Sequence Stratigraphic Approach To Exploration In Offshore Nigeria

Abstract

ABSTRACT The Dahomey Basin is characterized by stable progradation and a well-developed shelf-slope break. Systems tracts are similar to those along other stable progradational margins. In contrast, systems tracts in the Niger Delta Complex are similar to those in Neogene strata of the offshore Gulf oj Mexico. Different exploration strategies are necessary for the systems tracts in each area. INTRODUCTION Direct hydrocarbon indicators in seismic data along the coast of Nigeria (Fig. 1) suggest that thick, sand-rich strata occur in large undrilled structures in waters beyond the shelf margin (Fig. 2). In order to understand these reservoir sandstones, Cretaceous through Neogene strata in offshore Nigeria were divided into depositional sequences using seismic, wire line and biostratigraphic data. Each sequence was then divided into. systems tracts. Seismic facies were interpreted and mapped within each systems tract. Depositional sequences in the offshore Niger Delta resemble those in the Gulf of Mexico (Fig. 3). Strata in the Niger Delta Complex (Fig. 1) were deposited along an unstable progradational continental margin characterized by very high sedimentation rate, large growth faults, toe thrusts, large-scale slumping, and shale diapirs. The shelf-slope break is very poorly defined. In contrast, strata in the Dahomcy Basin (Fig. 1) were deposited along a stable progradational margin (Fig. 4). This margin is characterized by a more moderate sedimentation rate, a well-defined shelf-break, and limited growth-faulting. Systems tracts arc similar to those described by Vail,1 Van Wagoner and others,2 and Posamentier and others3 for stable progradational margins. Each of these provinces requires a different exploration strategy. The purpose of this paper is to discuss distribution of reservoir facies and seal facies within a sequence stratigraphic framework in each of these provinces to better develop those strategies. SEQUENCE STRATIGRAPHY Different criteria were used to differentiate systems tracts in the Niger Delta complex and the Dahomey Basin. Characteristics of systems tracts and associated facies in each province arc described below: Basin Floor Fan Basin floor fans arc similar in the Dahomey Basin and tic Niger Delta complex (Fig. 5). The upper boundary of these units is characterized by a well-defined continuous reflection that down laps in both directions against the sequence boundary. In the Niger Delta complex, basin floor fans commonly abut against the footwalls of growth faults in intraslope basins. fn the Dahomey Basin they arc best developed immediately down dip of submarine canyons. Similar basin floor fans in the Gulf of Mexico are characterized by thick, blockysands4,5,6 Mass-transport complexes7 arc also developed within this system tract in the Dahomcy Basin. They arc mounded features, Located immediately down dip of Large canyons, and Iic directly on the sequence boundary. Mass transport complexes commonly exhibit chaotic reflections and small listric normal faults. They arc probably comprised Largely of mud.7 Basin floor fan systems tracts developed during relative relative sea level fall1,3,8. As sea-level fell, sandy detritus prograded farther and farther toward the shelf margin.