Facies Description and Sedimentology of FABI Field, Coastal Swamp Depobelt, Niger Delta, Nigeria

Abstract

Sedimentary facies description and sedimentology of parts of the Coastal Swamp Depobelt have been studied using well logs and core data. Five wells designated FABI 001, 002, 003, 004, and 005 were correlated to establish the lateral continuity of the H7000 sand across the field, and using  gamma ray log to corroborate the gross depositional environments of the cored interval. About 51 ft of cored section of well 001 was described to identify lithology and delineate depositional environments based on facies types and sedimentary structures. The results of the analysis showed that the H7000 sand occurred between 12750 and 13500 ft (in well 001) across the five wells at different stratigraphic positions. The sand unit (cored interval) displayed a funnel shape log trend occurring in two prograding parasequences. The upper parasequence is well serrated whereas the lower unit below 13060 ft has little or no serration. Five facies types identified based on sedimentary structures, lithology, and paleontologic content include; cross-bedded sandstone, bioturbated cross-bedded sandstone, bioturbated interbedded sandstone and mudstone, bioturbated heterolithic sandstone/siltstone, and silty mudstone facies. Four environments inferred based on facies description include; tidal channel, upper shoreface, lower shoreface, and proximal shelf mudstone. Each facies unit was identified based on lithology and sedimentary structures of the core sample, textural characteristics and gamma ray log trend. The Tidal channel facies displays a fining upward log trend and overlies the upper shoreface facies. Upper shoreface facies is characterized by trough cross bedding, planar cross bedding, alternating parallel cross bedding, and double mud drape. The sandstone is light grey to light brown in colour, fine to medium grained, well sorted, rare hummocky laminations and free of shale successions; whereas lower shoreface is characterized by interbedded sandstone/mudstone facies. The offshore mudstone interval is composed of siltstone and very fine-grained sandstone showing parallel laminations, wavy and current ripple cross-laminations. This examination provided the framework for suggesting that the FABI field sediments were deposited within the shoreface shallow marine systems where both tide and wave activity influence the deposition of sediments.