Significance of the palynology and organic facies of the Abu Gabra no.1 well with respect to the petroleum geology of the Abu Gabra field, Sudan

Abstract

A petroleum geological investigation has been made of the Abu Gabra Field, Sudan, using cuttings samples, core chips, wire line logs and seismic sections. The lithostratigraphic studies, palynology, petroleum source rock evaluation and seismic interpretations permit a new understanding of the stratigraphic and structural history of the Abu Gabra Field.

Rich, well preserved palynomorph assemblages have been found. More than five hundred taxa have been recognised, with more than thirty of these taxa being new forms which do not appear to have been described in the literature. From the palynoflora found it has been possible to establish twenty new biozones, each with distinct assemblage characteristics and diagnostic palynomorph species of Middle Miocene to Neocomian ages. Detailed petroleum source rock evaluation, including Total Organic Carbon (TOC), visual kerogen types, elemental kerogen analysis, gas chromatography, vitrinite reflectance and spore colour have been made. The data of Total Organic Carbon indicate many organically rich sediments with an average > 2.0% of organic carbon. A particularly rich oil source unit is identified within the Abu Gabra Formation, lying between 9,300 and 10,600 feet (2793-3183m) with an organic carbon content averaging 2%, and reaching a maximum of 4% with a high content of algal-derived organic matter. This interval is within the oil window. A marked change has been found below 10,200 feet (3,060m) in spore colour and vitrinite reflectivity due to higher heat flow, resulting from the extensional movement which formed the basin. The interpreted seismic sections indicate that the Abu Gabra Field comprised both extensional and compressional movements. The multiphase tectonic history of the Abu Gabra area includes three discrete episodes of major crustal extension: an Early Cretaceous tectonic cycle, a Late Cretaceous tectonic cycle and Palaeogene tectonic cycle. Each cycle appears to consist of a rift initiation phase, an active rifting phase and a thermal sag phase.

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