Sedimentology and Reservoir Characterization of the Lower Tertiary Strata in Palouge-Fal Oil Field in Northern part of the Melut Rift Basin, Sudan

Abstract

Abstract

This study investigates the depositional environment, source area, sandstone

composition, diagenetic properties, reservoir quality and palaeogeography of the Lower

Tertiary strata at the Palouge-Fal Oil Field in the North Melut Rift Basin, Sudan. In this

study, the subsurface Lower Tertiary sediments were investigated essentially by five

sedimentological techniques. These included subsurface facies analysis, which was

based on 15 cutting samples and nine conventional cores description as well as on wire

line logs and two seismic section interpretations, petrographic analyses of sandstone

that included thin sections and scanning electron microscopic investigations as well as

clay mineral analyses.

The facies description and the analysis of conventional cores from the Samma

Formations and Yabous- sand member in the Palouge-Fal oil Field revealed the presence

of eight major lithofacies types, all of them are siliciclastic sediments. They can be

interpreted as deposits of fluvial and lacustrine environments.

Moreover, based on wire line logs, cores and cutting samples descriptions and

analyses and also on seismic sections interpretation, the Lower Tertiary strata in

Palouge-Fal Field can be classified into two different facies associations; fluvial facies

association (includes braided and meandering rivers sediments) and lacustrine facies

association. These facies associations most probably testify to environmental change in

response to main tectonic pulses. Seismic analysis revealed that the maximum thickness

in the deepest part in the study area reaches about 10km

The thin section investigations of the cores and cutting samples revealed that

feldspar accounts for 13.5 to 39.5% that of the quartz and the lithic fragments are

ranging between 64.1 to 85.5% and 0.0 to 6.7% respectively. Consequently, the

sandstones of the study area are classified as arkoses (feldspathic arenite) and sub

arkoses (sub feldspathic arenite). Moreover, the model analysis of the sandstones

revealed, that they stem generally from a continental provenance, transitional between

the stable interior of a craton and basement uplift, which is a basement area of relatively

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high relief along rifts. This allows the detrital components to be recycled and transported

for rather long distances and to be deposited in extensional and pull-apart basins. The

reservoir quality of the Samma Formations and Yabous- sand memberis better than that

of the Yabous member. The porosity of Samma Formation and Yabous- sand

memberranges between 16.7 to 30.0 % and 18.6 to 25.3%, respectively, whereas the

porosity of the Yabous memberranges between 16.3 to 23.75%. Moreover, thin section

investigations and the scanning electron microscope (SEM) analysis for the sandstones

of the Samma and Yabous formations have revealed that, their reservoir quality has

been affected positively and negatively by several diagenetic processes. The processes

which decreased the reservoir quality include: Clay Precipitation, compaction, quartz

overgrowths and carbonate cementation. On the other hand the processes which

increased the reservoir quality include; dissolution of feldspars and micas and partial

dissolution of carbonate cements and clays. . Furthermore, the reservoir quality of the

study intervals was not only affected by the above mentioned diagenetic processes, but

also in a large-scale by the type of depositional environment.

The clay mineral analysis revealed, that the studied strata are essentially

composed of kaolinite, illite, and illite/smectite mixed layer. The investigation of clay

minerals viewed that kaolinite has the greater amount in most of the reservoir zones, that

suggests most probably the intensity of chemical weathering and leaching processes

were under warm humid climate. The distribution of the clay minerals and the

concentrations of the chemical elements throughout the study area were controlled by

several processes and factors, which include: composition of the source rock, the

environmental conditions at the site of weathering and deposition, the nature of

transport, the physical and chemical conditions at the site of deposition, tectonic events

and diagenetic processes.