic lines (average spacing of 2km) with a total length of over 350km were shot. In addition, six wells were available: A- 1, A-2, A-3, A-4, A-5 and A-6; and check shots. a) b) BASIN DIRECTION STRUCTURAL MODEL BASED ON SEISMIC AND GEOLOGIC OBSERVATION SHOWING THE EFFECTS OF THE MAJOR AND SUBSIDIARY GROWTH FAULTS ON THE AGBADA FORMATION IN THE OSSU FIELD OF OML 124. DEFORMATION IS MORE PRONOUNCED IN THE HANGING - WALL SIDES OF THE MAJOR GROWTH FAULTS. S N MAJOR FAULT SUBSIDIARY FAULT MH MH MH MH MH MH Figure 1: a) Production and prospect map showing the location of the Ossu field of OML124 oil block, and the 3- D seismic survey coverage. b) Structural model based on seismic and geologic observation, showing the effects of the major and subsidiary growth faults on the Agbada Formation in the Ossu field of OML 124. Deformation is more pronounced in the Hanging-wall sides of the major growth faults. 1327 SEG Denver 2010 Annual Meeting © 2010 SEG Downloaded 10 Jan 2011 to 41.219.168.226. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/