Reconstruction of the Depositional Setting of Tortonian Sediments in the Yowi Field, Shallow Offshore Niger Delta, Using Wireline Logs

摘要

Well logs and ditch-cuttings information was used to study the subsurface of the Yowi oilfields, shallow offshore, in the eastern Niger Delta sedimentary basin. The essence of the study was to determine the gross depositional environments of selected and-bodies. Electrofacies trends from gamma ray log and mineral composition of ditch-cutting samples were used to reconstruct the depositional environments in the study area. A total of eleven (8) relatively thick sandstones reservoirs penetrated by five representative wells (A9, A12, A6, A4 and A2) occurred within the paralic Agbada sequence in the field. Seven log facies associations deduced from log trends revealed three major environments resulting from fluvial, marine and marginal marine processes. A coarsening upward log motif is interpreted to be deposits resulting from progradation, whereas fining upward trends are transgressive deposits. The presence of glauconite in all the wells in the field from about 1800ft down hole within the Agbada Formation shows that the environments were influenced by marine processes than continental. A predominantly marine, deltaic sequence strongly influenced by clastic output from the delta is inferred for the Yowi Field. Water depths fluctuated considerably and deposition occurred within a variety of littoral and neritic environments ranging from near shore barrier sand complexes to fully marine, outer shelf mudstones. An overall shallowing trend is observed on well log towards the base of the Qua Iboe Shale Member. The upper part of the well between the intervals of 2000 and 2450ft appears to have been deposited in a marginal marine setting. Below this depth down to about 5000ft may indicate relatively shallow marine conditions with normal salinities, well oxygenated bottom waters and periodic access to open marine conditions. Interval above 1800ft is the overlying continental Benin Formation. The associated environments based on log shapes and the presence of glauconite within the studied interval include; offshore/regressive bars, barrier foot, beach and shore face deposits.