Petrophysical and three-dimensional seismic data analysis of late Tertiary deltaic succession: The Agbada Formation, Sigma Field, offshore Niger Delta, Nigeria

A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology, University of Regina. xviii, 126 p. The Agbada Formation is the prolific portion and the prospective interval of the Niger Delta Basin, which is an extensional rift basin, located in the Gulf of Guinea, West Africa. The formation is a regressive offlap succession that formed under shallow-marine conditions, and contains the petroleum system that occurs in the Niger Delta Basin. The Niger Delta Basin occupies the southern end of the Benue Trough resulting from the failed rift which is related to the opening of the Atlantic Ocean since Late Jurassic Period. The evolution and sedimentation of the Niger Delta Basin are influenced by the extensional tectonic events since Cretaceous Period. The Cenozoic strata of the basin consist of very thick (>12km) succession dominated by deltaic depositional system that span from onshore delta plain (Benin Formation) through shallow marine delta front (Agbada Formation) to offshore prodelta (Akata Formation) accumulations. The strata are arranged in 11 biostratigraphically defined megasequences (MS-1 to MS-11), that straddle the world-wide second-order supercycles TA2 to TB3, of which MS-1 and MS-2 are Eocene, MS-3 to MS-5 are Oligocene, MS-6 to MS-10 are Miocene and MS-11 is Pliocene- Recent. This study addresses the subsurface distribution of the various facies of the upper Agbada Formation (MS-10 and MS-11, TB3) by analyzing its sedimentologic properties and geophysical characteristics based on wireline logs, core data and 3-dimensional seismic data. Wireline logs such as the gamma ray logs allowed delineation and correlation of the formation tops, distinguishing between shaly and reservoir sandstone intervals, and showing their lateral continuity across the study area, while the neutron-density and resistivity logs were used to delineate the hydrocarbon content of the reservoir sandstones as gas reservoirs. The well-toseismic tie aided the interpretation of the sandstone tops (horizons) on the seismic profiles. Six major lithofacies namely quartz sandstone (Lf-A), laminated mudrock (Lf-B), heterolithic facies PETROPHYSICAL AND 3D SEISMIC DATA ANALYSIS, TERTIARY NIGER DELTA N. J. OKORO (Lf-C), silty sandstone (Lf-D), carbonaceous mudrock (Lf-E) and gravelly sandstone lithofacies (Lf-F) were identified. Seismic stratigraphic analysis along with the nature of the stacking pattern identified with the aid of the gamma ray log define retrogradational and progradational cycles that delineate transgressive (TST), highstand (HST) and lowstand (LST) systems tracts. The sandstones make up the reservoirs, while the shales serve as seal/cap rocks for the reservoirs. Four reservoir intervals (A, B, C and D) were selected and mapped for this study. Seismic attributes like the root mean square (RMS) amplitude were used to identify bright spots which indicate possible gas accumulations within the study area. Structural features, such as extensional faults and rollover anticlines constitute the major hydrocarbon traps in the area. Depth structure maps aided the identification and the volumetric evaluation of the recoverable gas reserve in the study area. The volumetric recoverable gas reserve estimates for reservoirs A, B, C and D are 313 billion, 194 billion, 88.5 billion and 49.7 billion standard cubic feet of gas (SCF) respectively. Improved Stratigraphic Modified Lorenz (ISML) plot was used to delineate the flow unit distribution, identifying five hydraulic flow units (HFUs) ranging from barrier to semi-conductor to conductor zones within the cored interval of one of the wells used for the study (well 13). HFU1is within the barrier zone, HFU2 and HFU4 are within the semi-conductor zone, while HFU3 and HFU5 are within the conductor zone based on the slope angles of the identified hydraulic flow units (HFUs). Additionally, five hydraulic flow zones which correspond to the ones obtained from the ISML plot were delineated using the Winland’s R35 pore throat radius plots for sandstone reservoirs, thereby validating the ISML plot of flow unit distribution across the study area. The knowledge gained from this research is deemed to aid optimum reservoir characterization of other hydrocarbon fields and serve as a guideline for subsequent studies.