Abstract
Petroleum exploration and production of conventional reservoirs have been strongly developed over the years in Vietnam since 1987. However, with the decrease in petroleum production in recent years, a certain attention has started to be given to the unconventional reservoirs. This study presents the results of a comprehensive geochemical evaluation of three main shale formations in the northern Song Hong basin (also known as the Red River basin) of Vietnam. Over 200 cutting samples of Lower Miocene, Oligocene and Triassic shales from two onshore and two offshore wells were collected and tested for total organic carbon (TOC), hydrocarbon generation potential (S2), hydrogen index (HI), production index (PI), Tmax and vitrinite reflectance (Ro). Lower Miocene shales are characterized by HI of 94 to 591 mgHC/gTOC, TOC of 0.14 to 1.65 wt% and S2 of 0.34 to 4.62 mg HC/g rock, indicating a poor-to-fair source rock of mixed gas-prone kerogen types II and III. Their Tmax of 425 to 437 °C and Ro of 0.35 to 0.66% suggested an immaturity to early-maturity HC generation. Oligocene shales have HI of 164 to 602 mgHC/gTOC, TOC of 0.52 to 3.18 wt% and S2 of 0.63 to 5.61 mg HC/g rock, indicating a fair-to-good potential source rock with a gas- and oil-prone kerogen of mixed types II and III. Their Tmax values of 427 to 476 °C and Ro of 0.43 to 1.73% suggested an optimum thermal maturity. Triassic shales are characterized by HI < 50 mgHC/gTOC, TOC from 1.04 to 14.10 wt% and S2 from 0.03 to 1.11 mg HC/g rock, showing little remaining source or no potential for hydrocarbon generation in the late to post-mature stages with Ro of 1.79 to 2.10%. The newly obtained geochemical data and analysis results of this study are expected to improve the knowledge on the main source rocks in the northern Song Hong basin, in particular in the onshore part and to provide with the useful background information for further assessment of shale gas potential in Vietnam.
Highlights
Shale reservoirs have become more and more important and active exploration plays for many countries in the world, typically in the USA (Dejam et al 2018; Zhang et al 2018)
This study presents the results of a comprehensive geochemical evaluation of three main shale formations in the northern Song Hong basin of Vietnam
Organic matter derived from marine sediments can include plankton, marine algae, bacteria and lipidrich components of higher plants
Summary
Shale reservoirs have become more and more important and active exploration plays for many countries in the world, typically in the USA (Dejam et al 2018; Zhang et al 2018). A series of reports from EIA provided the initial assessments of world shale oil and shale gas resources. EIA (2013) reported a total of 46 countries, not including Vietnam, were technically assessed for recoverable shale gas unproved resources. The main steps of this study include a brief review of geological setting in which these shale formations are found, followed by a description of how the samples were taken from the wells, how the geochemical tests were conducted and their results interpreted. The study focused on three source rock formations in the petroleum system in the northern Song Hong basin, namely Lower Miocene, Oligocene and Triassic shales. The results from this study could be useful in future to calculate hydrocarbon potentially retained in shale for assessment of shale gas potential of a source rock in a sedimentary basin following an approach conducted by Sheikh and Giao (2017)
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