PALYNOFACIES, LITHOFACIES AND SEQUENCE STRATIGRAPHY OF THREE H-WELLS, OFFSHORE WESTERN NIGER DELTA BASIN, NIGERIA

Abstract

The study investigated the palynofacies, lithofacies and sequence stratigraphy of three wells located within shallow offshore of the Western Niger Delta Basin in Nigeria. The data utilised in the study were obtained from three wells named HA-001, HB-001 and HD-001. The study aimed at improving present correlatable biostratigraphic zones in the shallow offshore of the Western Niger Delta Basin. Seventy-six, ninety-six and seventy-seven ditch cutting samples within the depth intervals of 750-11610 ft, 4100- 13160 ft and 4875-12090 ft in HA-001, HB-001 and HD-001 respectively were analysed. The standard acid palynological method of sample administration for palynofacies, integration of wireline log motifs and distribution of index accessories for sedimentological attributes, paleoenvironmental and sequence stratigraphic studies, palynofacies colours for thermal maturity were employed. The study established two new assemblage biozones namely Cyperaceaepollis sp. – Nympheaepollis clarus, and Stereisporites sp. within the sedimentary intervals penetrated in the wells. The Cyperaceaepollis sp. – Nympheaepollis clarus biozones were further subdivided into Nympheaepollis clarus – Echitriletes pliocenicus and Cyperaceaepollis sp. – Elaeis guineensis subzones. The Late Miocene age is associated with the biozones. Colours of palynomacerals recovered indicated that thermal organic maturity ranges from optimum petroleum generation to early gas generation. The lithologic, textural and wireline log data indicated that the intervals studied in the HA-001 well belong to the Benin Formation and Agbada Formation, while HB-001 and HD-001 belong entirely to Agbada Formation. Index minerals and accessories are dominated by ferruginous materials, glauconite pellets, carbonaceous detritus, shell fragments and pyrites plus minor mica flakes. Lower delta plain, delta front and prodelta environment of deposition were inferred for the studied well intervals under changing dry and wet paleoclimate. The palyno-ecological groupings consisted of freshwater taxa 41.17%, mangrove taxa 37.64%, savanah taxa 16.38%, rainforest taxa 4.77% and montane taxa 0.07%; freshwater taxa 46.06%, mangrove taxa 27.38%, savanah taxa 21.26%, rainforest taxa 3.29% and montane taxa 1.07%; freshwater taxa 58.80%, mangrove 17.45%, savanah taxa 16.77%, rainforest taxa 4.77% and montane taxa 0.56% in the three studied wells respectively. Sand bodies which represent sub-environments within these settings are deposited in sequences. The systems tracts recognised are Lowstand Systems Tracts (LST), Transgressive Systems Tracts (TST) and Highstand Systems Tracts (HST) with stratigraphic surfaces such as Sequence Boundary (SB) and Maximum Flooding Surface (MFS). Four MFS (9.5 Ma), (7.4 Ma), (6.0 Ma) and (5.0 Ma) were identified within the studied intervals by matching with existing global palynostraticgraphic zonation scheme and global eustatic curve. The Lowstand Systems Tracts (LST) and the Highest Systems Tracts (HST) are potential hydrocarbon reservoirs. A good correlation existed in the wells due to similarities in the lithofacies, stratigraphic surfaces and palynostratigraphic zones. Two new biostratigraphic biozones were firmly established and they belong to Highstand Systems Tracts (HST) which are potential hydrocarbon reservoirs. These biozones are assigned Late Miocene age.