Assessment of Shale Gas Potential of the Triassic and Devonian Formations in the an Chau Basin (Vietnam)

An Chau Basin extends on the northeastern land part of Vietnam, with an approximate area of 10,000 km2. Devonian and Triassic core shale and outcrop samples, obtained from sixteen shallow wells and field works in this basin, were performed using geochemical and x-ray diffraction tests. This study aimed to predict shale gas potential based on the results of Rock–Eval pyrolysis, vitrinite reflectance, visual kerogen typing and mineral compositions. The shales consist of gas prone to inert terrestrial organic matter that reached the uppermost limit of the gas window (Ro > 2.33%, TAI > 3.3). The over maturity is likely to cause poor hydrocarbon generation potential of proposed type III/I kerogen, showing low to moderate TOC contents of 0.04–3.58 wt% and poor product yields S2 < 1 mg HC/g rock. Only about 40 m shale cores in the Triassic formation have average HI of about 229 mg HC/g TOC and entered dry gas window with an average vitrinite reflectivity of 2.35%, respect for shale gas potential. Most shales are dominated by brittle minerals such as quartz and carbonates, supposed to be favorable to shale brittleness. A clay-fraction is dominated by illite, chlorite and kaolinite, whereas, the smectite is almost transformed into illite–smectite and chlorite-smectite and organic matter predictably reached above 130 °C. The findings show that Triassic shales that are only present in local area show negligible shale gas potential. Therefore, the prospective areas need to be explored further for the complex geology structures of the neighborhood such as Ha Noi Trough and Song Hong Basin.