Development background of Mesozoic high-quality source rocks: Evidence from microfossils in North Carnarvon Basin, Australia

Abstract

Based on numerous microfossils in Mesozoic of North Carnarvon Basin in Australia, the Mesozoic paleoclimate and paleoenvironment were analyzed, the paleogeographic background and development model of Mesozoic high-quality source rocks were investigated and discussed. Variation of microfossil species and their relative contents in Mesozoic different intervals indicates changes of paleoclimate and paleoenvironment. The paleoclimate shows a cycle variation of arid–humid–arid–humid, the regression occurred in the Early Triassic–Late Triassic and the transgression occurred in the Late Triassic–Late Cretaceous. Four sedimentary facieses including delta facies, littoral facies, restricted sea facies and open sea facies were developed in the Mesozoic. The open sea was mainly developed in the Early Triassic, the delta was distributed in the Middle–Late Triassic, the restricted sea was especially well developed in the Jurassic, and the restricted sea and open sea were mainly distributed in the Cretaceous. Characteristics of microfossil assemblage in these four sedimentary facies are dramatically different. From the delta facies, littoral facies, restricted sea facies to open sea facies, content of pollen gradually decreases, but content of the dinoflagellate + acritarch gradually increases. The delta facies and littoral facies are dominated by the pollen. In the restricted sea facies, content of the pollen is equivalent to that of the dinoflagellate + acritarch. The open sea facies is dominated by the dinoflagellate and acritarch. Supply of sediment and formation of organic matter are influenced by the paleoclimate and paleoenvironment, and type of organic matter is controlled by the microfossil assemblage. Based on the palaeogeographic background, paleoclimatic condition and microfossil assemblage, two developmental models of the Mesozoic source rocks such as the development model of terrestrial organic matter under the background of large delta and the development model of mixed organic matter under the background of the restricted sea, were proposed.