Niger Delta Reservoirs

This study examines the Tertiary clastic sequence from the Niger Delta Basin containing sediments generally described as an upward and updip (south-to-north) transition from marine deep water and pro-delta sediments (Akata Formation), through alternating sand/shale paralic deposits (Agbada Formation), to continental deposits (Benin Formation) [Ejedawe, et al., 1984]. Production thus far has been principally from the Agbada Formation, which is at least 3,000 m thick. It is an alternating sequence of sands, silts and clays deposited principally in delta-front, tidal/estuarine, distributary channel, and delta-plain environments. Reservoir quality has been shown to be related to depositional environment (in association with lithology and geometry) and burial depth. The underlying and interbedded Akata Formation, which is reportedly up to 7,000 m thick, includes clays, silts and sands deposited in deeper water environments by slump/slide/debris flow, bottom current, turbidity current, and pelagic/hemipelagic processes, also locally exhibits reservoir potential (Shanmugam et al., 1995). The overlying and interfingering Benin Formation is composed of fluviatile sands and conglomerates, with local coaly deposits; and is about 2,000 m thick (Weber and Daukoru, 1975). The Benin Formation locally contains (heavy) oil and gas stringers, where the oil-bearing sands are associated with coal (Avbovbo and Ogbe, 1978).

The sedimentological study, “Phase I: Sedimentology”, was completed by Core Laboratories in 1996. “Phase II: Petrophysics”, is the second phase of the sedimentological study that characterizes the reservoir properties of the Tertiary Niger Delta reservoirs. The reservoir quality of each lithofacies will be determined by analysis of petrophysical properties. Results of the study together with the Phase I report results will be integrated to characterize hydraulic flow units and enhance reservoir modeling program. Phase II was completed in June 1999.

This reservoir study includes regional coverage of all of the lithofacies occurring in the Tertiary reservoirs of the Niger Delta region. More than 6,300 feet of conventional core from wells listed in Table 1, from both onshore and offshore areas have been described in detail. Each lithofacies has been sampled for petrographic, mineralogical, and grain size analysis. Lithofacies sequences were identified as an aid in interpreting depositional environment. In addition, a photographic atlas has been produced that exhibits the lithofacies present throughout the region, with their characteristics.

The primary study objectives of Phase I are as follows:

• Description (using a newly created, industry-approved standardized stratigraphic nomenclature) and characterization of all lithofacies present in the Tertiary reservoirs
• Relationships made between facies and depositional environments, diagenesis, and regional variations
• Creation of a photographic atlas of lithofacies with core, thin section, and SEM photographs; x-ray diffraction and sieve analysis data; and wireline logs over cored intervals

Other objectives include the identification of certain formation damage problems, such as fines migration, clay swelling, water sensitivity, and sanding that are related to the studied lithofacies. Sequence stratigraphy will be applied to a limited degree to the studied cores to fit them into the regional stratigraphic framework.

The primary study objectives of Phase II are to:

• Quantify petrophysical characteristics and reservoir quality for each facies and depositional environment both regionally and vertically
• Select and characterize hydraulic flow units using porosity, permeability and capillary pressure data (pore throat attributes)
• Compare horizontal to vertical permeabilities and obtain Kv/Kh relationships for lithofacies units
• Determine degree of compaction with burial and effects on reservoir quality
• Determine the Archie a, m and n values and CECs for each of the main lithofacies
• Illustrate reservoir characteristics with plug and thin section photographs

A secondary objective of the study is to compare porosity-permeability measurements made during this study with those measured immediately following coring to examine the possibility of core-shed diagenesis.

PRESENTATION OF RESULTS:

-Phase I

• Results of the study is presented in seven volumes. Volumes one (1) through four (4) will include the analytical database, with a rock catalog page for each sample, organized by well. XRD data will be available on CD. Volume five (5) and six (6) will include the detailed core descriptions, in log format, showing lithofacies designations, depositional environment interpretation and wireline logs. This information will all be available on CD. Volume six (6) will contain the interpretation with integration of all analytical results. It will include a photographic atlas of lithofacies, summary of observations, and recommendations pertaining to exploration objectives and strategies for enhanced production.

-Phase II

• Results of the study will be presented in rock catalog data volumes that include the petrophysical data as well as thin section and plug photographs. All of the data will also be supplied that correlates the petrophysical results with the lithofacies from the Phase I study. The hydraulic flow unit results will be summarized and illustrated in this volume as well.

Both Phase I and II are now available in HTML formats.