RESERVOIR EVALUATION: VANDJI FORMATION OF BERRIASIAN-VALANGINIANAGE IN LOWER CONGO BASIN.
Event name: Asia Petroleum Geoscience Conference and Exhibition 2019
Session: Oral Session 11: Modeling Sedimentary System
Publication date: 29 October 2019
Info: Extended abstract, PDF ( 699.57Kb )
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Lower Congo and South Gabon basin have seen many hydrocarbon successes since 1960’s. Recently, forgotten play in the lower rift section was revived with huge discovery in Lower Congo basin. This paper aims to provide a comprehensive reservoir analysis on one of the lower rift plays, Vandji Formation of Berriasian-Valanginian age in Lower Congo and South Gabon basin. Vandji Formation is deposited in the earliest stage of rift phase primarily in the basin between eastern and Atlantic Hinge as thick high energy alluvial and fluvial clastic sediments in lacustrine setting. This succession consists of conglomerate, fine to coarse grained sandstone. An analog of alluvial plain from Africa was used to better represent Vandji Gross Depositional Environment. Alluvial plain is generally 10-20km wide from basement highs and continents. The geometry of basement highs and shape of braided channel uses bouguer anomaly map as reference and consistent with wells that penetrated through braided channel facies. Rich well datasets of SEM, core data, mudlogs, electrical logs, reports and bouguer anomaly map were used to constrain the boundaries of gross depositional environment map for Vandji formations. Two models were established to capture the uncertainty between control points to investigate reservoir presence. Reservoir effectiveness was evaluated using porosity and permeability data to identify any correlation to the depositional environment. Vandji reservoir is perceived as poorly sorted and micropores are filled with interstitial clay such as illite filling the pore throat. Porosity depth trend for Vandji Formation shows high variability. Two trends were observed between alluvial plain and deltaic facies. This study also found that facies deposited in braided channel should belong to a different compaction curve. The wide range of porosity is also largely affected by diagenesis such as cementation, quartz overgrowth and clay filling. K-Phi trend shows that good permeability of more than 150mD seems to be attributed by intragranular dissolution pores associated with feldspars and deposited in high energy and constant movement creating void spaces. The porosity map is established underpinning isopach map between seabed-Top Vandji and most likely trend from porosity depth plot. Vandji reservoir quality is shown to have better porosity around basement highs with lower angle. Higher angle basement highs are more likely to erode and be deposited as alluvial plain.