A quick comparison of Pliocene and Upper Miocene shale resources in northern, central and southern parts of Song Hong basin with reference to their gas potential

Abstract

  This research is a follow-up of a more comprehensive PhD study on assessment of shale gas resources in the northern Song Hong basin that was conducted at the Asian Institute of Technology (AIT). The Song Hong basin, a typical pull-apart Cenozoic basin, had experienced a post-extensional stage accompanied by seafloor spreading from Upper Oligocene to Lower Miocene with its stratigraphy characterised by a fault-controlled syn-rift continental sequence followed by a post-rift marine sequence. In recent years, a number of gas fields have been discovered in the Song Hong basin with the Oligocene-Eocene and the Lower-Middle Miocene shales as the major and minor source rocks, respectively. On the other hand, the Pliocene and Upper Miocene shales, present in the stratigraphy from the north to the south of the Song Hong basin, have generally been considered as the seals, but not the source rocks in some previous studies. In July 2020, an exploration well (Ken Bau-2X) was drilled in Block 114 by ENI, reaching a total depth of 3,658 m and encountering a pay of about 110 m in several intervals of Upper Miocene sandstones interbedded with shales, confirming a considerable gas accumulations discovered in Vietnam so far. The interesting thing is that this well only encounter the Pliocene and Upper Miocene shales, the Oligocene-Eocene or Middle-Lower Miocene sediments underlying is absent or very thin. Therefore, potential source rock of these shales should be considered in the area, in particular with reference to petroleum system of the central Song Hong basin.

In this study, a preliminary comparison of the Pliocene and Upper Miocene shale resources in the northern, central and southern blocks in the Song Hong basin was conducted based on the analysis results of XRD, Rock-Eval pyrolysis, vitrinite reflectance, respectively. While the Pliocene and Upper Miocene shales in many areas of Song Hong basin, show a very low or no hydrocarbon generation potential, the very deep and thick Pliocene and Upper Miocene shales in the center and adjacent areas, deposited in a marine environment under the special conditions of abnormal pressure and high geothermal gradient, can be potential source rocks that have possibly generated and released a large amount of hydrocarbons. Further geochemical analyses and petroleum system modelling of the Pliocene and Upper Miocene shales in particular and for the whole central Song Hong basin are recommended.