Sequence Stratigraphic Framework and Depositional Architecture of Yowi Field, Shallow Offshore, Niger Delta, Nigeria

Abstract

Sequence stratigraphic analysis carried out in Yowi Field by integrating both well logs and biostratigraphic data revealed six depositional sequences within the Agbada Formation of about 7000 ft thick. Three systems tracts identified include highstand systems tract, transgressive systems tract and lowstand systems tracts. The highstand systems tract is majorly stacked regressive 4th - order shoreface sands that occurred in five parasequence sets, whereas the transgressive systems tract is a transgressive unit with both source rock and sealing potentials. The lowstand systems tract shows a boxcar log motif and is identified to be probably channel or barrier bar sand. Major shale units (maximum flooding surfaces) with regional significance mapped are designated MFS1 to MFS7, and tied to intervals with significant biostratigraphic records. This division is relevant to identifying genetic depositional units that contain one or more paleontologically distinct transgressive shale horizons. Six genetic sequences occurred in the field. The transgressive shales represent interruptions in the overall regressive sequence that is related to sea-level rise. Seven of these shales have been mapped and five of them correspond to five of the eleven genetic megasequences that occur delta wide. The ages of the sediments penetrated in the wells were inferred to range from Late middle Miocene to Pliocene using paleontological data. A predominantly marine and deltaic sequences strongly influenced by clastic output from the continent is inferred from both well log and paleontological data analysis. Paleowater depth is interpreted to fluctuate considerably and deposition occurred within a variety of littoral and neritic environments ranging from nearshore barrier sand complexes to fully marine outer shelf mudstones. The sediments are rapidly deposited within the shallow marine realm and reworked into longitudinal bars by wave action, strong longshore drift and tidal effects. The dominant depositional trend observed in the study area shows progradation.