SEQUENCE STRATIGRAPHIC ANALYSIS OF “AB” FIELD IN THE NIGER DELTA, NIGERIA USING WELL LOGS AND SEISMIC DATA

Abstract

A study was carried out in the ‘AB’ field of the Niger Delta, Nigeria using well logs and seismic data so as to understand the local stratigraphy, identify and describe the sedimentation patterns in the field in order to identify the depositional environments likely to generate hydrocarbon accumulation in the study area. Sequence stratigraphic framework for ‘AB’ field was constructed by combining well log data and seismic volume from three wells in the field. Interpretation of the gamma, resistivity and density logs was done using the petrel software, in which sequences were identified on well logs from its trace and sequence boundaries. System tracts were used to foretell the reservoir location and quality in order to infer the depositional environment. Key bounding surfaces and depositional sequences were delineated from the seismic profiles through reflection termination patterns. Results were presented in form of well logs, seismic sections and tables for the 3 wells -A, B and C. Three sequences were delineated with each of the sequences showing similarities in their log signatures to the others. Log patterns range from blocky to serrate, indicating intervals of abrupt decrease in gamma ray log response and intervals with increasing response. Log patterns conform to different environment of deposition in different system tracts, ranging from a basin floor fan in the lowstand to shoreface sands and fluvial incisions in the highstand system tracts. Reflection packages observed from the seismic profiles showed different depositional geometries. Reflection patterns showed variable amplitudes from high to low and high to low continuity. The prograding wegde complex is seen as high amplitude parallel reflections, high continuity of reflection patterns indicated uniform deposition of sediments while poor continuity showed non- uniform deposition. The alternation of highstand sands and transgressive shale was inferred to provide the desired combination of seal, source rock and reservoir necessary for hydrocarbon accumulation.