Abstract
Analysis of intermediate-scale reservoir heterogeneity in a well exposed outcrop analogue of a point bar deposit within the Enagi Formation was carried out in order to reveal its reservoir dimensions. The study area is located in northern part of Bida basin, Nigeria. Three lithofacies were identified: (1) conglomerate facies interpreted as channel lag deposits; (2) epsilon and trough cross-bedded, fine to medium grained, sandstone facies interpreted as products of lateral accretion and 3D migration of subaqueous dunes in a mixed-load channel system; and (3) mudstone facies recording overbank deposits. Outcrop measurements reveal reservoir dimensions as follows: 1) bankfull channel depth of 3.9 m; 2) bankfull channel width of 31.5 m; 3) reservoir width (meander amplitude) of 132 m; and 4) reservoir length (meander wavelength) of 911 m. Analysis of internal lithologic variability within the point bar deposit allowed the detection of three main internal reservoir heterogeneities. They include: a) vertical reservoir heterogeneity resulting from overall fining-up trend within the point bar deposits; b) small scale vertical heterogeneity owing to fining-up grain size within each lateral accretion unit; c) lateral reservoir compartmentalization due to mud-drapes observed on some of the lateral accretion surfaces. Information provided in this work has implications for improved computation of Stock Tank Oil in Place (STOIP) at the initial stage of exploration and in planning of well spacing during development and production stages. This work, also suggests that field measurements on outcrop analogues combined with empirical computation of reservoir dimensions provides valuable information useful at all stages of exploration and development of an oil field.
Highlights
A variety of scales of heterogeneity have been observed to control reservoir performance of fluvial sandstones [e.g. 1, 2, 3, 4, 5, and 6]
The fluvial sandstones of Bida and Enagi Formations are expected to serve as the main potential reservoir units within the Bida basin
Six levels of reservoir heterogeneity were recognized for the meandering river deposits of Mississippi river [21], which is often used as a classical modern analogue
Summary
A variety of scales of heterogeneity have been observed to control reservoir performance of fluvial sandstones [e.g. 1, 2, 3, 4, 5, and 6]. Difficulties associated with characterizing fluvial reservoirs have been highlighted [e.g. 7, 1]. Well exposed outcrop data provide one of the best information that may, perhaps aid proper characterization of fluvial reservoirs [8] for effective modeling purposes. This work aims at characterizing intermediate-scale heterogeneity of a well exposed point bar deposit within the Enagi Formation. This outcrop is situated on a road-cut located near Kawo village in Mashegu local government area of Niger state (Fig. 1)
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