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Rokdoc crossplot cutoff
Rokdoc crossplot cutoff












rokdoc crossplot cutoff

The sand percentage here is generally less than 30%. Characteristics of the reservoirs in the Agbada Formation are controlled by depositional environment and by depth of burial. Petroleum in the Niger Delta is produced from these unconsolidated sands in the Agbada Formation. The sediments were deposited in prodelta environments. The Akata Formation consists of clays and shales with minor sand intercalations. Outcrops of these units are exposed at various localities (Figure 1). The stratigraphic sequence of the Niger Delta comprises three broad lithostratigraphic units, namely, a continental shallow massive sand sequence, the Benin Formation, (2) a coastal marine sequence of alternating sands and shales, the Agbada Formation, and (3) a basal marine shale unit, the Akata Formation (Figure 2). Several models have been proposed by many researchers for shaly-sand analysis such as Juhasz model, dual water model, Indonesian model, Waxman and Smits model, and so forth. In fact, water saturations obtained from Archie’s equation have a tendency to overestimate the water in shaly sands. Therefore, Archie’s equation would not provide accurate water saturation in shaly sands. Archie’s equation was developed for clean rocks, and it does not account for the extra conductivity caused by the clay present in shaly sands.

rokdoc crossplot cutoff

Moreover, in clean sands, the irreducible water volume is a function of the surface area of the sand grains and therefore the grain size, but for shaly sands the addition of silt and clay usually decreases effective porosity due to poorer sorting and increases the irreducible water volume with the finer grain size. Using Archie’s equation in shaly sands results in very high water saturation values and may lead to potentially hydrocarbon bearing zones being missed. This gives rise to an excess conductivity compared with rock, in which clay crystals are not present, and this space might otherwise be filled with hydrocarbon. Several factors are to be considered when delineating petrophysical attributes for shaly-sand reservoirs because clay minerals add conductivity to the formation especially at low water saturations.Ĭlay minerals attract water that is adsorbed onto the surface, as well as cations (e.g., sodium) that are themselves surrounded by hydration water. Shales can cause complications for the petrophysicist because they are generally conductive and may therefore mask the high resistance characteristic of hydrocarbons. Core data is recommended for better characterization of these reservoirs. Empirical relationships apply with variable levels of accuracy with observation of the porosity-depth, water saturation-depth, and water saturation-porosity trends. The direction of deposition of the sands was thus inferred to be east west.

rokdoc crossplot cutoff

The pay zones are relatively inhomogeneous reservoirs as revealed from the buckle’s plot except in Apete 05. The reservoirs had high volume of shale (Vcl), high hydrocarbon saturation, low water saturation, and good effective porosity ranging 12.50–46.90%, 54.00–98.39%, 1.61–46.0%, and 10.40–26.80%, respectively. Ten sand units within the Agbada formation penetrated by the wells were delineated and correlated and their continuity was observed across the studied wells. Shale volume and porosities were calculated, water saturations were determined by the dual water model, and net pay was estimated using field-specific pay criteria. Wireline logs of four (4) wells from Apete field were studied to delineate petrophysical attributes of shaly-sand reservoirs in the field. If these departures are unrecognized, water saturation can be overestimated, and this can result in loss of opportunity. Conventional departures from Archie conditions for petrophysical attribute delineation include shaliness, fresh formation waters, thin-bed reservoirs, and combinations of these cases.














Rokdoc crossplot cutoff