Abdulquadri O . Alabere

Exploration onshore Niger Delta is moving into deeper HP/HT plays where pore pressure prediction is challenging due to secondary overpressure mechanisms and lack of data for calibration. Accurate pore pressure prediction is required to ensure safe and economic drilling campaign. This study aims at finding acceptable parameters and building models for predicting pore pressure where secondary overpressure mechanisms are observed in addition to undercompaction. Well logs and 3D seismic data were used to understand the evolution of pore pressure in the field. This was established from offset well data by making cross-plots of acoustic velocity versus density, temperature vs depth, geothermal gradient computation, computation of normal compaction trends, overburden gradients, shale pore pressure and fracture gradients. Then cube models were built for the field. Post-drilling pore pressure and fracture gradient estimation of eleven offset wells were done using Eaton's and Matthews & Kelly's methods respectively. Velocity analysis results from 3D seismic data were then used to predict 1D pore pressure models at six selected prospect locations. Analyses reveal depths shallower than 3800m TVD/MSL with geothermal gradient 3.0°C/100m and pressure gradient less than 1.50sg EMW are affected mainly by undercompaction and depths greater than 3800m TVD/MSL with geothermal gradient of 4.1°C/10m and pressure gradients reaching 1.82sg – 2.12sg EMW are affected by unloading. A general regression in pore pressure between 2000 – 3600m TVD/MSL interval were due to depletion by ongoing production from the developed reservoirs. The results of pore pressure modeling reveal a narrow drilling margin for the HP/HT prospect intervals.

This research investigates the palynomorph and palynofacies assemblages within the Maastrichtian Mamu Formation, Anambra Basin, shedding light on the vegetation and ecological conditions during this period. The diverse taxa identified, including Arecipites sp., Dictyophyllidites harissii, Monocolpites marginatus, Proxapertites operculatus, and Longapertites marginatus, reveal the prevalence of specific plant groups, offering valuable paleoenvironmental insights. Additionally, the presence of Amaranthaceae and Cyperaceae indicates wetland or marshy environments. Our study suggests a stable and diverse ecosystem with moderate temperatures and sufficient precipitation during the Maastrichtian in the Anambra Basin. The identification of terrestrial palynomorphs also hints at significant hydrocarbon potential within the Mamu Formation, warranting further investigation. Overall, this research contributes to our understanding of the Maastrichtian paleoenvironment and global changes during this critical phase of Earth's history. In conclusion, our study provides significant insights into the paleoenvironment and paleoclimate of the Maastrichtian in the Mamu Formation, Anambra Basin. The diverse palynomorph assemblage, along with the identification of specific palynofacies indicators, enables a robust reconstruction of the paleoflora, paleoenvironmental conditions, and hydrocarbon potential within the formation. These findings contribute to our understanding of past global changes during this critical phase of Earth's history and have implications.

Petrographic and petrophysical analyses of a Lower Cretaceous limestone reservoir, Abu Dhabi, United Arab Emirates provide new insights into the formation and diagenetic evolution of bimodal pore network. The most important diagenetic controls include: (i) widespread development of microporosity by micritization of allochems and partial cementation by grain-rimming calcite during eogenesis at or below the seafloor, (ii) partial to complete dissolution of peloids and concomitant precipitation of equant calcite cement during early mesogenesis, and (iii) stylolitization and development of tension gashes and concomitant massive cementation of the macropores by coarse-crystalline calcite and partial cementation of the micropores by calcite micro-overgrowths around micrite during late mesogenesis. Lateral compression owing to ophiolite obduction in late Cretaceous allowed flow of hot basinal brines along stylolites and tension gashes, which resulted in the precipitation of saddle dolomite and pyrite. Abundant depositional interparticle macropores were preserved in the grainstones by sparse grain rimming, dogtooth-like calcite cement that reduced the effect of mechanical compaction. Micro-and macropores are well interconnected yet the moldic, fracture pores, and intraparticle micropores within peloids are less well connected with the pore system. Hydrocarbon emplacement in the naturally oil/mixed-wet carbonate reservoir both increases the fluid pore pressure within the pore spaces of the rock thus reducing overburden stress and effective compaction; and reduces nucleation sites for cement growth thus retarding chemical diagenesis. However, the hydrocarbon emplacement occurred gradually and selectively filled the crestal limestones from the top to bottom in a way that its effect on rate of diagenesis differ vertically and spatially across the study area. Constraining the formation, evolution, and distribution of multimodal porosity network, including abundant micropores and mesopores allows better detection and evaluation of low-resistivity pay zones in carbonate successions.

Petrographic and petrophysical analyses of a Lower Cretaceous limestone reservoir, Abu Dhabi, United Arab Emirates provide new insights into the formation and diagenetic evolution of bimodal pore network. The most important diagenetic controls include: (i) widespread development of microporosity by micritization of allochems and partial cementation by grain-rimming calcite during eogenesis at or below the seafloor, (ii) partial to complete dissolution of peloids and concomitant precipitation of equant calcite cement during early mesogenesis, and (iii) stylolitization and development of tension gashes and concomitant massive cementation of the macropores by coarse-crystalline calcite and partial cementation of the micropores by calcite micro-overgrowths around micrite during late mesogenesis. Lateral compression owing to ophiolite obduction in late Cretaceous allowed flow of hot basinal brines along stylolites and tension gashes, which resulted in the precipitation of saddle dolomite and pyrite. Abundant depositional interparticle macropores were preserved in the grainstones by sparse grain rimming, dogtooth-like calcite cement that reduced the effect of mechanical compaction. Micro-and macropores are well interconnected yet the moldic, fracture pores, and intraparticle micropores within peloids are less well connected with the pore system. Hydrocarbon emplacement in the naturally oil/mixed-wet carbonate reservoir both increases the fluid pore pressure within the pore spaces of the rock thus reducing overburden stress and effective compaction; and reduces nucleation sites for cement growth thus retarding chemical diagenesis. However, the hydrocarbon emplacement occurred gradually and selectively filled the crestal limestones from the top to bottom in a way that its effect on rate of diagenesis differ vertically and spatially across the study area. Constraining the formation, evolution, and distribution of multimodal porosity network, including abundant micropores and mesopores allows better detection and evaluation of low-resistivity pay zones in carbonate successions.

Sonic logs are very essential for rock type identification, hydrocarbon typing, rock physics modelling, and reservoir characterization. However, they are seldom available due to high costs of acquisition or measurement errors. Empirical formulas and petroelastic models that are often used to predict missing sonic log data may not produce accurate velocity profiles and are limited to specific geologic settings. Using sonic logs from wells in the CONDA field located in the deep offshore Niger delta basin of the Gulf of Guinea, we demonstrate that machine learning algorithms can be used to predict sonic log data if suitable quantitative relationships exists between it and available well logs. Preprocessing such as outlier removal, missing data filling and normalization was applied to the well logs before using them as training datasets for the model prior to applying several machine learning algorithms to build a predictor model for missing DTP and DTS sonic logs. The results of the tr...

Background: The BidaBasin is the least studied of Nigeria's inland frontier basins but is believed to have oil and gas resources. No exploration wells have been drilled in the basin. However, it has recently become a subject of intense debate because of the renewed commitment by the state oil company, NNPC to commence exploration in the Basin in a bid to raise the country reserves by 40 billion barrels by 2023. This has made it imperative to increase research efforts to characterize the basin in terms of its petroleum system elements and understand the environment of deposition of its sediments. Materials and Methods: The research involves outcrop studies of sedimentary road cut sections and quarry sections of the Patti Formation, grain size analysis, petrographic analysis, foraminifera analysis and Total organic carbon (TOC). The field work exercise involved detailed section measurement, description of lithologic unit and sample collection. The equipment and the materials used for the successful mapping of the area include the Global Positioning System (GPS), clinometers compass, sample bag, masking tape, hammer and chisel, topographic map, measuring tape, hand lens, digital camera and field note. Results: The result of the grain size analysis reveals that the samples are fine grained (average values of 2.777, 2.165, 2.475), poorly sorted samples (average values of 1.661, 1.534, 1.887) and texturally mature. They are mainly platykurtic siltstones. The nature of the histogram is an indication of slow water current. Deduction from thin section petrographic analysis of the ironstone samples suggested the presence of goethite, siderite, quartz and some clay minerals. Diagenetic processes identified are compaction and cementation. Goethite is the predominating mineral of the ironstones. Structures observed in the section were massive bedding, laminations, fractures, concretions and bioturbations. Thin section petrography of the ironstone facies reveals that weathering and changes in the oxidation to reduction condition of the inter particle fluid between the pores of the ironstones during diagenesis lead to the ferruginisation of the precursor sandstones and clays in an oxidizing environment. Foraminifera results for the shale samples yielded generally low percentage of benthic foraminifera while the species observed include Bolivina spp., Bulimina spp., Miliammina spp. and Textularia spp. These benthic species dwells in an environment ranging from brackish to normal marine environment with oxic to dysoxic conditions. Total Organic Carbon (TOC) analysis of twelve shale samples was carried out which range from 1.08 wt. % to 4.07 wt. % revealed that the shale samples have a good potential for hydrocarbon. Conclusion:The Patti Formation consists of four lithofacies which are shale, siltstone, claystone and ironstone.The siltstone samples are fine grained, poorly sorted and majorly platykurtic which suggests that they were deposited in a low energy quiet water environment probably the fluvial environment. Deduction from the petrographic studies showed that the ironstones of the Patti Formation is composed of minerals such as siderite, goethite, quartz and clay matrix with the clay matrix serving as cementing minerals.Total Organic Carbon analysis of the shale samples revealed an average value of 2.51 wt. % which shows that the shales are good source rocks for hydrocarbon generation upon maturity.With the occurrence of species such as, Bolivina species, Bulimina species, Miliammina species and the Textularia species the shale samples are of shallow marine sources of anoxic conditions.