Reservoir quality characterisation of the upper sarir sandstone formation, Se Sirte Basin, Libya

Abstract

The Upper Sarir Sandstone Formation (USSF) is the main hydrocarbon reservoir in the southeastern Sirte Basin of Libya, which requires new techniques to enhance its recovery. An integrated study on high resolution core analysis and wireline logs that provides good understanding on sandstone reservoir quality has yet to be done in this field. The study is carried out to assess the reservoir quality and diagenetic imprints using integrated sedimentological and high resolution reservoir characterization to provide better understanding on the USSF sandstone reservoir quality. Objectives of the study are to determine the potential subsurface petroleum reservoir sandstone intervals in the USSF and the sedimentological and textural parameters that influence reservoir performance, to interpret the depositional setting and provenance of the sediments based on the textural and mineralogical datasets, to determine the reservoir quality variation by determining the sedimentological, textural and petrophysical / fluid – flow parameters as well as regional correlation across the Sirte Basin, to identify possible authigenic signatures and its influence to the porosity and permeability. and to determine the diagenetic stages of the Upper Sarir Sandstone reservoir. The study has been carried out using integrated wireline logs ( lithology logs, porosity logs, and resistivity logs ) and forty core plugs across three wells: N8-97 (10), N7-97 (15), and N4-97 (15). Wireline log data were digitized and qualitatively interpreted to lithological parameters. Core samples were subjected to petrographic, X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray, and Helium porosimetry analysis. The study found that, the USSF Sandstones constitute well to moderately- sorted, subrounded to angular, and coarse to fine – grained quartz lacking significant feldspar and lithics. The reservoir intervals are segmented by regional shale barrier across three wells, whereas two additional vertical fluid – flow barrier occurs between N7-97 and N4-97 wells. The reservoirs were deposited in a warm to humid paleoclimate non-marine system. The average fluid flow parameters of N8-97( ø 11.69%, K 119.07 mD), N7-97 ( ø 10.98%, K 41.52 mD) and N4-97(ø 7.75% K 55.49 mD) suggesting poor to good reservoir quality. The reservoirs were subjected to three paragenetic sequences, dominated by the early diagenetic (eodigenesis) and intermediate diagenetic (mesodiagenesis), with minor late diagenetic ( telodiagenesis) processes. The mesodiagenetic imprint including quartz overgrowths, quartz cementation, calcite cementation, and chemical compaction ( grain dissolution, recrystallization and reprecipitation) were identified as the major porosity reducers in the USSF reservoir. The eodiagenetic imprint including mechanical compaction, sericitisation, kaolinitisation, sideritisation, haematisation, pyritisation and chloritisation act as pore filling, grain coating and pore lining.