The purpose of this paper is to provide an overview of petroleum occurrence and provenance for the 1000 km West African Atlantic Margin from Cabinda to mid-Angola. Over this margin the Lower Congo Coastal and Kwanza provinces cumulatively account for reserves of c. 6 gigabarrels oil recoverable (GBOR). These are dominantly reservoired in Pinda carbonate traps of the former basin. However, with production from a range of aggradational wedge, carbonate platform and pre-salt reservoirs, a diversity in oil character presupposes complex hydrocarbon habitats charged by multiple sourcing. Each of these two major Atlantic margin salt basins constitutes a different, source rock driven, hydrocarbon habitat. As classic passive margin pull-apart basins, Early Cretaceous initiated rift events (Pre-rift, Syn-rift I, II, etc.) evolved into the drift phase opening of the southern Atlantic. A striking feature of this progression was widespread evaporite deposition of the Aptian Loeme salt. This separates two distinct sedimentary and tectonic domains of the Pre- and Post-Salt. The core Lower Congo Coastal habitat is dominated by the Pre-Salt Bucomazi Formation sourced ‘poly’ petroleum system. These lacustrine, often super-rich, sediments reveal considerable organofacies variations between their basin fill (Syn-rift I) and sheet drape (Syn-rift II) development, accounting for the compositional diversity in their progenic petroleums. Of crucial impact is a cognate diversity in their kerogen kinetic behaviour. This controls the conditions and timing of generation and realization of charge potential. With the Lower Congo Coastal habitat extending southwards towards the Ambriz Spur, the Bucomazi facies proper appears restricted to the northern and deeper proto-lake trend. Over the more weakly subsident margins such troughs host inferior sheet drape potential. Elsewhere, Upper Cretaceous-Palaeogene marine clastic Iabe Formation sourced petroleum systems are hydrocarbon productive, yielding petroleums of unique, and/or mixed Pre-Salt hybrid, source provenance. A less attractive Pre-Salt source system is reflected in the Kwanza habitat. Consequential effects of salt movement have proved important for halokinetic induced structuration, Cretaceous block rafting and pseudograben (Tertiary trough) source deposition. Marginal reserves of Infra-Cuvo and Cuvo derived petroleums are known. With limitations in charge potential at this level, the modest Kwanza reserves (0.1 GBOR) have a Post-Salt provenance deriving primarily form the Eocene Margas Negras and subordinate Aptian Middle Binga sourced petroleum systems. Cases of multi-sourced hybrid petroleums abound. In addition to the organic-rich Albian micrites, Upper Cretaceous black shales provide unproven possible offshore potential.
With cumulative reserves exceeding 23 gigabarrels oil recoverable (GBOR), the East Sirte Basin is a prolific oil province hosting supergiants such as the Amal, Augila-Nafoora and Sarir fields. Production from Precambrian-Oligocene reservoirs yields low sulphur and often highly waxy oils.The Late Mesozoic-Cenozoic Agedabia and older Hameimat, Maragh and Sarir troughs provide the main structural features of the habitat and control hydrocarbon prospectivity. Paleogene subsidence has facilitated the generative process with Mesozoic basin-fill sediments hosting source rocks for productive petroleum system(s). Traditionally the marine Upper Cretaceous Sirte Shale Formation source was thought to provide the dominant charge. Application of geochemical inversion procedures to oil data, however, indicates a greater diversity in oil provenance. Delineation of eight end-member generic oil families indicates a number of complex contributory petroleum systems, mixed-system hybrid oils also being evident. Non-marine (lacustrine) source inputs are also in evidence, enhanced waxiness differentiating petroleums of such provenance. Systematic screening of the stratigraphic section has additionally identified source potential in Nubian (Triassic and Lower Cretaceous), Rachmat-Tagrifet (Upper Cretaceous), Harash (Paleocene) and Eocene formations.Assignment of oil provenance has been achieved via multivariate oil data analysis and application of a carbon isotope-based source kerogen-oil correlation procedure. End-member petroleum systems have been definitively identified involving the Sirte Shale Formation, Rachmat-Tagrifet Formations and Nubian (Triassic) as the contributory sources. The remaining major systems rely upon Pre-Upper Cretaceous lacustrine sediments specific to the Hameimat and Sarir troughs. Whereas numerous archetypal Sirte Shale Formation oils were recognized (e.g. Messla, Hamid, Sarir-L etc.), reserves for many of the giant fields, including Amal, Augila-Nafoora and Sarir-C, rely on hybrid system charging.These results confirm that the prospectivity of the Sirte Basin is not exclusively dependent upon the Sirte Shale Formation, with other petroleum systems in operation, often involving hybrid-sourcing.
Laccaic acid contains a t least four components. Methods have been devised for separating these and for obtaining pure the major component, laccaic acid A,. This compound, of probable molecular coinposition C,,H,,NO,,, is a quinone-carboxylic acid containing four acetylatable hydroxyl groups (three arranged as in purpurin), one C-Me group, and a nonbasic nitrogen atom which can be estimated by the van Slyke method. The chromophore of laccaic acid A, is probably that of an anthraquinone modified by further conjugation.T H E insect Lnccifer Zacca Kerr, found on the twigs of certain trees native to India and South-East Asia, produces a resin known as stick 1ac.l Extraction of stick lac with dilute sodium carbonate solution removes pigments and leaves behind shellac. Treatment of the extract with chalk or alum precipitates lac-dye which, until the advent of synthetic dyes, was of considerable commercial importance. isolated material named laccaic acid. The isolation was effected by leaching the powdered dye with dilute acid. The crude pigment from the acid extract was purified by decomposing its lead salt with hydrogen sulphide and recrystallisation from water. Schmidt made preliminary studies of the chemistry of laccaic acid, but a more detailed investigation was carried out by Dimroth and Gold~chmidt,~ who isolated laccaic acid by extracting powdered stick-lac with water. Concentration of the extract and acidification with hydrochloric acid precipitated crude laccaic acid as a red solid which was recrystallised from formic acid. Chemical studies, summarised by Thorn~on,~ led Dimroth and Goldschmidt to a partial formulation of laccaic acid as a polysubstituted anthraquinone. This was elaborated by Mayer5 to the expression (I). Tschirch and Ludy6 obtained support for the presence of the anthraquinone nucleus from zinc dust distillation studies. Evidence for the presence of nitrogen in laccaic acid was obtained by Kamath and Potnis by analysis of the crude calcium salt of laccaic acid, but the H O 1 ~* ~C O M e nature of their preparation could not generate confidence HO \ Et in the result. Recently Venkataraman and his co-workers OH ,(TI found laccaic to be a mixture of pigments containing l-2yo of nitrogen present as -NH,.8 A second pigment isolated by Tschirch and Ludy6 from stick lac, and called erythrolaccin was recently formulated as 1,3,5,6-tetrahydro~y-8-methylanthraquinone.~ It is the purpose of this Paper to report work carried out on laccaic acid, isolated from stick lac essentially by the method of Dimroth and Gold~chmidt,~ in which the major component of laccaic acid has been obtained pure.Laccaic acid represented about ly0 by weight of stick lac, as was also found by DimrothFrom lac-dye Schmidt
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