Contrastive Study On Lacustrine Source Rock Characteristics Of Rift Strata In Mid-South Section Basins At South Atlantic

The four basins in the middle and south section of South Atlantic,Santos,Campos,Lower Congo and Kwanza,are today's hot oil and gas exploration areas with low levels of exploration and unclear distribution and geochemical characteristics of lacustrine source rock in rift strata.In this paper,sedimentology,organic petrology,elemental geochemistry and other subjects as the theoretical guidance,the use of seismic phase logging phase interpretation,elemental analysis and rock pyrolysis analysis and other technical methods,interpolation method and analytic hierarchy process and other mathematical methods to study the set of source rocks.The study on the rift basins on the central and southern parts of the South Atlantic shows:the facies belt types include three categories of the Lakeside belt,the shallow lake belt and the semi deep to deep lake belt;the Z value of the paleo-water salinity is around125-140;higher content of representative elements in terrigenous components,and in the west side,Fe/Mn is between 5 and 6.5,Sr/Cu is between 10.46 and 100.5,on the east side,Si/Al is between 2.8 and 5,and Al/K is between 3.5 and 6;organic matter types are I-II₁type,and there are differences in abundance and source rock maturity plus plane profile distribution characteristics;the potential hydrocarbon generation ability of source rocks was analyzed by using AHP that the weights of Santos Basin,Campos Basin,Kwanza Basin and Lower Congo Basin are 0.24,0.15,0.18 and 0.43 respectively.Finally,the following conclusions are drawn:During the rift period,the macro-sedimentary and palaeogeographic characteristics of both sides of the south-central South Atlantic are very similar.They are dominated by various sediments under the condition of arid saline water,and carbonate rocks in the west side are more developed;The similarity of palaeogeographic environment may lead to a high degree of similarity in organic matter types,although the abundance may vary with the salinity of water body,they are of medium or above quality.The thickness of source rocks varies with the sedimentary subsidence rate of each basin in rift period and the sedimentary time of clastic rocks on both sides.The horizontal distribution of source rocks is obviously controlled by the basement structure of the basin.According to the ranking results of potential hydrocarbon generation by AHP,the exploration potential of Lower Congo Basin and Santos Basin are relatively high,while those of Kwanza Basin and Campos Basin are relatively low.