Background:
Groundwater is the principal source of freshwater for many communities in the Niger Delta, where intensive petroleum exploration and production have raised increasing concerns regarding groundwater contamination. Although numerous hydrogeochemical investigations have been conducted within the region, most have focused on onshore environments, leaving offshore groundwater systems largely unexplored. Moreover, the combined application of complementary hydrogeochemical, statistical, index-based, and spatial analytical approaches remains limited, restricting comprehensive evaluation of groundwater quality and contamination patterns.

Objectives:
This study aimed to comprehensively assess groundwater quality and hydrogeochemical characteristics in an offshore petroleum-producing field of the Niger Delta and to identify the dominant natural hydrogeochemical processes and petroleum-related anthropogenic influences governing groundwater chemistry and contamination patterns.

Methods:
Groundwater samples were collected from ten monitoring wells during the dry season of 2024. Physicochemical properties, major ions, heavy metals, and total hydrocarbon content were determined following APHA standard methods with comprehensive QA/QC procedures. Hydrogeochemical facies were evaluated using Piper, Durov, and Stiff diagrams. Pearson correlation, Principal Component Analysis (PCA), and Hierarchical Cluster Analysis (HCA) were applied to identify relationships among hydrochemical variables and classify groundwater samples. Groundwater quality was assessed using the Metal Index (MI) and Water Quality Index (WQI), while Geographic Information System (GIS)-based inverse distance weighting (IDW) interpolation was employed to evaluate the spatial distribution of key groundwater quality indicators.

Results:
Groundwater was consistently characterized by a magnesium-sulphate (Mg–SO₄) hydrochemical facies across all sampling locations. Electrical conductivity (852–1,842 μS/cm), total dissolved solids (1,082–2,338 mg/L), magnesium (98–210 mg/L), sulphate (210–620 mg/L), and iron (2.84–12.45 mg/L) exceeded recommended guideline values in most samples. Principal Component Analysis extracted three components explaining 83.1% of the total variance, representing groundwater mineralisation, salinity-related processes, and sulphate-phosphate enrichment. Hierarchical Cluster Analysis classified the ten wells into four hydrochemically distinct groups with contamination severity increasing near petroleum infrastructure. Metal Index values ranged from 0.94 to 4.82 (Classes II–V), while Water Quality Index values ranged from 105 to 284, classifying all groundwater samples as unsuitable for drinking without treatment. Geospatial analysis identified contamination hotspots within approximately 500 m of petroleum facilities.

Conclusion:
The findings suggest that groundwater quality in Offshore Field X is governed by the combined influence of natural hydrogeochemical processes and petroleum-related activities. The integrated framework, incorporating hydrogeochemical facies analysis, multivariate statistics, water quality indices, and geospatial mapping, provides complementary lines of evidence for identifying contamination patterns and the processes influencing groundwater chemistry, thereby supporting more informed groundwater assessment and environmental management in petroleum-producing sedimentary environments.