Prediction of Reservoir Quality Continuity and Gross Depositional Environment Within The H-Sand Reservoirs in Parts of the Xin Field, Niger Delta Basin

Abstract

The search for more hydrocarbons in some old fields in the onshore and shelf parts of the Niger Delta Basin is gradually becoming a major avenue for increasing the nation’s reserves. Some asset teams now have new mandates to re-evaluate key hydrocarbon reservoirs in such fields for effective optimization and production. The major reason for the re-evaluation is that some of the reservoirs between the Late Eocene and Early Oligocene (34.0 Ma and 31.3 Ma) were wrongly tagged and mapped due to little understanding of the reservoir heterogeneity and distribution and these have caused several exploration failures including loss of man hours. To remedy this situation, integrated geological and geophysical techniques were applied to map ‘H-sands’ presence, continuity, quality, and their Gross Depositional Environment (GDE) across drilled and undrilled sections of the Late Eocene to Early Oligocene sequences in the chosen field. The specific objectives of the study were to: (i) establish a detailed sequence stratigraphic and seismic facies framework for the identification and correlation of important geological time surfaces across Northeast – Southwest (dip) direction for easy understanding of H-sand distributions and geological conditions that gave rise to their deposition (ii) use a rock physics diagnostic template (RTP) approach and multi-regression relationships from elastic properties to predict the effect of depositional and diagenetic trends on ‘H-sands’ distribution (iii) apply seismic simultaneous inversion techniques in mapping accurately the spatial and lateral distribution of the ‘H–sand’ reservoirs across the field and (iv) establish Gross Depositional Environment (GDE) and sand distribution models, using integrated approaches to reduce further exploration risks. This study adopted sequence stratigraphic, seismic facies, elastic attributes crossplot and seismic inversion techniques. Well logs, biostratigraphic/biofacies data, and 3-D Pre-stack time migration seismic data were quality checked (QC) before using them to build models for quantitative and qualitative delineation, correlation and prediction of temporal distributions of the ‘H-sand’ successions between 34.4 Ma and 31.3 Ma in the XIN Field. The results of the sequence stratigraphic correlation revealed the occurrence of three (3) regional markers as Maximum Flooding Surface (31.3_MFS, 33.0_MFS and 34.4_MFS) and two (2) Sequence Boundaries (32.4_SB and 33.3_SB) across the field. Seismic facies analysis across the objective interval revealed that the geological source of the H-sands were sourced from feeder and amalgamated channel systems. These channel systems and their associated deposits show that porosity values range from excellent to good quality though the intervals in OGU-002 well was not duly tagged. Analysis of the rock physics crossplot in velocity – porosity domains confirmed that the H1000 sands of the AS_N001 well possess common increase in grains that are non-cemented with good grain-to-grain contacts and sorting which showed that the H1000 sands are connected in both time and space with the sands of the OGU-002 well. Generally, these sediments are observed to be depositionally and diagenetically controlled. The results show that sediments with depositional control possess porosity values above 22%, while sediments with diagenetic control possess lower porosity values. On the other hand, log and seismic inversion results confirmed that Lithology Impedance (LI) attribute had similar or better discriminative strength when compared with the Gamma Ray attributes. An analysis of Lithology Impedance results from log and simultaneous inversion showed that values greater than -100 ft/s*g/cc represent shale facies while values less than -500 ft/s*g/cc represent sand facies. The heterolithic lithofacies comprising of sandy-shale and shaly-sand units were found to possess Lithology Impedance values between -100 ft/s*g/cc to -500 ft/s*g/cc. The result of the crossplot analysis between Lithology Impedance and porosity confirmed that the reservoir quality of the ‘H-sand’ increases with decreasing Lithology Impedance and increasing porosity. The gross depositional environment (GDE) of the ‘H-sand’ between 34.4Ma and 31.3Ma showed shoreface (inner shelf) through proximal to intermediate offshore (outer shelf) environments. Additionally, this study has demonstrated that techniques used in this study serve to minimize uncertainties during new search for bypassed hydrocarbons in already producing fields and can be used to more accurately determine the location of new wells during infill drilling for optimum production from the field.