Analysis of shaly sand reservoir rocks in the eastern Niger Delta Basin using geophysical well logs

Abstract

Shales in the reservoir causes complications for the petrophysicist because they generally are conductive and mask the high resistance characteristic of hydrocarbons. Data from a suite of well logs were used to estimate the effect of reservoir shaliness on petrophysical parameters of some reservoir rocks of the eastern Niger Delta Basin. The log section was digitized using Neuralog software. Delineation of the productive clean and dirty formations, as well as mapping of the fluid contents of the possible reservoir zones was carried out using Interactive Petrophysics software. Fifteen shaly sand bodies were identified. It was observed that, shale correction leads to a significant change in petrophysical parameters. The results obtained indicate that, the Simandoux and Indonesian models used for the study are both suitable for water saturation, and hydrocarbon saturation analysis in shaly sands of this part of the Basin. The porosity results for the Indonesian and Simandoux models gave, respectively 0.14-0.23 and 0.22-0.28, while the hydrocarbon saturation results are 0.650-0.908 and 0.650-0.911 with permeabilities values of 1487.442-8881.697 mD and 1568.532-7451.592 mD for uncorrected and corrected permeability, respectively. Thomas-Stieber model shows that shale distribution in sands of the eastern Niger Delta Basin is mainly structural with few of disperse and laminar ones.

Petrophysical Characterization of Shaly Sand Reservoirs in Well UK-05 Eastern Niger Delta Basin, Nigeria

“…According to Rider (1986) porosity and permeability classification, the porosities of the reservoirs are very good for reservoir P and excellent for reservoirs C, E, G and K, their permeability values is good for sand (reservoir P) and very good for sand (reservoir C, E, G and K) which are good enough to permit free flow of hydrocarbons. These results are similar to 3.42 to 29.09% of shale volume, 22.25 to 28.32% of effective porosity, 10 to 42% of water saturation and 58 to 90% of hydrocarbon saturation obtained by (Fozao et al 2019).…”

“…Results of hydrocarbon saturation (Sh), shows a decreasing but relatively constant trend with increasing shale volume (Fozao et al 2019). However, the values of hydrocarbon saturation increase simultaneously from Archie, Indonesian and then to Simandoux model at a given value of shale volume (Figure 6B).…”

“…From the log interpretation and calculations made, the general trend of porosity (effective porosity), the corrected effective porosity shows an increase with increase in shale volumes. The high values of Ф e, signify that the presence of shale in sandstone reservoirs, overestimates the porosity, that is, it causes the logging tool to read higher porosities than are the porosities available for the reservoir (Fozao et al 2019). This shows that the more the shale volume, the higher the uncertainty of actual porosity of the reservoir, which will affect the reservoir productivity potential.…”

“…Because of the low permeability of shales, their presence in the reservoir reduces the connectivity between pores. Thus an increase in shale volume reduces permeability in a reservoir (Fozao et al 2019).…”

A New Method To Estimate Resistivity Distribution Of Shaly Sand Reservoirs Using New Seismic Attributes

“…Nevertheless, resistivity property modelling has an additional challenge for a shaly sand reservoir due to the low resistivity contrast to its overburden lithology. The presence of clay minerals in shaly sand retains more water thus suppress the measurement of electrical resistivity of a shaly sand reservoir (Fazao et al, 2019). Archie's Law mathematically defines the resistivity of clean sand to its water saturation and porosity (Archie, 1942).…”