Sedimentology and Depositional Environment of D2 Sand in Part of Greater Ughelli Depobelt, Onshore Niger Delta, Nigeria

Abstract

The sedimentology and depositional environment of D2 sand in part of the Greater Ughelli depobelt have been studied using well logs and core data. Three wells were correlated to establish the lateral continuity of the D2 sand across the field and standard gamma ray log trends was used to infer depositional environments. Cored section of well B was described to identify lithology and delineate depositional environments based on facies types and sedimentary structures. Petrophysical characteristics of the reservoir of interest was generated using Archie Equation for water saturation, Timur formula for permeability computation and porosity values was determined quantitatively from density log. The results of the analysis showed that the D2 sand cut across the field. The reservoir displayed a funnel shaped coarsening upward gamma ray motif typical of a deltaic and progradational depositional profile. Four facies associations indicating four subenvironments within the deltaic front have been identified from the cored interval (3444.5 to 3458.5 m) of the D2 reservoir. Each facies unit was identified based on lithology and sedimentary structures of the core sample, textural characteristics and gamma ray log trend. The four lithofacies associations: A, B, C and D include muddy heterolithic sandstone, trough cross stratified sandstone, sandy/silty mudstone and crossbedded sandstones. These facies correspond to Lower Shoreface, Upper Shoreface, Tidal Flat and Channel sand respectively. Petrophysical analysis revealed the trend of reservoir quality within the facies. Porosity range from 0.79 to 10.32%, whereas permeability from 0.25 to 8.8 mD. Water saturation is high (0.8) within the poor quality facies and 0.3 in the highest porosity and permeability interval. Good porosity and permeability occurred within the channel and upper shoreface facies, whereas the tidal flat and lower shoreface have poor porosity and permeability values. The Shoreface facies has the best reservoir properties (10.32% porosity and 8.8 mD permeability) due to lack of shale intercalations and good sorting resulting from the sediments being properly reworked by wave action. The Channel facies (D) deposited by high energy current also has good reservoir properties, especially towards its base. The Tidal Flat facies has the lowest reservoir quality due to high proportion of shale/Clay that creates permeability barriers and occurred between the Upper shoreface facies and Channel sand facies. Sedimentology and depositional environments of facies have significant control on the quality of sand bodies.