Geochemical Characterization of Offshore Western Niger Delta Source Rock

Abstract

A geochemical characterization of rock samples obtained from three offshore fields, MJ, MF, and ML, was carried out. Offshore oil and onshore oil samples were also studied for correlation. The gas chromatographic data showed that the three offshore fields studied exhibited different and unique geochemical signatures. Most rock samples from the MJ field have moderate thermal maturity; n-alkane and their isoprenoid hydrocarbon distribution indicated predominant mixed organic matter with some terrestrial organic matter. The organic matters were deposited in alternating oxic and anoxic environments. In the MF field, sedimentation of the organic matter in the shallow depths was under reducing environments, whereas that of the deeper depth horizons was under oxidizing environments. The rock samples in the field had predominant input from marine organic matter with moderate thermal maturity. In the ML field, the geochemical data indicated that oxic conditions prevailed during sedimentation of the organic matter. About half of the rock samples from this field are thermally mature, with almost equal distribution of marine and terrestrial organic facies. Marine organic matter was prevalent in the shallow depth horizons, whereas terrestrial organic matter was prevalent in deeper depth horizons. The offshore oil samples and offshore rock extracts are also correlative. This study indicates that there are two petroleum systems in the offshore western Niger Delta—marine and terrigenous petroleum systems—though the terrigeonous system is more pervasive.