TECTONIC INFLUENCE ON RESERVOIR QUALITY ASSESSMENT OF DEEPWATER SANDSTONES FROM THE OFFSHORE NIGER DELTA, NIGERIA

Abstract

The research inferred the influence of tectonic setting on reservoir quality assessment of deepwater sandstones
(Q and R-reservoir sands) from the offshore Niger Delta with a view to delineating their tectonic setting,
provenance and climatic belt using results of thin-section photomicrographs, core-based porosity and
permeability data from three wells cored at various stratigraphic intervals. The study involved delineation of the
spatial distribution of framework mineralogy, their petrophysical properties and types of diagenetic alterations.
Point count modal analysis was employed, and involved counting up to 300 minerals per thin section slide. The
mineral percentages totalling 100% from each of the stained thin-section photomicrographs was used to infer
the sandstone composition and their provenance. The framework mineralogy was generally dominated by
monocrystalline quartz, small amount of feldspars and insignificant amount of rock fragments. An infiltrated
mudstone clasts were quite common. Traces of siderite overgrowth, quartz overgrowth, muscovite laths, zircon,
leached quartz, and heavy minerals were also common. Potassium feldspar dominated over plagioclase
feldspar. Some of the quartz grains were leached. The grains were poorly to moderately sorted with significant
effective porosities. The thin-section photomicrographs showed good interconnected pores and loose to point-
grain contacts. The reservoir sands have high porosity and permeability probably due to the unconsolidated
nature of the sediments and dissolution framework grains. The average modal compositions of the quartz,
feldspar and lithic fragment corresponded to 86.20%, 11.47% and 2.34% in Q-reservoir sand from Zebra-4
well; 88.14%, 9.68% and 2.18% in R-reservoir sand from Zebra-3well; and 97.47%, 2.03% and 0.50% in Q-
reservoir sand from Zebra-2ST3 well respectively.The predominance of monocrystalline quartz over
polycrystalline quartz in the thin-section were used to validate the derivation from a plutonic igneous rocks.
Thus, the bulk of the Zebra reservoirs are inferred to be derived from a similar parent rock type (plutonic
igneous rocks) with similar climatic conditions (hot and humid climate) and unique composition (quartz arenite
with subordinate arkosic arenite) with the same tectonic setting (craton interior). The open pores and loose
grain contacts were evidences that the studied reservoirs have undergone minor to insignificant mechanical
compaction while leached quartz, quartz overgrowth and siderite patches indicate intergranular pressure
dissolution.