Biosignals from the EA Field, Nigeria

Abstract

Abstract The EA Field study was undertaken to determine reservoir connectivity across growth faults. In this high subsidence area, conventional biostratigraphy does not have sufficient resolution to provide a production-scale time frame. Consequently, a new tool had to be developed which would provide such accuracy. This new high-resolution correlation tool, Biosignals, is based on the relationship ‘pollen-parent plant-ecology’. Pollen, thus related, have been grouped into ‘vegetation’ zones, which have been logged per well and summarized on saw blade diagrams. Sequences of vegetation zones could be recognized, showing an upward trend from spore dominated, via swamp, rain forest, savanna to montane. A number of potential regional time lines could be recognized: (1) a climate curve; (2) a change from the freshwater alga Pediastrum to the brackish water alga Botryococcus ; and (3) the top of frequent fungi. Based on this study the following conclusions may be drawn: (i) vegetational stacking patterns reflect fourth-order sequence stratigraphy; (ii) Biosignals suggest the tidal channels in well EA-13 to be lowstand deposits, although many of them belong to truncated sequences (only lowstand systems tract (LST) and transgressive systems tract (TST) preserved); (iii) on a third-order scale, the sandy middle part of the sequence (EA-1, 6000–6500 feet) correlates with dry climatic conditions and to low eustatic sea level. The two regionally extensive maximum flooding surfaces correlate with humid climatic intervals and eustatic high sea level; and (iv) the new high-resolution correlation tool provides a detailed biostratigraphical subdivision, within the existing conventional biozonation, and thus enables correlation of reservoir bodies across growth faults.