The geological and geochemical characteristics of source rocks in the Tertiary Nadu Formation of the Eastern Depression, Baise Basin, China

Dark mudstones are widely distributed in the Upper and Lower Sub-members of the Na1 Member and the Na2 Member of the Tertiary Nadu Formation in the Eastern Depression of the Baise Basin, with a cumulative thickness of 200–1000 m. There is more than 1% organic carbon in these three sets of dark mudstones in most areas of the Eastern Depression, indicating that they are good source rocks. The kerogen of these three sets of effective source rocks is mainly of Type IIA, but also with Types IIB and III. Distribution of the kerogen type was controlled by sedimentary environment. Ro values of the Na1 Member mudstones are 0.4–0.65%, showing that the organic matter is at the immature to low-mature stage, while those of the Na2 Member, which are deeper, are 0.7–0.8% or even higher, showing that the organic matter is at the early mature stage. The facies of thicker dark mudstones in the Depression suggest a semi-deep lake environment, and the areas with higher organic carbon and Ro values – the depositional centre and the subsiding centre of the basin – coincide with the hydrocarbon generation centre. The Na2 Member should be the most effective source rock in this area because of its maturity, thickness, the sedimentary environment, and the organic abundance and type. The Tiandong Sedimentary District, with its larger area, greater thickness of mudstone, wider range of semi-deep lacustrine environment, higher organic carbon amount, better organic matter type and higher maturity, has a greater potential for hydrocarbon generation and expulsion than the Tianyang Sedimentary District.