Geochemical and petrological properties of the Neogene shale gas in the central Song Hong basin, Viet Nam

Abstract

Song Hong basin or the Red River basin is generally considered to be composed up of three units, and namely, the northern, central and southern parts, among which the central Song Hong basin has been much less explored comparing to the other units. The recent discovery of gas in this part of Song Hong basin has raised the interests of the local and overseas explorationists studying and assessing to better understanding of hydrocarbon generation potential of source rocks in this area. In this study, the Pliocene and Upper Miocene shale samples, which taken from A-1X well in the central Song Hong basin, are analyzed for the source rock characteristics and petroleum generation potential based on geochemical, well log, X-Ray Diffraction data and 1D petroleum system modeling. It was found that the total organic carbon contents are poor for the Pliocene shales and poor to fair for the Upper Miocene shales ranging from 0.15-0.21 wt.% and 0.19-0.79 wt.%, respectively, indicating poor to fair source rock generation potential for both shales. The organic matters are mainly derived from higher plants of type III kerogen and possibly mixing a minor amount of type II kerogen from algae. Vitrinite reflectance values range from 0.45-0.65% for Pliocene and 0.72-0.93% for part of Upper Miocene shales, showing a good match with bottom hole temperature of 163oC. The burial and thermal history models indicate that the rapid sedimentation rate affected the increase in temperature in the Pliocene and Upper Miocene shales as mature source rocks and reached peak of oil generation. The models show the early hydrocarbon generation (Ro=0.60 %) in the Pliocene and the main phase of hydrocarbon generation (Ro=0.80 %) in the Upper Miocene. As results of this study, Pliocene and Upper Miocene shales can be considered having a potential to generate gas from both biogenic and thermogenic origins. It was estimated that the gas generation is about 17.88 Mtons, the gas expulsion approximately 4.3 Mtons/km2 and the remaining of kerogen of about 62.81 Mtons, gas initial in place of about 1.49 (Tcf). It is thought that the deeper shales are more effective source rocks, which need to be drilled more for new gas prospects.