Sequence stratigraphic analysis and reservoir quality assessment of an onshore field, Central Swamp Depobelt, Niger Delta Basin, Nigeria

Abstract

Sequence stratigraphy and reservoir quality studies were carried out by integrating well logs from five wells, biofacies data, and 3-D seismic reflection data of an onshore field, Niger Delta. Nine stratigraphic bounding surfaces including five sequence boundaries (SBs) dated 13.1 through 21.8 Ma and four maximum flooding surfaces (MFSs) dated 15.9 through 20.7 Ma were delineated. Four depositional sequences were identified, with an observed upward increase in thickness. Three major stacking patterns (progradational, retrogradational, and aggradational) were delineated and interpreted as lowstand systems tract (LST), highstand systems tract (HST), and transgressive systems tract (TST) using their bounding surfaces. The main seismic facies units identified are variable amplitude facies, high-amplitude continuous facies, and chaotic facies, which were tied to corresponding well log responses. Reservoir lithofacies analysis revealed the presence of barrier bar, shoreface, tidally influenced channel, and fluvial distributary channel sands, within the three systems tracts, deposited in an overall shallow marine setting. Well correlation also showed considerable net sand reduction along depositional dip (southward) and across the strike (eastward) directions. Well log-derived petrophysical parameters such as porosity and permeability were evaluated within the context of the reservoir depositional facies. The best quality reservoirs were seen to be the TST barrier bar sands and tidally influenced channels with average porosity (ɸ) and permeability (K) values of 0.18 v/v and 542 mD and 0.24 v/v and 2215 mD respectively, while the HST shoreface and LST fluvial distributary channel sands possess average ɸ and K values of 0.16 v/v and 437 mD and 0.16 v/v and 309 mD respectively. The results establish that the major control on the differences in petrophysical properties is the primary depositional facies.