Palynofacies, Lithofacies, and Sequence Stratigraphy Analysis of Three H-Wells in the Offshore Western Niger Delta Basin, Nigeria
Abstract:
This study focused on analyzing the palynofacies, lithofacies, and sequence stratigraphy of three wells located in the shallow offshore of the Western Niger Delta Basin in Nigeria. The wells, namely HA-001, HB-001, and HD-001, provided the data for the research. The objective was to enhance the existing biostratigraphic zones applicable to the shallow offshore area of the Western Niger Delta Basin. A total of 76, 96, and 77 ditch cutting samples were analyzed from depths ranging between 750-11,610 ft, 4,100-13,160 ft, and 4,875-12,090 ft in HA-001, HB-001, and HD-001, respectively. The analysis employed standard acid palynological methods to determine palynofacies, integrated wireline log motifs to study sedimentological attributes, paleoenvironmental conditions, and sequence stratigraphy. Additionally, palynofacies colors were used to assess thermal maturity. The study identified two new biozones: Cyperaceaepollis sp. – Nympheaepollis clarus and Stereisporites sp. within the sedimentary intervals of the wells. The Cyperaceaepollis sp. – Nympheaepollis clarus biozones were further divided into Nympheaepollis clarus – Echitriletes pliocenicus and Cyperaceaepollis sp. – Elaeis guineensis subzones. These biozones were associated with the Late Miocene age. The colors of recovered palynomacerals indicated a range of thermal organic maturity from optimum petroleum generation to early gas generation. The lithologic, textural, and wireline log data indicated that the studied intervals in HA-001 well belonged to the Benin Formation and Agbada Formation, while HB-001 and HD-001 were entirely within the Agbada Formation. The dominant index minerals and accessories included ferruginous materials, glauconite pellets, carbonaceous detritus, shell fragments, pyrites, and minor mica flakes. Based on the data, the depositional environments of the studied intervals were inferred to be lower delta plain, delta front, and prodelta, under varying dry and wet paleoclimate conditions. The palyno-ecological groupings showed the prevalence of freshwater taxa (41.17%), mangrove taxa (37.64%), savannah taxa (16.38%), rainforest taxa (4.77%), and montane taxa (0.07%) in HA-001; freshwater taxa (46.06%), mangrove taxa (27.38%), savannah taxa (21.26%), rainforest taxa (3.29%), and montane taxa (1.07%) in HB-001; and freshwater taxa (58.80%), mangrove taxa (17.45%), savannah taxa (16.77%), rainforest taxa (4.77%), and montane taxa (0.56%) in HD-001. The sand bodies observed represented sub-environments within these settings and were deposited in sequences. The identified system tracts included Lowstand Systems Tracts (LST), Transgressive Systems Tracts (TST), and Highstand Systems Tracts (HST), characterized by stratigraphic surfaces such as Sequence Boundary (SB) and Maximum Flooding Surface (MFS). Four MFS were identified at approximately 9.5 Ma, 7.4 Ma, 6.0 Ma, and 5.0 Ma within the studied intervals, by correlating with the existing global palynostratigraphic zonation scheme and global eustatic curve. The Lowstand Systems Tracts (LST) and the Highstand Systems Tracts (HST) were determined to be potential hydrocarbon reservoirs. Strong correlation among the wells was observed due to similarities in lithofacies, stratigraphic surfaces, and palynostratigraphic zones. Two new biozones were established within the Highstand Systems Tracts (HST), adding to the potential hydrocarbon reservoirs, and were assigned a Late Miocene age.
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