Geochemical Characterization of Rocks, Fluids and Solid Bitumens from Kwanza Basin, Angola.

Abstract

The Kwanza Basin of Angola is a South Atlantic Basin whose evolution was controlled by the Early Cretaceous rifting and later transition to a passive margin basin. This margin has been interpreted by several authors as lying over a Hyperextended Margin. In such setting thinning of the lithosphere is not linear from undeformed to oceanic domain like in the pure shear model (Mc Kenzie, 1978) but present “hyperextended” portions where crust or mantle lithosphere has been preferentially removed (Huismans & Beaumont, 2011). The objective of this study was to carry out the geochemical characterization of rocks, fluids and solid bitumens from Locosso-1ST1 and Catchimanha-1 wells. The geochemical evaluation of rock samples included Total Organic Carbon (TOC), Pyrolysis Rock-Eval, Visual Kerogen Analysis, Vitrinite Reflectance (Ro) and Thermal Alteration Index (TAI). Gas samples were analyzed by Gas Chromatography and Isotopic Analysis, and one oil sample was analyzed by Gas Chromatography – Mass Spectrometry (GC-MS) and Carbon Isotopes. Finally, the solid bitumens were analyzed using Pyrolysis-Gas Chromatography-Mass Spectrometry (Py-GC-MS) and Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS). Post-salt and pre-salt rocks contain mainly marine organic matter and very low quantities of terrestrial organic matter. Post-salt and pre-salt rocks in Locosso-1ST1 Well are immature to early mature, while in Catchimanha-1 Well, post-salt rocks are immature and pre-salt rocks are post mature (dry gas window). Locosso-1ST1 and Catchimanha-1 gases are mainly wet and thermogenic in origin. The isotope gas composition indicates that Locosso gases are likely to be late mature oil/condensate associated gas, and Catchimanha gases probably represent a mix of condensate associated gas and dry gas (Figure 1). It is possible that high quantities of thermogenic Catchimanha gases were generated from secondary cracking of liquid hydrocarbons. One oil sample from Locosso-1ST1 Well shows classical molecular characteristics of a lacustrine oil (Figure 2). Minor marine inputs are possible. This oil was generated by a highly mature clay-rich source rock. Calculated Rc% of 1,20 possibly reflects the maximum thermal maturity for the Locosso oil. The geochemical characterization of the pyroproducts from solid bitumens indicates different sources for Locosso and Catchimanha solid bitumens. The differences in biomarker and isotopic signatures for the various pyroproducts suggest that the solid bitumens are a mixture of lacustrine (Bucomazi Formation) and marine sourced oils. The pre-salt section in the Locosso-1ST1 Well is immature to early mature. Under this maturity conditions the formation of solid bitumens is not related to thermal cracking of oils. Locosso solid bitumen was probably generated by gas deasphaltening or oil mixing. On the other hand, the Catchimanha solid bitumen probably was generated by thermal cracking of oils. The maturity in the pre-salt section in Carchimanha-1 Well reached values up to 1,8-2,0Rc (equivalent to ~190-200ºC).

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018